Estimating groundwater infiltration into sewerages by using the moving minimum method a survey in Austria

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1 Estimating groundwater infiltration into sewerages by using the moving minimum method a survey in Austria Th. Ertl 1 *, G. Spatzierer², St. Wildt³ 1 University of Natural Resources and Applied Life Sciences Vienna, Department of Water, Atmosphere and Environment, Institute for Sanitary Engineering and Water Pollution Control, Muthgasse 18, A-1190 Vienna, Austria ² Amt der Burgenländischen Landesregierung, Abteilung 9 Gewässeraufsicht, Wulkawiesen, A-7041 Wulkaprodersdorf ³ Amt der Tiroler Landesregierung, Sachgebiet Siedlungs- und Industriewasserwirtschaft, Herrengasse 1-3, A-6020 Innsbruck *Corresponding author, thomas.ertl@boku.ac.at ABSTRACT The objective of this study was to find a method to get a quick estimation of I/I amount in a sewer system with more or less existing data, but exact enough to use it as PI for comparison of different WWTPs and sewer systems. For this study the moving minimum method has been chosen which is described and recommended by the German Wastewater Association (ATV-DVWK, 2003). Two indicators describing groundwater infiltration - Q inf /Q avdwf (%) and IWA PI wop32 (m³/km.a) - have been analysed in this study for more than 50 sewer systems in Austria by using the moving minimum method. The feedback quote of the survey behind was indeed quite low (ca. 6 %) and therefore the results cannot represent the situation all over Austria. The chosen method provides good reference values for groundwater infiltration. The method is applicable by operators very quickly and can be used as training tool for raising sensitivity to infiltration problems. KEYWORDS calculation method, groundwater infiltration, performance indicator, survey INTRODUCTION Infiltration is defined as water that enters the sewer via groundwater. This usually occurs through leaky sewer pipe joints, manholes and service connections. Being a function of groundwater head above the sewer leak, infiltration can result from stormwater and/or snowmelt infiltrating into the ground and into the sewer. Thus a wetweather event can trigger both inflow (usually a faster response to the system) and infiltration in the form of groundwater (EPA, 1998). In order to detect and quantify the amount of groundwater infiltration in combined sewer systems different methods (Bertrand-Krajewski et al., 2005) and performance indicators (Cardoso et al., 2005) have been developed. The objective of this study was to find a method to get a quick estimation of I/I amount in a sewer system with more or less existing data, but exact enough to use it as PI for comparison of different WWTPs and sewer systems. Ertl et al. 1

2 One method to estimate the infiltration flow without any additional measurements in the sewer system is to take the inflow measurements of a WWTP and apply the so called moving minimum method which is described and recommended by the German Wastewater Association (ATV-DVWK, 2003) to get a rather quick performance indicator as a reference value for further decisions. Originally this method calculates only the indicator Q inf /Q avdwf (%) as percentage of inflow at the WWTP. The authors suggest to extend the method by adding the length of the contributing mains of the sewer system in the official calculation file. So it can be used as denominator for the IWA PI wop 32 infiltration. METHODS The method can be applied whenever automatic flow meters are installed at the inlet of a WWTP. The method relies on the main assumption that infiltration is caused by rather slow oscillations of the groundwater level and quick changes of inflow are caused by storm runoff. Therefore a mathematical filter is applied to select the dry-weather-flow out of the whole dataset of 365 values of a year by taking the minimum daily inflow of a certain period around each day (21 days recommended) as the dry weather inflow. In a second step the domestic and industrial wastewater - as exactly as available from water supply measurements or calculations - is subtracted from the DWF-values. One big advantage of this method is the possibility of evaluation of seasonal behaviour of the system especially by graphical evaluation in a diagram. The method is originally intended only for combined sewer systems because in separated systems all infiltration/inflow can be calculated without this method, but when applied to sanitary sewers groundwater infiltration can be filtered from stormwater inflow as well as in combined sewers. To proof if this method is applicable for Austrian sewerage operators a survey has been carried through. At the yearly meeting of the speakers (coordinators) of the so called Neighbourhoods of WWTPs and sewer systems in 2006 the moving minimum method has been presented (see Afterwards an Excel-sheet prepared with all the necessary formulas and an accompanying letter have been sent by to all the speakers, so that they could spread it to their members and to collect them filled out and sent them back to the author. Finally 64 filled out sheets (of a potential of nearly 1000) have been received for evaluation. The feedback quote was indeed quite low (ca. 6 %) and therefore the results cannot represent the situation all over Austria. RESULTS AND DISCUSSION Results of the 64 WWTPs are presented as Q inf /Q avdwf (%) - infiltration flow expressed as a percentage of the daily mean dry weather flow in table 1. This indicator gives an estimate of the weight of infiltration flow on treatment costs (Cardoso et al., 2005). The levels indicating different recommended activities are taken by the german guidelines (ATV-DVWK, 2003). Hence it can be seen in table 1 that 32 operators should make investigations into the causes of the infiltration water (having more than 25%), whereas more than 50% of infiltration should be a driving force to reduce the amount of infiltration. In that situation 9 of the participating operators (see table 1) should apply more precise quantitative and qualitative methods for detection of the entry points. 2 Estimating infiltration flow by using the moving minimum method

3 Table 1. Summary of the survey for Q inf /Q avdwf (%) at the WWTPs Level Q inf /Q avdwf (%) Number of WWTPs (% of Recommended Activity total) low < 25 % 23 (36%) --- medium % 32 (50%) Investigations high % 7 (11%) Reduction very high > 67 % 2 (3%) Rehabilitation During the survey questions arose how to handle massive industrial wastewater contributions. As the method should help to evaluate groundwater inflow, a higher amount of industrial wastewater leads to minor percentage of groundwater infiltration arriving at the WWTP. But it has to be reminded that the indicator Q inf /Q avdwf (%) shows the impact of infiltration on the inflow of the WWTP and does not show the status of the sewer system according to infiltration. For analysing the status of the sewer system the performance indicator wop32 infiltration (m3/km/year) as proposed in the IWA manual (Matos et al., 2003) can be used. (Definition: Volume of water entering sewers from groundwater during the assessment period x 365 / assessment period) / total sewer length at the reference date). As most of the investigated sewer systems in this study comprise all types of sewers (combined, sanitary and storm) the denominator total sewer length must include only combined and sanitary sewers whereas storm sewers do not deliver wastewater and infiltration water to the WWTP. As many of the operators have not yet finished a detailed sewer database the lengths taken for further calculation are mainly quite good estimations in percentage of the well known total length of their systems. But nevertheless the indicator values can be taken as a reference value for describing the infiltration situation in the system. Please remind that stormwater inflow in sanitary sewers is not included in these values. The question now arises which levels can be used to give advice for recommended activities regarding the sewer status as in table 1 for the impact on WWTPs. The approach used for defining the levels is derived from the values recommended for sewer design in Austria. The guideline actually has been overworked and the final draft will be published in short (OEWAV, 2008). Unfortunately there is no design value considering the length of sewers. For infiltration a maximum design value of 1,0 l/s and 1000 Inhabitants is recommended. The mean value for the relative sewer length per population within the sample of 51 sewer systems (where the total length of contributing sewers was available) has been calculated as 9 km/1000 Inhabitants. Thus 0,11 l/s.km would be the mean corresponding design value for maximum infiltration within this sample. This value will be taken as the lower level for recommending investigation into the status of the sewer system. In the same proportion as the levels in table 1, the value for the medium level is 3 times as much infiltration water (0,33 l/s.km) as the low value and the upper level 6 times as much as the low level (0,66 l/s.km). It can be seen in table 2 that the pattern of distribution of the sewer systems in the different levels is nearly the same as in table 1. Ertl et al. 3

4 Table 2. Results of the survey for the IWA PI wop32 infiltration Level wop32 [m³/km.a] Number of sewer systems Recommended Activity (% of sample) low < (39 %) --- medium (39 %) Investigations high (14 %) Reduction very high > (8 %) Rehabilitation Beside the comparison of the distribution pattern in the tables a graphical comparison in figure 1 shows interesting details. The dashed line in figure 1 marks the balance between the levels of impact on the WWTP and the level of status of the sewer system. It has been drawn by starting at 0/0 and taking the lower, medium and upper levels for both indicators of infiltration. In the upper left part of the figure the impact of infiltration on the WWTP is highest although the sewers are still tight and in the lower right part the sewer systems are quite decayed but have nearly no negative impact on the WWTP. Considering the line of balance it can be seen that amongst the participants two thirds of the WWTPs have higher impact through infiltration than a decayed sewer system. 80,0 70,0 high impact on WWTP 60,0 Qinf/Qavdwf [%] 50,0 40,0 30,0 20,0 10,0 decayed status of sewer system 0, wop32 infiltration [m³/km.a] Figure 1. Comparison of the results of the 2 different indicators describing groundwater infiltration. Special interest has been taken to the details of the moving minimum method regarding the seasonal behaviour of the system. When examining the data files seasonal peaks of infiltration have been found for 30 sewer systems mainly occurring in April and for 8 WWTPs 4 Estimating infiltration flow by using the moving minimum method

5 additionally in September. These peaks correspond with the supposed oscillating groundwater levels. Adaptation of the moving minimum method Another special situation occurs in the highly touristic regions of Austria. The seasonal variety of the wastewater discharge has to be considered and implemented when analysing with the moving minimum method. Additionally the duration of the analysis has been extended 3 months each back and forward to have the 2 winter seasons included completely in one data set. Figure 2 gives an impression what it looks like when taking monthly values for the sanitary wastewater flow Fremdwasserbestimmung mit Gleitendem Minimum - KAN Year of investigation: 2006 Result Qinf/Qavdwf [%]: 20,2 measured inflow [l/s] wastewater discharge [l/s] calculated infiltration flow [l/s] [ l/s ] Figure 2. Resulting graph of the moving minimum method using seasonal wastewater discharge Critical reflexion of the moving minimum method It has to be considered that the moving minimum method tends to underestimate the real infiltration rates because of its inherent minimization filter. Therefore critical voices are against a countrywide implementation of the method. But it has to be repeated that the method gives a very quick indicator value with existing measurements. In addition to that the recommended level for further detailed investigations into infiltration problems is set quite low according to likely underestimations. Furthermore will the operators of WWTPs with high impact by infiltration do detailed investigations with or without results by the moving minimum method. A common use of the method by the operators gives a chance of countrywide indicators for infiltration with an acceptable level of accuracy. To compare the infiltraton rates by the described method the authors recommend to use the 50% percentile of Carbon loads and an inflow rate of 200 l/d and Inhabitant. Despite of counting only main sewers the infiltration rates calculated in the study naturally reflect the whole system including house connections too. Recent studies (e.g. Joannis and Calvar, 2006) have shown the heavy influence on infiltration of these parts of the system. But Ertl et al. 5

6 as there is a lack of data for the length of house connections at most of the operators, taking only the main sewers is a good compromise. Additionally all reference values e.g. the design recommendations are given for the main sewers only, too. CONCLUSIONS Two indicators describing groundwater infiltration - Q inf /Q avdwf (%) and IWA PI wop32 (m³/km.a) - have been analysed in this study for more than 50 sewer systems in Austria by using the moving minimum method. The feedback quote of the survey behind was indeed quite low (ca. 6 %) and therefore the results cannot represent the situation all over Austria. The average annual percentage of infiltration Q inf /Q avdwf (%) in the different municipal sewerages covers a wide range from 1 per cent up to over 70 per cent of the dry-weather flow. 14% of the participants of the survey are in charge of eliminating the causes for infiltration, considering the impact on the WWTP. Looking on the results for the IWA PI wop32 (m³/km.a) describing the status of the sewer system the range of values is also very broad, but the situation seems to be a little bit better. By using the moving minimum method the seasonal variation can be investigated as well. Resulting diagrams show an increased amount of infiltration occuring in spring and autumn in several sewer systems induced most probably by the oscillating groundwater levels. Lessons Learned Definition of the domestic and industrial amount of wastewater remains the critical point. Seasonal variations can be clearly documented and graphically presented Method provides good reference values for groundwater infiltration Method is applicable by operators quickly and can be used as training tool for raising sensitivity to infiltration problems Attention should be paid to long periods of rain or snowmelt, thus using a daily weather code is recommended The IWA performance indicator wop32 infiltration should be used by adding the length of the contributing sewer system in the data sheet ACKNOWLEDGEMENT The authors want to express many thanks to all the operators having contributed to the results with their infiltration analysis data. REFERENCES ATV-DVWK (2003) Fremdwassersituation in Deutschland (Situation of Infiltration/Inflow in Germany). Arbeitsbericht der AG ES-1.3 Fremdwasser (report of working group ES-1.3). Korrespondenz Abwasser, Abfall (50) Vol.1 (in german) Bertrand-Krajewski J.-L., Cardoso M.A, Ellis B., Frehmann T., Giulianelli M., Gujer W, Krebs P., Pliska Z, Pollert J., Pryl K. (2005) Towards a better knowledge and management of infiltration and exfiltration in sewer systems: the APUSS project. In: Global solutions for urban drainage. Proc. 10th Int. Conf. on Urban Drainage, Copenhagen, Denmark, August Cardoso M.A., Prigiobbe V., Giulianelli M., Baer E., De Bénédittis J., Coelho S.T. (2005) Assessing the impact of infiltration and exfiltration in sewer systems using performance indicators: case studies of the APUSS project. Proc. 10th Int. Conf. on Urban Drainage, Copenhagen, Denmark August Estimating infiltration flow by using the moving minimum method

7 EPA (1998) Innovative Urban Wet-Weather Flow Management Systems. EPA# EPA/600/R-99/029. download from on Joannis, C., Calvar, J. (2006) Controlling drains of housings in France. Proceedings of the 2nd International IWA Conference on Sewer Operation and Maintenance Oct., Vienna. OEWAV (2008) ÖWAV Regelblatt 11 Richtlinien für die abwassertechnische Berechnung und Dimensionierung von Abwasserkanälen. (Guideline for hydraulic design of sewers). Final draft. Vienna. Ertl et al. 7