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A method for the evaluation of pollution loads from urban areas at river basin scale
Physics and Chemistry of the Earth 29 (2004) 831–837 www.elsevier.com/locate/pce
A method for the evaluation of pollution loads from urban areas at river basin scale I.I. Nafo *, W.F. Geiger FB10, Urban Water Management, University of Duisburg-Essen, Universitaetsstr. 15, 45141 Essen, Germany Accepted 24 May 2004 Available online 8 July 2004
Abstract The European water framework directive (WFD) requires the identification of significant human pressures and impacts on water bodies. This is part of the wider analysis of river basins required by Article 5 (and Annex II) of the WFD, which must be completed by the end of 2004. The human pressures on surface waters include point and diffuse source pollutions, water abstraction, water flow regulation, morphological alterations and land use patterns. In order to be able to complete the analysis of significant pressures and impacts before the WFD deadline it is necessary to maximise the use of existing information. A method using only existing information for the evaluation of point source pollution at river basin scale, especially the pollution loads from drained urban areas into surface waters was therefore proposed. This method includes an approach to estimate imperviousness in urban areas and to calculate sewer overflow volume at river basin scale. The imperviousness in urban areas is estimated using information on land use in CORINE Landcover or ATKIS (Official Topographic–Cartographic Information System). Data used for the evaluation of sewer overflows, such as rainfall depth, sewer lengths, dry weather flow volume and total volume of combined wastewater discharged to wastewater treatment plants, are available from German Weather Survey, or they are, like in North Rhine-Westphalia, stored in databases. Imperviousness in the Wupper basin and nutrient inputs from urban areas into the Wupper river system were calculated in a case study and compared with the results of other investigations. 2004 Elsevier Ltd. All rights reserved. Keywords: Pollution loads; Imperviousness; Sewer overflow volume
1. Introduction For the identification of significant pressures from point sources on surface waters it is necessary to evaluate the pollution loads from Wastewater Treatment Plants (WWTP) and urban areas, i.e. separate sewer and combined sewer overflows (SSO and CSO). The pollution loads from WWTP can be easily calculated. The final effluent of this point source is continuously measured and sampled. An alternative simple method is needed for the estimation of pollution loads from urban areas, especially from CSOs at river basin scale. The method must be able to estimate total imperviousness in river basins as this is unknown in most cases. To fulfil * Corresponding author. Tel.: +49-201-183-2885; fax: +49-201-1833793. E-mail address: [email protected] (I.I. Nafo).
1474-7065/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2004.05.010
the requirements of the WFD before the deadline, it is necessary that the simple method maximises the use of existing information. The proposed method (MESAN) uses a simple approach for the evaluation of mean annual pollution loads at river basin scale based on calculated annual runoff volume from sewer systems and concentration values from literature. Data on annual rainfall, annual dry weather flow volume and total annual volume of combined wastewater discharged to WWTP are needed for the estimation of the annual sewer overflow volume. Diffuse pollution loads from urban areas into surface waters, e.g. through infiltration, were not addressed in this study. The size of paved urban areas within the river basin is estimated using an empirical relation between the proportion of artificial areas (from CORINE Landcover or ATKIS) and imperviousness.
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2. Methodology The pollution loads are calculated at river scale for a substituted drainage system which represents all drained urban areas in the investigated river basin. Components of the substituted drainage system are (1) a WWTP supposed to drain all urban areas in the river basin, (2) a separate sewer system and a combined sewer system, and (3) a combined sewer overflow structure representing all CSO’s in the river basin (Fig. 1). The proposed approaches for the calculation of the mean annual pollution loads, the sewer overflow volume and the required impervious areas at river basin scale are explained in the following. 2.1. Approach for the calculation of the mean annual pollution loads at river basin scale Annual pollution loads from drained urban areas can be calculated (1) by summarizing the pollution loads of the different emission sources connected to sewer systems through their specific pollution loads (e.g. specific nitrogen deposition on paved surfaces or daily nitrogen load per capita), and (2) by assuming that the pollution load from the whole sewer system can be described through a mean annual flow volume and a sewer specific substance concentration. Because of the different specific loads of the emission sources a distinction between pollution loads from roofs, from streets and other artificial areas and from domestic and industrial wastewater is necessary for the
first approach. The required data on specific pollution loads of wastewater can be derived from information on dry weather flow to WWTP which is known as a result of continuous sampling. Whereas the required data on artificial areas is not available in most of the cases at river basin scale. A generalisation of data from literature on specific pollution loads of emission sources, e.g. nutrient loads from roofs and streets, is not possible due to different dry and wet deposition rates according to site specific factors like the volume of traffic. Furthermore, the mean annual efficiency of the different CSO structures in the existing drainage systems in the river basin is also required. This efficiency may vary for different years and also for different structures depending on the rainfall-runoff and other specific characteristics of the drained area and the drainage system like steepness of terrain and sewer system or the total length of the sewer system. The second approach mentioned above is proposed in this study to calculate pollution loads from drained urban areas. Mean annual pollution loads of a substance resulting from sewer overflows and flow to WWTP are simply calculated for each pathway at river basin scale by multiplying the mean annual sewer overflow volume and the mean annual total flow volume to WWTP with the substance concentrations in these pathways, which are known as a result of continuous sampling (in the case of WWTP’s) or can be taken from literature (e.g. Brombach and Fuchs, 2002; Fuchs et al., 2004). Different substance concentrations are used for the overflow volumes of separate and combined sewer sys-
River Basin
Mean Annual Rainfall hN Paved Areas in Combined System Ai,CS Runoff Coefficient Ψm Paved Areas in Separate System Ai,SS+ND
TARCS + DWF
Waste Water Treatment Plant
DWF + ARWWTP
Overflow Structure QCSO
TAF
QSS+ND
Total Annual Runoff (TARCS)
Ai,CS * Ψm * hN
Annual Runoff Volume from Separate System QSS+ND = Ai,SS+ND * Ψm * hN Annual Combined Sewer Overflow Volume
QCSO = TARCS - ARWWTP
Annual Runoff to WWTP (ARWWTP ) = Total annual flow volume to WWTP (TAF) - Annual Dry Weather Flow to WWTP (DWF)
Fig. 1. Approach for the evaluation of runoff volumes from urban areas at basin scale.
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Table 1 Ammonia, nitrate and total nitrogen concentration in sewer overflows (according to Brombach and Fuchs, 2002) Parameter [mg/l] Separate sewer overflow NH4 -N NO3 -N Ntot Combined sewer overflow NH4 -N NO3 -N Ntot
Worldwide data
Central European data
Mean
Min, max
Mean
Min, max
1.49 1.62 3.67
0.02, 21.93 0.05, 12.83 0.65, 8.82
1.81 2.39 4.19
0.20, 21.93 0.20, 12.83 2.10, 6.50
2.80 1.65 10.90
0.30, 12.60 0.03, 6.05 3.50, 17.20
2.39 1.47 14.94
0.30, 8.48 0.03, 4.07
tems (see Table 1). The concentration of separate sewer overflows is likely the same as the resulting concentration of the overall surface runoff from artificial areas. Whereas the concentration of combined sewer overflow must reflect the resulting mixing concentration of runoff from paved areas and wastewater from domestic and industrial uses, and also the cleaning effect of the CSO structure. This approach is adequate for the estimation of the pollution loads from drained urban areas at river basin scale in order to evaluate their significance in comparison to other river pollution sources (e.g. diffuse sources). When used at scale of a single drainage system, the approach also provides adequate results for the comparison of the pollution loads from different drainage systems in a river basin. However, the design of water protection measures cannot be derived from the results of this approach. 2.2. Approach for the estimation of combined sewer overflow volume at river basin scale The mean annual runoff volume from paved areas (total annual runoff volume TAR) is the product of annual rainfall hN , the size of paved areas Ai and mean annual runoff coefficient Wm (see Fig. 1). The so calculated value represents the required mean sewer overflow volume QSSþND in case of areas drained by separate sewers. In difference to that, the amount of combined wastewater discharged to WWTP needs to be considered for the estimation of the annual combined sewer overflow volume (Fig. 1). The mean annual combined sewer overflow volume QCS is the difference between the total runoff from areas drained by combined sewers TARCS and the mean annual runoff to WWTP (ARWWTP ). The mean annual runoff to WWTP (ARWWTP ) is calculated as the difference between the total wastewater flow volume TAF and the dry weather flow volume to WWTP (DWF), which are measured or calculated continuously. The calculation of the exact mean runoff coefficient value requires monitoring data on rainfall-runoff activity in the investigated urban area. However, by consideration of the urban area as a whole when evaluating mean annual runoff, a runoff coefficient in a range of
0.7–0.85 is adequate for surface runoff volume from paved areas according to the gained experiences in urban runoff simulation in the investigated river basin. 2.3. Approach for the estimation of paved urban areas at river basin scale The imperviousness in urban areas is best evaluated by terrestrial survey or using aerial photos. Both methods are costly and time consuming when used for larger areas. Furthermore, it is sufficient to use estimates of paved drained areas instead of ‘exact’ values of this parameter for the evaluation of pollution loads with the above mentioned simple approach. An empirical relation (1) between the proportion of artificial areas (settlements and roads pSR) according to ATKIS or CORINE and imperviousness in urban areas It is therefore proposed for the estimation of the total paved drainage areas Ai;t in river basins with a basin area ARB . A first empirical relation was found in this study by analysing the survey data of 11 drainage areas in the investigated Wupper river basin. But more data on imperviousness was needed for a transferability of the results to other urban areas. Comprehensive data on imperviousness in urban areas with different population densities was available from the investigations of Arlt et al. (2001) and Dahlmann et al. (2002). The empirical relation used in the study (1) was then developed trough non-linear regression using the imperviousness data from the investigations of Arlt et al. (2001) and the corresponding proportion of artificial areas which were extracted from ATKIS or CORINE. An alternative relation was also established in the same way using the imperviousness data from the investigations of Dahlmann et al. (2002) and compared to (1). Fig. 2 shows the comparison of the empirical relations. There is only a little difference between the size of total paved areas calculated with both approaches for the usual proportion of artificial areas in river basins between 10% and 50% of the river basin area. Ai;t ¼
0:24 pSR1:14 ARB 100
ð1Þ
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Ratio of imperviousness in the district [%]
40 Approach based on data from ARLT et al. (2001) R² = 0,89 n = 116 30 Approach based on information from DAHLMANN et al. (2002) R² = 0,94 n = 111 20
10
0 10
Proportion of settlements and roads in the district [%]
100
Fig. 2. Comparison of proposed empirical relations between the proportion of artificial areas and imperviousness in urban areas based on imperviousness data from literature and information extracted from ATKIS.
Due to their resolution CORINE Landcover do not include road areas, and roads are only included in ATKIS as lines. Therefore the information on artificial surfaces (pSRCORINE ), on settlements and roads (pSRATKIS ) taken from these sources must be corrected for the estimation of paved areas in river basins, i.e. correction of CORINE data using (2) and ATKIS data using (3). The Eqs. (2) and (3) are also empirical and were developed trough non-linear regression. pSRCORINE 0:04 þ 0:24 lnðpSRCORINE Þ pSRATKIS pSR ¼ 0:6 þ 0:09 lnðpSRATKIS Þ
pSR ¼
ð2Þ ð3Þ
A correlation found in the study trough linear regression between the ratio of combined sewer length lCS to the total sewer length ðlCS þ lSS Þ and the proportion of paved areas drained by combined sewer systems Ai;CS is used to assign the estimated total impervious area to the combined and separate drainage systems (4). lSS is the length of foul sewers in the separate sewer system. The size of drained paved urban areas Ai;D may be lower than the total paved areas Ai;t . The fraction of drained paved urban areas can be assumed to be equal to the connectivity percentage of population to the sewer system. Information on sewer lengths and paved areas used for this analysis is obtained from a survey of drainage systems in North Rhine-Westphalia (Dohmann and Coburg, 2003). Ai;CS ¼ 0:97
lCS Ai;D lCS þ lSS
and
Ai;SS ¼ Ai;D Ai;CS ð4Þ
3. Results As an example, the results of the estimation of paved areas, sewer overflow volumes and total nitrogen loads from sewer systems into the Wupper river for the period 1998–2000 are presented as follows. The total paved areas in the Wupper river basin is estimated to be 9886 ha based on a proportion of settlements and road in the river basin of 30.6%. Taking in account the connectivity percentage of population to the sewer system in the river basin of about 96%, the drained paved urban areas is about 9490 ha which is drained to 43% by combined sewer system. The annual rainfall in the river basin is about 1384 mm/yr for the period 1998–2000. An overall mean runoff coefficient of 0.7 is assumed for the estimation of surface runoff volume. Assuming a concentration of 10 mg Ntot /l for combined sewer overflow and 4 mg Ntot /l for separate sewer overflow according to the database of Fuchs et al. (2004), total nitrogen loads from drained paved areas in the Wupper river basin are about 385 t Ntot /yr (39 kg/ ha yr) in the period of 1998–2000. These nitrogen loads from sewer overflows represent only 17% in comparison to the total nitrogen emissions from urban sources (sum of loads from sewer overflows and WWTP effluents, Fig. 3). By considering the single drainage areas of the WWTP’s in the Wupper river basin for the period 1998– 2000, the drainage area of the four biggest WWTPs situated in the densely populated parts within the river basin are identified as the most important pollution sources regarding total nitrogen nutrient inputs (Fig. 4). However, a more detailed analysis of the considered
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1500 1449 1000
176 209
500
in t Ntot/yr Loads from separate sewer overflows Loads from combined sewer overflows
0
478 385
in t Ntot/yr Loads from sewer overflows Loads from WWTP WTP due to surface runoff Loads from WWTP due to dry weather flow
Fig. 3. Calculated total nitrogen loads from sewer overflows in comparison with emissions from WWTP’s into the Wupper river system for the period 1998–2000.
Fig. 4. Calculated total nitrogen loads from sewer overflows in the drainage areas of WWTP’s into the Wupper river system for the period 1998– 2000.
urban pathways is needed for the setting of priorities for measures to restore or protect the good status of the river.
4. Discussion Due to the assumptions regarding runoff coefficient, the substance concentrations for sewer overflows from literature (mean concentrations values) and the empirical relations for the estimation of paved areas, the simple approach for load calculation may not reproduce the exact emissions from sewer overflows for the investigated time period. However, a validation of the calculated loads failed due to lack of adequate monitoring data on sewer overflow activities. A validation of the results is in
fact feasible using river water quality monitoring data and gauge data. However, an overall consideration of all emission sources (e.g. diffuse sources) is needed for this purpose, which was not part of the study. Assuming that runoff coefficient and the substance concentrations are adequate for the objectives of the study, it is important to assess the reliability of the approaches for paved area evaluation. The following comparisons were made for this purpose: (1) comparison of estimated paved urban areas with investigations based on aerial photos (mostly situated outside of the Wupper river basin, Table 2), and (2) comparison of the estimated paved areas, sewer overflow volumes and loads with the results of MONERIS (Behrendt et al., 2000, 2002) and calculated values for the drainage area of 11 WWTP’s in the Wupper river basin as reference (Fig. 5).
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Table 2 Comparison of calculated proportion of impervious area in urban districts and a river basin with results of investigations on basis of aerial photos Urban district or river basin
Proportion of impervious areas [ha]
Urban district 1 Urban district 2 Urban district 3 River basin 1
Difference [%]
Aerial photos
Calculated
1834 195 1121 502
1686 214 1012 430
)8.1 9.7 )9.7 )14.3
1.000
Calculated at river basin scale
MESAN (1998-2000) MONERIS (1998-2000)
SS+CS CS SS CS
100 CS
SS+CS SS
SS
SS 10
+30% 1:1
SS 1
-30%
SS : seperate sewer system CS : combined sewer system : paved area [km²] : sewer overflow [m³/s] : mean annual N-load [t]
SS+CS
CS SS
CS 0 0
1
10
100
1.000
Reference data for drainage area of WWTP's
Fig. 5. Comparison of the calculated paved areas, sewer overflow volumes and nitrogen inputs with other investigations.
• Paved urban areas and the runoff coefficient of these areas are estimated in MONERIS using empirical approaches of Heaney et al. (1976), which were developed by analysing survey data on population density, imperviousness and runoff of many US American urban areas. Annual pollution loads from drained urban areas are calculated in MONERIS by summarizing the pollution loads of the different emission sources connected to sewer systems by assuming that the specific pollution loads of paved surfaces in a urban area, e.g. streets and roofs, have the same value. • The imperviousness of the drainage area used as reference represents the actual situation in the case study. This reference data was obtained from a survey on the sewer systems in the Wupper basin as answers of the different sewerage operators (Geiger and Nafo, 2002). As there is no measured data on sewer overflow activities, the sewer overflow volumes and the nutrient loads from reference status are also calculated with the above mentioned approaches. The comparison of estimates of paved areas in some urban districts or river basin obtained by using the empirical approach (1) with investigations based on
aerial photos shows differences up to ±15% (Table 2). However, the higher the percentage of sealed roads in the urban district outside of settlements, the wider the difference between the estimated and the actual impervious area. But, this proportion of paved areas outside of settlement is not drained by sewers in most cases. The comparison of predicted values of the variables paved areas, sewer overflow volume and total nitrogen loads with the calculated values for the drainage area of 11 WWTP’s in the Wupper river basin as reference is presented in Fig. 5. Each variable in the figure is only to be compared among themselves for the different types of sewers systems (separate system or combined system) or for the overall sewer system (separate system and combined system). The figure does not intend to show any relation between the variables. The comparison of the variables among themselves shows that the estimations for the overall sewer system (SS + CS) are about the same, except for sewer overflow from MONERIS. The predicted total imperviousness with both methods only differs from reference status by ±3%. However, the developed method MESAN overestimates the size of paved areas drained by combined sewers by 19% and underestimates the size of paved areas connected to separate sewers by )9%. The differences between the
I.I. Nafo, W.F. Geiger / Physics and Chemistry of the Earth 29 (2004) 831–837
predicted sizes of paved areas in the separate sewer and the combined sewer systems using MONERIS and the reference status exceed ±30%. This high difference, when MONERIS is used, is probably due to data quality in MONERIS, as both the proposed approaches in MESAN and MONERIS use sewer lengths to assign total paved areas to the different sewer systems. The differences in the predicted sizes of paved areas also lead to some differences in the estimates of sewer overflows and nutrient loads. These differences can exceed ±30% for the different types of sewer systems in comparison to the reference status when MONERIS is used. It is remarkable that despite the high differences between sewer overflow volumes estimated with the two methods, both for the different types of sewer systems and for the overall sewer system, the calculated total nitrogen load for the overall sewer system are about the same (difference around 10%). The reason for this is that the higher sewer overflow volume estimated in MONERIS (due to the overestimation of the paved areas and the higher discharge rate of CSO) is compensated both by the small separate sewer overflow volume in MESAN (due to the underestimation of the paved areas), and by the nitrogen concentrations assumed in MESAN, which are higher than the nitrogen concentrations calculated in MONERIS. However, except the predicted sizes of paved areas drained by combined or separate sewer systems, the overall results of MESAN do not exceed the reference status by more than 10%.
5. Conclusion In order to identify significant human pressures and impacts on surface water bodies by the end of 2004 as required by the European Water Framework Directive, a simple method was proposed with approaches for the estimation of paved areas, sewer overflow volumes and nutrient inputs from drained urban areas at river basin scale. Pollution loads can also be estimated for different drainage areas of WWTP’s within a river basin helping to identify the most important emission sources with respect to pollutant inputs from urban areas. A validation of the results failed due to lack of adequate monitoring data on sewer overflow activities. However, the comparison of the predicted total paved areas for three urban districts and one small river basin with investigations based on aerial photos shows a discrepancy of ±15%. The total paved areas in the Wupper basin, the sewer overflow volumes and pollution loads to the
837
Wupper river system calculated at river basin scale using the developed method differs from the reference status by not more than 10%. In the case study the paved areas drained by combined sewers are overestimated and, the paved areas connected to separate sewers slightly underestimated in comparison with the reference status.
Acknowledgements The project at the University Duisburg-Essen was funded by the Ministry for Environment in North Rhine-Westphalia, Germany. We would like to thank Mr. Mertsch and Mr. B€ urgel for their support. References Arlt, G., G€ ossel, J., Heber, B., Hennersdorf, I.L., Thinh, N.X., 2001. Auswirkungen st€adtischer Nutzungsstrukturen auf Bodenversiege€ kologische Raumentwicklung lung und Bodenpreis. Institut f€ ur o € e. V., IOR-Schriften Bd. 34. Behrendt, H., Huber, P., Kornmilch, M., Opitz, D., Schmoll, O., Scholz, G., Uebe, R., 2000. Nutrient Emissions into river basins of Germany. UBA-Text 23/00. Behrendt, H., Bach, M., Kunkel, R., Opitz, D., Pagenkopf, W., Scholz, G., Wendland, F., 2002. Quantifizierung der N€ahrstoffeintr€age der Flussgebiete Deutschlands auf der Grundlage eines harmonisierten Vorgehens. Forschungsvorhaben im Auftrag des Umweltbundesamtes. FB 29922285. Brombach, H., Fuchs, S., 2002. Datenpool gemessener Verschmutzungskonzentrationen von Trocken- und Regenwetterabfl€ ussen in Misch- und Trennkanalisation. Abschlussbericht, ATV-DVWKForschungsfonds 2001, Projekt 1-01. Dahlmann, I., Gunreben, M., Tharsen, J., 2002. Fl€achenverbrauch und Bodenversiegelung in Niedersachsen. Bodenschutz 3, 79 pp. Dohmann, M., Coburg, R.C., 2003. Abflussrelevante Fl€achen in NRW––Zusammenstellung verschiedener Fl€achendaten. Institut f€ ur Siedlungswasserwirtschaft, Rheinisch-Westf€alische Technische Hochschule Aachen. Forschungsbericht i.A. des Ministeriums f€ ur Umwelt und Naturschutz, Landwirtschaft und Verbraucherschutz des Landes Nordrhein-Westfalen, F€ orderkennzeichen: IV-9-042 241 0010. Fuchs, S., Brombach, H., Weiß, G., 2004. New database on urban runoff pollution. Proc. Novatech Conf., 6–10 June 2004, Lyon, France. Geiger, W.F., Nafo, I.I., 2002. Entwicklung einer einfachen Methode zur Ermittlung der Stoffeintr€age in Fliessgew€asser aus Regen- und Mischwassereinleitungen zur Umsetzung der Bestandsaufnahme und Erf€ ullung von Anhang VII der EU-Wasserrahmenrichtlinie. Fachgebiet Siedlungswasserwirtschaft und Abfallwirtschaft, Universit€at Essen. Forschungsbericht i.A. des Ministeriums f€ ur Umwelt und Naturschutz, Landwirtschaft und Verbraucherschutz des Landes Nordrhein-Westfalen, F€ orderkennzeichen: IV-9-042 241. Heaney, J.P., Huber, C.W., Nix, S.J., 1976. Storm water management model level I––preliminary screening procedures. Environmental Protection Technology Series, EPA-600/2-76-275.
A method for the evaluation of pollution loads from urban areas at river basin scale I.I. Nafo *, W.F. Geiger FB10, Urban Water Management, University of Duisburg-Essen, Universitaetsstr. 15, 45141 Essen, Germany Accepted 24 May 2004 Available online 8 July 2004
Abstract The European water framework directive (WFD) requires the identification of significant human pressures and impacts on water bodies. This is part of the wider analysis of river basins required by Article 5 (and Annex II) of the WFD, which must be completed by the end of 2004. The human pressures on surface waters include point and diffuse source pollutions, water abstraction, water flow regulation, morphological alterations and land use patterns. In order to be able to complete the analysis of significant pressures and impacts before the WFD deadline it is necessary to maximise the use of existing information. A method using only existing information for the evaluation of point source pollution at river basin scale, especially the pollution loads from drained urban areas into surface waters was therefore proposed. This method includes an approach to estimate imperviousness in urban areas and to calculate sewer overflow volume at river basin scale. The imperviousness in urban areas is estimated using information on land use in CORINE Landcover or ATKIS (Official Topographic–Cartographic Information System). Data used for the evaluation of sewer overflows, such as rainfall depth, sewer lengths, dry weather flow volume and total volume of combined wastewater discharged to wastewater treatment plants, are available from German Weather Survey, or they are, like in North Rhine-Westphalia, stored in databases. Imperviousness in the Wupper basin and nutrient inputs from urban areas into the Wupper river system were calculated in a case study and compared with the results of other investigations. 2004 Elsevier Ltd. All rights reserved. Keywords: Pollution loads; Imperviousness; Sewer overflow volume
1. Introduction For the identification of significant pressures from point sources on surface waters it is necessary to evaluate the pollution loads from Wastewater Treatment Plants (WWTP) and urban areas, i.e. separate sewer and combined sewer overflows (SSO and CSO). The pollution loads from WWTP can be easily calculated. The final effluent of this point source is continuously measured and sampled. An alternative simple method is needed for the estimation of pollution loads from urban areas, especially from CSOs at river basin scale. The method must be able to estimate total imperviousness in river basins as this is unknown in most cases. To fulfil * Corresponding author. Tel.: +49-201-183-2885; fax: +49-201-1833793. E-mail address: [email protected] (I.I. Nafo).
1474-7065/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2004.05.010
the requirements of the WFD before the deadline, it is necessary that the simple method maximises the use of existing information. The proposed method (MESAN) uses a simple approach for the evaluation of mean annual pollution loads at river basin scale based on calculated annual runoff volume from sewer systems and concentration values from literature. Data on annual rainfall, annual dry weather flow volume and total annual volume of combined wastewater discharged to WWTP are needed for the estimation of the annual sewer overflow volume. Diffuse pollution loads from urban areas into surface waters, e.g. through infiltration, were not addressed in this study. The size of paved urban areas within the river basin is estimated using an empirical relation between the proportion of artificial areas (from CORINE Landcover or ATKIS) and imperviousness.
832
I.I. Nafo, W.F. Geiger / Physics and Chemistry of the Earth 29 (2004) 831–837
2. Methodology The pollution loads are calculated at river scale for a substituted drainage system which represents all drained urban areas in the investigated river basin. Components of the substituted drainage system are (1) a WWTP supposed to drain all urban areas in the river basin, (2) a separate sewer system and a combined sewer system, and (3) a combined sewer overflow structure representing all CSO’s in the river basin (Fig. 1). The proposed approaches for the calculation of the mean annual pollution loads, the sewer overflow volume and the required impervious areas at river basin scale are explained in the following. 2.1. Approach for the calculation of the mean annual pollution loads at river basin scale Annual pollution loads from drained urban areas can be calculated (1) by summarizing the pollution loads of the different emission sources connected to sewer systems through their specific pollution loads (e.g. specific nitrogen deposition on paved surfaces or daily nitrogen load per capita), and (2) by assuming that the pollution load from the whole sewer system can be described through a mean annual flow volume and a sewer specific substance concentration. Because of the different specific loads of the emission sources a distinction between pollution loads from roofs, from streets and other artificial areas and from domestic and industrial wastewater is necessary for the
first approach. The required data on specific pollution loads of wastewater can be derived from information on dry weather flow to WWTP which is known as a result of continuous sampling. Whereas the required data on artificial areas is not available in most of the cases at river basin scale. A generalisation of data from literature on specific pollution loads of emission sources, e.g. nutrient loads from roofs and streets, is not possible due to different dry and wet deposition rates according to site specific factors like the volume of traffic. Furthermore, the mean annual efficiency of the different CSO structures in the existing drainage systems in the river basin is also required. This efficiency may vary for different years and also for different structures depending on the rainfall-runoff and other specific characteristics of the drained area and the drainage system like steepness of terrain and sewer system or the total length of the sewer system. The second approach mentioned above is proposed in this study to calculate pollution loads from drained urban areas. Mean annual pollution loads of a substance resulting from sewer overflows and flow to WWTP are simply calculated for each pathway at river basin scale by multiplying the mean annual sewer overflow volume and the mean annual total flow volume to WWTP with the substance concentrations in these pathways, which are known as a result of continuous sampling (in the case of WWTP’s) or can be taken from literature (e.g. Brombach and Fuchs, 2002; Fuchs et al., 2004). Different substance concentrations are used for the overflow volumes of separate and combined sewer sys-
River Basin
Mean Annual Rainfall hN Paved Areas in Combined System Ai,CS Runoff Coefficient Ψm Paved Areas in Separate System Ai,SS+ND
TARCS + DWF
Waste Water Treatment Plant
DWF + ARWWTP
Overflow Structure QCSO
TAF
QSS+ND
Total Annual Runoff (TARCS)
Ai,CS * Ψm * hN
Annual Runoff Volume from Separate System QSS+ND = Ai,SS+ND * Ψm * hN Annual Combined Sewer Overflow Volume
QCSO = TARCS - ARWWTP
Annual Runoff to WWTP (ARWWTP ) = Total annual flow volume to WWTP (TAF) - Annual Dry Weather Flow to WWTP (DWF)
Fig. 1. Approach for the evaluation of runoff volumes from urban areas at basin scale.
I.I. Nafo, W.F. Geiger / Physics and Chemistry of the Earth 29 (2004) 831–837
833
Table 1 Ammonia, nitrate and total nitrogen concentration in sewer overflows (according to Brombach and Fuchs, 2002) Parameter [mg/l] Separate sewer overflow NH4 -N NO3 -N Ntot Combined sewer overflow NH4 -N NO3 -N Ntot
Worldwide data
Central European data
Mean
Min, max
Mean
Min, max
1.49 1.62 3.67
0.02, 21.93 0.05, 12.83 0.65, 8.82
1.81 2.39 4.19
0.20, 21.93 0.20, 12.83 2.10, 6.50
2.80 1.65 10.90
0.30, 12.60 0.03, 6.05 3.50, 17.20
2.39 1.47 14.94
0.30, 8.48 0.03, 4.07
tems (see Table 1). The concentration of separate sewer overflows is likely the same as the resulting concentration of the overall surface runoff from artificial areas. Whereas the concentration of combined sewer overflow must reflect the resulting mixing concentration of runoff from paved areas and wastewater from domestic and industrial uses, and also the cleaning effect of the CSO structure. This approach is adequate for the estimation of the pollution loads from drained urban areas at river basin scale in order to evaluate their significance in comparison to other river pollution sources (e.g. diffuse sources). When used at scale of a single drainage system, the approach also provides adequate results for the comparison of the pollution loads from different drainage systems in a river basin. However, the design of water protection measures cannot be derived from the results of this approach. 2.2. Approach for the estimation of combined sewer overflow volume at river basin scale The mean annual runoff volume from paved areas (total annual runoff volume TAR) is the product of annual rainfall hN , the size of paved areas Ai and mean annual runoff coefficient Wm (see Fig. 1). The so calculated value represents the required mean sewer overflow volume QSSþND in case of areas drained by separate sewers. In difference to that, the amount of combined wastewater discharged to WWTP needs to be considered for the estimation of the annual combined sewer overflow volume (Fig. 1). The mean annual combined sewer overflow volume QCS is the difference between the total runoff from areas drained by combined sewers TARCS and the mean annual runoff to WWTP (ARWWTP ). The mean annual runoff to WWTP (ARWWTP ) is calculated as the difference between the total wastewater flow volume TAF and the dry weather flow volume to WWTP (DWF), which are measured or calculated continuously. The calculation of the exact mean runoff coefficient value requires monitoring data on rainfall-runoff activity in the investigated urban area. However, by consideration of the urban area as a whole when evaluating mean annual runoff, a runoff coefficient in a range of
0.7–0.85 is adequate for surface runoff volume from paved areas according to the gained experiences in urban runoff simulation in the investigated river basin. 2.3. Approach for the estimation of paved urban areas at river basin scale The imperviousness in urban areas is best evaluated by terrestrial survey or using aerial photos. Both methods are costly and time consuming when used for larger areas. Furthermore, it is sufficient to use estimates of paved drained areas instead of ‘exact’ values of this parameter for the evaluation of pollution loads with the above mentioned simple approach. An empirical relation (1) between the proportion of artificial areas (settlements and roads pSR) according to ATKIS or CORINE and imperviousness in urban areas It is therefore proposed for the estimation of the total paved drainage areas Ai;t in river basins with a basin area ARB . A first empirical relation was found in this study by analysing the survey data of 11 drainage areas in the investigated Wupper river basin. But more data on imperviousness was needed for a transferability of the results to other urban areas. Comprehensive data on imperviousness in urban areas with different population densities was available from the investigations of Arlt et al. (2001) and Dahlmann et al. (2002). The empirical relation used in the study (1) was then developed trough non-linear regression using the imperviousness data from the investigations of Arlt et al. (2001) and the corresponding proportion of artificial areas which were extracted from ATKIS or CORINE. An alternative relation was also established in the same way using the imperviousness data from the investigations of Dahlmann et al. (2002) and compared to (1). Fig. 2 shows the comparison of the empirical relations. There is only a little difference between the size of total paved areas calculated with both approaches for the usual proportion of artificial areas in river basins between 10% and 50% of the river basin area. Ai;t ¼
0:24 pSR1:14 ARB 100
ð1Þ
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I.I. Nafo, W.F. Geiger / Physics and Chemistry of the Earth 29 (2004) 831–837
Ratio of imperviousness in the district [%]
40 Approach based on data from ARLT et al. (2001) R² = 0,89 n = 116 30 Approach based on information from DAHLMANN et al. (2002) R² = 0,94 n = 111 20
10
0 10
Proportion of settlements and roads in the district [%]
100
Fig. 2. Comparison of proposed empirical relations between the proportion of artificial areas and imperviousness in urban areas based on imperviousness data from literature and information extracted from ATKIS.
Due to their resolution CORINE Landcover do not include road areas, and roads are only included in ATKIS as lines. Therefore the information on artificial surfaces (pSRCORINE ), on settlements and roads (pSRATKIS ) taken from these sources must be corrected for the estimation of paved areas in river basins, i.e. correction of CORINE data using (2) and ATKIS data using (3). The Eqs. (2) and (3) are also empirical and were developed trough non-linear regression. pSRCORINE 0:04 þ 0:24 lnðpSRCORINE Þ pSRATKIS pSR ¼ 0:6 þ 0:09 lnðpSRATKIS Þ
pSR ¼
ð2Þ ð3Þ
A correlation found in the study trough linear regression between the ratio of combined sewer length lCS to the total sewer length ðlCS þ lSS Þ and the proportion of paved areas drained by combined sewer systems Ai;CS is used to assign the estimated total impervious area to the combined and separate drainage systems (4). lSS is the length of foul sewers in the separate sewer system. The size of drained paved urban areas Ai;D may be lower than the total paved areas Ai;t . The fraction of drained paved urban areas can be assumed to be equal to the connectivity percentage of population to the sewer system. Information on sewer lengths and paved areas used for this analysis is obtained from a survey of drainage systems in North Rhine-Westphalia (Dohmann and Coburg, 2003). Ai;CS ¼ 0:97
lCS Ai;D lCS þ lSS
and
Ai;SS ¼ Ai;D Ai;CS ð4Þ
3. Results As an example, the results of the estimation of paved areas, sewer overflow volumes and total nitrogen loads from sewer systems into the Wupper river for the period 1998–2000 are presented as follows. The total paved areas in the Wupper river basin is estimated to be 9886 ha based on a proportion of settlements and road in the river basin of 30.6%. Taking in account the connectivity percentage of population to the sewer system in the river basin of about 96%, the drained paved urban areas is about 9490 ha which is drained to 43% by combined sewer system. The annual rainfall in the river basin is about 1384 mm/yr for the period 1998–2000. An overall mean runoff coefficient of 0.7 is assumed for the estimation of surface runoff volume. Assuming a concentration of 10 mg Ntot /l for combined sewer overflow and 4 mg Ntot /l for separate sewer overflow according to the database of Fuchs et al. (2004), total nitrogen loads from drained paved areas in the Wupper river basin are about 385 t Ntot /yr (39 kg/ ha yr) in the period of 1998–2000. These nitrogen loads from sewer overflows represent only 17% in comparison to the total nitrogen emissions from urban sources (sum of loads from sewer overflows and WWTP effluents, Fig. 3). By considering the single drainage areas of the WWTP’s in the Wupper river basin for the period 1998– 2000, the drainage area of the four biggest WWTPs situated in the densely populated parts within the river basin are identified as the most important pollution sources regarding total nitrogen nutrient inputs (Fig. 4). However, a more detailed analysis of the considered
I.I. Nafo, W.F. Geiger / Physics and Chemistry of the Earth 29 (2004) 831–837
835
1500 1449 1000
176 209
500
in t Ntot/yr Loads from separate sewer overflows Loads from combined sewer overflows
0
478 385
in t Ntot/yr Loads from sewer overflows Loads from WWTP WTP due to surface runoff Loads from WWTP due to dry weather flow
Fig. 3. Calculated total nitrogen loads from sewer overflows in comparison with emissions from WWTP’s into the Wupper river system for the period 1998–2000.
Fig. 4. Calculated total nitrogen loads from sewer overflows in the drainage areas of WWTP’s into the Wupper river system for the period 1998– 2000.
urban pathways is needed for the setting of priorities for measures to restore or protect the good status of the river.
4. Discussion Due to the assumptions regarding runoff coefficient, the substance concentrations for sewer overflows from literature (mean concentrations values) and the empirical relations for the estimation of paved areas, the simple approach for load calculation may not reproduce the exact emissions from sewer overflows for the investigated time period. However, a validation of the calculated loads failed due to lack of adequate monitoring data on sewer overflow activities. A validation of the results is in
fact feasible using river water quality monitoring data and gauge data. However, an overall consideration of all emission sources (e.g. diffuse sources) is needed for this purpose, which was not part of the study. Assuming that runoff coefficient and the substance concentrations are adequate for the objectives of the study, it is important to assess the reliability of the approaches for paved area evaluation. The following comparisons were made for this purpose: (1) comparison of estimated paved urban areas with investigations based on aerial photos (mostly situated outside of the Wupper river basin, Table 2), and (2) comparison of the estimated paved areas, sewer overflow volumes and loads with the results of MONERIS (Behrendt et al., 2000, 2002) and calculated values for the drainage area of 11 WWTP’s in the Wupper river basin as reference (Fig. 5).
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Table 2 Comparison of calculated proportion of impervious area in urban districts and a river basin with results of investigations on basis of aerial photos Urban district or river basin
Proportion of impervious areas [ha]
Urban district 1 Urban district 2 Urban district 3 River basin 1
Difference [%]
Aerial photos
Calculated
1834 195 1121 502
1686 214 1012 430
)8.1 9.7 )9.7 )14.3
1.000
Calculated at river basin scale
MESAN (1998-2000) MONERIS (1998-2000)
SS+CS CS SS CS
100 CS
SS+CS SS
SS
SS 10
+30% 1:1
SS 1
-30%
SS : seperate sewer system CS : combined sewer system : paved area [km²] : sewer overflow [m³/s] : mean annual N-load [t]
SS+CS
CS SS
CS 0 0
1
10
100
1.000
Reference data for drainage area of WWTP's
Fig. 5. Comparison of the calculated paved areas, sewer overflow volumes and nitrogen inputs with other investigations.
• Paved urban areas and the runoff coefficient of these areas are estimated in MONERIS using empirical approaches of Heaney et al. (1976), which were developed by analysing survey data on population density, imperviousness and runoff of many US American urban areas. Annual pollution loads from drained urban areas are calculated in MONERIS by summarizing the pollution loads of the different emission sources connected to sewer systems by assuming that the specific pollution loads of paved surfaces in a urban area, e.g. streets and roofs, have the same value. • The imperviousness of the drainage area used as reference represents the actual situation in the case study. This reference data was obtained from a survey on the sewer systems in the Wupper basin as answers of the different sewerage operators (Geiger and Nafo, 2002). As there is no measured data on sewer overflow activities, the sewer overflow volumes and the nutrient loads from reference status are also calculated with the above mentioned approaches. The comparison of estimates of paved areas in some urban districts or river basin obtained by using the empirical approach (1) with investigations based on
aerial photos shows differences up to ±15% (Table 2). However, the higher the percentage of sealed roads in the urban district outside of settlements, the wider the difference between the estimated and the actual impervious area. But, this proportion of paved areas outside of settlement is not drained by sewers in most cases. The comparison of predicted values of the variables paved areas, sewer overflow volume and total nitrogen loads with the calculated values for the drainage area of 11 WWTP’s in the Wupper river basin as reference is presented in Fig. 5. Each variable in the figure is only to be compared among themselves for the different types of sewers systems (separate system or combined system) or for the overall sewer system (separate system and combined system). The figure does not intend to show any relation between the variables. The comparison of the variables among themselves shows that the estimations for the overall sewer system (SS + CS) are about the same, except for sewer overflow from MONERIS. The predicted total imperviousness with both methods only differs from reference status by ±3%. However, the developed method MESAN overestimates the size of paved areas drained by combined sewers by 19% and underestimates the size of paved areas connected to separate sewers by )9%. The differences between the
I.I. Nafo, W.F. Geiger / Physics and Chemistry of the Earth 29 (2004) 831–837
predicted sizes of paved areas in the separate sewer and the combined sewer systems using MONERIS and the reference status exceed ±30%. This high difference, when MONERIS is used, is probably due to data quality in MONERIS, as both the proposed approaches in MESAN and MONERIS use sewer lengths to assign total paved areas to the different sewer systems. The differences in the predicted sizes of paved areas also lead to some differences in the estimates of sewer overflows and nutrient loads. These differences can exceed ±30% for the different types of sewer systems in comparison to the reference status when MONERIS is used. It is remarkable that despite the high differences between sewer overflow volumes estimated with the two methods, both for the different types of sewer systems and for the overall sewer system, the calculated total nitrogen load for the overall sewer system are about the same (difference around 10%). The reason for this is that the higher sewer overflow volume estimated in MONERIS (due to the overestimation of the paved areas and the higher discharge rate of CSO) is compensated both by the small separate sewer overflow volume in MESAN (due to the underestimation of the paved areas), and by the nitrogen concentrations assumed in MESAN, which are higher than the nitrogen concentrations calculated in MONERIS. However, except the predicted sizes of paved areas drained by combined or separate sewer systems, the overall results of MESAN do not exceed the reference status by more than 10%.
5. Conclusion In order to identify significant human pressures and impacts on surface water bodies by the end of 2004 as required by the European Water Framework Directive, a simple method was proposed with approaches for the estimation of paved areas, sewer overflow volumes and nutrient inputs from drained urban areas at river basin scale. Pollution loads can also be estimated for different drainage areas of WWTP’s within a river basin helping to identify the most important emission sources with respect to pollutant inputs from urban areas. A validation of the results failed due to lack of adequate monitoring data on sewer overflow activities. However, the comparison of the predicted total paved areas for three urban districts and one small river basin with investigations based on aerial photos shows a discrepancy of ±15%. The total paved areas in the Wupper basin, the sewer overflow volumes and pollution loads to the
837
Wupper river system calculated at river basin scale using the developed method differs from the reference status by not more than 10%. In the case study the paved areas drained by combined sewers are overestimated and, the paved areas connected to separate sewers slightly underestimated in comparison with the reference status.
Acknowledgements The project at the University Duisburg-Essen was funded by the Ministry for Environment in North Rhine-Westphalia, Germany. We would like to thank Mr. Mertsch and Mr. B€ urgel for their support. References Arlt, G., G€ ossel, J., Heber, B., Hennersdorf, I.L., Thinh, N.X., 2001. Auswirkungen st€adtischer Nutzungsstrukturen auf Bodenversiege€ kologische Raumentwicklung lung und Bodenpreis. Institut f€ ur o € e. V., IOR-Schriften Bd. 34. Behrendt, H., Huber, P., Kornmilch, M., Opitz, D., Schmoll, O., Scholz, G., Uebe, R., 2000. Nutrient Emissions into river basins of Germany. UBA-Text 23/00. Behrendt, H., Bach, M., Kunkel, R., Opitz, D., Pagenkopf, W., Scholz, G., Wendland, F., 2002. Quantifizierung der N€ahrstoffeintr€age der Flussgebiete Deutschlands auf der Grundlage eines harmonisierten Vorgehens. Forschungsvorhaben im Auftrag des Umweltbundesamtes. FB 29922285. Brombach, H., Fuchs, S., 2002. Datenpool gemessener Verschmutzungskonzentrationen von Trocken- und Regenwetterabfl€ ussen in Misch- und Trennkanalisation. Abschlussbericht, ATV-DVWKForschungsfonds 2001, Projekt 1-01. Dahlmann, I., Gunreben, M., Tharsen, J., 2002. Fl€achenverbrauch und Bodenversiegelung in Niedersachsen. Bodenschutz 3, 79 pp. Dohmann, M., Coburg, R.C., 2003. Abflussrelevante Fl€achen in NRW––Zusammenstellung verschiedener Fl€achendaten. Institut f€ ur Siedlungswasserwirtschaft, Rheinisch-Westf€alische Technische Hochschule Aachen. Forschungsbericht i.A. des Ministeriums f€ ur Umwelt und Naturschutz, Landwirtschaft und Verbraucherschutz des Landes Nordrhein-Westfalen, F€ orderkennzeichen: IV-9-042 241 0010. Fuchs, S., Brombach, H., Weiß, G., 2004. New database on urban runoff pollution. Proc. Novatech Conf., 6–10 June 2004, Lyon, France. Geiger, W.F., Nafo, I.I., 2002. Entwicklung einer einfachen Methode zur Ermittlung der Stoffeintr€age in Fliessgew€asser aus Regen- und Mischwassereinleitungen zur Umsetzung der Bestandsaufnahme und Erf€ ullung von Anhang VII der EU-Wasserrahmenrichtlinie. Fachgebiet Siedlungswasserwirtschaft und Abfallwirtschaft, Universit€at Essen. Forschungsbericht i.A. des Ministeriums f€ ur Umwelt und Naturschutz, Landwirtschaft und Verbraucherschutz des Landes Nordrhein-Westfalen, F€ orderkennzeichen: IV-9-042 241. Heaney, J.P., Huber, C.W., Nix, S.J., 1976. Storm water management model level I––preliminary screening procedures. Environmental Protection Technology Series, EPA-600/2-76-275.