Heat Recovery from Untreated Wastewater a Case Study of Heat Recovery from Sewer Line to District Heating Network

Heat recovery from untreated wastewater A case study of heat recovery from sewer line to district heating network

Master of Science Thesis KTH School of Industrial Engineering and Management Energy Technology EGI-2017-0014 Division of Applied Thermodynamics and Refrigeration SE-100 44 STOCKHOLM

Examensarbete EGI 2017-0014 MSC Heat recovery from untreated wastewater - A case study of heat recovery from sewer line to district heating network

Ola Vestberg

Godkänt Examinator Handledare

Björn Palm Björn Palm

Uppdragsgivare Kontaktperson Käppalaförbundet & Annabella Hultman & Norrenergi AB Staffan Stymne

Abstract High amounts of heat energy are today deposited into the urban wastewater system. The current society-wide development for energy efficiency has so far barely touched upon the area of wastewater heat conservation, which is why the share of total energy consumption from water use is increasing. Through this master thesis a case study was performed, assessing in particular the heat recovery potential from untreated wastewater in the common sewer line upstream from a wastewater treatment plant (Käppalaverket) for supply onto the local district heating network (Norrenergi) by the use of a heat pump solution. The current wastewater treatment process is using temperature dependant biological treatment for denitrifying the wastewater before it is disposed to the Baltic Sea, which poses limitations on upstream heat extraction.

The purpose of the study was to assess the heat recovery potential and possibilities when using untreated wastewater compared to what is done traditionally using treated wastewater after the treatment plant. Furthermore a technological review was done over the area of heat recovery from untreated wastewater and also an evaluation of potential equipment and technology suppliers.

Hydraulic modelling and thermodynamic simulations of the wastewater system were performed. Results showed that during a majority of the year approximately 4 MW of heat could be extracted while staying within conservative limits in regards to a minimum influent temperature as well as a maximum upstream temperature decrease. During wet season however, no or very limited heat could be recovered as the influent temperatures are already in a rather sensitive range in regards to the biological treatment process. At this level, through analysis of available equipment for heat recovery from untreated wastewater, a maximum heat amount of approximately 18 GWh per year could be supplied to the district heating network. Furthermore, it was found that reducing the amount of supplementary water in the system would be highly beneficial, both regarding HR potential but also for the treatment process in the plant. Also, if extensive HR performed by water consumers would occur, the model shows that this would probably have a negative effect on downstream temperature and the treatment process.

Through this study it was concluded that even though the theoretically available heat in the system is very large, the practical heat recovery potential is very limited under current conditions. The strongest reason is the limitation posed by the temperature requirements of the influent wastewater. If also cooling is considered, the heat recovery prospects might be better due to the lower net energy extraction from the wastewater.

Regarding the economic feasibility of an installation for heat recovery from untreated wastewater, the assessment made in this project showed that it may actually be comparable to projects using other types of waste heat.

The results and conclusions from this study should not be considered as a green light, or as motivation, for performing any upstream heat recovery installations. Such projects must be done in consensus with local authorities and especially the wastewater treatment plant in question. Further analyses in this area is considered essential before exploring it further, such as assessing the transient behaviour of the surrounding rock walls when heat is recovered upstream. The model used in this study also needs confirmation through actual temperature measurements within the system, which do not exist at the moment. Furthermore, a complete life cycle analysis should be carried out for the entire urban water system, which should find an optimal way of where to use and to recover the energy.

Sammanfattning Stora mängder värme sköljs idag ner i avloppsnätet. Den pågående samhällsbreda utvecklingen för energieffektivisering har så här långt knappt rört vid området för spillvatten, varför andelen av den totala energikonsumtionen från vattenanvändning ökar. Inom detta examensarbete har en fallstudie genomförts där värmeåtervinning från orenat avloppsvatten längs avloppsnätets samlingsledning uppströms från Käppalaverkets reningsverk har utretts. Detta med hjälp av en värmepump för att förse Norrenergis fjärrvärmenät. Den nuvarande vattenreningsprocessen baseras på temperaturberoende biologisk rening för att denitrifiera spillvattnet innan det släpps ut i Östersjön, vilket sätter begränsningar på uppströms värmeuttag.

Studien syftade till att utreda potentialen och möjligheterna för att använda orenat avloppsvatten jämfört med att som traditionellt använda redan renat avloppsvatten efter reningsverket. Därtill utfördes en teknikinventering över området samt en utvärdering av potentiella teknik- och utrustningsleverantörer.

Hydraulisk modellering och termodynamiska simuleringar av avloppssystemet utfördes. Resultaten visade att under en majoritet av året så kan ungefär 4 MW värme extraheras från det orenade vattnet inom konservativa gränser i förhållande till tillåten minsta inloppstemperatur till reningsverket samt en maximal uppströms temperatursänkning. Under vintern dock så kan väldigt lite, eller till och med ingen, värme återvinnas på grund av att inloppstemperaturen till reningsverket redan befinner sig inom ett relativt kritiskt område i förhållande till den biologiska reningsprocessen. På denna nivå så kan uppskattningsvis maximalt 18 GWh per år förses till fjärrvärmenätet. Det fanns också att en reduktion av mängden tillskottsvatten skulle vara väldigt gynnsamt, både för värmeåtervinnings skull men även för själva reningsprocessen. Därtill, om utbredd uppströms värmeåtervinning hos konsumenterna skulle tillåtas, så visar modellen att detta skulle ha en negativ påverkan på nedströms reningsprocess.

Genom denna studie dras slutsatsen att trots att den teoretiskt tillgängliga värmen är stor i systemet så är de praktiska möjligheterna väldigt begränsade under nuvarande förhållanden. Den starkaste orsaken till detta är begränsningen som utgörs av temperaturkravet som reningsprocessen har. Om även kyla anses möjligt så ökar även möjligheterna för värmeåtervinning på grund av den lägre nettoeffekten från värmeåtervinning som sker.

Då den ekonomiska genomförbarheten analyserades fanns att en installation med värmeåtervinning från orenat avloppsvatten faktiskt är jämförbar med andra spillvärmeprojekt.

Det är viktigt att poängtera att resultat och slutsatser i denna studie inte bör anses som någon form av grönt ljus, eller som motivering, för att genomföra någon sorts uppströms installationer för värmeåtervinning. Projekt av sådan natur bör genomföras i samförstånd med lokala myndigheter, och i förlängningen även med avloppsreningsverket i fråga. Fortsatta analyser inom detta område betraktas som absolut nödvändiga före man exploaterar detta område vidare. Sådana studier är till exempel att analysera det transienta beteendet hos den omgivande bergväggen i avloppsnätet då värme börjar återvinnas uppströms. Modellen i denna studie behöver också vidare bekräftelse genom faktiska temperaturmätningar i avloppssystemet, vilka idag ej existerar. Vidare bör en fullständig livscykelanalys över hela det urbana vattensystemet göras, var man bör finna ett optimerat sätt att använda samt återta energin.

List of Contents 1. Introduction ...... 7 2. Technology review - Wastewater Heat Recovery (WWHR) ...... 10 2.1 The concept of WWHR – system overview ...... 10 2.2 WWHR from sewer line ...... 13 2.3 Existing installations of large scale heat recovery from untreated wastewater ...... 14 2.4 Existing installations of heat recovery from treated wastewater in Sweden ...... 17 2.5 Suppliers of external HEX for untreated WW ...... 18 3. Case study – Heat recovery from sewer line to district heating (DH) network ...... 21 3.1 System description ...... 21 3.1.1 The WW system of Käppalaverket ...... 21 3.1.2 The DH and DC networks of Norrenergi ...... 22 3.1.3 Pre-requisites for the case study...... 23 3.1.4 Case study in relation to existing installations for WWHR from sewer line...... 23 3.3 Models ...... 25 3.3.1 Hydraulic model ...... 25 3.3.2 Heat pump model ...... 26 3.3.3 Performance model ...... 27 3.4 Results ...... 28 3.4.1 Heat resource side...... 28 3.4.2 Heat recipient side ...... 36 3.4.3 Technical alternatives ...... 37 3.5 Conclusions ...... 38 3.6 Recommendations & Further work ...... 39 4. A widened perspective – Wastewater heat recovery in Sweden ...... 40 4.1 Upstream heat recovery – The perspective of a WWTP process unit ...... 40 4.2 Legal and financial implications ...... 42 4.3 Temperature relationship – other large scale WWTP in Sweden ...... 43 References ...... 44 Appendix A – Flow measurement data used for connection points ...... 48

List of Figures

Figure 1. The diversified system of heat recovery from wastewater...... 10 Figure 2. The two concepts of HR from the sewer line...... 13 Figure 3. Heat recovery (HR) system, energy facility...... 18 Figure 4. HUBER ThermWin concept...... 19 Figure 5. Flow scheme of Sharc system...... 20 Figure 6. Flow scheme of Sandvika wastewater heat recovery plant...... 20 Figure 7. Annual variation in influent volumetric flow rate and temperature to Käppalaverket WWTP, for the year 2014...... 22 Figure 8.Influent flow rate and temperature during spring 2014...... 22 Figure 9. HR potential, considering boundary for minimum influent temperature...... 28 Figure 10. HR potential, considering limit for actual temperature decrease...... 29 Figure 11. Temperature profile in the main tunnel, January-March...... 29 Figure 12. Temperature profile in the main tunnel, April-June...... 30 Figure 13. Temperature profile in the main tunnel, July-September...... 30 Figure 14. Temperature profile in the main tunnel, October-December...... 30 Figure 15.Temperatures and flow rates of main and connection tunnels, January-March average...... 31 Figure 16. Accumulated flow in main tunnel and the individual contributions, Jan-Mar average...... 32 Figure 17. Losses in the WW system, monthly basis...... 33 Figure 18. Decreased losses due to lowered tunnel temperatures...... 33 Figure 19. Temperature change relationship, Danderyd and influent...... 34 Figure 20. Influent water temperature if supplementary water was reduced...... 34 Figure 21. Influent water temperature when HR is extensively performed by consumers...... 35 Figure 22. Supplied power (left) and total heat (right) to DH net if heating only is considered. . 36 Figure 23. Supplied power (left) and total heat (right) to DH net if both heating and cooling are considered...... 36 Figure 24. Influent temperatures at WWTPs at different locations in Sweden...... 43

List of Tables

Table 1. Pros and cons of performing heat recovery (HR) at different sections of the WW system...... 12 Table 2. VEAS WWTP, general data...... 14 Table 3. SHR plant in Sandvika, general data...... 14 Table 4. SHR plant in Sköyen Vest, general data...... 15 Table 5. SHR plant False Creek energy centre, general data...... 16 Table 6. SHR facility at Borders College, general data...... 17

Nomenclature Abbreviation Definition Coefficient of Performance COP Ratio between heat and electric power. A measure of how well a heat pump or refrigeration machine works. District heating DH Concept of heat provided to consumer from large central heat production facility. District heating network DH net Piping network that transports and supplies the centrally produced heat. District cooling DC Concept of cold provided to consumer from large central cold production facility. District cooling network DC net Piping network that transports and supplies the centrally produced cold. Ground water GW Water that is naturally maintained in the ground. Heat exchanger HEX Apparatus where heat is transferred between medias. Heat pump HP Apparatus consisting of several components, used for lifting/lowering temperatures of medias. Heat recovery HR Installation where heat is being recovered in the wastewater system. Other supplement water OSuppW Collective expression, all supplementary water except from ground water. Sewage heat recovery SHR Same meaning as WWHR, but used where more suitable in regards to how it is related where the facility in question exist. Sewage water SW The water in the sewer system that actually comes from consumers. Sewer system SS The system, i.e. the tunnel network that transports sewage water from consumers to the wastewater treatment plant. Supplement water SuppW All additional water in the sewer system besides sewage water from consumers. Wastewater WW Collective expression, including water from all sources. Wastewater heat recovery WWHR Concept of heat being recovered in wastewater system. Wastewater treatment WWTP Facility where wastewater is cleaned from plant residuals and substances.

1. Introduction It is the year of 2016, the climate threat along with its awareness is greater than ever and an increasing number of climatic catastrophes are deduced to global warming and environmental pollutions. Mitigating strategies are many, in which energy efficiency has become a fashionable expression and its concept has today deeply infiltrated the mentality of politics, industry and city planners. The importance of energy conservation is today getting high priority, both at process level and system level. At system level, waste heat from industry as well as commercial and municipal facilities has long been used as an energy resource. Lately, focus has also turned to wastewater as a potential resource of waste heat, which has been only sparsely explored so far. In Sweden, it is estimated that as much as an annual 9 TWh of heat (BOFAST, n.d.) are flushed down the drain from water consumers to the joint sewage network. Compared to the estimation that industrial waste heat potential in Sweden amounts to up to 7.9 TWh (Cronholm, et al., 2009) and is being considered a substantial resource should indicate that residual heat in wastewater is a highly unused resource. To put this in perspective, the total energy supply in Sweden was 565 TWh in 2013 (Statens energimyndighet, 2015). However, the technical ability to recover the existing waste heat from wastewater is not accounted for in these numbers as the resource depends on local temperatures and flow rates with the wastewater.

Applications for heat recovery (HR) from wastewater (WW) do already exist, at different levels of the WW system. The majority of them as relatively small scale solutions in proximity to, or inside, buildings where the water was primarily used in the first place. These applications use e.g. heat exchangers (HEX) in shower floor drain, at outgoing pipes or in the joint pipes from several apartments (Jonsson, 2015). In larger scale, heat could be recovered using heat pumps from already treated water downstream of the waste water treatment plant (WWTP), which could be internally used in the WWTP or externally as e.g. district heating (DH). Recovery in these respective locations of the sewer system (SS) has their pros and cons but both provide a relatively uncomplicated situation in terms of technology and retrievable energy. Lastly, HR along the common sewer lines is today an unexplored area in Sweden, however approximately 100 projects exist outside of Sweden in varying capacity, the majority of them in Switzerland and Germany but also in China and Japan (Suda & Carmichael, 2015). As a large scale example in Sandvika, Norway, a sewage heat recovery plant (SHRP) was employed in 1987 for providing heating and cooling to the Baerum municipality after a study showing that a SHRP would be the most economical solution. Sewage is pumped from the sewage into the plant where two heat pumps and a four-pipe system are used to extract the energy. The heat pumps each have a capacity of 6.5 MW for heating and 4.5 MW for cooling. This covers 80% of the required peak load capacity and 52% of the total energy supply (Friotherm, n.d.). In a similar manner in Vancouver, Canada, a SHRP was constructed in 2010 which today provides 70% of domestic hot water and space heating demand of the region it serves, producing approximately 30’000 MWh in 2015, including supplementary natural gas firing (City of Vancouver, 2016). These two successful projects have been carried out in cities not dissimilar to e.g. Stockholm, Sweden, in terms of average temperatures and precipitation which may motivate for the investigation of conducting a similar SHR project there as well.

The available heat and potential for HR are not easily determined. The available heat is directly dependant of temperature and flow rate of the water, factors that in turn are decided by a wide range of parameters that differs within the specific SS in question, between different local SSs and also between geographical regions of the SSs. The potential for HR has been subject to numerous studies, with a seemingly common conclusion that every opportunity for HR projects is unique and must be pre-assessed under a longer time range in terms of temperature and water flow. This is expressed by e.g. (Suda, 2016) and (Knippenberg, 2014), both involved in different SHR projects concluding that no modelling can replace a thorough feasibility study. Studies from Germany and Switzerland show that WW could provide the basis for heating and cooling of up to 3% of all buildings (Schmidt, n.d.). In Vancouver, Canada, with 2.4 million inhabitants and WW temperature range of 10-25°C, an analysis has shown that 100 MW could be recovered from the SS region wide (Suda & Carmichael, 2015). According to (Hepbasli, et al., 2014), in any city in Turkey, 40% of the produced heat ends up

7 as wasted heat in the sewer system. Compared to other countries, the potential in Sweden should at sight be great regarding its vastly expanded DH network for making use of the retrievable heat in the sewer system.

It could be argued about the fact that heat is lost along the stretch of the sewer tunnels. In Sweden, the ground temperature is fairly constant between 8 and 12 °C around the year. As the temperature in the influent water ranges from 8 to 18°C, it is implied that heat exchange occur between sewage water and surrounding grounds of the tunnel. From a study made by (Abdel-Aal, et al., 2014) measured values for Antwerp, Netherlands, show that the temperature drops by as much as 4 K per km of sewer pipe during periods. Another source tells that after travelling 10 km in sewer pipes, the sewage will have the same temperature as surrounding ground (Elias-Maxil, et al., 2014). The distance will be smaller with smaller pipes and temperatures due to the lower energy distribution of the mass. The issue of heat losses along the sewer line is recognized as highly relevant and in need of modelling.

Attempts of modelling HR potential has been made in different ways. Work has been done by (Dürrenmatt & Wanner, 2014) to create a mathematical model for predicting WW temperature downstream of a HR plant. From the model, it was found that heat exchange between WW and pipe wall is the most important process of heat transfer. However, lateral inflows are not accounted for which are normally not negligible, which limits the general applicability of the model. Another modelling tool, developed by KWL, is a heat-seeking sewer model (Suda & Carmichael, 2015). It creates an overview of HR potential at every location within the entire sewer network. The model accounts for a vast range of factors, including groundwater infiltration and rainfall, and adopts many field measurement points and is calibrated against WWTP influent flow rates and temperatures with a maximum temperature error of 7% in the winter season. The strength of the model is to quickly identify areas with significant heat potential (Suda, 2016). Further modelling, based on data from conditions in Amsterdam was performed by (Maxil, 2015) to assess energy content in the sewer network and implications of cooling the network. This model is built up by sub-models such as a stochastic model for WW discharge from households and a temperature estimating model, TEMPEST. The model is admitted not fully coherent with field data but needs further development, but substantial work has been done and the model could be used as a foundation for specified simulation.

As for the requirements that HR from the sewer network would be successful, it has been identified that a minimum requirement of 10 litres per second and minimum width of 600 mm is needed at the sewer location where heat is extracted, referring to heat exchangers placed inside the sewage tunnel. Furthermore, (Maxil, 2015) claims from other reliable literature that economic feasibility is reached only if heat demand is above approximately 1 MW. Also, a bivalent system is preferred before a monovalent, i.e. that the heat pump provides the base load and an external burner or boiler is used for peak requirements (Knippenberg, 2014).

In Sweden, resistance against HR along the common sewer line exists. One of the major concerns when extracting heat from the sewer line is the feared effect that this will have on the nitrification process in the WWTP due to the implied temperature decrease of the influent water to the plant. This matter is highly relevant as one of the major tasks of the WWTP is to clean out nitrogen from the sewage and this process must not be compromised in regards to Swedish legislation in the extension of EU directives (European Union, 1991). Specifically the Baltic Sea is a sensitive area and subject of eutrophication which requires high levels of temperature dependant nitrogen removal from the WW. Previous research on downstream temperature effect tells different stories. Conclusions were made by (Wanner, et al., 2005) through modelling of ammonium levels in the WWTP that occurrence of reduced influent temperature under only a few hours’ time should not affect the nitrification because of hydraulic inertia in the large basins of the WWTPs. Permanently reduced influent temperatures however will have an effect on the nitrification process. In numbers, this effect is expressed as a 1°C decrease in influent temperature will result in 10% reduced net specific growth rate as well as the safety factor. This effectively leads to the need of either a 10% larger sludge tank which in extension would require a general capacity increase throughout the entire plant layout. This research suggests to stay on the conservative side when extracting heat from the sewer line. In Germany and Switzerland,

8 where WWHR installations already exist, regulations demand that temperature is only decreased a maximum of 0.5 K and that the final daily average of the influent temperature to the WWTP must not be below 10°C, which is the normal design temperature of the WWTPs in Switzerland (Maxil, 2015). Conclusively, it is difficult to generalize the findings why it all comes down to a necessity for an evaluation and analysis on the local level and the specific sewage system in question.

Background and objectives of the study As awareness and knowledge about energy conservation increases, actors on the energy market are acknowledging a growing number of sources of waste heat. DH company Norrenergi (NE) together with Käppalaförbundet (KFB), a wastewater treatment company has been introduced to the possibility of recovering heat from the wastewater in KFBs sewer line and supply it to NEs DH net. This concept would potentially be a great use of otherwise wasted heat and an increased energy efficiency within the urban water cycle and the WW system. To determine the potential and possibilities for a WWHR installation, a techno-economical pre-study was performed. The pre-study was carried out as a master thesis project which is presented in this report. The core of the pre-study was the site specific case study for a realized promising site in the municipality of Danderyd. The specific objectives of this project pre-study were to;

 Investigate the energy potential in the sewage, within the limits of what could be extracted without affecting the treatment process at the WWTP.  Determine specific technical and practical possibilities to perform HR along the sewer line.  Perform and present a technology review over the field of HR from WW.  Assess the economic investments of a potential WWHR project.  Assess the environmental impact of a potential WWHR project.

After this introductory chapter, a technology review of WWHR concepts, techniques, suppliers and existing installations are presented in chapter 2. Next in chapter 3, the case study is presented including a general description of the interested parties (KFB and NE) and their respective systems of WW and DH, the pre-requisites and questions set for the study, the methodology and model for finding answers and finally the results, technical evaluation and conclusions of the case study. After the case study is presented, a more general discussion is presented regarding the potential for WWHR around the country, in the light of the information retrieved as well as the problems and possibilities identified through the case study.

2. Technology review - Wastewater Heat Recovery (WWHR) This chapter describes first the overview of the wide concept of performing HR from different stages in the WW system, from a general perspective. More focus is then put on HR from the sewer line, directed towards the case study related to this thesis. Then, a number of existing projects of large scale WWHR projects around the world are described. Lastly, required technical equipment and a number of technologies and suppliers are presented in this chapter.

2.1 The concept of WWHR – system overview The concept of HR from WW could be considered highly diversified and the ways of making use of the available heat are many. In this section, a system overview is presented. First, the system is divided into three levels. Level 1 represents the WW system, all the way upstream from the water consumption to the final effluent from the WWTP, where the potential heat resource exists. Level 2 is the transferring system that connects Level 1 and Level 3, i.e. the technical system that provides heat from resource to final demand. Finally, Level 3 is the receiving system, i.e. where the heat is finally to be reclaimed. Figure 1 depicts an overall view of the different system levels and its diversified sections.

Figure 1. The diversified system of heat recovery from wastewater. From top to bottom, Level 1 represents the energy resource within the WW system, Level 2 the transferring technologies and Level 3 the recipient system. Red lines denote heat transfer and blue lines cold transfer. The dotted line indicates the focus area of this project.

In regards to a possible HR project, the heat resource in Level 1 is what decides the potential of the HR project as in how much heat could be recovered in terms of power (peak load) and total produced heat (capacity). On the other side, the heat demand of the recipients in Level 3 is also a deciding factor for the economic viability of the HR project, as in what heat power and capacity is required for the supply to be economically advantageous from other heating alternatives. Lastly, the Level 2 technologies must comply with the restrictions and requirements from the resource and the recipient demands, in terms of economy and technical efficiency. It could also be the case that the Level 2 technologies available are insufficient and becomes the limiting factor. Perhaps supplementary equipment is needed for providing external heating to meet the demand if the resource is too small. All in all, the success of a HR project depends on the specific project case but also on the matching and balancing of the three levels.

Heat resource (Level 1) At Level 1, HR could be performed at different locations within the WW system. In what way and to what extent this is possible depends on the temperature and flow rate with the WW at the specific location in question. The highest temperatures in the WW system is found upstream from the WWTP, closest to the consumers, where the flow consists of a high share of high-temperature sewage water (SW) from the consumers. The further downstream the WW runs in the system, the temperature is lower. This is partly because of the mixing with low-temperature supplement water (i.e. a mix between storm water, drainage water and ground water) which is in fact generally lower than the SW and also than the influent temperature to the WWTP and also because the WW becomes subject to heat losses to the inside and to surrounding grounds of the tunnels. However, even though the inflow of supplementary water (SuppW) decreases the average WW temperature, SuppW still contributes with some amount of energy and also provides a higher total flow rate. Thus, the total energy resource is increased by the SuppW. The inflow and infiltration of supplement water is far from fully mapped in regards to its flow rates, amounts and what locations in the WW system it occurs. Estimations for the WW systems in Stockholm are that it accounts for up to 40% of the total WW influent to the WWTP (Käppalaverket, n.d.) and that the temperature is somewhat close to the ambient outdoor temperature (Käppalaverket, n.d.). Projects are in motion from the WWTPs attempting to decrease the amount of supplement water in the WW system as it also carries a lot of species that are not subject of cleaning in the WWTP. Furthermore, diurnal fluctuations in the water consumption and seasonal trends of the WW temperatures and flow rates bring additional dimensions to the system.

It is quickly realized that it is a rather complex task to determine the available heat at a specific location, especially the further downstream the site is located. It is usually in need of field measurements at the specific site. Understandably, the resource is more predictable far upstream, closer to the consumer deposit into the WW system, or at post-treatment downstream of the WWTP where temperature and flow rate of the effluent is usually relatively well known. To sum up, the WW resource potential depends on factors such as;

 Available heat in the WW tunnel. Varies with temperature and flow rates which are dependent on daily and seasonal patterns.  Allowable temperature decrease from the WW, as decided by WWTP from its accepted lowest influent temperature.  Accessibility of the tunnel section, for withdrawal of sewage and installation of equipment.  Distance to recipient, as heat losses occur.  Distance to electricity network, for installation of equipment with electricity demand.

From the overall perspective, the HR opportunities and applications at different locations from the WW system have their pros and cons. Advantages and disadvantages discussed above, are summarized in Table 1 below.

Table 1. Pros and cons of performing heat recovery (HR) at different sections of the WW system. Advantages Disadvantages In-house  High water temperature  High fluctuations  Short heat transport (Low losses)  Difficult to match peak demand  Producers = Consumers  Decentralized system, high operative  No impact from supplement water expenses From sewer  Large amounts of WW  Dependant on sewage network line  Relatively short heat transport  Potentially impacts WWTP processes  Moderate WW temperature   Untreated WW inside equipment, potentially retrieves otherwise lost higher downtime and equipment costs heat along sewer From treated  No impact on WWTP processes  Plants not close to consumer water  Large effluent flow rate  large heat  Low WW temperature supply  Clean water through equipment  Cools WW deposited to environment Source: Adapted and extended from (Culah, et al., 2015)

Heat recipients (Level 3) As for the heat recipients in Level 3, HR downstream of the WWTP could be done for the purpose of using the heat internally in the processes of the WWTP such as sludge drying or providing pre-heating for the digestion process in the digestion tanks. It could also be utilised externally to provide to either a DH net, e.g. as performed at Henriksdalsverket in Stockholm lowering the exit temperature to 0.5-4 °C (Stockholm Vatten, 2016), or used for heating of local neighbourhoods which is carried out at Käppalaverket on the island of Lidingö, providing 9 GWh a year (Käppalaförbundet, 2016). In direct connection to the consumers, heat in the WW could be recovered through so called in-house applications. The recipients of the recovered heat are the consumer themselves as the heat through these applications is normally recirculated by heat exchanging of outgoing water in different ways. In this manner, the producer becomes the consumer as well as the least amount of heat is lost to the sewer system. However, in Sweden, regulations on municipal level apply that the temperature of the outgoing water to the WW system must not be lower than the cold fresh water provided to the consumer in the first place (Danderyds kommun, 2008). This limits the heat harvesting with in-house applications. For HR projects the heat recipient may pose conditions for design and dimensions in terms of;

 Demand of delivered power, i.e. peak demand (kW)  Demand of delivered amount of heat (kWh)  Demand of delivered temperature level.  Whether the HR project should provide for the entire power and heat demands and temperature level or if supplementary supply (from e.g. firing) is considered.

The possibility also exist, as mentioned in Figure 1, of using heat storage such as accumulators or so called phase change materials (PCM) for buffering heat and use at other time spans (and potentially other locations) from when the heat is harvested. Heat storage also occurs in a DH net, which utilises an accumulator in order to cover the diurnal demand changes.

Heat transferring technology (Level 2) The heat transferring technologies of level 2, between source and recipient, are directly related to where the heat is recovered within the WW system. Within the in-house HR applications, HEXs in different forms and stages are used to recirculate heat, which has been mapped and illustrated by e.g. (Jonsson, 2015). Within the areas of HR from the sewer line or from treated water, the use of a heat exchanger and a connected heat pump are commonly used to lift the final temperature to the recipients. An indicative number for heat pumps are that it uses around 20-33% (Hart, 2011) electrical power of the thermal output, yielding a coefficient of performance (COP) of 3-5 depending on temperature

12 operating points. For the situations of HR from either sewer line or from treated water, the transferring technologies could differ in ways of;

 Type of transferring unit, depends on specific demands of Level 1 and Level 2.  If heat transfer is divided into a single or multiple steps. Depends on what parts are in contact with the untreated WW.  Type of refrigerant and heat transferring media.  Biofilm mitigation technique and maintenance requirements.

The technology for HR from the sewer line from untreated water is further described in the next section.

2.2 WWHR from sewer line As described in the previous section, HR could be performed at different locations in the WW system, in Level 1. Focusing deeper on HR from the sewer line, it is common that a HEX together with a heat pump is used for recovering and supplying the heat. The heat exchanging could be performed either within the sewer or by withdrawing water from the sewer and perform external heat exchanging outside the tunnels (Schmidt, n.d.). The principle for HR from the sewer line includes a number of characteristic steps that are common for all existing projects that have been performed.

The most critical technology requirement is for the specific heat exchanging from the untreated WW to the heat receiving medium. This requires contact with the untreated WW which impose the issue of solid particles handling and the compliance with greases and sludge. The heat exchanging could be performed in two ways, as depicted through Figure 2 below.

Figure 2. The two concepts of HR from the sewer line. To the left, direct HR inside tunnel and to the right, external HR outside the tunnel.

On the left hand side, heat exchanging is performed directly inside the tunnel through HEXs integrated in the tunnel segments or through surface HEXs. A connecting heat pump is used to lift the temperature up to the desired level at the recipient. To the right, the WW is diverted from the sewer line for performing the heat exchange externally.

2.3 Existing installations of large scale heat recovery from untreated wastewater The idea of large scale HR from untreated sewage where external heat exchanging is performed, has not been widely explored. Only a few installations have been carried out of which the majority of them are described here. The descriptions here are regarding the WW systems that are connected, the equipment used for HR and some operation parameters.

SHR Sandvika, Oslo (Norway) VEAS WWTP treats WW from 600’000 P.E. in 3 municipalities in Oslo, Norway. It has a dry weather flow of minimum 1900 litres per second (l/s) and a maximum flow of 9000 l/s during wet weather flow. The minimum requirement for nitrogen cleaning is 70% (VEAS, 2016), but no effluent nitrogen limits exist. The last years, the nitrogen treatment has been quite poor, but exemption has been allowed from authorities due to adjustments in the process (VEAS, 2014), (VEAS, 2015). The treated water is finally disposed to the Oslo fjord. Table 2 below summarizes the data.

Table 2. VEAS WWTP, general data. Quantity Unit Source Influent flow rates (min/avg/max) 1900/3390/9380 l/s [1] Influent temperature (min/avg/max) 6/11/19 °C [1] Nitrogen removal limit (min) 70 % [2] Nitrogen removed 60 (2014) & 53 (2015) % [2], [3] Treatment method Biofilm reactors - [4] Sources: [1] (Ryrfors, 2016) [2] (VEAS, 2014), [3] (VEAS, 2015), [4] (VEAS, 2016)

Upstream from VEAS WWTP, a SHR plant is located. Oslofjord Fjarnvarme AS, owned by the Fortum group, is providing DH and DC to the local communities using raw sewage as heat source at the Sandvika plant since 1987. In the planning of the SHR plant it was realized that HEXs submerged in the sewer would not be sufficient for the large extraction capacity. The WW is therefore extracted from the sewer and pre-treated (screened) through filtration and sedimentation before entering the HEX. The sedimentation chamber is a relatively large one. The HEXs are of shell and tube type. The two heat pump units are working in serial operation and each has a capacity of 6.5 MW for heating and 4.5 MW for cooling. The chilled water (sea water) is cooled in the first heat pump from 8 to 4°C, going out to the DC net. In the second heat pump, the WW is cooled from 10 to 6°C and the output temperature from the HP is 78°C (Friotherm, n.d.). More detailed data are presented in Table 3 below.

Table 3. SHR plant in Sandvika, general data. Quantity/Type Unit Source Type of heat pump (HP1/HP2) UNITOP 28 C / 28 [1] WW flow rate (HP1/HP2) n/a l/s [1] Heated water flow (HP1/HP2) 160 l/s [1] WW temp In-Out (HP1/HP2) 8.0-4.0(seawater)/10.0-6.0 °C [1] Heated water temp In-Out (HP1/HP2) 57.0-78 °C [1] COP (COP incl. cooling) 3.1 (5.22) - [1] Refrigerant (HP1/HP2) R134a [1] Heating capacity (HP1/HP2) 13 (6.5/6.5) MW [2] Cooling capacity (HP1/HP2) 9 (4.5/4.5) MW [2] Heat source (supplementary) Oil burners [2] Heating capacity (supplementary) n/a MW n/a Power at terminal 4.5 MW [1] Energy from WW 24.4 GWh [1], [2] Annual heat production 47 GWh [2] Annual cold production 11 GWh [2] Production share (WW/Suppl.) of total 52/48 % [2] Sources: [1] (Pietrucha, 2012); [2] (Friotherm, n.d.)

SHR Sköyen Vest, Oslo (Norway) Upstream from VEAS WWTP is also second application installed that uses raw sewage as heat source in the Sköyen heat pump plant, owned by Hafslund Fjernvarme AS, formerly Viken Fjernvarme (Bush & Shiskowski, 2008). In fact, this is the largest of its kind in the world since the commissioning in 2005 and expansion in 2007. It provides both DH and DC and has a heating capacity of almost 28 MW from WW alone. The remarkable with this plant, apart from its size, is that it yields a high output temperature of 90°C from the heat pumps to the DH net (Bailer & Pietrucha, 2006). The WW is pre- treated through filtration and sedimentation but with a smaller sedimentation chamber than in Sandvika which requires more frequent cleaning. Both applications in Oslo are using reversed flow for avoiding biofilm formation in the evaporator (WasteWaterHeat, 2010). General data for the plant is presented in Table 4.

Table 4. SHR plant in Sköyen Vest, general data. Amount Unit Source Type of heat pump (HP1/HP2) UNITOP 50 FY / 34 FY [2] WW flow rate (HP1/HP2) 650/390 l/s [2] Heated water flow (HP1/HP2) 150/80 l/s [1],[2] WW temp In-Out (HP1/HP2) 10.0-5.76 / 10.0-6.3 °C [1] Heated water temp In-Out (HP1/HP2) 75.5-90 / 60-75.5(90) °C [1] COP (HP1/HP2) 2.83-3.5 (2.80/2.89) - [2] Refrigerant (HP1/HP2) R134a/R134a [2] Heating capacity (HP1/HP2) 27.6 (18.4/9.2) MW [1] Cooling capacity 12.134 MW [2] Heat source (supplementary) Electricity [2] Heating capacity (supplementary) 12 MW [1] Power at terminal 6.62 MW [2] Energy from WW 90 GWh [3] Annual heat production 130 GWh [4] Annual cool production n/a GWh n/a Production share (WW/Suppl.) 91/9 % [4] Sources: [1] (Pietrucha, 2012); [2] (Bailer & Pietrucha, 2006); [3] (Hart, 2011); [4] (Hafslund, 2016)

Hafslund AS is purchasing the heat that is extracted from the WW from the inter-municipal company VEAS which owns the SS and the WW. The Sköyen Vest project was funded by a 20% support from the Oslo energy efficiency fund Enova (Nilsen, 2016).

SHR False Creek energy centre, Vancouver (Canada) Iona Island WWTP provides primary treatment of sewage from around 600’000 P.E. Local treatment regulations are allowing a maximum 130 mg/l biochemical oxygen demand and 100 mg/l for total suspended solids (Metro Vancouver, 2016). It has an influent flow rate of 4500 l/s in dry season and 8500 l/s wet season with WW temperatures ranging between 12 and 24 °C (Metro Vancouver, 2014), while no nitrogen treatment limits apply. Therefore no nitrogen treatment is performed at this stage but an upgrade to secondary treatment is planned by 2030. The SS is a combined one that collects both SW from consumers and SuppW. The SS consist mainly of concrete pipes laid in the ground (Hunt, 2016).

From the sewer line upstream from the WWTP in the False Creek neighbourhood energy centre, untreated sewage are screened and passed through HP evaporators, recovering heat for providing DH to the local community. This is the first installation in North America and was installed in 2010 (Comeault, 2011) and is owned and operated by the city of Vancouver (City of Vancouver, 2016). The facility will be expanded in 2018 and increase its SHR capacity from 3 MW to approximately 8 MW. This due to that the actual WW temperatures were higher than what was designed for and there is potential for expansion (McConnell, 2013). Two heat pumps are in operation today, with the

15 possibility to work in series or in parallel for turndown and operating range. In this way, hot water flow and discharge temperature could be varied in a wide range. In this installation, reversed flow is used for avoiding biofilm formation on the HEX tubes. Also quarterly de-fouling is performed (WasteWaterHeat, 2010). General data for False Creek energy centre are summarized in Table 5 below.

Table 5. SHR plant False Creek energy centre, general data. Quantity/Type Unit Source Type of heat pump (HP1/HP2) Tecsir [1] WW flow rate 100-120 l/s [1] Heated water flow 20-45 l/s [1] WW temp In (Summer/Winter) 16-20/23-25 °C [1] Heated water temp Out (Par./Ser.) 70/82 °C [1] COP 2.9-3.4 - [1] Refrigerant (HP1/HP2) R134a [3] Heating capacity 3 (8 in 2018) MW [1] Cooling capacity 0 MW n/a Heat source (supplementary) Natural gas boiler [1] Heating capacity (supplementary) 16 MW [1] Power at terminal n/a MW n/a Energy from WW 44 GWh n/a Annual heat production 63 GWh [2] Annual cool production 0 MWh n/a Production share (WW/Suppl.) of total 70/30 % [1] Sources: [1] (McConnell, 2013); [2] (Comeault, 2011), [3] (Baber, 2017)

The City of Vancouver owns and operates all facilities within the WW system, i.e. everything from WW and WWTP to HR facility and heat distribution system.

SHR Borders College, Galashiels (Scotland) In Galashiels, Scotland, a project was launched in 2015, where heat is recovered from the SW at the Borders College, close to the city of Edinburgh, for providing local heating to the college campus. Galashiels wastewater treatment works (WWTW) receives and treats the WW downstream with a P.E. of 27000. Nitrogen treatment limitation of 15 mg/l is set and is reached using secondary treatment including biological filters, humus tanks and a final natural treatment in a lagoon before releasing the treated water into the River Tweed (East of Scotland Water, 2001).

The SHR project is carried out by Scottish Water and Sharc energy systems, who also set up a range of measuring equipment for establishing that the downstream temperature and ultimately the biological process of the WWTP was not affected (Brockett, 2016). The general data for this SHR plant is summarized below in Table 6.

Table 6. SHR facility at Borders College, general data. Quantity/Type Unit Source Type of heat pump Carrier [1] WW flow rate 32 l/s [1] Heated water flow n/a (to match demand) l/s [1] WW temp In (Cold/Warm season) 7-8/13-16 °C [1] Heated water temp Out 50-60 °C [1] COP 4.8 - [1] Refrigerant (HP1&HP2) R134a (shifting to HFOs) Heating capacity 800 (2*400) kW [1] Cooling capacity 0 MW n/a Heat source (supplementary) Natural gas boiler [1] Heating capacity (supplementary) n/a MW n/a Power at terminal 500-600 kW [1] Energy from WW 1.9 GWh n/a Annual heat production 1.9 (+5% from Suppl) GWh [1] Annual cool production 0 MWh n/a Production share (WW/Suppl.) of total 95/5 % [1] Sources: [1] (Dunsmore, 2016)

The facility is on land owned by the college but the equipment is owned by Sharc energy systems which sells the heat to the college over a 20 year heat sell agreement. The investment also has support from Equitix and UK Green Investment Bank (Borders College, 2016).

The energy facility at Borders College has installed sewage HP units parallel to the old existing natural gas boiler and an added valve on the water supply line to the boiler that diverts the heated water so that it passes through the HPs instead of the boiler for most of the time. The heated water is thereby distributed between the two heat sources according to the heat demand. The operation is simple, as everything is automatically regulated and only remote supervision is required. Prior to the project start, simple pre-calculations were performed by staff at the WWTP downstream regarding the feared temperature drop in the influent WW to the WWTP. The rough estimates, as in percentage energy extraction, showed that the potential temperature decrease of the influent WW to the WWTP would be at most a few degrees. After installations, post-measurements were performed. Since the installation in December 2015 no temperature decrease has so far been detected downstream, at least none that could be directly related to the HR occurring upstream. Also in further planned projects, such as a large scale installation of 20 MW heating and 14 MW cooling, it is assumed that the limits for wastewater treatment will not be exceeded. It is believed that heavy rainfalls still have larger influence on the influent water than HR will have. Scottish water sees continuing success of further projects by having a strong belief in the equipment they are utilising for the HR as well as having very supportive governmental relationships.

2.4 Existing installations of heat recovery from treated wastewater in Sweden As comparative information, this section presents some installations in Sweden that uses the already treated wastewater after the WWTPs. Information was collected from installations at Käppalaverket, Solnaverket, Hammarbyverket and Ryaverket (Gothenburg).

Käppalaverket From the treated WW in K-verket, Käppalaförbundet recovers heat for the local community of Gåshaga. The WW holds an average outlet temperature of 13-14°C from the WWTP to the heat pump inlet and the heat pump using R134a refrigerant increases the temperature to suitable level for heating nearby dwellings. The total production is around 9 GWh annually. However, the ambition is to increase this number (Käppalaverket, n.d.).

Solnaverket As part of NEs heat production, they use treated water from Brommaverket. They use four heat pumps, in total 75-100 MW and the COP is between 2.6 and 3.1, using R134a as refrigerant. The annual production is over 550 GWh (Norrenergi AB, 2015).

Hammarbyverket Fortum Värme uses treated WW from Henriksdals WWTP for producing heat onto their DH net. It has seven heat pumps in total, with a peak capacity of totally 225 MW. Some are using R134a and some R22 as refrigerant. Over 900 GWh is recovered from the treated WW. It performs a temperature lift from 10-19°C up to 60-90°C onto the DH net, while decreasing the WW temperature correspondingly from 10-19°C down to between 0.5 and 11°C depending on the season (AB Fortum Värme, 2012).

Ryaverket (Gothenburg) At Ryaverket in Gothenburg, Göteborgs Energi both treats the WW and recovers heat from it. Four heat pumps have a capacity of 160 MW and produce annually 440 GWh, all using R134a as refrigerant. The temperature lift is from 12 to 70-90°C while the corresponding temperature decrease of the WW is from 12 to 3°C (Göteborg Energi AB, 2014).

To summarize, the current installations which use already treated WW for HR in Sweden have much higher production levels than the installations abroad (presented in the previous section) using untreated WW from the sewer line. It could also be noted that the outlet WW temperatures from the WWTPs are relatively high. It does in fact increase slightly from the inlets of the WWTPs due to the biochemical activity which is exothermic.

2.5 Suppliers of external HEX for untreated WW Looking at the area where sewage is withdrawn from the sewer in order to extract heat outside of the sewer tunnel, the necessary stages for an energy facility is represented in Figure 3 below.

Figure 3. Heat recovery (HR) system, energy facility.

From left to right, water is diverted through a runoff channel from the sewer line to a collective shaft, often called a wet well. The wet well is used for initial pre-treatment in terms of a strainer but also for maintaining a controlled flow for the water pump. The actual pre-treatment often contains a sedimentation step and could be performed to different extent depending on how clean the WW needs to be for passing through the HEX. In the HEX, heat is transferred from the WW to an intermediate fluid, e.g. brine, before led to the heat pump. The heat pump is depicted as a simple vapour compression cycle, where in the heat pump evaporator the intermediate fluid exerts heat to a refrigerant which then goes through a phase change (from liquid to gas) at the low pressure side. The heat pump compressor provides a higher pressure and a higher temperature of the refrigerant. Through the higher pressure, the refrigerant boiling point becomes higher, why it becomes possible to exert heat to the DH water in the condenser at higher temperature while phase changing back to liquid state. The heat pump expansion valve then decreases the pressure of the refrigerant gas. Regarding the HEX, the intermediate step is not always necessary. In some cases, the WW is passed directly through the heat pump evaporator, which in that case works as the HEX that absorbs heat from the WW.

Auxiliary equipment is also required which must also be mentioned. Such equipment are pumps and piping (in order to transport WW and heat transferring media around the system) but also a control system that enables automatic and dynamic operation as well as the possibility to supervise and keeps track of the operation.

A few manufacturers exist that has tried to comply with the issue of biofilm formation and clogging in HEXs. A number of them have been investigated and are presented below.

Huber Technology Huber’s technology withdraws sewage from the sewer channel through a strainer system that clears out solid particles and into a wet well culvert. The greywater is then pumped up through a HEX (RoWin) where a secondary fluid absorbs the heat. An automatic cleaning system prevents formation of biofilm on the HEX pipes. Residual sludge and sedimentation are passed out from the floor of the HEX via a screw transporter (Huber Technology, n.d.). Figure 4 shows the flow scheme of a representative system utilising Huber ThermWin technology.

Figure 4. HUBER ThermWin concept. Sewage is diverted from the sewer tunnel through a strainer to the sewage drum. Source: (Huber Technology, n.d.)

The HUBER technology has been adopted in several applications in especially Germany, normally used for small to medium size installations. The specific treatment technology is regarded patented property, but is based on mechanical removal through a built-in device that periodically removes sludge from the HEX pipes. The heat pump in connection to the RoWin heat exchanger is not of Sharc manufacturing but must be adapted from other suppliers. Thereby, an intermediate stage is used before the heat pump in relation to Figure 3 earlier.

Sharc Technology Most of the installations that Sharc has been involved in so far is in direct relation to larger facilities, such as multi-resident buildings, community centres etc. The outgoing water from the buildings are collected in a wet well. The Sharc technology extracts sewage from the wet well, passes it through a macerator that reduces the size of particles in the sewage before filtering out the solids along with the greases and sludge in the next stage. This stage is the specific treatment technology that is regarded patented property and which is claimed to be in need of no periodic cleaning. This would mean that little maintenance is required. This has been of specific focus of the technology. The now clean water is led into a HEX that interacts with the loop of the building in question. Figure 5 shows the flow scheme of the Sharc technology.

Figure 5. Flow scheme of Sharc system. Source: (Arcadis, 2013)

Normally, a heat pump is placed separately after the heat exchanger to raise the temperature to the heat recipient, as performed at Borders College described in previous section. The heat pump is not of Sharc manufacturing but must be adapted from other suppliers. The wet well exists, if required, as a separate tank unit which means no construction work is needed for setting up a wet well shaft. Also Sharc utilises an intermediate step before operating the heat pump.

Friotherm Friotherm has been involved in many large scale HR projects, among which the two SHR installations in Oslo is using Friotherm equipment, described in previous section. With the Friotherm equipment (called Unitop), the HEX where the sewage is passed could be used directly as a heat pump evaporator (or condenser), meaning no intermediate HEX stage is needed. In Sandvika for example, a site construction is carried out where both mechanical filtration and sedimentation is performed. The flow scheme is shown in Figure 6 below.

Figure 6. Flow scheme of Sandvika wastewater heat recovery plant. Source: (Friotherm, n.d.) Friotherm equipment also utilises the concept of backflow in their heat pumps in order to avoid biofilm formation. The Unitop units could be operated over a wide range of heating power demands, with stage-less part load operation. Heat pump capacities of up to 20 MW exists (Friotherm, n.d.).

3. Case study – Heat recovery from sewer line to district heating (DH) network As a part of this master thesis, a case study was performed where the possibilities of connecting the respective systems of the two project providers, Käppalaförbundet (KFB) and Norrenergi (NE), were assessed. The specific case was regarding the geographical area around Danderyd’s hospital in the municipality of Danderyd on the northern side of the city of Stockholm. This area is favourable for a HR project between the interested parties as their respective networks, i.e. the WW tunnel and the DH net, close to overlap there. The case study treated the following perspective questions;

Heat resource  What is the heat recovery potential in the sewage tunnel at the specific site over the year? In regards to heating power (MW) and heat amount (MWh).  What are the heat losses in the system and what potential energy savings is thereby possible through extracting heat upstream?  How does upstream heat recovery compare to downstream heat recovery from treated WW in terms of amount of heat recovered over the year?  What happens to temperatures and heat recovery potential if; o Extensive heat recovery is performed at consumer side? o Supplement water could be avoided in the sewer system to a large extent?

Heat recipient  How much heat could be supplied to the DH network under the current conditions by the use of a heat pump solution?  If also cooling is considered, how does the heat recovery potential and supplied heat change?

Technical solutions – Alternative ways to go  Find suitable technical concepts and layouts for different alternatives.  From equipment supplier alternatives find energy provided, general investment costs and environmental performance.

3.1 System description The overall system consists initially of two major parts, the WW system of Käppalaverket (K-verket) where the heat resource exists and the DH net of Norrenergi being the heat recipient. A connection between these systems was performed by designing an energy facility with adequate equipment for transferring heat between the two systems.

3.1.1 The WW system of Käppalaverket K-verket is handling and treating WW from 11 municipalities within the district of Stockholm. This estimates to about 635’000 people equivalents (P.E.), including industries. The WWTP is located on the island of Lidingö and the total length of the tunnel network is 65 km. A 0.1% inclination makes the water fall by gravitational forces along the sewer line and is lifted by pumps 20 m at three positions within the system. The majority of the tunnel network consists of natural rock walls where the tunnels are penetrating the bedrock. The network is a combined SS, meaning it also receives and carries supplement water in terms of ground water, storm water, drainage water and infiltrating surface water.

Yearly variations in influent WW flow rates to the WWTP are between 500-2500 dm3/s, having a temperature range of approximately 8-18°C. Figure 7 shows the annual distribution of influent flow rates and corresponding temperatures to the K-verket WWTP. The amount of supplement water in the system is estimated to around 40% of the total WW but strongly varies over the year and is particularly high during heavy rainfall. This could be observed in more detail in Figure 8, showing the flow rates and temperatures during the wet winter season. It is also indicative from the clear trends that

21 the higher flow rates from heavy rain weather decrease the average temperature of the influent WW to the WWTP.

Figure 7. Annual variation in influent volumetric flow rate and temperature to Käppalaverket WWTP, for the year 2014.

Figure 8.Influent flow rate and temperature during spring 2014.

Regarding the WW flow rate at the site if interest, i.e. Danderyd’s hospital, this is a fraction of the total WW influent flow rate. This amount varies between 50-57% of the total WW flow rate over the year, the highest during July and the lowest during January.

3.1.2 The DH and DC networks of Norrenergi Norrenergi provides DH to over 100’000 people in the municipalities of Solna and Sundbyberg in the Stockholm district and owned is municipally owned. The total stretch of the DH net is about 190 km long and the heat is produced mainly through incineration of different biofuels in Solnaverket and waste heat from treated WW at Bromma WWTP. NE also provides DC to commercial districts such as hospitals, server halls and offices. The total stretch is 35 km and the cold is provided from treated WW in Bromma (after being cooled by the DH net), but also from sea water and cooling machines (Norrenergi AB, 2015). The DC net does not stretch pass the specific site of the case study though. The supply temperature of the DH net varies over the year between 70 and 120°C, depending on the outdoor temperature. A higher supply temperature is needed due to more total heating power demand in the DH net, higher required radiator temperatures in the consumer dwellings, but also due to higher heat losses.

3.1.3 Pre-requisites for the case study The given requirements for the case study and the possible technical solution to the application, as expressed by KFB and NE, were as follows;

 A minimum of 5 MW usable heating power was considered as the limit for the HR project to be interesting in terms of making the heat contribution to the DH net substantial. This was also pre-assessed to be possible at the specific site. It was also established during the project that potential cooling could also be of interest.  A desired inlet water temperature to the DH net of 72°C was expressed, in order to provide onto the DH net forward line. It was further established that providing lower inlet temperatures onto the return line would also be acceptable under certain circumstances.  A boundary value of 10-12°C as minimum temperature with the influent water to the WWTP.  An existing area inside a building of 12x15 m and a height of 9 m is available for placing equipment. Here, high voltage connections are already installed. Both the DH net and the main tunnel is within 100 m range from the specific area.

3.1.4 Case study in relation to existing installations for WWHR from sewer line When comparing factors between the case study situation and the existing installations described in previous chapter, it is found that none of the large scale installations is a perfect match to the case study situation. Hence no precise reference exists today when attempting to construct a feasible HR installation for the case study system. The following notes could be made, comparing the installations to the case study;

 Looking at the two installations in Oslo, where climatic conditions are fairly similar to Stockholm, it proves that technically large scale heating and cooling from WW is possible. However, the output WW temperatures from these HR plants are much lower than what could possibly be operated in the case study, as low as 6 °C. The low temperatures operated in Sandvika and Sköyen Vest is possible because the WW treatment requirements are much lower and the nitrogen removal is not of the same focus as in Sweden. The downstream WWTP do operate active sludge treatment but seem to have problems with poor sludge qualities and low nitrogen removal results. Perhaps this could be related to the upstream HR as the outlet temperatures are well below the critical interval for nitrification.  Another lesson learned from the two Oslo installations is that direct use of untreated sewage in the heat pump heat exchangers have caused more downtime and higher costs for maintenance than predicted. Also lower income due to poor heat transfer occurred.  The installation in Vancouver has a much greater heat resource in terms of flow rates and temperatures in the current sewer than the case study, making direct relations between the situations difficult.  The HR project at Borders College in Scotland is currently at only 800 kW, which would rather be considered a medium scale installation. In this case, the effect of potentially decreasing the WWTP influent temperatures by recovering heat upstream was actually considered and measured, as the nitrogen removal level is of high importance here as well. But even though no temperature effect has been detected, it is quite difficult to apply the same conclusion to the conditions of the case study since the heating power is lower as well as the water flow at the site of installation. Also, the supply temperature from the heat pump to the heating system is quite low (50-60°C) compared to the desired 72°C of the case study. Parameters simply don’t match and thus no suitable reference example exists.

As for the situation of Switzerland, regulations for sewage heat extraction have in fact been established and many small scale installations are in place. However, if regarding the limit of exactly 10°C as a set boundary for allowing heat extraction, this implies that either the technology for nitrogen removal is different from the active sludge treatment used in Sweden or that the treatment results are poorer. This since 10°C would be quite risky in relation to the temperature interval for biological nitrogen removal.

If also making a short comparison to the existing installations were HR is performed using WW that has already been treated in a WWTP, such as described in section 2.4, the following observations can be made;

 As seen in the installations, the temperature decrease is much larger than what is possible when using untreated WW, due to the limitation in outlet temperature.  The same COP should be possible to accomplish as the temperature levels are almost the same in the case study.  Comparing the installed power and total heat production, this is certainly way beyond the possible when using untreated WW, also due to the resource limit.  The productions that respective interested party (i.e. KFB and NE) already have using treated WW, which is 9 GWh in Käppala and above 550 GWh in Solnaverket.

3.2 Methodology In order to find answers to the questions related to the case study, a certain flow scheme was followed of which the specific activities are explained below.

System definition When starting up the project, it was a rather time-consuming task to read into, map and understand the very technical area of WW treatment in Sweden but also the specific WW system of KFB, including tunnel system and field data. Defining the system at this stage included looking into and organize field data of flow rates and temperatures on both WW and DH side. Also local WW treatment requirements and WWTP layout were looked into and defined.

Background research With the initial conditions for the case study set, research was carried out for scanning the field and gaining up to date information on the topic, both for the purpose of the analysis and for supplying information to the project providers. Looking into installations already in place today it was found that not many large scale installations exist around the world. This motivated to dig deeper into these installations as it is a rather uncommon concept for finding out information about;

 Local conditions for WW treatment, how the concern of temperature decrease of the influent WW was considered.  Energetic and physical conditions of the WW, flow rates and temperatures.  Technical equipment and performance  Boundaries of ownership

Information was gathered through visiting web sites, reading official documents and through personal communication with the few existing HR facilities as well as their respective downstream WWTPs.

A literature review was carried out looking into previous scientific research on local circumstances on WW systems, energetic analyses of sewer systems and attempts of determining HR potential within SSs through modelling. This included browsing of scientific reports and communication with some researchers and their research institutions for finding information that could relate to the case study. Found from this were results regarding heat losses and temperature drop within SSs but also of limitations for HR within the system.

Creation of the models An initial modelling phase was carried out based on the key indicators of interest for finding quantitative results for the questions related to primarily the heat resource side in the case study. At the first modelling stage, limited information was available regarding the WW composition at different locations of the WW system which limited the possibility of modelling the heat resource. At a later

24 stage in the project, such data was revealed which opened up for re-design and expansion of the hydraulic modelling for improved results.

Analysis Analysis of the results was performed parallel to the modelling process as results appeared. Through this working manner a somewhat iterative process was developed with re-considerations and alterations of the model from the analysed results.

Conclusions Conclusions were drawn from the modelling results and comparisons to background information regarding other facilities and concepts for HR. Some early conclusions also called for the need of expanded analyses in some areas, causing an interactive working manner and iterative process here as well.

3.3 Models This section describes the different models that were used in this study. A hydraulic model was created for mapping the current flow and temperature situation in the wastewater system and for assessing the effects that different upstream events cause in the system. A heat pump model was created for analysing what heat and cold could be provided to the DH and DC net. Finally, an expansion of the heat pump model, i.e. a performance model was used to assess the performance of different solutions and equipment from actual suppliers.

3.3.1 Hydraulic model As mentioned, the hydraulic modelling was performed at different stages of the project of which the structure of the final version is presented here. The SS of KFB collects water from 11 different municipalities, which is in total collected from and measured by 25 WW connection points around the SS. Upstream from the respective measuring point, local WW networks exist but are not in the regime of KFB and hence were not modelled here.

In general, all calculations and simulations were performed based on monthly averages, i.e. 12 different values for every parameter over the year. Calculations were performed assuming steady state conditions, i.e. transient behaviour of e.g. tunnel walls were not considered. The continuous data available were for flow rates and share of WW sources at the connection points. As source for flow rates the internal software aCurve was used and for quantities of WW sources a Sweco report (Adrup, et al., 2014) was used. Temperature data was only available for the inlet to the WWTP. At every connection point, the temperature was calculated through

, (1) where is the sewage푇 푊푊 water, temperature= ∗ 푇푆푊 based+ ∗ 푇 on푆푊 an average outlet water temperature from consumers and is the temperature of the supplementary water. and are the flow shares of 푆푊 respective푇 WW source based on data together summing up to 1 (one). was further calculated 푆푊 in a similar manner푇 from the shares and temperatures of other supplementary water ( ) based 푆푊 on local monthly average outdoor temperatures and ground water ( ) 푇based on average temperature. 푂푆푊 Also the mixture temperatures where the flows from connection points are mixed into 푇the main tunnel are calculated using the mixture formula. 푇퐺푊

The mixture temperature along the main tunnel was calculated all the way down to the influent stage of the WWTP, incorporating flows from connection points along the stretch of the main tunnel. In order to quantitatively determine the heat losses and furthermore the temperature distribution in the SS, the heat from every connection point were initially treated as maintained through its entire travel downstream to the WWTP. A final supposed theoretical mixture temperature could then be compared to actual influent temperature data, yielding a suggested total amount of heat loss on a J/s basis throughout the SS. The heat loss value was then distributed over the entire WW flow and also the

25 entire flow length, finally yielding a heat loss number on a W/m3*m (watt per cubic metre of water and tunnel metre) basis. This allowed calculating heat losses (and corresponding temperature losses) of the specific connection flows in regards to its actual flow rate and flow length, which then provided a simulated temperature distribution throughout the stretch of the main tunnel.

Constructing the model in this way allowed for assessing the downstream effect of upstream HR by manipulating the calculated value at the point of interest, i.e. Danderyd, by using the calculated temperature distribution and heat losses as reference values. This also made it possible to construct different scenarios through alterations of certain parameters, e.g. the amount and share of supplementary water and the disposed temperature from the consumers if extensive HR should be performed through building related applications. As upstream temperature changes are simulated through any scenario, the heat losses were changed accordingly as

, 푐,푟−푐,푤 (2) ,푤 , ∆ = ∆ ∗ (1 − 푐,푟 ) where is the temperature loss between two connection points along the main tunnel stretch, is the temperature in °C at the connection point at reference case and is the , suggested∆ new temperature from upstream scenarios. This is based on the simplified assumption that , ,푤 heat transfer changes proportionally to the temperature difference in the heat equation

, ∆ (3)

where is the volumetric flow rate, = 푉 the ∗ 훿 density ∗ ∗ ∆of the media and the specific heat at constant pressure of the media. Equation 3 was furthermore also used for calculating the heat carried with the WW at푉 different locations in the SS. The훿 field data used for the flow rates at the connection points are presented in Appendix A for reference.

3.3.2 Heat pump model In the supposed energy station required for transferring heat from the SS to the DH network, theoretical calculations were made for assessing the amount of heat that could be supplied to the DH net based on the parameters of;

 Available heat in the WW as limited by a minimum influent temperature to the WWTP  Actual WW temperature decrease due to heat extraction at the Danderyd site  Flow rate at the Danderyd site  Coefficient of performance (COP) of the heat pump

The available heat in the WW was calculated through equation 3, but with a here being the spare temperature, i.e. , of the influent temperature available for HR in regards to a minimum temperature limit, , for maintaining proper biological treatment in∆ the WWTP. For this, 푖 minimum temperature∆ levels of 9-13°C were examined. From the actual influent temperature at the WWTP, based on statistical푖,푖푖 average, the allowed temperature decrease was simply calculated through the subtraction

(4)

Through simulations of the ∆ hydraulic푖 = 푖,푎푎 model, an− empirical푖,푖푖 relationship was developed for the WW temperature decrease at Danderyd and the influent WW flow respectively. The temperature decrease at the Danderyd site is allowed to be larger than the decrease in influent temperature since the flow in Danderyd is only a partial flow of the total influent flow rate. This partial flow rate was presented in section 3.1.1. The temperature decrease relationship was calculated as the ratio

(5)

∆푖,푎 푖,푎푎 − 푖,푎 휃∆ = = where was the simulated∆퐷푎 temperature∆ 퐷푎 decrease at Danderyd which then caused a corresponding temperature decrease of the total influent WW temp , as calculated by the 퐷푎 difference∆ between the actual statistical influent temperature and the simulated influent 푖,푎 temperature . A specific value for each month of the year was∆ calculated for this temperature 푖,푎푎 decrease relationship. This relationship was then used for determining the allowed temperature decrease at ∆ Danderyd푖,푎 in regards to the earlier mentioned influent WW temperature limits. As explained prior to equation 3 earlier, temperature losses are assumed smaller when temperatures are decreasing downstream from Danderyd as an effect of the simulated heat recovery at Danderyd.

Finally, a was assigned to a supposed heat pump, which includes the power required for running the compressor in order to determine the corresponding heat that could be supplied to the DH net. was 퐶calculated through the equation

퐶 (4) 퐶 = , meaning the ratio between heat supplied to the DH퐸 network ( ) and the compressor power ( ).

In the case where cooling was also considered, i.e. providing also a heat sink for the DC net within퐸 the energy station, a net temperature decrease was considered at the Danderyd site. This means that an assumed heat transferred to the SS from the DC network could be equally withdrawn from the SS for providing to the DH network through the heat pump. Where the cooling machine (i.e. a reversed heat pump) would be used, a in terms of

퐶

(5) 퐶 = was assigned, indicating the ratio between heat absorbed퐸 at the cold side of the machine ( ), i.e. from the DC system and the compressor power ( ). 3.3.3 Performance model 퐸 As additional information to the project providers, an extended performance model was created and run in order to compare different technical alternatives. In this model, technical information and operational data was collected from equipment suppliers and used for calculations in regards to;

 Energetic performance in terms of potential heating power (MW) and produced heat (MWh/year)  Costs in terms of investment costs and O&M costs where applicable, presented in SEK/kW and SEK/kWh.  Environmental pollution (in terms of CO2e due to refrigerant leakage)

Since equipment suppliers are in general restrictive with case specific numbers being published, the results from the performance model are not presented in this report but are provided separately to the project providers. Merely a summary is referenced in this report.

Costs and space requirements were determined respectively through summation of necessary expense items and the total number of proposed units required. The pollution was determined as

, (6)

푇 퐶 푒푞푖푒 = ∗ 퐶,푖푎

27 where the mass flow rate is the estimated refrigerant leakage rate and (also known as GWP, Global Warming Potential)is the carbon dioxide equivalent of the specific refrigerant used, ,푖푎 given in kg CO2/kg released refrigerant into the atmosphere. Among the퐶 analysed alternatives, some suppliers point to an industrial standard of 5% annual leakage rate while others claims as low as 0,5% annual leakage rate, accomplished through intensive development work regarding sealings and connections as well as shortening of internal pipings to reduce leakage.

3.4 Results This section presents the results that were found through the case study and are expressed in regards to the specific questions that were stated in the beginning of the chapter.

3.4.1 Heat resource side The available heat in the WW tunnel was assessed in terms of recovery potential (related to overall temperature decrease) as well as current and avoided heat losses through upstream HR. Furthermore, mapping of temperature distribution in the system, heat contributions from different connection tunnels as well as potential scenarios were looked into.

Recovery potential Going straight to the overall HR potential, Figure 9 shows the total heating power that could be allowed to be extracted within the entire tunnel network, regarding the boundary value of minimum influent water temperature to the WWTP.

Figure 9. HR potential, considering boundary for minimum influent temperature.

The baseline here is considered to be 12°C as limit for extracting any heat at all, i.e. if the influent temperature is to be lowered to this level. Again, this level of 12°C is considered as it would avoid the rather volatile range of decreased biological activity. Note that this is the general case, disregarding where heat is actually extracted in the tunnel system. Certainly, in a partial flow such as the Danderyd site, the actual temperature decrease is allowed larger, if also no additional extraction occurs in the system. It is quite clear that during the period Dec-Apr, the allowed heat extraction is very limited, while the rest of the year show very good potential. As the potential differs widely between seasons, it seems difficult to find a HR level that works around the year. This is of course discouraging as heat is most valuable during the winter for DH companies. If considering 10°C as the lower limit, the recovery potential becomes fairly good around the year but with the risk of compromising the WW treatment process as the range of 9-12°C is a very sensitive and unpredictable interval to operate in. If hypothetically allowing temperature decrease down to as low as 9°C would seem to open up the recovery potential widely.

In Figure 10, the available heat is shown when regarding a limit for the actual temperature decrease of the influent water. Here, the simulation using 12°C as minimum influent temperature is presented.

Figure 10. HR potential, considering limit for actual temperature decrease. Here, 12°C is the influent water temperature boundary for allowing HR.

In this case, 0.5°C was considered as the baseline limit for temperature decrease, as used as a general limit for WWHR in Germany and Switzerland. At this value, the average available heat is exceeding 3 MW during May-Nov. With higher extraction limits, the HR increases proportionally in Jun-Nov but remains the same during May and Dec as no further temperature decrease is allowed due to already low initial temperatures. Jul and Aug show a bit lower values despite high temperatures since the WW flows are relatively low during these months. During the wet season, Jan-Apr, results show that HR potential is in fact zero here as no temperature decrease is allowed if 12°C is considered as the minimum influent temperature for HR.

Temperature distribution in main tunnel From modelling of the heat sources upstream from the WWTP, the temperature distributions were calculated along the main tunnel of the SS. These are shown in Figure 11 - Figure 14, the four yearly quarters being presented in their respective figures. The location of MR22 represents the site of interest for the case study, about 12.8 km upstream from the WWTP of Käppala. The temperature nodes are representing the mixture temperatures in the main tunnel at the sites of connecting tunnels. The last point of every profile (Käppala) is the calculated influent temperature to the WWTP, which was at most occasions very close to the actual measured influent temperatures to the WWTP.

Figure 11. Temperature profile in the main tunnel, January-March.

Figure 12. Temperature profile in the main tunnel, April-June.

Figure 13. Temperature profile in the main tunnel, July-September.

Figure 14. Temperature profile in the main tunnel, October-December.

It should be noted that the flows from the municipalities of Nacka and Värmdö are not presented here, but were calculated into the final temperature at the Käppala node. Through these figures, some observations could be made of which the most important ones are;

 Temperature at the site of interest (MR22) is around 10-11°C in the wet season, January to April, which give little room for HR if boundary values in the influent flow should be kept. This is due to a high share of supplement water during this period which lowers the average temperature.  Around the year, temperatures at MR22 are relatively lower than downstream temperatures, in fact even lower than the final influent temperature to the WWTP.  It could be noted that the connecting flow from MR26 is providing a great temperature increase, all year around. Especially during wet season, this contribution becomes vital as the influent temperatures are in critical nitrification regions here. The reason is that this connection contains a relatively high share of SW in the total WW, between 78-100% around the year, as well as a rather high total WW flow. See also the upcoming section. Understandably, the inflow of SuppW is then relatively low, but an investigation of this was out of the scope of this study.  Observing the temperature drop/increase between the node (MR30+MR27) and the last node (Käppala), both at 0 km distance from the WWTP, this is due to the connection of the flows from Värmdö and Nacka municipalities, which carries comparatively low temperatures.

Temperature and flow contribution from different connections In further detail, the contributions made by the connecting flows could be observed in Figure 15, where the main tunnel temperature is depicted as reference along the stretch from far upstream down to the WWTP inlet.

Figure 15.Temperatures and flow rates of main and connection tunnels, January-March average.

The values in Figure 15 are presented as averages for the wet season, i.e. where low temperatures and high flows occur. The blue line depicts the temperature profile of the main tunnel, in the same way as in Figure 11-Figure 14. This profile has sections without values since some connections, such as MR15-MR18 joins together in a separate branch before connecting to the main tunnel. In these cases, the value following the empty section is representing the final connection of that branch to the main tunnel. As reference, the total accumulated flow is presented below in Figure 16.

Figure 16. Accumulated flow in main tunnel and the individual contributions, Jan-Mar average.

Observations noted through this analysis were;

 The connection with the highest temperature influence is MR26, as also noted in previous section. Compared to other connections with similarly high flow rates, such as e.g. MR01, MR26 has a relatively high share of WW in the total flow of that connection which provides a much higher temperature increase from that specific connection. Comparatively high connective temperatures could be found in MR03 and MR18 and MR29, but the corresponding flow rates are rather small in these connections.  A similar temperature increase as by MR26 could be observed at the height of MR03, even though the flow rate is much smaller than at MR26. This is because the relative flow rate in MR03 to the total flow in the main tunnel at that point is quite high as it is far upstream and the total flow is yet rather small.  The temperature influence from the tunnels of Nacka and Värmdö seems to be relatively small as it contains low temperatures that are close to the influent temperature in the tunnel from Lidingö side.

Heat losses throughout system, before and after heat extraction Looking into the subject of heat losses throughout the system, it is not an easy task to determine. Through the modelling, quantitative numbers was calculated in order to assess the overall heat loss situation on a basis of energy loss per cubic metre of water and per meter of flow distance, (W/m3•m). The specific numbers were used within the model for estimating for example temperature distribution, but are of little interest here. In Figure 17, the calculated total heat losses throughout the system are shown on a monthly basis in terms of MW.

Figure 17. Losses in the WW system, monthly basis.

Observing Figure 17, the heat losses seems to be highest in March and April, typically months with high water flow rates. However, if heat losses are expected to be highest during the period when the outdoor temperatures are the lowest, i.e. January and February, these results would be surprising. This could point to the fact that there is a thermal inertia in the rock walls, of up to as much as a couple of months. This suggests for example that heat is stored in the rock walls during warmer periods, i.e. summer months, where warmer water inside the tunnel is slowly heating them. There is even a period, September and October, where heat seems to be released from the rock wall and transferred to the water. The opposite occurs during the winter where the rock walls are slowly cooled down by the decreasing water temperatures, finally creating a large temperature difference between the walls and the water when spring begins and spring water (rain and melted snow) rather suddenly starts to flow in the tunnel.

When the temperature in the water flow decreases due to heat extraction upstream, so do the losses downstream of that point too. In Figure 18, the decreased losses are shown for temperature decreases in Danderyd by 0.1-0.5°C.

Figure 18. Decreased losses due to lowered tunnel temperatures. As can be observed there is a proportional increase in the energy lost for the incremental temperature decrease for each month. The pattern is quite natural from the assumption in the model that the heat transfer is proportional to the temperature difference between the water and the surroundings, i.e. the tunnel rock walls and the air flow in the tunnel. The highest amount of energy saved could thus be made during winter season and especially in April, with a high value of approximately 900 kW at 0.5°C temperature decrease. During the period August-November, the same reasoning tells that the heating of the water instead decreases when the temperature upstream is decreased, observed from the negative values of this period.

Temperature relation in regards to heat extraction The purpose of the modelling was ultimately to determine how much heat could be extracted while still maintaining adequate influent temperatures to the WWTP in Käppala. When a sudden temperature decrease was simulated within the model at the Danderyd site, the corresponding downstream temperatures were monitored. In this case, it was necessary to account for the decreased temperature losses that occur due to the smaller temperature difference between the water and the tunnel rock wall. In Figure 19 below, the ratio between a temperature decrease at Danderyd site and influent water to the WWTP could be observed.

Figure 19. Temperature change relationship, Danderyd and influent.

Figure 19 tells on a monthly basis what the effect of a hypothetical temperature drop in Danderyd would infer on the end temperature downstream at the WWTP. The solid line shows the ratio as it looks like today, ranging between 0.58-0.62. This ratio follows exactly the flow ratio between Danderyd site and the total flow to the WWTP. The dashed line is accounting for the heat loss correction. This means that a heat extraction in Danderyd would effectively lead to lower downstream losses, and the corresponding temperature decrease in the influent water decreases. As the heat transfer is proportional to the temperature difference, the relationship in Figure 19 is valid for different levels of temperature decrease.

Scenario – decrease of supplement water (SuppW) If SuppW were to be reduced, the flow and temperature contribution from that part would be smaller. As SuppW is assumed to be overall lower than the WW temperature from consumers, the total temperature would then effectively increase. This was assessed through modelling, with the results in terms of influent temperature shown in Figure 20 below.

Figure 20. Influent water temperature if supplementary water was reduced.

The SuppW reduction was simulated as percentage diversion of other supplementary water (OSuppW), meaning all SuppW except for ground water (GW) which was considered difficult to

34 reduce in practice. During wet season, especially in Jan-Feb, the SuppW reduction would increase the influent water temperature substantially as the OSuppW is quite cold compared to WW temperatures during that period. A smaller effect is also seen in March. An interesting observation is made when looking at April. Here, hardly any influence is seen even though it would seem that an initially high share of SuppW in the system would mean a potentially drastic temperature increase as the SuppW is reduced. The reasons for this are, first of all, that the OSuppW reduced is holding a rather high temperature depending on relatively high outdoor temperatures at this time of year compared to for example Jan-Feb. Also, the potential temperature increase due to SuppW reduction is eaten up by further increased losses during that period, in the way the model was expressed. As Figure 20 shows, April would then be a critical month were influent temperatures are quite much lower than rest of the year. However, as not accounted for in the model, reducing SuppW during winter and thereby increasing overall temperatures, would effectively also lead to less cooling down of the surrounding rock walls and smaller losses would then occur during April. During summer, OSuppW and WW temperatures are relatively similar leading to very low effects on final influent temperatures. During fall, the potential temperature increase is again high, resulting from a larger temperature difference between OSuppW and WW.

Scenario - Consumer heat recovery in large extension Another scenario, which is currently in direct focus, is where consumers are beginning to explore the HR possibilities within or related to buildings and facilities. Figure 21 below shows the simulation of extensive upstream heat recovery in consumer vicinity.

Figure 21. Influent water temperature when HR is extensively performed by consumers.

The simulation was performed for an incremental temperature decrease, 1-5 K, of the average WW temperature that leaves the consumer dwellings. The bars shown in the figure is depicting the temperature change ratio between the WW and influent water, the trend being somewhat related to the one in Figure 19. It could be observed that the effect is strongest during periods where the WW share of the total SW, i.e. the summer. Looking at the wet season, the absolute numbers tell that a 1 K temperature decrease by consumers would lower the influent temperature by approximately 0.4-0.5 K, implying that every incremental temperature decrease from consumer HR leads to a rather challenging development, as the influent temperatures are decreasing during already sensitive temperature intervals. However, the temperature decrease simulated at consumer side might be overdramatic as this would require every consumer to install equipment for this matter which would imply a long time period ahead.

3.4.2 Heat recipient side Looking at the other side of the HR system, the recipient in the shape of a DH net was considered. In relation to the available heat for recovery on the resource side, a corresponding heating power supplied to the DH net was simulated using typical operational numbers. Also the corresponding produced heat to the DH net, based on the heating power and number of days of the specific month was calculated. Figure 22 and Figure 23 below shows the supplied heating power (in terms of a monthly average value) and total heat delivered to the DH net, when considering heating only and heating and cooling respectively. Both cases is regarding conservative parameters for heat extraction, i.e. 12°C minimum influent temperature and 0.5 K maximum temperature decrease. The power and energy from WW and compressor is together what is supplied by the heat pump machine to the DH net.

Figure 22. Supplied power (left) and total heat (right) to DH net if heating only is considered.

Figure 23. Supplied power (left) and total heat (right) to DH net if both heating and cooling are considered.

Here, a COP of 4 was assumed, which is quite generous in regards to actual operational performance. It should be noted that a lower COP, i.e. a poorer performance, would potentially lead to a higher supplied power from the heat pump to the DH net, but would mean that a higher share of electrical power is used which in the end is undesired. If heating only is considered, a rather high share must be provided by supplementary heating over the year. During dry season just over 4 MW could be provided by the heat pump except from July due to relatively low WW flows. Over the year, a maximum of approximately 18 GWh could be provided from WW amounting to 50% of the total heat supply from the heat pump at a level of 5 MW required power.

When also considering cooling as a possibility, the average heating power supply is higher as the cooling compensates for the primary heat extraction from the sewer line. This fact also opens up the potential for HR during the wet season. Also, the more cooling power installed, the more HR from the sewer could be performed. The supplied power is here between just below 5 MW up to a high 6 MW from the heat pump, during dry season. During wet season the supplied heating power is estimated to equal the corresponding amount of cooling power plus the compressor power, in total approximately

1.5 MW. The total heat provided from WW is 28 GWh, providing 75% of the basis of 5 MW requirement from the heat pump.

Comparing these two alternatives with existing installations at K-verket and Brommaverket WWTPs, providing 9 GWh and 568 GWh of heat annually from treated WW, it could be realized that both alternatives are providing higher amounts of heat than the KFB installation. However, if the potential for HR from treated WW in K-verket is explored fully, estimated to 20-30 MW, HR from untreated WW upstream would not be advantageous in that sense.

3.4.3 Technical alternatives Looking closer into the technical solutions, the different supplier alternatives described in section 2.5 was analysed for providing a minimum of 5 MW of heat to the DH net at a temperature of minimum 72°C. Since equipment suppliers are in general relatively restrictive with publically publishing data and costs, the results presented here are not supplier specific. Through technical and budgetary proposals sent from the suppliers upon request, analyses were made for examining the following;

 Energy recovered and supplied  Operational performance and data  Equipment investment costs  Carbon footprint

The results should be considered more of a general indication of what could be expected in the field for heat recovery from untreated wastewater.

Energetic The installed capacity could be adapted according to the demand. Technology is also available for performing stage-less operation if load needs to be altered due to the varying level of available heat. The issue of untreated wastewater in contact with heat exchanger surfaces is constantly present. However, a high 95% availability could be reached using an adequate maintenance program and preventive measures.

Regarding the COP, at least 3.2 could be guaranteed through some suppliers in a trimmed operation when lifting the temperature up to 72°C.

Environmental/Carbon footprint The environmental effect is directly related to the refrigerant leakage rate and also to what refrigerant is actually used in the heat pump. With the suppliers with longer experience in using wastewater for heat recovery, a lot of effort has been invested into minimizing leakage. Thereby, a leakage rate of as low as 0.5% of the total refrigerant amount annually has been reached. Also, the use of new refrigerants such as R1234ze has been initiated, leading to even lower environmental effect. The total amount of CO2e becomes as low as only 262 kg per year. Certainly, this is at a very early stage and needs further testing and confirmation. Looking at the indicators of CO2e per installed kW and produced kWh, these may land at best on numbers such as 0.031 kg CO2/kW and 0.0000037 kg CO2e/kWh, which are of course very low.

Cost As for the costs, these were rather difficult to assess since they were expressed in different forms by the suppliers and were also covering different portions of the required equipment. The indicative costs per installed capacity (kW) and produced heat (kWh), these may land at best on numbers such as an investment cost of 7500 SEK/kW and 0.06 SEK/kWh if combining equipment for both pre-treatment, heat exchanging and heat pump. This is regarding the total investment cost per produced kWh over an estimated 15 year life-time, while disregarding running costs and interest rate. In discussion with NE, these numbers are at the higher level of production from other waste heat sources. Of course, the heat pump constitutes a large portion of the costs, believed to be normally about 50% of the total

37 installation costs. The largest uncertainty lies in the estimation of the necessary construction work and excavation of a runoff shaft. As for the operational costs, some suppliers have expressed the O&M costs to be in the size of 1% of the total investment cost annually.

3.5 Conclusions In regards to the questions posed in the beginning of the chapter, the following conclusions were possible to outline through the case study;

Regarding heat recovery potential  The theoretical HR potential in the WW flow is very large. However, for the specific WW system at hand, the actual HR potential is only a fraction of the theoretical according to the simulations performed.  Regarding actual HR potential, the available heat in the WW system is very seasonally dependant. It was found that when staying within conservative limits, there is no available heat for recovery during the wet season of Jan-Apr. The reason is that the current average temperatures of the influent water is already within a highly sensitive interval, where heat extraction and further temperature decrease would possibly have very bad effect on the biological treatment process in K-verket WWTP. This is unfortunate as heat would be most valuable during the winter when heat production is large for NE. During other periods of the year, HR potential is abundant and up to a 10 MW heating power might be possible. At current situation, between 9-10°C of allowed influent temperature could be considered as a limit for making the HR project feasible in regards to create a year around operating energy station.  Performing actual temperature measurements might show that temperatures are in reality in fact higher than simulated, which would imply overall better HR potential.  If cooling is also considered, HR possibilities are extended due to a compensating effect that this have on the net withdrawn heat from the WW in the sewer line. Combined heating and cooling would open up HR possibility also during winter.  Heat losses to the surrounding rock walls and ventilation air throughout the WW system is peaking at 35 MW in April, due to a rather sudden increase of average WW temperatures upstream in combination with cold rock walls from the winter and increasing flow rates from spring water. Through heat extraction upstream, up to 900 kW could be saved during winter.  During fall, the WW is actually heated by the surrounding rock walls as colder upstream SuppW comes in contact with the walls that have been heated during warm summer months.  Reducing SuppW would be highly beneficial, both for the treatment process as it moves average temperatures higher above the critical temperature interval (10-12°C) during winter season and for the HR potential as it may even allow recovery during wet season. The exception here is the month of April, where very little effect is seen of reducing SuppW as the effect is eaten up by increased losses.  Allowing consumer HR in large extension would, according to the model used, decrease influent water temperatures and affect the biological treatment process negatively. How large extension however has not been established as it would require more sophisticated models taking into account transient behaviours of the system. A conservative standpoint is considered sound at the moment regarding allowing consumer HR.

Regarding technical solution  Results show that it seems difficult to find a year-around solution to produce high heating power during all seasons, due to the low available heat during the wet season.  Compared to existing HR installations from treated WW at K-verket and Brommaverket, available heat during conservative conditions is still well above what is today recovered at K- verket. HR upstream should then not affect potential extension of HR downstream of the WWTP under these conditions. However, in relation to what is produced at NE (over 500 GWh of heat per year), this would be in no comparison.

 The investment cost for the imagined WWHR project could possibly be in the range of other waste heat projects. However, as long as the production is allowed only at part-time of the year, the energy cost will be quite high.

3.6 Recommendations & Further work A few recommendations and suggestions for further work is here expressed, based on the findings and experiences from this project.

 In general, a great difficulty and limitation in this project was the lack of temperature data. Before continuing with the HR project, it seems vital that temperature and flow measurements are performed at the specific site considered for HR. This is considered as a minimum for gaining reliable input data for design and dimensioning of equipment.  In the model used for all calculations, temperature data from only one single point was available, i.e. the influent water temperature. It is suggested that temperature measurements are performed at more locations throughout the WW system in order to confirm and calibrate the model but also to more precisely predict potential consequences of upstream HR. A common observation from projects of this kind is the importance of raw data, which could at times instantly confirm questions otherwise being spent days on speculations.  In regards to the boundary value of minimum influent temperature, where 12°C was considered critical for this project, a thorough process evaluation on the temperature dependence might show a more nuanced picture for this boundary value. This also relates to the necessity of a system-wide optimization analysis of the WWTP to assess where most energy is conserved.  System optimization of the entire urban waste water cycle. It might even be the case that expanding the WWTP capacity in order to energetically optimize the system will be the most economically viable situation.  It may be an interesting case study to explore the possibility of performing HR at the site of an existing pump station within the system.  Some questions have been treated in this study but far from fully answered. Also, further questions do constantly arise around the topic of upstream HR. One suggestion is to set up a smaller scale pilot project, in the order of a couple 100 kW, at e.g. the site of the case study or where consumer requests are lying. Through this pilot demonstration facility, some questions should be possible to answer regarding e.g. temperature dependence of upstream HR. This may also be a good opportunity to show good will and objectiveness to the issue and establish a transparent and open relationship between the treatment plant, the authorities and the consumers.

 All in all, there is a great need of continuing pre-studies and research on understanding the potential risk of lowering downstream influent temperatures from different upstream events. As a suggestive continuing study, the following may be performed; o Install temperature measurement equipment at the existing measuring points for flow rate. (Required for acquiring a better view of the vast WW system, building and confirming models for sensitivity analyses) o Perhaps it is necessary to recalibrate the WW sources from the subsystems connected to every measuring point. o Build a model that includes the entire urban WW system, i.e. even the sources of WW and the effluent WW being released into the sea. o Perform a comprehensive sensitivity analysis and adjoining risk analysis of the entire urban WW system using indicators of CO2 equivalents and costs and including every parameter that may affect the performance of the WWTP.

4. A widened perspective – Wastewater heat recovery in Sweden In regards to the potential and possibilities for HR along the sewer line, the specific sewer system as well as the specific site within the system are all unique and needs individual treatment. The theoretical heat potential in the WW systems could be very large, possibly up to the order of several 10s of MW. In practice however, the heat that could actually be recovered is often limited as the treatment process requires that the influent water keeps adequately high temperatures, primarily to provide sufficient nitrogen removal if biological treatment is used. Actual site and system measurements for temperatures and flow rates are absolutely necessary in order to both determine the HR potential but also for assessing the effect of lower influent temperatures to the downstream WWTP. This chapter is divided into isolated sub-sections, where specific issues related to potential wastewater HR in Sweden are treated, such as the perspective of the very process unit of a WWTP, legal and financial implications and variations in influent WW temperatures around the country. 4.1 Upstream heat recovery – The perspective of a WWTP process unit This project did not focus on the factual mechanisms of the treatment process within the WWTP, but were merely considering boundary values as set by limits for wastewater treatment. However, these boundary values are a delicate matter as they are not easily set but at the same time have an immense impact on the activity of the WWTP, why this becomes perhaps the strongest limiting factor for the potential of HR along the sewer line in a country like Sweden. Considering this fact, it was deemed that a few paragraphs were dedicated here for explaining the background of the temperature boundary values and also include the perspective from the WWTP process unit at K-verket. This section could be related to by other WWTPs who are being faced with the question, or are in the progress, of implementing large scale HR within their sewer system.

Whether HR from wastewater along the common sewer line is possible, and to what extent, is ultimately determined by the spare capacity of the WWTP in terms of what possible temperature decrease in the influent water that could be allowed. For every sewer system with its connecting WWTP, this becomes a unique situation. The only imaginable scenario where this would not be the case seems to be if the influent water were to be heated at the inlet of the WWTP. To determine such spare capacity is not easy. Both current and future treatment requirements must be considered and more importantly, the potential risks must be carefully treated.

Preconditions for Käppalaverket WWTP For the specific sewer system and WWTP (K-verket), certain preconditions apply and are listed below;

 First of all, K-verket utilizes active sludge treatment which is not very common in these regions. This means sludge is suspended for a certain amount of time in order to allow the nitrification bacteria to work efficiently.  K-verket has experienced that with influent temperatures of 11-12°C, the nitrification process becomes very slow and at 9°C it is basically non-existent.  The yearly average of effluent nitrogen concentration is around 9 mg/L water. The current limit is 10 mg/L, as stated by authorities.  The effluent from K-verket is disposed to the Baltic Sea which today is strongly subject to eutrophication. The nitrogen removal demands are therefore even stricter, and larger compromises in the removal process cannot be allowed.  With over 600000 P.E., K-verket is one of the largest WWTPs in the region. This means that basically no back-up in terms of sludge transfer from other WWTPs is available if the activity in K-verket is disturbed and the organic sludge is eliminated.

Related to these, other preconditions may apply to other WW systems and WWTPs around the country. It should be declared that the nitrogen removal in K-verket is at a very high level and the technology is far reached to what is physically possible, due to the high treatment demands.

Lower influent temperature – Problems and mitigation measures The bottom line here is that nitrogen removal must be performed through biological treatment where nitrification bacteria are working. As all biological processes, this is temperature dependant. A decrease in influent temperature would therefore influence the rate of which bacteria reproduce and nitrogen is removed. It is also the case that the sludge sinks slower at lower temperatures. A decrease in temperature effectively means that more biomass is required in order to remove the same amount of nitrogen. This could be carried out through mainly three different measures;

 Increased volumes of the basins where nitrification bacteria are suspended.  Add and keep more biomass in the existing basins together with adopting so called membrane technology. The risk with maintaining higher sludge volumes (per surface area) is that the sludge is having problems to sink in the post-sedimentation, which becomes the bottleneck in the process line. The sinking is important in order to recirculate the sludge for an efficient and regenerative process. The membranes are used as physical barriers so that the sludge does not escape the basins and is allowed to sink properly.  Adding transport materials with large surface area which sinks through the water and onto which bacteria can grow. These are recirculated and thereby an efficient process is created.

All three alternatives would furthermore require ethanol additives which come with very large costs. As an example, KFB has estimated that the membrane alternative would cost approximately 2 billion SEK (approximately 3150 SEK/P.E.).

Future development – Higher treatment requirements In the future, even higher treatment requirements will apply. In 2017, the effluent nitrogen limit will be set to 6 mg/L, which in practice would demand levels of 5 mg/L for the WWTP to dimension its activity for. The strategy of K-verket to comply with the stricter demands is to add certain additives. Factors that would promote staying on the conservative side and limiting upstream HR would be;

 With potentially lower influent temperatures from upstream HR, the spare capacity would then be even sparser.  An increasing number of P.E. will be connected to the sewer system in the future which would include a higher load on the WWTP.  The decreasing amount of water consumption on a per capita basis would, (although a promising step in the direction of energy efficiency behaviour), mean on average lower influent temperatures while nitrogen level with main source from human disposals remains. This also together with the increasing amount of supplement water from an aging sewer system, higher infiltration of ground water and increased precipitation.

Necessary analyses From the view of the WWTP and its process, it is necessary to determine a few key questions in regards to the implied influent temperature decrease from upstream HR before approving further exploration, namely;

 Optimization of the energy use throughout the entire urban WW cycle.  Quantitative analysis of the process values as effect of lower influent temperatures.  Risk analysis, what if the WWTP is completely shut down due to unexpected and unmanageable low influent temperatures? What would be the environmental impact?

4.2 Legal and financial implications This section relates to general issues in the nature of legal and financial complications that needs to be pondered and resolved before a HR project of this kind could be initiated.

When considering the location of HR in the WW system, either locally in building relation or centrally at the common sewer line, a general standpoint must be determined before exploring the HR and perform installations as interest has been shown by property owners to install their own HR equipment. Consumer HR is today regulated by law in terms of “outlet water from the consumer must not be colder than the inlet cold water temperature” and also that “any larger alterations to the existing local WW system must be in consensus with the authorities” (Svenskt Vatten, 2016). Today, a conservative standpoint regarding upstream HR has been maintained, but technical development has opened up HR possibilities and hence a new assessment might need attention. As interest, such as e.g. the setup of this project, from WWTP is growing regarding energy optimizing the urban WW system, this issue falls into this legal (and also somewhat ethical) conundrum. It may be difficult with the current standpoint to defend a large scale HR plant at the common sewer line while preventing property owners from performing local HR.

Looking more close-up and regarding boundaries for ownership of the energy facility, this becomes a separate issue to resolve. Either the DH company (here NE) or the WWTP company (here KFB) could be the sole owner of the equipment and the facility while the heat is sold by KFB to NE. It may also be the case that an equipment supplier is the owner (alternatively part investor) and heat is sold over a purchase agreement. As for the specific situation, KFB is legally held to the water and sewer legislation (ABVA). In practice this means they are publically owned through taxation and the investments they do must be beneficial to the inhabitants of the served municipalities. Normally, the financial risks that KFB may take are within the investment costs while the running costs must be well known and cannot be risked (Nielsen, 2016). On the other side, NE is owned by Solna Stad and Sundbybergs Stad through the shareholding company Norrenergi & Miljö AB. NE is to a larger extent governed by the law about public purchase within the energy section, (LUFS). In the specific situation where a DH company and a WWTP company are collaborating, the presence of both legislations might be both limiting and enabling the cooperative project depending on where boundaries of facility ownership are drawn.

Related to both above paragraphs, it becomes a specific problem related to the case study if a HR agreement is set by the WWTP and the DH company for heat purchase and local HR installations are extensively carried out. In that case, the delivered heat to the DH company may be compromised as the overall temperatures in the SS is lowered, potentially leading to breaking the contract for the heat delivery. Hence, it is of great importance that an overall perspective is regarded in order to determine standpoints and legislation where society could benefit most from HR in the urban WW system.

In addition to above mentioned, some practical issues have been identified and/or presented by others to be fundamental for projects regarding waste heat recovery, such as (Källman & Petterson, 2014) and (Havtun & Bohdanowicz, 2014). These are for example;

 How to value the recovered heat? This would in this case relate to how the heat can be claimed as property of either part. Strictly, consumers connected to DH are purchasing heat supplied to heating water, using some of it and “wasting” the rest to the sewer. It may then pose a complicated situation where heat is recovered from the sewer, sold and provided to the DH net and at the end sold back to the consumers. This issue might be in need of further pondering.  Limits of ownership.  Confidence between the parties.  Both parties (heat seller and buyer) see long-term guaranties and gains of the project. The last three points are related to what is mentioned in above paragraphs.

4.3 Temperature relationship – other large scale WWTP in Sweden In this project, the analysis was to a wide extent regarding the boundary values of influent WW temperatures to the WWTP. These temperatures were further investigated through data received from several large WWTPs in Sweden, from north to south. Figure 24 below show the monthly averages of influent WW temperatures for eight different WWTPs in Sweden for the year 2015, including K- verket.

Figure 24. Influent temperatures at WWTPs at different locations in Sweden.

What could be read out from the lines are for one thing that, not surprisingly, the trend is very similar across the country around the year with rather low temperatures during wet season and substantially higher during the summer period. If regarding the conditions of K-verket with active sludge treatment, most of the WWTPs face the same issue during wet season with temperatures within the critical range for the biological treatment. However, not all WWTPs are using active sludge treatment for which HR potential might be higher. The more northern WWTPs in Luleå and Gävle here show very low relative temperatures around the year, possibly due to more and colder SuppW from colder outdoor temperatures. In January and February, the SuppW is very low in Luleå as the temperature rarely reaches above zero degrees, resulting in a temperature closer to the more southern located WWTPs. At the same time, snow melting occurs more or less through the entire winter (wet season) in the southern regions. As spring starts large amounts of snow starts to melt in the northern parts why the influent WW temperatures drop quite a lot in e.g. Luleå. Without having more detailed information about SuppW flow rates and shares, it is difficult to draw more certain deductive conclusions on the curves of specific WWTPs. However, it could be stated that both the incoming melted snow (SuppW) as well as the general climate, i.e. outdoor temperature both are decisive factors for the total influent temperature. The shifting in the curves in vertical direction should (more, but not completely) indicate the general climate conditions of the site, while the more quick changes of the individual curves are due to SuppW inflows.

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Appendix A – Flow measurement data used for connection points This appendix present the actual numbers used for flow rate at the connection points in the hydraulic model. These numbers are monthly averages based on field data over a five year period, given in litres per second.

MR01 MR02 MR03 MR06 MR09 MR15 MR16 MR17 MR18 MR20 MR21 Jan 320,3 15,2 64,6 119,6 23,1 32,4 172,2 126,5 9,5 89,6 13,6 Feb 333,1 14,6 63,3 120,7 22,2 31,5 186,7 120,7 9,2 75,6 15,2 Mar 269,9 14,5 65,0 120,7 22,5 32,1 177,5 123,3 9,4 77,7 14,2 Apr 252,9 15,5 67,3 132,7 23,6 33,9 175,5 131,8 10,1 82,8 12,3 Maj 232,3 14,4 61,4 93,3 20,6 27,9 140,6 95,9 8,7 59,0 10,7 Jun 214,6 13,7 59,2 95,3 19,7 25,2 137,1 84,9 8,1 53,2 8,0 Jul 191,3 11,8 52,8 77,3 15,2 19,9 106,0 67,9 7,2 39,6 6,4 Aug 207,1 12,7 53,2 81,5 17,7 21,7 118,1 80,7 7,5 42,1 8,7 Sep 240,2 14,7 60,7 103,9 20,7 26,8 137,2 102,2 8,5 55,4 10,7 Okt 254,4 14,1 59,2 104,0 20,7 25,8 140,3 96,6 8,5 58,2 10,8 Nov 261,8 14,3 59,6 104,8 21,1 27,4 153,6 101,1 9,2 72,2 10,9 Dec 273,9 14,5 61,9 115,1 22,6 30,9 155,7 120,4 10,3 99,2 12,4

MR22 MR23 MR26 MR27 MR28 MR29 MR30 MR31 MR32 MR33 Jan 19,0 41,5 236,3 6,7 38,9 21,6 38,2 59,6 9,1 9,3 Feb 19,2 39,9 254,0 6,3 35,4 21,5 34,8 54,6 8,8 9,9 Mar 18,4 38,8 238,3 6,5 33,2 19,5 31,8 54,3 8,3 8,9 Apr 17,7 43,3 239,8 6,9 34,3 21,7 37,1 58,7 9,3 9,6 Maj 16,4 29,6 212,3 5,2 28,0 17,1 26,1 57,0 6,8 7,3 Jun 16,4 27,2 195,9 5,5 30,2 17,6 28,4 46,2 5,8 5,8 Jul 12,5 18,3 180,3 3,7 23,5 13,1 18,5 40,8 4,3 4,8 Aug 14,8 23,0 203,2 4,9 28,5 14,2 19,7 42,6 5,8 5,8 Sep 16,4 28,5 227,3 5,7 30,8 16,9 25,9 53,7 7,1 7,3 Okt 16,5 30,0 221,8 5,8 31,1 18,6 27,9 42,4 7,1 7,3 Nov 16,4 33,1 218,7 6,1 33,3 20,1 31,3 54,0 7,7 8,1 Dec 18,0 39,2 210,9 6,9 37,9 20,4 37,5 57,4 8,8 8,8

Värmdö Nacka Bonästunneln MR10 EG00 Jan 56,8 216,9 28,1 200,9 635,8 Feb 59,5 215,9 31,1 197,2 636,8 Mar 52,7 184,7 31,7 206,8 644,5 Apr 61,1 210,5 37,0 195,8 672,9 Maj 45,4 150,2 29,3 215,1 594,4 Jun 47,5 163,2 30,1 218,4 567,2 Jul 38,2 109,9 26,8 186,5 490,3 Aug 41,8 128,0 27,3 197,4 523,7 Sep 49,3 148,0 31,6 197,2 580,3 Okt 53,1 188,2 26,1 214,8 589,9 Nov 62,7 196,0 25,2 193,6 579,2 Dec 66,0 201,7 27,1 183,9 601,6