What Effect Does Rehabilitation of Wastewater Pipelines Have on the Share of Inﬁltration and Inﬂow Water (I/I-Water)?

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Article What Effect Does Rehabilitation of Wastewater Pipelines Have on the Share of Inﬁltration and Inﬂow Water (I/I-Water)?

Kristin Jenssen Sola 1,2,*, Jarle Tommy Bjerkholt 1,3, Oddvar Georg Lindholm 1 and Harsha Ratnaweera 1

1 Faculty of Mathematical Sciences and Technology, Norwegian University of Life Sciences, 1430 Ås, Norway; [email protected] (J.T.B.); [email protected] (O.G.L.); [email protected] (H.R.) 2 The Municipality of Asker, 1372 Asker, Norway 3 Faculty of Technology, Natural Sciences and Maritime Sciences, University of South-Eastern Norway, 3603 Kongsberg, Norway * Correspondence: [email protected]; Tel.: +47-41632184

Abstract: Infiltration and inflow water (I/I-water) is a big challenge in sewage systems in many countries. I/I-water above an acceptable level indicates that the sewage system is not functioning properly. I/I-water leads to increased pumping costs and increased sewage overflow, leading to increased pollution of the receiving waters. Many rehabilitation projects are driven by the need to reduce the share of I/I-water and common measures are to replace pipes and manholes. The share of I/I-water is predominantly driven by rainfall. This makes it difficult to document the efficiency of mitigating measures. One way to address this issue is to compare data from rehabilitation areas to areas where no measures have been implemented. Three rehabilitation areas in Asker Municipality, Norway, were successfully assessed by applying this approach. Asker has a 100% separate system. The strategy to reduce I/I-water in Asker Municipality was to rehabilitate sewage mains, either by full replacement or lining the old pipes, and replacement of manholes. The assessment shows that rehabilitation of selected municipal pipes, pipes proven to be in bad condition through closed circuit Citation: Sola, K.J.; Bjerkholt, J.T.; Lindholm, O.G.; Ratnaweera, H. TV inspection, reduced the share of I/I-water only to a limited extent. Since the rehabilitation done What Effect Does Rehabilitation of was not a complete replacement of all pipes and manholes, the limited effects are assumed to be Wastewater Pipelines Have on the caused by the water finding other ways into the system. In separate systems other measures than Share of Infiltration and Inflow Water renovations of pipes should be considered when aiming to reduce I/I-water. (I/I-Water)? Water 2021, 13, 1934. https://doi.org/10.3390/w13141934 Keywords: infiltration and inflow water (I/I-water) measures; measurements; wastewater

Academic Editor: Caetano C. Dorea

Received: 26 May 2021 1. Introduction Accepted: 10 July 2021 Wastewater systems are composed of components such as manholes, pipes, and Published: 13 July 2021 pumping stations. All these components are affected by multiple factors such as aging and deterioration, and climate change, with an increasing amount of rainfall and urbaniza- Publisher’s Note: MDPI stays neutral tion [1,2]. Infiltration and inflow water (I/I-water) is common in sewage systems. However, with regard to jurisdictional claims in I/I-water represents unwanted water in the sewage system. Combined sewage systems are published maps and institutional affil- iations. normally designed to transport a certain volume of stormwater in addition to sewage, as for separate systems all stormwater entering the system is unwanted. The share of I/I-water in a sewage system is an indicator for how well the system is serving its original purpose. I/I-water originates from groundwater, rainfall, rivers, and drinking water leakages [3–7]. I/I-water enters the sewage system thorough leaky pipes and manholes and stormwater Copyright: © 2021 by the authors. pipes incorrectly connected to the system. The sources that contribute the most to the share Licensee MDPI, Basel, Switzerland. of I/I-water vary geographically. However, in most places, rainfall strongly influences This article is an open access article the share of I/I-water in the wastewater pipeline system [3,8,9]. This means that climate distributed under the terms and conditions of the Creative Commons change most likely will contribute to larger amounts of I/I-water. This also means that Attribution (CC BY) license (https:// in two otherwise identical localities, the area with higher annual rainfall is likely to have creativecommons.org/licenses/by/ higher annual rates of I/I-water. 4.0/).

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Excessive I/I-water has both social, economic, and environmental impacts. The economic consequences are associated with increased pump operation and less efficient treatment at wastewater treatment plants (WTP). The environmental consequences are linked to overflow discharge, increased outflow of pollutants due to increased flow of water through the WTP, and lower treatment rates at the WTPs [6–10]. The social aspects are linked to the number of basement floodings and potentially reduced access to clean water [8]. A simple method to determine the share of I/I-water in wastewater systems is to measure the flow in the wastewater pipeline system, and compare this measured flow to the number of person equivalents (PE) connected and drinking water use in that same system [7,11]. This method is called the water balance method. Another simple and commonly used method is to calculate the level of I/I-water using the dilution method [8,9]. The input to this method is measurements of, for instance, total phosphorus (TOT-P) entering the wastewater treatment plant, the number of connected PEs, and assumptions of TOT-P production per person per day [11,12]. Both methods have drawbacks. The water balance method does not include water that disappears out of the system. The dilution method does not include the inflow of TOT-P into the system. The water balance method depends on measured figures of drinking water consumption; the dilution method depends on figures related to TOT-P production/PE/day. In both methods, there are uncertainties related to equipment/measuring methods. We may also find more advanced methods aimed at quantifying I/I-water. The use of isotopes is an example of such a method [13]. The use of distributed temperature sensing (DTS) has also proven to be an efficient method when aimed at identifying entry points of inflow water [6,14]. However, because both water flow data and data of TOT-P often are easily available to most decision-makers, the water balance method and the dilution method are widely used in Norway. Renovating sewer pipes, establishing retention basins, and upsizing undersized components (bottlenecks) in the wastewater system are all traditional ways of dealing with unwanted impacts from I/I-water on the wastewater system [8]. Rehabilitating wastewater pipes may result in improving both the condition and the functionality of the pipes. To identify which pipes to renovate, a closed circuit TV inspection (CCTV) can be used [6]. As the amounts of I/I-water are largely influenced by rainfall, water flow measurements carried out before and after measures have been implemented are unlikely to be comparable. Rainfall varies greatly, both in terms of geography and time, and it is challenging to identify two completely identical precipitation events. One workaround for this challenge would be to compare I/I-water levels prior to and after rehabilitation during dry weather events, but such comparisons would not provide an accurate picture of the situation, as the share of I/I-water is likely to be higher during rainfall. Another approach may be to compare water flows before and after measurements in a rehabilitation area, with a control area in which no interventions are made [15]. Using a control area gives a reference point that is independent of any precipitation events that may occur. The control area should be comparable to the rehabilitation area in terms of length of pipes, numbers of manholes, age of pipes/manholes, paved areas, and the number of connected PE. However, most importantly, the areas should be located close to each other. A number of indicators are used in order to assess the state of the water and wastewater systems. Indicators commonly used are volumes and frequency of overflows, amount of infiltration and inflow water, the number of basement floodings, and the renovation rate [7,16]. These indicators are divided into two main categories: indicators aimed at measuring the functionality of the system and indicators aimed at measuring the condition of the system. Infiltration and inflow water (I/I-water) reflects wastewater system malfunc- tions and may therefore be used as an indicator of the functionality of wastewater systems. The purpose of the presented study is to calculate the share of I/I-water in separate wastewater systems in selected areas in Asker Municipality in Norway, both before and after implementing measures. The study also aims at assessing whether introduced mea- Water 2021, 13, x FOR PEER REVIEW 3 of 16

Water 2021, 13, 1934 after implementing measures. The study also aims at assessing whether introduced3 of 16 measures within the areas have had an impact on the level of I/I-water. In order to examine the impact of the completed measures to prevent I/I-water, two control areas are in- cludedsures withinin the thestudy. areas The have main had angoal impact of the on pr theoject level is of to I/I-water. test out In the order mentioned to examine method the to understandimpact of the whether completed it can measures be useful to prevent on a gene I/I-water,ral basis two controlfor this areas type are of includedanalysis in for I/I- water.the study. The main goal of the project is to test out the mentioned method to understand whether it can be useful on a general basis for this type of analysis for I/I-water. 2. Materials and Methods 2. Materials and Methods 2.1.2.1. Study Study Area Area AskerAsker Municipality Municipality isis located located southwest southwest of Norway’s of Norway’s capital, capital, Oslo. Asker Oslo. has Asker seasonal has sea- sonalvariations variations with with cold winters,cold winters, heavy heavy rain during rain during spring and spring autumn, and autumn, and dry summers. and dry summers.Asker’s Asker’s location location in Norway in Norway is shown is inshown Figure in1. Figure 1.

FigureFigure 1. 1. MapMap of of Norway andand section section of of Asker. Asker.

Most of the wastewater system in Asker was built in the 1960s and 1970s. The Most of the wastewater system in Asker was built in the 1960s and 1970s. The wastewater network consists of concrete pipes and PVC. The renovation rate for the past wastewater5 years has network been approximately consists of 1.5%,concrete which pipe is lowers and than PVC. the The estimated renovation necessary rate ratefor the of past 5 years2% [17 has]. The been renovation approximately method is,1.5%, in most which cases, is lower sliplining than old the concrete estimated pipes. necessary The pipes rate of 2%to [17]. be renovated The renovation are chosen method based is, on in the most calculation cases, sliplining of I/I-water old and conc CCTVrete pipes. inspection. The pipes to Thebe renovated entire wastewater are chosen system based is a on separate the calculation system. Even of I/I-water so, the sewer and network CCTV inspection. acts more The entirelike awastewater combined system. system In is 2020 a separate the share system of I/I-water. Even amounted so, the sewer to about network 66% of acts the totalmore like a combinedamount of watersystem. delivered In 2020 to the the wastewatershare of I/I-water treatment amounted plant (WTP). to about These calculations66% of the total amountare based of water on the waterdelivered balance to the method wastewater [11]. treatment plant (WTP). These calculations The strategies for the water and wastewater business in Asker are anchored in the are based on the water balance method [11]. master plan for water and wastewater services in Asker from 2017. In this plan, I/I-water wasThe identified strategies as one for ofthe the water main and challenges wastewater [17]. At business a tactical in level, Asker the are utility anchored defines in the masterseveral plan renovation for water zones. and These wastewater zones are services defined basedin Asker on the from identified 2017. In needs this of plan, both theI/I-water waswastewater identified pipes as one and drinkingof the main water challenges pipes. Bad [17]. functionality At a tactical of sewer level, pipes the is weightedutility defines several renovation zones. These zones are defined based on the identified needs of both the wastewater pipes and drinking water pipes. Bad functionality of sewer pipes is

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weighted high when defining these zones. Errors like cracks, staggered joints, and obvi- oushigh leakages when deﬁning will lead these to renovation. zones. Errors Trying like cracks,to combat staggered the challenges joints, and posed obvious by I/I-water, leakages measurementswill lead to renovation. have been Trying implemented to combat in the seve challengesral of the posed renovation by I/I-water, zones. Since measurements the sys- temhave is beena 100% implemented separate system, in several the main of the measure renovation has zones. been the Since rehabilitation the system of is asewer 100% pipes,separate as well system, as troublesh the mainooting measure for faulty has been connected the rehabilitation stormwater. ofThree sewer of these pipes, rehabil- as well itationas troubleshooting zones are assessed for faulty in this connected study: Dæli stormwater., Vakås, and Three Vestre of these vei. rehabilitationThe areas were zones se- lectedare assessed based on in previous this study: assessments Dæli, Vakås, of andI/I-wa Vestreter that vei. indicated The areas that were the selected areas contribute based on toprevious a large extent assessments to the ofhigh I/I-water level of that I/I-water indicated in the that utility. the areas The contributelevel of I/I-water to a large prior extent to implementingto the high level the ofmeasures I/I-water varied in the between utility. The40 and level 90%, of I/I-waterdepending prior on the to implementingweather situation.the measures The rehabilitation varied between areas and 40 the and sewer 90%, system depending in Asker on theare weathershown in situation. Figure 1. The The locationsrehabilitation of the areas two rain and thegauges sewer are system marked in in Asker Figure are 2, shown along inwith Figure the 1two. The areas locations used as of controlthe two areas. rain gauges are marked in Figure2, along with the two areas used as control areas.

FigureFigure 2. 2. OverviewOverview map map of of the the investigated investigated area areass as as well well as as rain rain gauge gauge locations. locations.

PriorPrior to to the the rehabilitation, rehabilitation, in in 2015, 2015, 2016, 2016, and and 2017, 2017, water water flow flow meters meters were were installed installed inin the the rehabilitation rehabilitation areas areas and and the the control control ar areas.eas. The The first first area area in in which which measures measures were were implementedimplemented was was Dæli. Dæli. Vakås Vakås is is an an area area where—as where—as of of 1 1 January January 2021—rehabilitation 2021—rehabilitation is is ongoing,ongoing, but manymany measuresmeasures have have already already been been implemented. implemented. Measures Measures were were implemented imple- mentedin the Vestre in the veiVestre area vei in 2019.area in J. R.2019. Wilhelmsens J. R. Wilhelmsens vei (JRWs vei vei) (JRWs and vei) Holmen and Holmen were included were includedas control as areas, control as noareas, measures as no weremeasures implemented were implemented in those areas in those during areas the timeduring periods the timecovered periods in this covered study. in The this rain study. gauge The in the rain north gauge iscalled in the “Mellom north is Nes”called and “Mellom is used Nes” when andlooking is used at Vestrewhen vei/Holmen.looking at Vestre The vei/Holmen. rain gauge between The rain Vakås/Dæli gauge between and JRWsVakås/Dæli vei is called and “the fire station.” All assessed areas are residential areas with no industrial activity. The JRWs vei is called “the fire station.” All assessed areas are residential areas with no indus- calculations do not account for commuting. trial activity. The calculations do not account for commuting. There are no overflow points in JRWs vei, Dæli, or Vakås, but there are overflow points associated with sewage pumping stations in Holmen and Vestre vei. Both overflow points

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Water 2021, 13, 1934 There are no overflow points in JRWs vei, Dæli, or Vakås, but there are overflow 5 of 16 points associated with sewage pumping stations in Holmen and Vestre vei. Both overflow points and pumping stations are located downstream of the measuring points of discharge and didand not pumping influence stations the calculations. are located downstream of the measuring points of discharge and There is nodid tradition not inﬂuence of measuring the calculations. the groundwater table in Asker, so there are no data on how the wastewaterThere is no discharg traditiones of are measuring affected the by groundwater groundwater. table in Asker, so there are no data on how the wastewater discharges are affected by groundwater.

2.2. Method to Assess2.2. Method the Effect to Assess of Rehabilitation the Effect of Rehabilitation To investigateTo the investigate impact of the the impact renovations, of the renovations, water flow water levels ﬂow and levels I/I-water and I/I-water volumes volumes were examinedwere and examined compared and under compared different under weather different conditions, weather conditions, including including both dry both and dry and wet weather events.wet weather Rehabilitated events. Rehabilitated areas were areasexamined were examinedand compared and compared to the control to the controlareas areas before and afterbefore the andrehabilitation. after the rehabilitation. Figure 3 illustratesFigure the3 illustrates method the utilized method to utilized investigate to investigate the impact the of impact the renovations. of the renovations.

FigureFigure 3. Schematic 3. Schematic drawing drawing of the method of the used method to investigate used to theinve impactstigate of the measures impact in of the measures rehabilitation in the areas. rehabilitation areas. In order to carry out the evaluation of the effect from the rehabilitation, it was essential In order toto carry obtain out reliable the evaluation flow data from of the all areas.effect from the rehabilitation, it was essential to obtain reliable2.3. Measuring flow data Equipment from all areas. In the analyses conducted for this study, data were retrieved from two rain gauges 2.3. Measuringlocated Equipment in Asker. These gauges have been in operation since March 2015 (Mellom Nes) and In the analysesMarch 2016conducted (Asker for fire this station) study, [18 ].data The were rain gauges retrieved also from measure two snowfall rain gauges during the located in Asker.winter These season. gauges have been in operation since March 2015 (Mellom Nes) and March 2016 (AskerTable1 fireoutlines station) the characteristics [18]. The rain of gauges the water also flow measure measuring snowfall devices during used [19 ]. the winter season. Table 1. Characteristics of the measuring devices used in the different zones. Table 1 outlines the characteristics of the water flow measuring devices used [19]. Dimension of Pipes in Which Measurement Uncertainty Area Type of Measuring Device Table 1. Characteristicsthe Gauge Wasof the Installed measuring devices used in the different zones. (during Good Conditions) Dæli 315 mm PCM4 with POA sensor 0–10% Dimension of Pipes in which Measurement Uncertainty Area Type of MeasuringPCM4 with Device CSM-P sensor and Vakås 250 mm 5–10% the Gauge Was Installed air ultrasound (during Good Conditions) Dæli Holmen 315 mm 400 mm PCM4 with POA PCM4 sensor with POA sensor 0–10% 0–10% PCM4 with CSM-P sensor and air Vakås JRWs250 vei mm 290 mm PCM-F with KDA sensor5–10% 0–15% Vestre vei 230 mmultrasound PCM4 with POA sensor 0–10% Holmen 400 mm PCM4 with POA sensor 0–10% JRWs vei 290 mm The PCM4PCM-F and with PCMF KDA sensors sensor both use the same 0–15% principle to determine the flow Q. Vestre vei 230 mm The method PCM4 is shown with inPOA Formula sensor (1). 0–10%

The PCM4 and PCMF sensors both use theQ = same v (average) principle× A to determine the flow Q. (1) The method is shown in Formula (1).

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where: Q = flow; v = flow velocity; A = flow cross-section. The flow cross-section A is determined by continuously measuring the filling level, taking the channel shape as well as the channel dimensions into account. The filling level in the pipe is measured with two independent level sensors, a pressure sensor at the bottom of the pipe and an ultrasonic level sensor at the top of the pipe. The flow velocity is determined with the PCM4 equipment based on an ultrasonic reflection principle, whereas the PCMF equipment has a velocity sensor based on the classic Doppler method [20–22]. Several conditions need to be met to achieve reliable flow data. The pipe in which the meter is installed should be straight without bends and have a constant diameter. The water flow should be as steady as possible, meaning that the pipe should maintain a steady and limited incline and not intersect with feed pipes. Experiences from Asker Municipality in Norway suggest that the ideal water flow rate for measurements is around 2 l/s. Normally, flows of 2 l/s correspond to areas servicing 1000 PE, which is a relatively large area. The limited sensitivity of current measurement technology makes it difficult to get good data from smaller drainage areas. Generally, pipeline renovations are conducted on small segments of the pipeline system, usually where the need is considered the greatest based on CCTV inspections. Since such a large area is required to get reliable water flow measurements, the share of rehabilitated pipes will be relatively small compared to the total pipe length within the area observed. The flow meters used are intended to log data continuously. However, there are often operational outages that disrupt the data series. Since timeouts are relatively common in the time series, it can be difficult to find periods with reliable measurement data for several areas at the same time. This poses a challenge to comparing measurements from different areas. Conducting extended measurement periods is, therefore, crucial to obtain reliable data that can be used in comparative studies. The series selected for this study meets the abovementioned criteria. In the data used for this study, there are frequent timeouts. The use of water flow measurements to evaluate I/I-water requires close monitoring of the measuring devices.

2.4. Calculation Method In the calculations performed in the presented study, measured water flow was used to calculate the share of I/I-water by using the water balance method. Water consumption is not measured in all households in Asker. The calculations utilize a stipulated daily water consumption of 140 l/PE based on experience from houses where water use is measured [11]. Since no overflow points were identified upstream of the measuring points, the calculations did not take into account any water losses. The water balance method is shown in Formula (2) [11,12].

(% share of I/I-water) = (Qtot − PE × Qap)/(Qtot) × 100 (2)

where: I/I = I/I-water in the wastewater system (%); Qtot = total amount of water transported to the measuring point (l/day); PE = the number of persons situated within the catchment area; Qap = the amount of wastewater each person produces a day (l/PE/day).

2.5. Description of the Selected Catchments Characteristics of the sewer pipes and drinking water pipes in the renovation zones (Dæli, Vakås, and Vestre vei) are shown in Table2. The table also includes some characteristics of the control zones (JRWs vei and Holmen). Water 2021, 13, 1934 7 of 16

Table 2. Characteristics of the wastewater systems in Dæli, Vakås, and Vestre vei.

Rehab. Rehab. Total Length Number Average Age of Area Sewer Pipes Manholes Average Age of Remaining Pipes of Pipes (m) of PE Selected Pipes %% 24% of pipes less than 20 years old Dæli 12,181 3136 13 12 About 50 Remaining pipes: about 40 years old 34% of pipes less than 20 years old Vakås 9480 1886 11 13 About 60 Remaining pipes: about 40 years old JRWs vei 6520 1026 Control area Unknown—not 31% of pipes less than 20 years old Vestre vei 15,138 3087 7 10 registered Remaining pipes: about 40 old Holmen 6380 1538 Control area

By including the proposed measures, the remaining pipes were assessed to be in good condition in all zones except Vakås. Some additional pipes were selected for renovation in this zone after the extraction of data was finished. The CCTV inspection and site inspection did not reveal additional pipes that should be renovated either in Dæli or in Vestre vei. The control areas are smaller than the areas they were compared to in terms of the number of inhabitants and the total length of the pipes. This was not ideal, but the selection of perfect measuring points was limited. The municipality has a strategy to demand rehabilitation of all private sewer pipes connected to the municipal sewer pipes that are being rehabilitated. This is a time-consuming process. In the assessed rehabilitation areas, no private pipes had been rehabilitated before the data used in the study were extracted. For all areas, primarily old concrete pipelines/manholes had been rehabilitated. Only pipes identified in bad condition through CCTV inspection were selected to be renovated. In Norway, all water and sewage pipes are installed below the maximum frost depth. For Asker Municipality, the minimum depth of installation is 1.7 m [23]. The main soil type in Asker is marine deposited clay. In addition to being residential areas with only simple road structures, the following characteristics for each of the rehabilitation areas were identified: Vakås: • Parts of the sewer system are situated near a small stream. • Several wrongly connected stormwater pipes where discovered, but none of these were corrected by the time of data collection. • The work mainly required full excavations and replacement of the concrete pipes with new PVC pipes. • The manholes selected for rehabilitating were fully replaced. All the wastewater pipes in Vakås, the pipes selected for renovation, and the rivers in the area are shown in Figure4. • The rehabilitation of the pipes was initially prompted by surface water that had been erroneously connected to the wastewater pipeline system. This was fixed. During the process, pipes in bad condition were discovered. These pipes were selected for renovation. • The work mainly entailed sliplining. • Limited work was performed on manholes. Vestre vei: • Frequent overflow runoff necessitated the redevelopment of wastewater pipelines in the zone. The overflow point was situated downstream of the renovation zone; • The work mainly entailed sliplining. • Limited work was performed on manholes, with some manholes being fully encased. Water 2021, 13, x FOR WaterPEER2021 REVIEW, 13, 1934 8 of 16 8 of 16

Figure 4. A map of the Vakås area. Pipes selected for rehabilitation are marked in the pink color. Figure 4. A map of the Vakås area. Pipes selected for rehabilitation are marked in the pink color. 3. Results and Discussion • The rehabilitationAll theof the main pipes characteristics was initially (urbanization, prompted by soil surface type, and water installation that had depth been of pipes) erroneouslyof connected the selected to areas, the wastewater apart from size, pipeline are similar. system. Unlike This the was other fixed. areas, During the groundwater the process, pipeslevel in in bad the Vakåscondition area iswere affected discovered. by the river. These This pipes makes were the sewer selected system for inreno- Vakås more vation. susceptible to infiltration of groundwater than in the other areas. Groundwater could • The work mainlyinfiltrate entailed both pipes sliplining. and manholes. The amounts of I/I-water in the rehabilitation areas in the following were compared • Limited work was performed on manholes. to the levels in the control zones. If the level sank equally in the control area and the Vestre vei:rehabilitation area, it is likely that changes in precipitation caused the reduction and • Frequent overflownot the renovation runoff necessitated of the pipes the and redevelopment manholes. The of results wastewater are based pipelines on the followingin the zone. Theprerequisite: overflow The point assessed was situated rain events downstream occur after a of minimum the renovation of three dayszone; of dry weather. This comparison takes into account neither rainfall conditions before the event in question • The work mainly entailed sliplining. • nor the temperature in the compared periods. Limited work wasSome performed of the assessed on manholes, data sets were with extracted some manholes during wintertime. being fully During encased. dry weather this did not influence the results. During periods with precipitation the results may have 3. Results and Discussionbeen influenced if the precipitation fell as snow. This problem is discussed in more detail All the mainin thischaracteristics section where (urbanization, appropriate. soil type, and installation depth of pipes) of the selected areas,3.1. Dry apart Weather from size, are similar. Unlike the other areas, the groundwater level in the Vakås3.1.1. area Dæli is affected and Vakås by the river. This makes the sewer system in Vakås more susceptible to infiltration of groundwater than in the other areas. Groundwater could in- Figure5 shows dry weather periods prior to the implementation of the measures. The filtrate both pipesfigure and shows manholes. the percentage of I/I-water in the total measured water flow. Even though The amountsthe of entire I/I-water wastewater in the system rehabilitati is a separateon areas system, in the we following found a large were portion compared of I/I-water. to the levels in the controlFor Vakås, zones. the shareIf the of leve I/I-waterl sank variedequally between in the control 84% and area 91%. and In JRWs the reha- vei, the share bilitation area, ofit is I/I-water likely that varied changes between in approximatelyprecipitation 69%caused and the 79%, reduction whereas the and share not ofthe I/I-water renovation of thein pipes Dæli ranged and manholes. between approximately The results are 59% based and 76%.on the In following Vakås, the prerequisite: wastewater pipelines The assessed raintraverse events a riveroccur at after several a minimu points, whichm of three is a likely days contributorof dry weather. to the This high groundwatercom- parison takes intotable account and the neither high shares rainfall of I/I-water. conditions before the event in question nor the temperature in the compared periods. Some of the assessed data sets were extracted during wintertime. During dry weather this did not influence the results. During periods with precipitation the results may have been influenced if the precipitation fell as snow. This problem is discussed in more detail in this section where appropriate.

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3.1. Dry Weather 3.1.1. Dæli and Vakås Water 2021, 13, x FOR PEER REVIEW Figure 5 shows dry weather periods prior to the implementation of the measures.9 of 16

The figure shows the percentage of I/I-water in the total measured water flow. Even though the entire wastewater system is a separate system, we found a large portion of I/I- 3.1.water. Dry Weather 3.1.1. Dæli and Vakås 100 Figure 5 shows dry weather periods prior to the implementation of the measures. 90 Water 2021, 13The, 1934 figure shows the percentage of I/I-water in the total measured water flow. Even 9 of 16 though 80the entire wastewater system is a separate system, we found a large portion of I/I- water. 70 Dæli 60 100 JRWs vei 50 90 Vakås 40 80 Linear (Dæli) 30

Infiltration/inflow, % 70 20 DæliLinear (JRWs vei) 60 10 JRWsLinear vei (Vakås) 50 0 Vakås 40 Linear (Dæli) 30 Infiltration/inflow, % Linear (JRWs vei) 20 2016/2/12 2016/2/13 2016/2/14 2016/2/15 2016/2/16 2016/2/17 2016/2/18 2016/2/19 2016/2/20 2016/2/21 2016/2/22 2016/2/23 2016/2/24 2016/2/25 2016/2/26

10 Linear (Vakås) Figure 5. Calculated share of I/I-water in a dry weather period, before the implementation of the measures.0

For Vakås, the share of I/I-water varied between 84% and 91%. In JRWs vei, the share

of I/I-water varied2016/2/12 2016/2/13 between2016/2/14 2016/2/15 2016/2/16 approximately2016/2/17 2016/2/18 2016/2/19 2016/2/20 2016/2/21 69%2016/2/22 and2016/2/23 2016/2/24 79%,2016/2/25 whereas2016/2/26 the share of I/I-water in Dæli ranged between approximately 59% and 76%. In Vakås, the wastewater pipelines FiguretraverseFigure 5. Calculateda river at share shareseveral of of I/I-water I/I-waterpoints, in which ain dry a dry weather is weathe a likely period,r period, contributor before before the implementation theto implementationthe highof groundwater the measures. of the measures.table and the high shares of I/I-water. Figure6 shows dry weather periods after the implementation of the measures. Figure 6 shows dry weather periods after the implementation of the measures. For Vakås, the share of I/I-water varied between 84% and 91%. In JRWs vei, the share of I/I-water varied between approximately 69% and 79%, whereas the share of I/I-water 100 in Dæli ranged between approximately 59% and 76%. In Vakås, the wastewater pipelines traverse90 a river at several points, which is a likely contributor to the high groundwater table and80 the high shares of I/I-water.

Figure70 6 shows dry weather periods after the implementation of the measures. Dæli 60 100 JRWs vei 50 90 40 Vakås 80 30 Linear (Dæli)

Infiltration/inflow, % 70 20 DæliLinear (JRWs vei) 60 10 JRWsLinear vei (Vakås) 50 0 40 Vakås 30 Linear (Dæli) Infiltration/inflow, %

20 2017/12/7 2017/12/9 Linear (JRWs vei) 2017/12/11 2017/12/13 2017/12/15 2017/12/17 2017/12/19 2017/12/21 10 Linear (Vakås) Figure 6. Calculated0 share of I/I-water during a dry weather period, after the implementation of the measures. In this period, JRWs vei was the area with the highest share of I/I-water, between 82% and 87%, which is an increase from the assessed period in 2016. In the Vakås area, the share

2017/12/7 varied2017/12/9 between 77% and 82%, whereas the share in Dæli varied between an estimated 51% 2017/12/11 2017/12/13 2017/12/15 2017/12/17 2017/12/19 2017/12/21 and 63%. Despite increased shares of I/I-water in the control area when comparing shares of I/I-water before and after the implementation of the measures, we registered a decrease

Water 2021, 13, x FOR PEER REVIEW 10 of 16

Figure 6. Calculated share of I/I-water during a dry weather period, after the implementation of the measures.

In this period, JRWs vei was the area with the highest share of I/I-water, between 82% and 87%, which is an increase from the assessed period in 2016. In the Vakås area, the share varied between 77% and 82%, whereas the share in Dæli varied between an estimated 51% and 63%. Despite increased shares of I/I-water in the control area when comparing shares of I/I-water before and after the implementation of the measures, we regis- Water 2021, 13tered, 1934 a decrease in the share of I/I-water in Vakås. This may indicate that the implemented 10 of 16 measures had an impact on I/I-water during dry weather periods in this renovation area. Four different dry weather periods were extracted and analyzed closely: 1. 12.02.2016–26.02.2016in the share to of 04.05.2018–13.05.2018 I/I-water in Vakås. This may indicate that the implemented measures had an impact on I/I-water during dry weather periods in this renovation area. 2. 12.02.2016–26.02.2016 to 16.05.2018–27.05.2018 Four different dry weather periods were extracted and analyzed closely: 3. 31.08.2016–10.09.2016 to 04.05.2018–13.05.2018 1. 12.02.2016–26.02.2016 to 04.05.2018–13.05.2018 4. 31.08.2016–10.09.2016 to 16.05.2018–27.05.2018 2. 12.02.2016–26.02.2016 to 16.05.2018–27.05.2018 The reduction3. in I/I-water31.08.2016–10.09.2016 for the areas to of 04.05.2018–13.05.2018 Vakås and Dæli is summed up as follows: Vakås: dry weather4. = 9–22%31.08.2016–10.09.2016 to 16.05.2018–27.05.2018 Dæli: dry weather = 0 The reduction in I/I-water for the areas of Vakås and Dæli is summed up as follows: ComparisonsVakås: between dry weatherthe control = 9–22% areas and rehabilitation areas in dry weather suggest that the I/I-waterDæli: quantities dry weather declined = 0 only in the Vakås area. Comparisons between the control areas and rehabilitation areas in dry weather suggest 3.1.2. Vestre Vei that the I/I-water quantities declined only in the Vakås area. Figure 7 shows3.1.2. a dry Vestre weather Vei period in the summer of 2018. No rainfall was registered during the entireFigure period,7 shows meaning a dry that weather the periodpeak shown in the summer for 4666-Holmen of 2018. No rainfallmust have was registered been due to factorsduring other the than entire rain. period, Most meaning likely the that meter the peak was shown out of for order 4666-Holmen this day. must An- have been other option is thatdue the to meter factors was other temporarily than rain. Most covered likely with the meterparticles was or out sludge. of order Approx- this day. Another imately 30 mm ofoption rainfall is that occurred the meter during was temporarily three days covered before with the particlesassessed or dry sludge. weather Approximately period. 30 mm of rainfall occurred during three days before the assessed dry weather period.

50 4666-Holmen

40 12440-Vestre vei 30 Linear (4666-Holmen) 20 Infiltration/inflow, %

10 Linear (12440-Vestre vei) 0 2018/7/4 2018/7/9 2018/6/19 2018/6/24 2018/6/29 2018/7/14 2018/7/19 2018/7/24

FigureFigure 7. Calculated share share of of I/I-water I/I-water in ain dry a dry weather weathe period,r period, before before the implementation the implementation of the measures. of the measures. Figure8 shows the estimated share of I/I-water in a dry weather period after the Figure 8 showsimplementation the estimated of share the measures of I/I-water in the in Vestre a dry vei weather area. The period period after prior the to im- the examined plementation of thedry measures weather period in the was Vestre relatively vei area. wet, The with period approximately prior to 74the mm examined of rainfall dry over 11 days, which is the reason for the high share of I/I-water at the start of the period.

Water 2021, 13, x FOR PEER REVIEW 11 of 16

Water 2021, 13, 1934 11 of 16 weather period was relatively wet, with approximately 74 mm of rainfall over 11 days, which is the reason for the high share of I/I-water at the start of the period.

60 4666-Holmen 50

40 12440-Vestre vei

30 Linear (4666-Holmen)

Infiltration/inflow, % 20

10 Linear (12440-Vestre vei) 0 2019/9/12 2019/9/14 2019/9/16 2019/9/18 2019/9/20 2019/9/22 2019/9/24 2019/9/26

FigureFigure 8. 8. CalculatedCalculated share share of of I/I-water I/I-water in in a adry dry period period,, after after the the implementation implementation of of the the measures. measures.

In the period afterIn the the period measures after were the measures implemented, were implemented,the lowest estimated the lowest I/I-water estimated I/I-water level in Vestrelevel vei was in Vestre approximately vei was approximately 50%. In Holmen, 50%. In the Holmen, lowest thelevel lowest was levelapproxi- was approximately mately 44%. The44%. share The of share I/I-water of I/I-water in the Vestre in the Vestrevei zone vei was zone generally was generally higher higher than in than in Holmen. Holmen. The shareThe share of I/I-water of I/I-water for the for Holmen the Holmen zone zone was wasthus thus higher higher during during this this period period than during than during thethe period period prior prior to tothe the measures measures being being completed completed in inVestre Vestre vei. vei. The The share share of of I/I-water in I/I-water in VestreVestre vei vei remained remained rather rather stable stable before before and and after after the the implementation implementation of of the the measures. measures. Three different dry weather periods were extracted an analyzed closely: Three different1. 08.12.2017–25.12.2017dry weather periods were to 06.08.2019–09.08.2019 extracted an analyzed closely: 1. 08.12.2017–25.12.20172. 29.05.2018–09.06.2018 to 06.08.2019–09.08.2019 to 06.08.2019–09.08.2019 2. 29.05.2018–09.06.20183. 19.06.2018–08.07.2018 to 06.08.2019–09.08.2019 to 15.09.2019–19.09.2019 3. 19.06.2018–08.07.2018The reduction to 15.09.2019–19.09.2019 in I/I-water during dry weather for the areas of Vestre vei is likely to The reductionhave in been I/I-water 0. during dry weather for the areas of Vestre vei is likely to have been 0. 3.2. During Rainfall 3.2. During Rainfall3.2.1. Dæli and Vakås 3.2.1. Dæli and VakåsFigure 9 shows rainfall periods prior to the implementation of the measures. The figure shows the percentage of I/I-water in the total measured water flow. Figure 9 shows rainfall periods prior to the implementation of the measures. The fig- The measured water flow in Dæli shows that the area was heavily affected by rainfall, ure shows the percentage of I/I-water in the total measured water flow. as the proportion of I/I-water peaked during precipitation events, whereas JRWs vei and Vakås had a consistently high proportion of I/I-water. Vakås and JRWs vei are presumably affected by I/I-water to such a degree that the levels remained generally high. In Dæli, the variations in I/I-water were greater than in the other two areas. The share of I/I-water declined to approximately 50% in Dæli towards the end of the period. In other words, Dæli is quickly influenced by rainfall and dry weather alike, and the primary sources of I/I-water are presumably directly linked to surface water, and to a lesser extent to infiltration. JRWs vei and Vakås are constantly influenced by groundwater and infiltration is likely to be constantly at a high level. Figure 10 shows the proportion of I/I-water in all three zones during rainfall after the implementation of the measures.

Water 2021, 13, x FOR PEER REVIEW 12 of 16

100 40

90 35 80 30 70 Precipitation 60 25 Dæli 50 20 JRWs vei 40 15 Vakås Rainfall, mm Water 2021, 13, 1934 12 of 16 Water 2021, 13, x FOR PEER REVIEW 30 Linear (Dæli) 12 of 16 Infiltration/inflow, % 10 20 Linear (JRWs vei) 10 5 Linear (Vakås) 1000 040

90 35 80 2016/5/1

2016/5/15 2016/5/29 2016/6/12 2016/6/26 2016/7/10 2016/7/24 30 70 Precipitation Figure 9.60 Calculated share of I/I-water in a wet weather period,25 before the implementationDæli of the measures. 50 20 JRWs vei The40 measured water flow in Dæli shows that the15 area was heavily affectedVakås by rainfall, as the proportion of I/I-water peaked during precipitationRainfall, mm events, whereas JRWs vei and 30 Linear (Dæli) VakåsInfiltration/inflow, % had a consistently high proportion of I/I-water.10 Vakås and JRWs vei are presumably 20 affected by I/I-water to such a degree that the levels remained generallyLinear high. (JRWsIn Dæli, vei) the 10 5 variations in I/I-water were greater than in the other two areas. The Linearshare (Vakås)of I/I-water declined0 to approximately 50% in Dæli towards the0 end of the period. In other words, Dæli is quickly influenced by rainfall and dry weather alike, and the primary sources of I/I-water are presumably directly linked to surface water, and to a lesser extent to infiltra-

tion. JRWs vei2016/5/1 and Vakås are constantly influenced by groundwater and infiltration is 2016/5/15 2016/5/29 2016/6/12 2016/6/26 2016/7/10 2016/7/24 likely to be constantly at a high level. FigureFigureFigure 9.9. CalculatedCalculated 10 shows share share the of of I/I-waterproportion I/I-water in ain wetof a wetI/I-water weather weathe period, inr period,all before three before the zones implementation the during implementation rainfall of the measures.after of the the implementationmeasures. of the measures.

The100 measured water flow in Dæli shows that the area35 was heavily affected by rainfall, as the proportion of I/I-water peaked during precipitation events, whereas JRWs vei and 90 Vakås had a consistently high proportion of I/I-water. Vakås30 and JRWs vei are presumably affected80 by I/I-water to such a degree that the levels remained generally high. In Dæli, the variations70 in I/I-water were greater than in the other25 two areas. ThePrecipitation share of I/I-water declined to approximately 50% in Dæli towards the end of the period. In other words, 60 Dæli Dæli is quickly influenced by rainfall and dry weather20 alike, and the primary sources of I/I-water50 are presumably directly linked to surface water, and to a lesserJRWs extent vei to infiltra- 15 tion. JRWs40 vei and Vakås are constantly influenced by groundwater Vakåsand infiltration is likely to be constantly at a high level. Rainfall, mm 30 10 Linear (Dæli) FigureInfiltration/inflow, % 10 shows the proportion of I/I-water in all three zones during rainfall after the implementation20 of the measures. 5 Linear (JRWs vei) 10 Linear (Vakås) 1000 035 90 30 80 2018/1/5 2018/2/2 2018/2/9 2018/1/12 2018/1/19 2018/1/26 2018/2/16 2018/2/23 70 25 Precipitation Figure 10. Calculated share of I/I-water in all surveyed points during rainfall, after the implemen- Figure 10. Calculated60 share of I/I-water in all surveyed points during rainfall,20 after the implementationDæli of the measures. tation of the measures. 50 Figure9 shows that the I/I-water share in JRWs vei remainedJRWs vei quite stable at approximately 80%, regardless of rainfall. For Vakås,15 the share varied roughly between 70% and 40 Vakås 91%. The share of I/I-water in the Vakås area wasRainfall, mm generally lower than the corresponding 30 share in JRWs vei. For Dæli, the share of10 I/I-water lay betweenLinear 47% (Dæli) and 83%. In Figure8 Infiltration/inflow, % 20 we saw that the level of I/I-water was approximately the same for the Vakås area and the Linear (JRWs vei) JRWs vei area. However, in Figure9, we5 see that for most of the time JRWs vei was the 10 area with the highest share of I/I-water. Linear (Vakås) 0 Only two periods with reliable and comparable0 data were found: 2018/1/5 2018/2/2 2018/2/9 2018/1/12 2018/1/19 2018/1/26 2018/2/16 2018/2/23

Figure 10. Calculated share of I/I-water in all surveyed points during rainfall, after the implementation of the measures.

Water 2021, 13, x FOR PEER REVIEW 13 of 16

Figure 9 shows that the I/I-water share in JRWs vei remained quite stable at approximately 80%, regardless of rainfall. For Vakås, the share varied roughly between 70% and 91%. The share of I/I-water in the Vakås area was generally lower than the corresponding share in JRWs vei. For Dæli, the share of I/I-water lay between 47% and 83%. In Figure 8 we saw that the level of I/I-water was approximately the same for the Vakås area and the JRWs vei area. However, in Figure 9, we see that for most of the time JRWs vei was the area with the highest share of I/I-water.

Water 2021, 13, 1934Only two periods with reliable and comparable data were found: 13 of 16 1. 18.05.2016–01.06.2016 to 04.05.2018–13.05.2018 2. 18.08.2016–02.09.2016 to 07.02.2018–25.02.2018 From these two1. periods18.05.2016–01.06.2016 it seems that there to 04.05.2018–13.05.2018 was a minor reduction in I/I-water in both areas: 2. 18.08.2016–02.09.2016 to 07.02.2018–25.02.2018 Vakås: rainfall = 3–8%From these two periods it seems that there was a minor reduction in I/I-water in Dæli: rainfall = 7–9%both areas: Vakås: rainfall = 3–8% In the time period from 05.01.2018 to 23.02.2018, as shown in Figure 10, the air tem- Dæli: rainfall = 7–9% perature changed from below to above zero on 23.01. This change in temperature explains In the time period from 05.01.2018 to 23.02.2018, as shown in Figure 10, the air temper- the delay in I/I-water in the system, as the precipitation stored as snow on the ground in ature changed from below to above zero on 23.01. This change in temperature explains the the beginning of thedelay period in I/I-water melted in in the the system, second as part the precipitationof the period. stored The asmaximum snow on themeas- ground in the ured on 25.01 wasbeginning caused by of melted the period snow melted entering in the the second sewer part system. of the period. The maximum measured The temperatureson 25.01 in wasthe time caused period by melted from snow 07.02.2018 entering to the 25.02.2018, sewer system. which was one of two periods selected forThe comparison temperatures toin the the time time period period from from 18.8.2016 07.02.2018 to to 02.09.2016, 25.02.2018, whichfluc- was one tuated below andof above two periodszero. During selected this for period, comparison the snow to the melted time period and entered from 18.8.2016 the sewer to 02.09.2016, system on days withﬂuctuated temperatures below and above above zero. zero. However, During thisthe period,temperature the snow and melted snow cover and entered the situation were thesewer same system in the oncontrol days witharea temperaturesand the rehabilitation above zero. area However, and the the results temperature and and snow cover situation were the same in the control area and the rehabilitation area and the results trends are therefore comparable. In the selected period the reduction in the share of I/I- and trends are therefore comparable. In the selected period the reduction in the share of water was 5% in theI/I-water control was area. 5% In in the the reha controlbilitated area. Inareas the rehabilitatedof Vakås and areas Dæli, of the Vakås reduc- and Dæli, the tions were 13% andreductions 14%, respectively, were 13% and for 14%, the respectively,same period. for the same period.

3.2.2. Vestre Vei 3.2.2. Vestre Vei Figure 11 shows differentFigure 11 showsrainfall different periods rainfall before periods the implemented before the implemented measures. measures.

90 60

80 50 70 Precipitation

60 40 4666-Holmen 50 30 40 12440-Vestre vei

30 20 Rainfall, mm

Infiltration/inflow, % 20 Linear (4666-Holmen) 10 10 Linear (12440-Vestre 0 0 vei) 2018/9/5 2018/9/10 2018/9/15 2018/9/20 2018/9/25 2018/9/30 2018/10/5

FigureFigure 11. Calculated 11. Calculated share of I/I-watershare of I/I-water in Holmen in and Holmen Vestre veiand in Vestre a wet weathervei in a period, wet weather before theperiod, implementation before of the measures.the implementation of the measures.

There was no clear pattern during rainfall periods with regards to which site had the highest shares of I/I-water. Both areas were clearly influenced by rainfall. In Figure 10, we can see that the two areas were roughly similarly impacted by rainfall. Holmen had the highest volumes of I/I-water in the first part of the period, whereas Vestre vei had the highest volumes in the latter part of the period. The highest rainfall quantities came in the first part of the period. Water 2021, 13, x FOR PEER REVIEW 14 of 16

There was no clear pattern during rainfall periods with regards to which site had the highest shares of I/I-water. Both areas were clearly influenced by rainfall. In Figure 10, we can see that the two areas were roughly similarly impacted by rainfall. Holmen had the Water 2021, 13highest, 1934 volumes of I/I-water in the first part of the period, whereas Vestre vei had the 14 of 16 highest volumes in the latter part of the period. The highest rainfall quantities came in the first part of the period. By looking at rainfallBy looking in conjunction at rainfall inwith conjunction the estimated with the share estimated of I/I-water share of for I/I-water all pe- for all peri- riods, the volumesods, of I/I-water the volumes were of I/I-watergreatest in were the greatestHolmen in zone the Holmen during zonehigh during precipitation high precipitation periods. Holmen periods.was impacted Holmen more was impactedslowly by more rainfall slowly than by Vestre rainfall vei. than Despite Vestre vei. that, Despite dry that, dry weather readingsweather showed readings that the showed share of that regular the share infiltration of regular was inﬁltration greatest was in greatestVestre vei. in Vestre vei. Figure 12 shows theFigure share 12 shows of I/I-water the share in ofHolmen I/I-water and in Vestre Holmen vei and in Vestre different vei inrainfall different rainfall periods after the implementedperiods after the measures. implemented measures.

100 60 90 50 80 Precipitation 70 40 60 4666-Holmen 50 30 40 12440-Vestre vei

20 Rainfall, mm 30 Infiltration/inflow, % 20 Linear (4666-Holmen) 10 10 Linear (12440-Vestre 0 0 vei) 2019/9/27 2019/10/2 2019/10/7 2019/10/12 2019/10/17 2019/10/22

FigureFigure 12. 12. CalculatedCalculated shareshare of of I/I-water I/I-water during during rainfall rainfa periods,ll periods, after after the implementation the implementation of the measures. of the measures. During all rainfall events, we can see that the share of I/I-water was larger in Holmen During all rainfallthan in events, Vestre we vei. can In Holmen,see that the the share share of I/I-waterI/I-water variedwas larger between in Holmen 63% and 93%. In Vestre vei, the share varied between 40% and 89%. than in Vestre vei. In Holmen, the share of I/I-water varied between 63% and 93%. In Ves- Three different periods were extracted an analyzed closely: tre vei, the share varied between 40% and 89%. 1. 05.11.2017–10.11.2017 to 28.08.2019–29.08.2019 Three different periods were extracted an analyzed closely: 2. 28.07.2018–12.08.2018 to 10.08.2019–12.08.2019 1. 05.11.2017–10.11.20173. 05.09.2018–11.09.2018 to 28.08.2019–29.08.2019 to 27.09.2019–04.10.2019 2. 28.07.2018–12.08.2018The reduction to 10.08.2019–12.08.2019 of I/I-water for the areas of Vestre vei was likely to be between 11 and 43%. 3. 05.09.2018–11.09.2018 to 27.09.2019–04.10.2019 4. Conclusions The reduction of I/I-water for the areas of Vestre vei was likely to be between 11 and 43%. The only area where there was a clear reduction of I/I-water in dry weather was Vakås. This was also the only area where the renovation method was a full replacement of selected pipes and selected manholes. Since the Vakås area was under the inﬂuence of a 4. Conclusions river, and therefore also groundwater, it is reasonable to assume that this was the source The only areabeing where reduced there during was drya clear weather. reduction of I/I-water in dry weather was Vakås. This was also theThe only main area conclusion where ofthe this renovation study is that method rehabilitating was a full only replacement small proportions of of selected pipes andpipelines selected and manholes. manholes leads Since to the a small Vakås but area not proportional was under the reduction influence in I/I-water. of The a river, and thereforeaverage also agegroundwater, of the wastewater it is reasonable pipes in allto assume three renovation that this was zones the is source relatively young. being reduced duringHowever, dry weather. even after completing the renovation of all pipes assumed to be in bad condition, the level of I/I-water was high. Rehabilitating pipelines with the sole purpose of reducing The main conclusion of this study is that rehabilitating only small proportions of I/I-water cannot be recommended. In the investigated areas, the rehabilitation of sewer pipelines and manholespipes showed leads onlyto a limitedsmall but results. not Itproportional is likely to be reduction that by leaving in I/I-water. some pipes, The even if they average age of theare wastewater not proven to pipes be in badin all condition, three renovation I/I-water will zones ﬁnd itsis wayrelatively into some young. of them. Water may be transported along the trench to other weak points, entering the system at these

Water 2021, 13, 1934 15 of 16

points instead. Pipes not proven to be in bad condition with CCTV inspection may, after all, not be completely sealed. Private pipes and connection points between municipal pipes and private pipes are also examples of such weak points. Due to the results in this study, it is clear that indicators aiming at documenting the condition and functionality of the sewer system should be chosen carefully. It may seem that the indicator renovation rate may be misguiding when used to assess the functionality of the pipes. Even when investing large sums of money in renovating pipes, this may not improve the total functionality of the system. The scope of the presented study was to investigate the effect rehabilitation of municipal pipes has on I/I-water. It is recommended to perform a new study when the rehabilitation of private pipes is completed. It is also recommended to start a groundwater level measurement program in different parts of the municipality to find out how the groundwater influences the level of I/I-water. Every manhole is a puncture in the piping system. By including manholes in the wastewater system, we allow the I/I-water to enter the system at numerous weak points. The extent to which a full replacement of manholes alone, not in combination with the renovation of pipes, will reduce the level of I/I-water should be investigated. Whatever measure is chosen, the effect of the investments should always be evaluated. Evaluating the impact of measures to control I/I-water is important but challenging. Because the volumes of I/I-water are so closely related to rainfall, it can be difficult to find two similar—and thus comparable—situations before and after the implementation of measures. Even if two or more rainfall periods have equally high volumes of rainfall, the ratio of rainfall to dry weather before those periods will affect the results. To get around this challenge, the proposed method of comparing results from a project area with a control area where no measures have been implemented may be appropriate. This was done successfully in this study. The method requires long series of data, which in and of itself can pose challenges.

Author Contributions: Conceptualization, K.J.S.; methodology, K.J.S.; investigation, K.J.S.; writing— original draft preparation, K.J.S.; writing—review and editing, J.T.B., O.G.L. and H.R.; supervision, J.T.B., O.G.L. and H.R.; project administration, K.J.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Asker Municipality and the Research Council of Norway. Grant number 272969. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors want to thank Asker Municipality for willingly sharing information and for giving access to data. The authors also want thank Asker Municipality and the Research Council of Norway for financing this study. Conflicts of Interest: The authors declare no conflict of interest.

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