Combining thermal hydrolysis with drying and downstream thermal processes to optimize energy recovery from sewage sludge

W. P. F. Barber*, and P. Christy*, * Cambi Inc., 279 Great Valley Parkway, Malvern, 19355, PA, USA (E-mail: [email protected]; [email protected])

Abstract Thermal hydrolysis of sewage sludge prior to subsequent is a well-established process with nearly 90 facilities worldwide. Cambi, the market leader, has two thirds of the installations ranging in size from 11 – 450 t DS/d. The majority of plants are combined with land use of the biosolids as a low carbon fertilizer due to the biosolids meeting Class A standard under the US EPA 503 regulations. In addition, the dewatered biosolids are friable, have low odor and high dry solids which make it a desirable product. However, there are instances where land application is limited due to regulations or topography, or clients have existing thermal infrastructure and want to maximize its use. The aim of this paper is to demonstrate how combining thermal hydrolysis with digestion prior to thermal systems such as drying and incineration can be beneficial and highlights full-scale case studies to quantify the influence in the field. Keywords Anaerobic Digestion; thermal hydrolysis; drying; incineration

INTRODUCTION Thermal processing of sludge, such as drying or incineration, can play an important role within the development of a water company’s strategic plan for biosolids management. In addition to regulatory requirements which may exist, there may be insufficient local land available (due to topographical limitation for example), or the sludge may contain other materials limiting its use. Furthermore, water utilities may already have existing infrastructure based on thermal treatment and prolonging their use may provide an economic long-term option compared to installing new infrastructure. Additionally, some dried biosolids materials have had sustained use as a fertilizer product (Clark, 1930). In spite of the advantages of thermal processing – primarily sludge reduction, there are several disadvantages and these have led to a trend in Europe where many drying and incineration systems have been, or are being decommissioned. Table 1 highlights the advantages and disadvantages of both drying and incineration. Table 1. Advantages and Disadvantages of sludge drying and incineration Advantages Disadvantages Drying Proven at full-scale for sewage sludge High capital and operating costs compared with other methods of water extraction High volume reduction compared with Needs auxiliary fuel such as natural (or bio) gas dewatering Meets Class A as defined by US EPA 503 Potentially complex operation regulations Storage and handling of product may be easier Sludge goes through a “sticky phase” which causes than sludge cake (especially if pelletised) handling problems in the drier and may require recycling of dry sludge. However, this may be overcome by use of indirect driers Long storage times possible Very sensitive to fluctuations in load. If this happens

2797 Advantages Disadvantages either dry solids output is compromised or sludge throughput must be reduced Larger range of disposal options than biosolids Issue with fibres cake Increases heating value of sludge prior to thermal Chemicals (such as ferric) may have adverse effects on destruction the thermodynamic properties of the dried sludge. This has caused explosions in stored dried material. Material can be used as a fuel With direct driers, production of gas which may require further treatment. Use of digestion or heat recovery can partially Critically reliant on gas prices and may become offset the energy demand of drying uneconomical with slight fluctuation in gas price Rewetted dried pellets may be odorous, especially for raw dried sludge Not a final disposal option, will compete with high quality Class A dewatered cake for agricultural outlets Expensive to turn into fuel so third party can exploit value of at negligible or no cost Can be high carbon footprint in absence of on-site renewable energy availability.

High dry solids sludge can be dusty which reduces its desirability in certain reuse markets Incineration Highest Volume Reduction Very low on hierarchy Potential for energy recovery Requires complex gas abatement technology Ultimate disposal option Generates carbon dioxide which could incur future taxes Fly ash may be recycled, and nutrients extracted Negative publicity (especially on current sites) Not from viewed by public as acceptable solution. Environmental groups calling for incinerator tax schemes to be implemented Standard technology Removes phosphorous (a non-renewable resource) from ecosystem Reduces reliance on landfill Produces a number of , most of which are classified as hazardous requiring special treatment options High capital and operating cost compared to alternatives

THEORY Energy requirements of drying One of the main issues with drying is the energy required to evaporate water. As well as dryer type, the quantity of energy required by a sludge dryer is critically dependent on the temperature and type of sludge which is being dried. The type of sludge influences both thermodynamic properties and dewaterability (i.e. water content) of the sludge being dried. Typically, the greater the fraction of waste , the higher the water content within the material to be dried and the higher the energy demand for the dryer. From theory, assuming an ambient temperature of 20°C, the energy required to evaporate one metric tonne of water, is 2.34 MMBTU. In addition to the water, the dry matter in the sludge has to be

2798 heated. At the same ambient temperature, based on the specific heat capacity for dried sludge (Ref), heating one tonne dry matter of sludge would require an additional 0.10 MMBTU. These numbers assume no heat loss and need to be adjusted to account for dryer heat losses and potential heat recovery both of which are dependent on type of dryer. From the literature (refs), dryers typically require 800 kWhr to 1100 kWhr of energy for each metric tonne water evaporated. In imperial units this can be in the range of 3.4 MMBTU/t water evaporated, although operational data – as we will see – is typically higher. To put this number into context, this is orders of magnitude higher than the energy required to remove a ton of water by dewatering and then transporting it. The US Department of Energy (Ref) provides data on the energy use of transport with respect to hauling weight (ref). From that report, a heavy truck consumes 3357 BTU energy to move one ton one mile, therefore on an energy basis it is possible to move a ton of water over 1000 miles for the same energy expenditure as evaporating it in a dryer. Using rail, it is possible to transport a ton of water over 10,000 miles to the other side of the world.

Fortunately, at a wastewater treatment works, some of the energy required for sludge drying can be recovered by use of waste heat or by using biogas produced from anaerobic digestion. In addition to producing biogas, a further benefit of anaerobic digestion is reducing the quantity of sludge requiring drying.

Thermal hydrolysis and sludge drying

As with anaerobic digestion, thermal hydrolysis can increase capacity of existing drying plants, or significantly reduce the size of new-build facilities. In order to demonstrate the influence of anaerobic digestion with and without thermal hydrolysis pre-treatment, a hypothetical example based on drying a dewatered sludge cake containing 100 dry tons/day of either primary, waste-activated (WAS) or mixed (60-40 ratio primary-WAS) sludge to 90% dry solids from ambient temperature is used. The example is based on expected dewaterability (Higgins et al., 2015; Phothilangka et al., 2008; Everett, 1972) and digestion performance (Van Dijk & de Man 2010; Stuckey & McCarty, 1984; Haug et al., 1978) Figure 1, shows the impact of digestion with and without thermal hydrolysis pretreatment on water evaporation requirements (size) for the hypothetical dryer.

2799

Figure 1. Influence of digestion and digestion with thermal hydrolysis pre-treatment on water evaporation of drying. Key: () primary sludge; () mixed sludge (60:40 primary:WAS); () waste activated sludge.

What is immediately noticeable is the influence of sludge type on the dryer size. Regardless of presence of digestion or pre-treatment, a dryer processing waste activated sludge will be approximately double the size of one processing an equivalent amount of primary sludge. This is due to the poor dewaterability of waste activated sludge which is limited by presence of extracellular polymeric material (refs), compared to primary sludge. Depending on sludge type, anaerobic digestion reduces dryer size by between 30% (for pure WAS) to 40% (for pure primary). This reduction is due to conversion of sludge to biogas upstream. When thermal hydrolysis is added, a larger quantity of sludge is converted to biogas, but importantly regarding drying, the dewaterability is significantly improved. This improvement is typically 10% points (Barber, 2016). The combination of further enhanced volatile solids destruction and improved dewaterability reduces the drying by 65 – 70% in size compared to the raw sludge dryer. Even compared to the option with digestion, drying requirements are reduced by half when thermal hydrolysis is introduced. Figure 2, summarizes these findings in a normalized way for the mixed sludge, based on the size of a raw sludge dryer being the baseline.

2800

Figure 2. Influence of digestion with and without thermal hydrolysis pre-treatment on size of sludge drying

Figure 3 shows how the energy requirements of drying can be offset by the production of biogas from anaerobic digestion. In this example using the data for mixed sludge, the dryer requires a little under 700 MMBtu/d of energy to run. Anaerobic digestion reduces the energy demand to approximately 580 MMBtu/d, whilst the addition of thermal hydrolysis lowers it further to 330 MMBtu/d. In this instance, anaerobic digestion can produce a biogas with a heating value of circa 750 MMBtu/d, which meets the dryer demand and leaves a further 170 MMBtu/d which can be used elsewhere or to generate renewable energy. Whilst thermal hydrolysis increases the biogas production a further 170 MMBtu/d to 920 MMBtu/d (approx. 20%) compared to standard digestion, the dryer only requires 330 MMBtu/d, meaning a surplus of nearly 600 MMBtu/d which is nearly triple that for the digestion option when thermal hydrolysis is absent. This shows the main benefit of thermal hydrolysis when coupled to drying as a combination of increased biogas production with a reduction in dryer energy demand.

2801

Figure 3. Energy required by sludge drying compared to energy produced during anaerobic digestion. Baseline: Dryer required to dry 100 tDS/d sludge cake to 90% dry solids. Key: () energy required by dryer (60:40 primary:WAS); () energy produced by anaerobic digestion; () net energy.

The results of the analysis depend on how much waste activated sludge is in the sludge mix, as this not only digests worse (i.e. less biogas production to offset the dryer energy needs), but also, it dewaters less well (more water to evaporate therefore dryer energy demand increases). For waste activated sludge, the energy demand for drying increases from 800 MMBtu/d to nearly 950 MMBtu/d whilst gas production reduces drastically from approximately 800 MMBtu/d to below 400 MMBtu/d for standard digestion. From a surplus of 400 MMBtu/d for mixed sludge, there is now a deficit of over 550 MMBtu/d which is necessary for standard digestion. Whilst biogas production also drops with thermal hydrolysis from 950 to 550 MMBtu/d, the energy required by the equivalent dryer also reduces to 440 MMBtu/d, which maintains a surplus of energy, albeit a little over 100 MMBtu/d.

Figure 4 summarizes the influence of both digestion and thermal hydrolysis on the energy balance of drying.

2802 a) 75% of biogas required by dryer

b) 23% more biogas of which 35% required by dryer plus 10% for steam production

3 times more biogas available for co-gen than standard digestion

c)

10% more biogas of which 50% required by dryer plus 1% for steam production

2.4 times more biogas available for co-gen than standard digestion

Figure 4. Influence of thermal hydrolysis on energy balance of digestion with drying. a) no thermal hydrolysis; b) thermal hydrolysis of both primary and waste activated sludge; c) thermal hydrolysis of only waste activated sludge. For 100 dry ton/d mixed sludge comprising 60% primary and 40% waste activated sludge.

Although thermal hydrolysis produces between 23% and 10% more biogas for full- and WAS-only processing respectively, the smaller requirements of the dryer mean that between 240 and 300 percent more biogas is available for other uses, such as renewable energy generation or production of biomethane compared with standard digestion.

2803 The analysis clearly highlights the importance of dewatering. Although most emphasis has been given to use of the technology to improve biodegradability of sludge with process efficacy being inversely proportional to the initial biodegradability of the material (Wilson & Novak, 2009), thermal hydrolysis was originally seen as a means to improve sludge dewaterability (Lumb, 1940, 1951). Interest gathered pace in the early 1970s when significant improvements in dewaterability were correlated with heat application to various sludges (Everett, 1972). There is currently one full-scale facility where thermal hydrolysis is intentionally installed after anaerobic digestion in order to further enhance dewaterability and volatile solids destruction (Kovolos et al., 2016). At that site, in Amperverband, volatile solids destruction is between 70 and 75%, with dewaterability over 40% dry solids from a centrifuge. When the numbers from that plant are used in the current analysis, installing thermal hydrolysis downstream of digestion can actually provide a volume reduction with just dewatering equivalent to that of a raw sludge dryer. In addition to not requiring drying with its concomitant gas demand (860 MMBtu/d), the downstream thermal hydrolysis option would produce in the region of 1070 MMBtu/d biogas.

Thermal Hydrolysis with incineration Incinerators are sized based on physical throughput and quantity of energy being fed to the system, and both of these are influenced by anaerobic digestion and thermal hydrolysis. Whether or not spare capacity is potentially available is dependent on a combination of volatile solids content and increased dry solids in the cake being burnt. Figure 4 shows how digestion influences heating value of sludge on both a dry (Fig. 5a) and wet (Fig. 5b) basis.

Here, raw sludge has a heating value of between 7000 and 8000 Btu/dry lb, and typically anaerobic digestion will lower this value by 15%, increasing to reductions of 20% and 25% when thermal hydrolysis is added for WAS-only and all sludge. This reduction will have implications when dried biosolids is used directly as a fuel source, for example in cement kilns. However, the change in heating value alters when looking at a wet cake. The reduction due to digestion increases slightly to 20% due to slight changes in dewaterability. However, improvements in dewatering gain back heating value when digestion is preceded by thermal treatment. For hydrolysis of waste activated sludge, the cake has a heating value approximately 10% lower than raw, cake, whilst thermally hydrolysed digested cake has similar heating value to raw material.

2804

5a)

5b)

Figure 5. Influence of digestion with and without thermal hydrolysis on the higher heating value of dry sludge (a) and wet cake (b)

It is possible to calculate the value of both volatile and dry solids with respect to energy content. From this analysis, one percentage point of volatile solids is equivalent to 100 Btu/lb, whilst one percentage point improvement in dewatering is worth 325 – 350 Btu/lb. Therefore, as thermal hydrolysis decreases volatile solids content by over 15 % points, the energy lost is equivalent to 1500 Btu/lb. In order to offset this energy loss, the dewatering would need to improve by (1500/350 =) 4.3 % points. The better dewatering and higher energy content means that less energy is required to pre-heat the sludge prior to burning, resulting in more of the heat energy being used for power generation in the incinerator. This is shown in Figure 6.

2805

Figure 6. Energy required by incinerator for burning 100 tDS/d raw sludge equivalents. Key: () energy required by incinerator; () auxiliary fuel required in addition to fuel value of sludge.

Figure 6 is calculated from mass and energy balance for use of a fluidized bed incinerator compliant with Waste Incineration Directive (Ref). The major outputs from the incinerator are all influenced by digestion and thermal hydrolysis as demonstrated in Table 2.

Table 2. Influence of digestion and thermal hydrolysis on incineration of sludge Raw Digested TH-WAS TH-All Combustion air required [scfm] 13283 66% 61% 57% Flue gas [scfm] 20625 61% 55% 51% Scrubber effluent [lbs/hr] 18592 53% 46% 41% Steam production [lbs/hr] 6766 130% 67% 47% Ash production [lbs/hr] 1917 100% 100% 100% Generation at incinerator [MWe] 1.10 0.00 0.02 0.37 Biogas [MWe] 0.00 2.95 3.35 3.71 Note: Percentages relate to data for raw sludge. E.g. Steam production for raw sludge is 6766 lbs/hr. With thermal hydrolysis the figure is 47% of this, i.e. 3180 lbs/hr steam.

Whilst the majority of parameters reduce with increasing digestion and thermal hydrolysis, power generation can increase. For digestion, there is no generation at the incinerator due to the need for auxiliary fuel, however, with thermal hydrolysis there is 0.37 MW generation possible compared with 1.10 MW for raw sludge. However, when the energy generated upstream of incineration is included, it is possible to see the value of anaerobic digestion, with each digestion option extracts more energy overall than raw incineration.

2806 Case-studies There are numerous facilities where Cambi thermal hydrolysis has been used prior to downstream thermal processing of sludge. These are summarized in Table 3.

Table 3. List of thermal hydrolysis facilities where biosolids undergo downstream thermal processing Size Plant Year Sludge processed Outlet [tDS/d] Borregaard 1999 12 Paper/pulp Incineration Ringsend 2002 244 Raw Drying/dewatering with land application of biosolids Bruxelles Nord 2006 99 Raw** Wet air oxidation Amperverband 2007* 22 Digested Incineration Vilnius 2011 99 Raw Drying with land application of biosolids Davyhulme 2011 364 Raw Incineration or land application of biosolids cake Crossness 2014 129 Raw Incineration or land application of biosolids cake Beckton 2014 129 Raw Incineration or land application of biosolids cake Tilburg 2014 110 Raw Incineration Vigo 2014 66 Raw Drying with land application of biosolids Psytallia 2015 79 WAS-only Drying Hengelo 2015 39 WAS-only Incineration Bakdal 2016 110 Raw, foodwaste Drying and/or incineration Jurong 2016 58 WAS-only, FOG Drying and incineration * Thermal hydrolysis on site since 2007. Downstream configuration since 2014 ** Raw refers to primary and waste activated sludge prior to digestion

In addition to the facilities in Table 3, other plants have used thermal hydrolysis to shut down drying plants as shown in Table 4.

Table 4. Facilities where thermal hydrolysis has been used to shut down sewage drying plants Size Sludge Plant Year Outcome [tDS/d] processed Drum dryers decommissioned and replaced by thermal Afan 2012 86 Raw hydrolysis with agricultural recycling of cake Santiago de WAS- Originally used solar drying, now thermal hydrolysis with 2012 124 Chile only agricultural recycling of cake 4 streams of drum drying decommissioned and replaced Bran Sands 2012 173 Raw by thermal hydrolysis with agricultural recycling of cake Drum dryers decommissioned and replaced by thermal Cardiff 2012 129 Raw hydrolysis with agricultural recycling of cake Drum dryers decommissioned and replaced by thermal Five Fords 2017 67 Raw hydrolysis with agricultural recycling of cake

There is a trend in the UK where drying plants and incinerators are being shut down and replaced for more environmentally and financially sustainable options. This was highlighted recently (Jolly & Taylor, 2017) where Yorkshire Water proposed a biosolids strategy to decommission their four fluidized bed raw

2807 sludge incinerators with advanced anaerobic digestion with land application of recycled cake. There now follows real examples of where thermal hydrolysis has been used to either eliminate thermal processing altogether or at least compliment it.

Northumbrian Water. Replace raw dryers with thermal hydrolysis and keep dryers as contingency In 2001, Northumbrian Water – a UK Water Company, installed 7 x 6.5 metric tonnes water evaporation Andritz drum dryers, to provide a dried pellet material for land application. However, the energy requirements of the dryers were high, and the facility consumed 17.47 MW natural gas in addition of 1.96 MW of electricity to dry approximately 40,000 metric dry tonnes of sludge per year (Rawlinson et al., 2009). These numbers correspond to energy demands of 4.1 MMBtu/t and 0.5 MMBtu/t water evaporation for natural gas and electricity respectively. Almost 10 years later, the biosolids strategy was reviewed, and instead of paying for dryer upgrades and refurbishments, it was decided to install anaerobic digestion with thermal hydrolysis. Although there was no regulatory requirement to move away from raw sludge drying, the client identified a number of environmental, and client-specific reasons for decommissioning the drying plant as follows (taken from Rawlinson et al., 2009):

• Environmental reasons: lower carbon footprint, reduced consumption of fossil fuels, lower energy requirements, production of renewable energy, beneficial use of dewatered cake for agriculture, negligible odor, and more sustainable than maintaining drying; • Client-specific reasons: natural gas consumption reduced by over 90%; elimination of imported electricity at the plant; Class A biosolds; significant reduction in overall operating costs; electricity produced could achieve renewable credits; reliance on electricity and gas significantly reduced helping to move to self-sufficiency; maximised use of existing assets; addressed issues with FOG and wear and tear, could use drying plant as back up if required.

A Cambi thermal hydrolysis plant (2 trains of 4 B12 reactors) including all ancillary equipment, co- generation (3 x Jenbacher JMC 420 rated at 1.415 kWe, and 1 x Jembacher JMC312 rated at 526 kWe) and digestion (3 x 1.67 million gallon each) was installed (shown in Figure 7) for 5 million GBP less than the upgrades which would have been required by the drying plant. Following the installation of thermal hydrolysis (shown in Figure 6), gas demand reduced from 17.47 MW to 1.4 MW (0.3 MMBtu) which was required for steam generation. Subsequently, the client installed a second Cambi plant at another site to reinforce the new strategic direction of advanced anaerobic digestion combined with land application of the biosolids. The Bran Sans plant has been running well since 2010, during which time it has saved almost 4,000,000 MMBtu of natural gas at time of writing.

2808

Figure 7. Cambi thermal hydrolysis at Bran Sands, Northumbrian Water

Dŵr Cymru, Welsh Water. Shut down drying plant and replace with thermal hydrolysis A similar revision of biosolids strategy was performed by another UK Water Company, Dŵr Cymru, also known as Welsh Water (Bowen et al., 2009). Up to 2009, Welsh Water had three main processes for its 260 tds/d sludge, namely, liming using either hydrated or quicklime (21%), standard mesophilic anaerobic digestion followed by storage to attain Class B biosolids (43%), and thermal drying (36%). As with Northumbrian Water, Welsh Water were keen to maximize production of renewable energy, which also had the benefit of reducing carbon footprint and operating costs. Up until 2009, the Water Company was operating drying plants at Cardiff, Afan and Nash which were drying approximately 100 dry tons/d between them of raw sludge from SBRs. However, the operating costs of the dryers were very high and sensitive to energy prices (Bowen et al., 2009). Along with sustainability, reducing operating costs were key drivers for Welsh Water in the absence of a regulatory requirement to otherwise change their practices. The Water Utility undertook a study with sustainability as the main focus. Incineration was ruled out as an option as it was considered problematic to obtain planning nor did it fit with the focus of the study. After looking at other options, it was ultimately decided to go down the route of advanced anaerobic digestion. The new biosolids strategy would eliminate drying altogether, significantly reduce the use of liming and introduce advanced digestion as shown in Figure 8.

2809 a) b)

Figure 8. Welsh Water’s change in biosolids management plan, from a) liming and drying to b) advanced digestion.

The new approach resulted in a reduction in operating costs of $12.5 Million USD (based on historic exchange rates), a reduction in carbon footprint of over 37,000 tonnes/yr and renewable energy generation of 6 MW. The new strategy (Oliver, 2011) initially delivered Cambi thermal hydrolysis at Cardiff (2 trains of 3 x B12 reactors – Figure 9), and Afan (1 train of 4 x B12 reactors – Figure 10). In line with Welsh Water’s continued commitments of sustainable sludge management, a further Cambi thermal hydrolysis plant was installed at Five Fords (2 trains of 3 x B4 reactors) in 2017.

Figure 9. Cambi thermal hydrolysis at Cardiff, Dŵr Cymru

2810

Figure 10. Cambi thermal hydrolysis at Afan, Dŵr Cymru

Psytallia, Greece. Thermal hydrolysis of WAS-only to increase existing capacity of drying and reduce dryer operating costs Psytallia is a Greek island in the Mediterranean Sea and treats an average daily flow of 160 mgd. As well as primary settlement, the plant has nitrogen removal using activated sludge treatment. Sludge is then thickened and digested in 8 x 2.2 million gallon mesophilic digesters. Sludge is dewatered and dried in 4 x 9.5 ton water evaporation/hr drum driers. The co-generation plant was set up to preferentially provide biogas for drying, with surplus. being used to generate renewable energy (Zikakis, et al., 2017). The client was undergoing a number of initiatives to make the facility more energy efficient, and contracted AKTOR in 2014 to undertake the work. A feasibility study determined that it was preferable to install thermal hydrolysis rather than new digestion capacity to help with improving energy efficiency. The anaerobic digesters are set up in two groups of four (labelled Group A and B) and it was decided to install thermal hydrolysis on only the WAS fraction of the sewage sludge processed by just one group of four digesters (Group B) as shown in Figure 11.

2811

Figure 11. Configuration of WAS-only thermal hydrolysis at Psytallia, Athens, Greece

Sludge composition at the plant is 60:40 primary:waste activated sludge and digestion retention time 30 days. A Cambi plant consisting of 1 train of 4 x B6 reactors was installed for the waste activated sludge from Group B digestion plant and sized for average throughput of 47 tons/d dry solids with maximum capacity of approximately 80tons/d (Zikakis, et al., 2017). Installation of the THP and steam generator plant was in 2015 with start-up and plant modifications between August 2015 and February 2016. Stable operation was attained at a loading rate into the digester is 0.018 lbs VS/gallon.d in May 2016 which was preceded by a four month start up period. Prior to installation, the drying plant consumed nearly three quarters of the biogas produced leaving little for renewable energy generation. With thermal hydrolysis installed, volatile solids destruction increased from 45% to 52% even though only half of the WAS (only 20% of the entire flow) was processed. This coincided with an increase in 3 3 biogas yield form 393 Nm /tonne DSfed to 452 Nm /tonne DSfed (Zikakis, et al., 2017), a 16% increase.

However, a large benefit in improving the energy demand of the facility was due to improvements in dewatering, clearly shown in Figure 12.

2812

Figure 12. Dewatering performance before (blue dots) and after (orange dots) application of WAS-only thermal hydrolysis (Taken from Zikakis, et al., 2017).

However, a large improvement was observed in the dewaterability of the digested biosolids as shown in Figure a2. Prior to thermal hydrolysis, the sludge dewatered to approximately 22% dry solids prior to being dried to 92%. The average dewaterability of the sludge since installation is 31% using the same dewatering equipment and polymer. This has resulted in a reduction in gas demand for the dryer of nearly 40% (Zikakis, et al., 2017). The overall benefits of improved digestion and dewatering on overall energy balance at Pystallia are shown in Figure 13.

2813 a)

b)

Figure 13. Influence of installation of thermal hydrolysis on waste activated sludge on half of the total sludge flow, on digestion and drying energy balance at Psytallia, Greece. Key: a) before, b) after installation

Without thermal hydrolysis, 73% of biogas diverted to dryer. Although biogas production following thermal hydrolysis only showed a modest increase by 16%, when combined to a reduction in dryer energy demand this resulted in over 2.5 times more biogas being available for renewable energy compared to the previous case.

Davyhulme, United Utilities. Use of thermal hydrolysis to increase digestion and incineration capacity, reduce operating costs, increase flexibility and lower carbon footprint

In the late 1990s, restrictions on nitrogen application by the Nitrates Directive (91/676/EC); concerns over metals; changes to farming practices; public perception, and reduction in brown-field reclamation were major factors influencing use of biosolids to agriculture in the UK. Previous predictions on the influence of these parameters (Hall, and Dalimier, 1994) suggested that the quantity of Europe’s biosolids incinerated would increase from 23% to 38% from 2000 to 2005. Independent studies commissioned by the UK Water Company United Utilities confirmed the potential landbank reduction for its catchment area in the North West of England. Consequently, in 2002, United Utilities formulated the first of several strategies to reduce its reliance on land application from around 70% to circa 40% by the installation of additional incineration. This was to be achieved by upgrading capacity at its existing digested biosolids incineration facility (Shell Green) from approximately 30,000 TDSA (metric) to 75,000 TDSA and building a second incineration plant in Lancashire for raw sludge. However, major concerns grew within the team tasked with delivering the Lancashire incinerator regarding the project’s long-term sustainability. Incineration – even based on energy recovery – is discriminated against in the waste hierarchy (CEN Report PD 13846:2000) which places priority on waste avoidance, minimization, and

2814 recycling above incineration and discriminates against disposal. Furthermore, increased awareness of climate change, new incentives on renewable energy production, elevated energy costs (subsequently increasing cost of fertilizers) and changing perceptions made the decision of incineration less clear. Subsequently, United Utilities, undertook various studies looking at many potential solutions for the project and developed ten potential options involving various combinations of standard and advanced digestion systems with and without incineration. These were reduced to two and, along with the existing solution (of lime addition) investigated in more detail. The eventual solution had to be one which (Lancaster, 2015):

• Maximized use of existing assets; • Significantly increased renewable energy generation; • Reduced the need for incineration; • Reduced United Utilities’ operational carbon footprint and environmental impact; • Produced a consistently high quality biosolids product suitable for recycling in the North West of England; • Reduced customers’ bills; • Provided full contingency during maintenance periods; and • aligned fully with long term sludge strategy

Key to maximising existing assets was the necessity to increase capacity of the existing Shell Green incineration plant. This was to be achieved by increasing capacity of its largest digestion plant at Davyhulme which contributed approximately 2/3rds of the flow to the incinerator via a 100 km long pipeline. The pipeline was initially built to take liquid digested sludge at 3.5% dry solids from seven digestion plants and pump the sludge to Liverpool from where it was dumped in the sea. When this was outlawed in 1998, the Mersey Valley Processing Centre (Shell Green) was commissioned on the pipeline route which combined dewatering with incineration of the digested sludge. Subsequently, the seven sites although geographically at different locations, were fundamentally connected to the incineration outlet.

Therefore, to free up capacity for burning at the incinerator, more biosolids would have to be destroyed upstream (by converting to biogas at Davyhulme) and the biosolids had to have a higher energy content. Sensitivity analysis demonstrated that energy content in biosolids cake was intrinsically linked to its dry solids (Barber, 2009, 2007), therefore if dewaterability improved, cake energy content concomitantly increased so more could be burnt. It was found that advanced digestion with no dewatering improvement decreased capacity of the incinerator due to a reduced heating value in the cake material. Therefore, of advanced digestion options studied, only thermal hydrolysis was capable of meeting the fundamental drivers of the project. Separate studies were undertaken to allay concerns surrounding dewaterability of mixtures of thermally hydrolysed (Davyhulme) and non-hydrolysed sludges (other sites feeding pipeline), and the capacity of the pipeline to pump higher dry solids for which it was designed. The results of those studies were favourable, and the board gave approval for the SBAP (Sludge Balanced Asset Program) in 2007. The development of United Utilities’ sludge management program is summarized in Figure 14.

2815

Figure 14. Development of United Utilities’ sludge strategy. Prior to 1998 digested liquid sludge from 7 sites was fed into a pipeline and dumped at sea. When this was outlawed it was burnt in a purpose built facility based on dewatering and incineration. Subsequent project after 2005 looked at developing new incinerator for 7 sites where liming was employed. That project evolved into one involving thermal hydrolysis to digest all sludge proposed for burning at an existing digestion plant at Davyhulme from where it could be recycled to land as Class A material, or sent into pipeline to Incineration plant where it could be dewatered and recycled as Class B material or burnt.

The program of works is based on sludge cake deliveries being made to Davyhulme, where the sludge is thermally hydrolysed and digested alongside the Davyhulme sludge, then dewatered in a new facility for Class A recycling, or pumped as a liquid to the incineration plant where the sludge is dewatered as Class or burnt. The dewatered cake import reception facility is based on 2 x 22,400 gal cake reception hoppers taking deliveries from 30 ton trucks. Cake is transferred by elevated Chainlink conveyors (used also at the incinerator) to 2 x 211,000 gallon glass lined storage vessels. The thickened sludge and cake is then blended within a range of 16% to 19% dry solids and fed to a Cambi thermal hydrolysis plant made up of 4 trains of 5 x B12 reactors. The design throughput of the thermal hydrolysis plant is approximately 370 dry ton/d. Treated sludge is transferred to the existing digesters which have had additional spray polyurethane insulation to maintain the temperature differential between the inside and outside surfaces of the structures due to the higher operating temperatures of the sludge after it has passed through the THP plant (McNeill & Thornton, 2011). Two 315,000 ft3 flexible membrane gas holders store biogas prior to treatment for siloxane and other contaminants before burning in the cogeneration plant comprising 5 x 2.4 MWe Jenbacher CHP engines (McNeill & Thornton, 2011). The project has enabled approximately 45 tds/d spare capacity in the incinerator (15 and 30 tds/d due to improved digestion and dewatering respectively) and 165 tds/d spare capacity in the Davyhulme

2816 digestion system, without building any further incineration nor digestion capacity. The entire project based on delivering sludge reception and associated equipment, thermal hydrolysis and ancillaries, thickening for Davyhulme sludge, gas storage, 2 new CHP engines and movement of 3 existing engines and reinstallation and configuration for steam generation and post-digestion dewatering plant cost approximately $120 million. The detailed design, construction and installation took 3.5 years, followed by 6 months commissioning and 1 year optimization (Edgington et al., 2016). The plant has exceeded all guarantee parameters, notably a gas production of over 400 m3/t DS fed, biosolids cake in excess of 31% dry solids, and no requirement of auxiliary fuel for steam generation. The latter was due to the development of specific operations and knowhow developed by United Utilities (Lancaster, 2015). The award-winning facility1 is the largest thermal hydrolysis plant in Europe, has reduced the utilities’ carbon footprint by 8% (Barber 2009)), is the UK’s greatest generator of renewable energy from sludge, and has resulted in a significant reduction in the need for chemicals and makes operational savings of between $3 and $6.5 million (USD) annually (Lancaster, 2015). Subsequently a biogas upgrading facility to pipeline quality has been installed (Lissett et al., 2016), and United Utilities has installed two further Cambi thermal hydrolysis plants at Leigh and Burnley. Figure 15 shows the Davyhulme thermal hydrolysis plant as installed.

Figure 15. Cambi thermal hydrolysis plant with dewatered cake import facility installed at Davyhulme, United Utilities.

1 Awarded “Most innovated green energy scheme in the world” by the Institution of Chemical Engineering, 2013, and Major Civil Engineering Construction Project 2014, by the British Construction Industry

2817 Amperverband, Use of downstream thermal hydrolysis to reduce operating costs on site and at off-site incinerator

With downstream thermal hydrolysis the dewatering benefits are not deteriorated by the anaerobic digestion process (early Refs). In this way dry solids remain higher, and this is beneficial for further thermal processing downstream. One full-scale plant operating downstream hydrolysis is at Amperverband in Germany where it has been in operation over two years (ref oda). The plant processes 4900 tons dry solids annually with a high volatile solids content of 83%. The sludge make is typical of a European plant, and comprises 60% primary with 40% waste activated sludge, based on biological phosphorous removal. The plant has mesophilic anaerobic digestion with retention time of 22 days and managed a volatile solids reduction of approximately 50%. A little over 13,000 wet tons biosolids cake were produced annually and these were transported at distance to an incinerator due to lack of land application in Germany. Figure 16 summarizes the plant performance at Amperverband before and after installation of downstream thermal hydrolysis (taken from Barber et al., 2017).

a)

b)

2818 Figure 16. Influence of downstream thermal hydrolysis at Amperverband. a) before, and b) after installation

Following downstream thermal hydrolysis, the cake dry solids is between 38 and 43% from a centrifuge. However, whilst the cake is drier, it has lower volatile solids contained within it and this could influence the incinerator energy balance. The energy content of the sludge was calculated using methods presented previously whereby a correlation was found between volatile solids content and calorific value which was within +/- 9% of measurements taken by bomb calorimetry (Barber, 2007). These results are as shown in Table 5.

Table 5. Calculated sludge energy values on a dry and wet basis WAS-only Cambi Sludge type Unit Control Cambi SolidStream® Dry basis [BTU/lb] 6980 6197 5725 Wet cake [BTU/lb] 1347 1549 2095

As expected, the energy value drops on a dry basis due to more volatile material being converted to biogas. Here the energy value drops by approximately 10 and 20% for WAS-only thermal hydrolysis and SolidStream® respectively. Installation of downstream thermal hydrolysis has resulted in annual operating savings of 550,000 euros.

CONCLUSIONS

• Although there is a trend in Europe where many thermal processing systems are being decommissioned due to environmental and financial factors, there may be limited instances when thermal systems may be beneficial to maintain. These include treatment plants surrounded with few local agricultural recycling opportunities or with poor topography, occasions where regulations prohibit use of biosolids to agriculture, or instances where it may be more economical to keep existing infrastructure. • Thermal drying has an extremely high energy demand, in the region of 3 – 4 MMBtu/ton water evaporated. It is possible to transport the same quantity of water 1000 miles for the same expenditure of energy. Anaerobic digestion can be used to partially or totally offset the energy demand of drying. • As with anaerobic digestion, thermal hydrolysis optimizes and increases capacity in downstream thermal systems through a combination of improved volatile solids destruction but far more importantly, enhanced dewatering. • Anaerobic digestion can reduce the size of a raw sludge dryer by approximately a third whilst the addition of thermal hydrolysis reduces it by two thirds. Even compared to combining digestion with drying, thermal hydrolysis reduces water evaporation by half.

REFERENCES

Barber, W. P. F (2009) From raw incineration to advanced digestion – influence of sustainability on meeting a sludge strategy, Paper IWA Specialist Conference on Sustainability and Sludge Treatment, Harbin China.

2819 Barber, W. P. F (2009) United Utilities’ biosolids strategy reduces carbon footprint, Article, World Water, March/April, 51 – 53. Barber, W. P. F, Nilsen, P. J. and Christy, P. (2017) Cambi Solidstream: Thermal hydrolysis as a pre-treatment for dewatering to further reduce operating costs, Proceedings of WEFTEC, Chicago, USA, p 5070 – 5083. Barber, W.P.F., 2007, June. Observing the effects of digestion and chemical dosing on the calorific value of sewage sludge. In Paper IWA Specialist Conference: Moving forward Wastewater biosolids sustainability: Technical, managerial, and public synergy (pp. 24-27). Barber, W.P.F., 2016. Thermal hydrolysis for : a critical review. Water Research, 104, pp.53-71. Bowen, A., Evans, B., Oliver, B., Evans R., and Merry, J. (2009) Advanced digestion at Cardiff and Afan, Dwr Cymru Wels Water drive for lowest sustainable cost of sludge treatment and a 15% reduction in Carbon Footprint, (2009) Advanced digestion plant at Bran Sands design and construct experiences, In Proceedings of Aquaenviro’s 14th European Biosolids and Organic Resources Conference and Exhibition, Leeds, UK. Edgington, R., Belshaw, D., and Jolly, M. (2014) Commissioning of United Utilities Thermal Hydrolysis Digestion Plant at Davyhulme Wastewater Treatment Works. 18th European Biosolids and Organic Resources Conference. Edgington, R., Belshaw, D., Lancaster, R., and Jolly, M. (2015) Thermal Hydrolysis at Davyhulme, One year on. 19th European Biosolids and Organic Resources Conference. Environmental Protection Agency (2003) Environmental Regulations and Technology – Control of Pathogens and Vectors in Sewage Sludge; EPA/625/R-92/013, Revised July, 2003. U.S. Environmental Protection Agency: Washington, D.C. Everett, J. G. 1972 Dewatering of wastewater sludge by heat treatment. Journal Water Pollution Control Federation , 92- 100. Hall, J.E. and Dalimier, F. (1994) Waste management - sewage sludge. Part 1. Survey of sludge production, treatment, quality and disposal in the European Union. Contract report to the European Commission DG XI. WRc report No. EC 3646, Medmenham, SL7 1FD, England. Haug, R. T. 1977 Sludge processing to optimize digestibility and energy production. Journal Water Pollution Control Federation, 1713-1721. Jolly, M., and Taylor, T. (2017) From incineration to digestion in Ten Years, In Proceedings of Aquaenviro’s 21st European Biosolids and Organic Resources Conference and Exhibition, Leeds, UK. Kolovos, A., Kjorlaug O. and Nilsen, P.J. 2016 Development and assessment of a model for Cambi´s SolidStream process using “SUMO” WWTP simulation software, Proceedings of the 21st European Biosolids and Organic Resources Conference,15-16 November, Edinburgh, Scotland. Lancaster, R. (2015) Thermal Hydrolysis At Davyhulme - 'A Vision To Reality', Proceedings of WEF, Residuals and Biosolids Conference 2015, The Next Generation of Science, Technology, and Management, Washington DC. Lancaster, R. (2015) Thermal Hydrolysis at Davyhulme WWTW, One Year On.Water Environment Federation (WEF) Residuals and Biosolids 2015, Water and Environment Federation; Washington DC. Lissett, T., Barnes, L., Edgington,R., Horne, P., and J. Taylor (2016) Davyhulme Biomethane Utilisation Project maximising value for customers through integrated biogas energy generation, UK Water Projects 2016, pp 1 – 5. McNeill, J., and Thornton, J (2011) Davyhulme WwTW delivering United Utilities’ sludge balanced asset programme, UK Water Projects, pp 105 – 108. Oliver, B. (2011) Cardiff & Afan Advanced Digestion Plants, Dŵr Cymru Welsh Water delivers AD plants early, and moves towards power self-sufficiency in wastewater treatment, UK Water Projects, pp 45 – 51. Rawlinson, D., Halliday, S., Garbutt, S., and Jobling, I. (2009) Advanced digestion plant at Bran Sands design and construct experiences, In Proceedings of Aquaenviro’s 14th European Biosolids and Organic Resources Conference and Exhibition, Leeds, UK. Rudolfs, W., and Heisig, H. M. 1929 Digestion of activated sludge. Sewage Works Journal, 146-159. Torpey, W.N. and Melbinger, N.R., 1967. Reduction of digested sludge volume by controlled recirculation. Journal (Water pollution control federation), pp.1464-1474. Wilson, C. A., and Novak, J. T. 2009 Hydrolysis of macromolecular components of primary and secondary wastewater sludge by thermal hydrolytic pretreatment. Water Research, 43 (18), 4489-4498. Zikakis, D., Chauzy, J., Droubogianni, I. and Georgakopoulos, A. (2017) The application of WAS-only Thermal Hydrolysis at Psyttalia WWTP, Proceedings of IWA-Sludgetech Conference, June, London.

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