INTEGRATED MEASURES FOR THE REDUCTION OF STORM WATER AND COMBINED SEWER OVERFLOW IMPACT ON AN URBAN LAKE

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1 Hydrological Processes and Water Management in Urban Areas (Proceedings of the Duisberg Symposium, April 1988). IAHS Publ. no. 198, INTEGRATED MEASURES FOR THE REDUCTION OF STORM WATER AND COMBINED SEWER OVERFLOW IMPACT ON AN URBAN LAKE Thorkild Hvitved-Jacobsen Environmental Engineering Laboratory University of Aalborg, 9000 Aalborg, Denmark Stig Jensen Viborg Metropolitan Sewerage District 8800 Viborg, Denmark ABSTRACT For two years an intensive monitoring programme was carried out in order to establish a basis for a wastewater management plan for the city of Viborg, Denmark. In this plan special attention is paid to reducing the impact of stormwater runoff (SWR) and combined sewer overflows (CSO) from the 506 ha paved drainage area of the city on an urban twin-lake of 266 ha. Based on the results of the investigation a wide variety of both structural and non-structural measures will be implemented during a five year planning period, which started in 1987 based on a financial plan which amount to $ 12 million. In the attempt to reduce the impact from CSO on the lake special attention was paid to evaluating possible negative effects of the increased hydraulic load on the wastewater treatment plant. Exemplified by phosphorus, the total non-point-urban loadings on the twin-lake will be reduced from 0.35 to about 16 g m yr during the planning period. INTRODUCTION The sewerage systems which are in operation in many Danish cities have been established during the past 100 years. The requirements for the function of these systems have in the meantime gradually undergone considerable changes. Besides the ability to transport the sewage to a wastewater treatment plant, special attention has in recent years been paid to the wet weather problems: flooding, and receiving water impacts from SWR and CSO. In many cities these new requirements are both a problem and a challenge. A recurring matter of discussion is the high level of resources required in terms of expertise and financial support. In order to overcome these problems Viborg Metropolitan Sewerage District established a framework consisting of procedures to ensure consultation and review of a wide variety of structural and non-structural measures during the planning process. This paper presents an example of how the impact from different urban non-point pollutant sources under the limitations of an existing infra-structure can be reduced. Furthermore, it is shown how different measures interact and which important negative effects must be taken into account. Due to the limited degrees of freedom, cost efficiency and the possibility of practical implementation play an important role. 163

2 Thorkild Hvited-Jacobsen and Stig Jensen THE SEWERAGE AND LAKE SYSTEM The sewerage system in Viborg dates back to the 1880s and comprises today about 375 km of sewer pipes. Although the last combined sewers were built more than 30 years ago, they still serve 65% of the total paved area in the city (Table 1). Table 1. Distribution of the sewered area in Viborg Sewer type Total sewered Paved area area (ha) (ha) combined Separate In particular the concrete pipes constructed during the period need rehabilitation. This problem can be exemplified by the fact that 98 sewer sections are washed regularly in order to maintain proper function. About 40,000 inhabitants live in the city, which is located around a twin-lake - lake Noerresoe and lake Soendersoe (Figure 1). This lake system is of very high recreational value for the inhabitants. Therefore, 15 years ago a sewage treatment plant discharging primary treated wastewater to the twin-lake was replaced by a biological wastewater treatment plant which discharges to a different receiving water system, a river located about 3 km SE of lake Soendersoe. Phosphorus is the limiting nutrient in the lake system. Due to external as well as internal phosphorus loadings on the twin-lake, eutrophication is still a problem. Measures to restore the lake will be taken, but restoration requires a reduction in loadings from a wastewater treatment plant (located upstream) and from urban non-point discharges. Physical characteristics of the twin-lake are summarized in Table

3 Integrated measures for the reduction of storm water Figure 1. Sewered area of the city of Viborg. Table 2. Physical characteristics of the twin-lake, lake Noerresoe and Lake Soendersoe Noerresoe Soendersoe 2 area, km, volume, m mean depth, m max. depth, m residence time, d

4 Thorkild Hvited-Jacobsen and Stig Jensen COMBINED SEWER OVERFLOW LOADINGS Loadings on the twin-lake from the combined sewered areas (Figure 1) where supposed to contribute to the following two main effects: Eutrophication due to the phosphorus content in the overflow water. Bacterial contamination. In order to quantify these loadings an intensive programme comprising the collection of sewer data, monitoring pipe flow variations and sampling for analyses of wastewater and urban runoff characteristics was carried out. Based on these findings a runoff model covering the entire sewer network was set up. Computations were based on a 33 years continuous historical rain record and a combination between a kinematic wave model (ILLUDAS) and a time-area model (SAMBA), Johansen et al. (1984). Discharge to the twin-lake takes place from 10 overflow structures (Figure 2). Based on the runoff model, phosphorus loadings were calculated from each of the overflow structures (Table 3). As seen from Table 3, CSO-loadings on the Northern part of the twin-lake are negligible compared with the loadings on the Southern part. Utilization of free capacity in the sewer pipes and basins in the Northern part of the city in order to reduce the total annual load of phosphorus to the lake is possible. Based on model simulations it appears that bacterial deterioration during the summer season of beaches located along the Northwestern shoreline can be avoided. Based on analysis of the output from the runoff model a combined set of measures can be used in order to reduce the total CSO-loadings of phosphorus: Optimal function of the two overflow structures: Borgvold and Rundebaenken, i.e. start and stop discharges at the same time. Extensioruof the basin at Sygehus oest (figure 3). Based on a cost efficiency analysis a 1500 m storage basin was chosen m storage volume in the main sewers south and west the lake can be used during a storm event. Measures in order to route the wastewater to the sewage treatment plant at the beginning of a storm event will increase the storage capacity in the sewerage system for the remainder of the event. 166

5 Integrated measures for the reduction of storm water Kaervaengetj Erik MenvedsVej Borgvold Rundebaenken Golf Hotel Sygehus Oest Kokildedalen Oegaardshoejen Dyssedalen Asmildhoejen Sygehus Vest Lyngvej to waste water treatment plant, Bruunshaab Figure 2. Location of overflow structures with loadings on the twin-lake. Table 3. Overflow structures Phosphorus loadings on the twin-lake from CSO Phosphorus loadings (kg yr ) summer winter total Lake Noerresoe: Kaervaenget Kokildedalen Oegaardshoejan Dyssedalen Asmildehoejen Lake Sœndersœ: Borgvold Golf Hotel Rundebaenken Sygehus oest Sygehus vest Lyngvej Total

6 Thorkild Hvited-Jacobsen and Stig Jensen kgyr-ij k P-reduction volume L, 1,,_ 1 1 W m 3 Figure 3. Reduction of the CSO-loadings on the lake at Sygehus oest versus the extended storage capacity. These measures will not increase the loading on the Northern part of the lake; on the Southern part the. phosphorus loading from the overflow structures will be reduced from 470 to 141 kg yr", i.e. 517 kg yr" (Figure 4). 168

7 Integrated measures for the reduction of storm water kg yr Figure 4. Reduction in CSO-loading of phosphorus on the Southern part of the twin-lake resulting from the integrated measures, cf. text. WET WEATHER LOADINGS ON THE WASTEWATER TREATMENT PLANT The reduced CSO-loadings on the twin-lake is followed by a corresponding increased amount of wastewater routed to the wastewater treatment plant, Bruunshaab. This increased loading may cause overflow or reduced treatment at the plant. Under extreme conditions this dual problem may result in an overall negative effect, Lindholm (1985). In order to maintain proper function at the wastewater treatment plant the maximum allpwed inlets to the primary and secondary treatment steps are 5,000 m hr and 2,250 m hr respectively. 3 However, the storage capacity in existing detention basins is only 2,600 m. Under these conditions more than 400 kg P yr would be discharged to the river Noerreaa due to overflow before secondary treatment and reduced removal at the wastewater treatment plant. This is not acceptable. Therefore, based on the results, from Figure 5 the storage capacity at the treatment plant will be increased by 11,200 m which will reduce the number of overflows to 1-2 times per year. Unlike the detention basins in urban areas these basins are open and therefore relatively inexpensive. 169

8 Thorkild Hvited-Jacobsen and Stig Jensen overflow phosphorus volume. discharged m 3 -yr" 1 "kg-yr Figure 5. 10,000 15,000 m 3 Effect of increased storage capacity in open basins at the wastewater treatment plant. Still, the increased amount of wastewater routed to the wastewater treatment plant will increase the amount of phosphorus discharged here. A calculation shows 115 kg P yr. Therefore, the total phosphorus reduction for the combined sewered area amounts to: reduced P-loading at the lake - increased P-loading at the river = = 214kgyr STORM WATER LOADINGS Based on data originating from other separate sewered catchments in Denmark it was expected that the loadings on the twin-lake from storm sewer outlets would be of relative importance compared with other non-point urban sources. Therefore, an intensive monitoring programme was carried out during the spring/summer and the fall season of Nutrients as well as heavy metals were monitored. With phosphorus as an example the loadings on the twin-lake are given in Table 4. The numbers correspond to a specific loading of 2.3 kg P ha yr. Table 4. Loadings with phosphorus from separate sewered areas on the twin-lake Part of the lake Noerresoe Soendersoe Paved area (ha) Phosphorus -1, loading (kg yr ) total

9 Integrated measures for the reduction of storm water Compared with the original loading from combined sewered areas (Table 3) the outlets from storm sewers contribute about 45 % of the total non-point urban loadings. Measures in order to reduce the impact from that specific source are therefore very important. Several methods were evaluated, the following were found the most attractive: Soil infiltration of rainwater Construction of detention ponds for removal of pollutants. As to soil infiltration of rainwater, it may be noted that there is no interest in the main part of the separate sewered catchment in using of groundwater supplies for drinking water purposes. Therefore, infiltration from e.g. roofs and parking lots seems to be a possible method for reduction of SWR. However, it is not clear to what extent the Sewerage District can require that rainwater be infiltrated. The concept of pollutant removal from SWR in detention ponds is new in Denmark. By chance, a wet detention pond, (i.e. a pond which is also wet during dry weather periods) is located east of lake Noerresoe, where it is integrated in a recreational area. The detention pond receives SWR from a residential catchment area of 22.2 ha, comprising single family units, and having a runoff coefficient of 0.3. The pond forms an on-line basin for a separate stormwater system and controls discharges to the Northern patl of the twin-lake. The pond has an average water depth of 0.7 m, a surface area of 900 m and a dry weather storage volume of about 500 m. In order to determine pollutant removal efficiencies in the pond an intensive monitoring programme was set up (Hvitved-Jacobsen et al., 1987). Some of the main results from this investigation are shown in Table 5. Table 5. Average mass removal efficiencies in the stormwater detention pond, (%) Season Number of events SS tot.p Zn Cd Pb Cu Spring/summer Fall NA The efficiency of pollutant removal in a detention pond depends on the residence time of the stormwater. Therefore, the ratio between the total volume of each storm event diverted to the pond and the dry weather storage volume is an important parameter. Exemplified by phosphorus this phenomenon is illustrated (Figure 6) for each of the 10 events investigated (Table 5). Figure 6 indicates that proper design of the detention pond may lead to 60-70% mass removal of phosphorus. 171

10 ThorJdld Hvited-Jacobsen and Stig Jensen 1.0 mass removal fraction of tot. P 0.5 i i i i i i i i i i Vinput _ volume of storm input Vpond pond volume Figure 6. Mass removal of phosphorus versus volume ratio V. 7 V,. input pond Although there is a lack of space in the existing separate sewered area, it is possible to provide 60-70% of the area with detention ponds, many of them integrated in parks. Using detention ponds the calculated total phosphorus loading from the separate sewered areas to the twin-lake can be reduced from 405 kg yr (Table 4) to about 230 kg yr. LOADINGS FROM THE NON-SEWERED AREA In the unsewered parts of the catchment area of the twin-lake, wastewater from singlefamily houses is treated in septic tanks. The effluent from these septic tanks is disposed of mainly to nearby ditches and brooks. Inspections showed that the majority of these septic tanks were badly operated and maintained. Therefore, the municipality initiated a programme in order to reduce the phosphorus loading on the twin-lake from these singlefamily houses. One of the main elements was a compulsory yearly emptying of septic tanks. The result was a calculated phosphorus reduction to the lake of kg yr. CONCLUSIONS In order to reduce the phosphorus loadings from urban non-point sources into a lake (as the first step in a restoration process) a wide variety of measures are planned to be carried out over a five-year period. In combined and separate sewered areas the phosphorus loading to the lake will be reduced by about 1 65% and 45% respectively, corresponding to an average reduction of about 500 kg P yr or about 1 kg P ha yr. It is important to notice that the reduction of overflows is partly counterbalanced by increased loading on the wastewater treatment plant resulting in an phosphorus discharge of 115 kg P yr" from the wastewater treatment plant to a different receiving water system. 172

11 Integrated measures for the reduction of storm water ACKNOWLEDGEMENTS The authors acknowledge the financial support from the city of Viborg. Also, the authors wish to thank P. Poulsen, Hedelskabet, and B. Jensen, Palle Christensen, Consulting Engineers, for their help and contributions during the entire planning process. REFERENCES Johansen, N.B., T. Hvitved-Jacobsen and P. Harremoës (1984). Simulation of the impact of combined sewer overflows on rivers, proceedings of the 8th Nordic Hydrological Conference on Hydrology and Water Quality, Nyborg, Denmark, Aug. 6-8, 1984, Nordic Hydrological Programme, report no. 5, vol. 2, Lindholm, O.G. (1985). May retention basins have an overall negative effect?, Vatten, vol. 41, no 1, Hvitved-Jacobsen, T., K. Keiding and Y.A. Yousef (1987). Urban runoff pollutant removal in wet detention ponds, proceedings of the 4th International Conference on Urban Storm Drainage, Lausanne, Switzerland, Aug. 31-Sep. 4, 1987,

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