Sewerage Management System for Reduction of River Pollution

Transcription

1 Sewerage Management System for Reduction of River Pollution Peter Hartwig Germany Content page: 1 Introduction 1 2 Total emissions 3 3 Discharge from the wastewater treatment plants 4 4 Discharge from the sewerage system overflows 6 5 Sewerage management systems General remarks Combined storage/ equalization/ sedimentation tank Control of the outflow from the stormwater storage tanks 8 6 Recommended procedure for the adaption to other sewerage systems 9 7 Conclusion 10 1 Introduction The sewerage system of a settlement has the task to transport rainand drainage water and municipal and industrial sewage to the treatment plant or to the discharge points. Regarding the structure of the sewerage system combined and separate systems will be used:

2 Combined system Municipal and industrial sewage and rainwater is transported in one channel. Separate system Municipal and industrial sewage is transported in one channel, the rain- and drainage water in another channel. Combined systems needs stormwater overflows, because the rainwater can be much bigger than the sewage. The rainwater from rainfalls with less intensity or duration can be treated in the wastewater treatment plant. In separate systems the rainwater is fed directly to a river, usually without any treatment. In German cities the old parts of a town have often a combined system. New settlements in the surroundings are dewatered with separate systems, where the sewage is fed to the combined system. This leads to a higher concentration in the combined system and following also to a higher discharge of loads into the river. The influent point of higher polluted industrial wastewater has to be considered for the technical calculations of the discharge loads. In some cases a separate pipe from the industry to the wastewater treatment plant is a suitable measure to reduce the direct discharge of loads to a river. To reduce the negative impact to the receiving rivers by the stormwater overflows from combined systems, storage tanks or further measures can be installed. The hydraulic loads of the sewerage system and also the concentration of the relevant components, which have an impact to the river quality, are changing in a wide range.

3 Because of the structure of a sewerage system, with many different pipes and channels, storage tanks and treatment facilities and the big differences in loads and quantities, by a management procedure for the sewerage system, significant reduction of the discharged loads can be achieved. Some examples will be explained following. 2 Total emissions The situations before passing a settlement and the impact of the several discharges points are relevant for the river quality. The total emissions are the sum of emissions, which are fed by all technical discharge points to a river. The main discharge comes from the stormwater overflows, the rainwater discharge points and the effluent from the wastewater treatment plant. Following parameters are mainly relevant

4 for the decrease of the water quality in a river by discharge from sewerage systems: Parameter Main impact to the river flow erosion of the river bottom suspended solids clogging of the porous river bottom system nitrogen - ammonia NH 4 -N decrease of the oxygen concentration - nitrate NO 3 -N eutrophication oxygen buffer in high loaded river systems phosphorus eutrophication organics, measured as BOD 5 or COD reduction of the oxygen control, production of sludge The goal of a management of a sewerage system is to minimize the negative impact to the river as a sum of all discharges. As consequence of the improved water quality in the rivers by using advanced technologies for the wastewater treatment the negative impact of the direct discharge is increasing. 3 Discharge from the wastewater treatment plants In Germany most parts of the wastewater is treated in wastewater treatment plants. The requirements for the effluent quality are defined in an EU-regulation corresponding to the capacity of the wastewater treatment plant. The German regulation takes as reference sample instead of the daily average value a grap sample, because of this the values are higher as defined in the EU-regulations. The total efficiencies are comparable for both regulations.

5 Capacity [PE] COD [mg/l] BOD 5 [mg/l] < 1, < 5, NH 4 -N [mg/l] < 10, N tot [mg/l] P tot [mg/l] < 100, > 100, Table 1: Requirements for the effluent of wastewater treatment plants in Germany Average values for the effluent concentration of wastewater treatment plants are significantly lower than the required. Usual effluent concentration can be estimated as chemical oxygen demand: COD mg/l biological oxygen demand: BOD mg/l ammonia: NH 4 -N 1 2 mg/l nitrate: NO 3 -N 6 mg/l organic nitrogen: org. N 2 mg/l phosphorus: P 1 mg/l Table 2: Average concentrations in the effluent of municipal wastewater treatment plants in Germany During stormwater runoff and increased inflow to the wastewater treatment plant, the effluent values can increase at the beginning of the event according to the load situation of the treatment plant. Main results of stormwater runoff are higher ammonia concentrations and, if the final clarifier s efficiency is decreasing, higher amounts of suspended solids and COD (1 g SS 1.2 g COD). After the first shock load, the dilution of the sewage by the rainwater leads to a decrease of the concentration in the effluent of the plant. Main factors which are influencing the actual capacity of a wastewater treatment plant are the temperature and the sludge settlement, characterized by the sludge volume index SVI.

6 4 Discharge from the sewerage system overflows Characteristic for the overflow is that the flow has a wide range between zero and maximum values, which are factor 100 1,000 of the sewage flow at dry weather conditions. The loading of the rainwater with the relevant parameters BOD 5, COD, N, P, SS is a function of the duration of the dry weather period before. Also the sewer cleaning period and the street cleaning periods are relevant. As an average of measurements under different conditions, an organic concentration of COD = 107 mg/l can be estimated. 5 Sewerage management systems 5.1 General remarks The natural retention volume in a catchment area is limited and has usually only a capacity of a few mm rainfall. All other rainwater has to be treated in the wastewater treatment plant, has to be stored in a storage tank or in the channel system, or has to be discharged. A reduction of the direct discharge into the river can be achieved by an increase of the inflow to the wastewater treatment plant or by an increase of the storage volumes. The increase of the inflow to the wastewater treatment plant is limited due to the biological system, which can stand inflow changes between dry weather peak load and stormwater runoff peak load until up to factor 3, if the design and the technical components are suitable for that. The retention volume in the catchment area is a question of the required investment, technical problems with very large retention volumes are not given. There are a wide range of different strategies known how to manage the sewerage system to find the optimized operation condition. Two examples are shown in the following chapters.

7 5.2 Combined storage/ equalization/ sedimentation tank Wastewater treatment plants which have a separate sludge stabilization (> 50,000 PE) are using primary sedimentation tanks as one component of the mechanical treatment. The task of this tank is to remove the sludge, which can be settled and which is coming with the inflows to the plant. The removal efficiencies are dependent from the retention time in the sedimentation. Typical removal rates are: Parameter retention time h h BOD 5 25 % 33 % COD 25 % 33 % SS 50 % 64 % N 9 % 9 % P 11 % 11 % Table 3: Efficiencies of primary sedimentation tanks Good operation results could be found with primary sedimentation tanks, which have an additional retention volume to equalize the daily fluctuations of the inflow during dry weather and a storage volume during stormwater runoff. The equalization during dry weather simplifies the operation of the biological stage. The machines like pumps or aeration systems can be operated more continuously, the effluent concentrations can be reduced. The hydraulic load of the final clarifier is equalized which leads to less suspended solids in the effluent because of stable fluid conditions. The additional storage volume for stormwater runoff has the function to keep the first high loaded flush after the start of a strong rain. Compared with separate storage tank the operation is much easier because no cleaning or additional maintenance is required.

8 The dry weather equalization volume is only used some part of a day. If the rainfall just starts if the dry weather equalization volume is free, this volume can be used additionally. To be able to take into account this behaviour for the technical calculations to define the capacity of a sewerage system, a specific tool was developed, which works with a longterm continuous simulation of the sewerage system runoff and which includes this multi-functional storage and equalization tanks. 5.3 Control of the outflow from the stormwater storage tanks In greater catchment areas, several storage tanks and storage channels can be integrated. Usually, each storage tank has a fix outflow. The suitable flow is a result of long-term simulation or other design procedures. But the rainfall is not equal in duration and intensity over the whole catchment area. Sometimes some storage tanks have an overflow, while other are still empty. This situation is shown in the flow scheme for the dewatering infrastructure of the city of Hildesheim, 240,000 PE.

9 The outflow from an individual storage tank can be increased if other storage tanks are not feeding the design capacity, if the wastewater treatment plant can handle the additional flow and loads. The regulation will be done by a management system, which is just under installation. As main elements of this sewerage management system the rain intensity, the water levels in the stormwater storage tanks and the individual effluents are used in the optimization procedure. Later a rainfall prediction tool will be implemented. The wastewater treatment plant is evaluated by using relevant parameters like turbidity in the effluent of the final clarifier, the sludge layer heights in the final clarifier and continuously measured NH 4 -N and no 3 - N concentrations in the effluent of the biological stage. A possible maximum flow will be calculated and used by the sewerage control system. For the prediction of the effectiveness of this sewerage management tool hydrological and hydro-dynamical simulation programs are used. The simulation calculations show that a reduction of 25 % of the direct discharge can be achieved by adapting the outflows from the storage tanks. 6 Recommended procedure for the adaption to other sewerage systems At first a basic investigation is required to collect the relevant data from the wastewater treatment plant and the sewerage system. Representative rainfall data are required.

10 First estimations about a possible reduction of the direct discharge can be done by using a hydro-dynamical long-term simulation model (in this case-study: KOSIM, ITWH Hannover). Following the maximum capacity of the sewer system and the wastewater treatment plant have to be checked. It is required to have access to the outflow control from a central operation, where also the main data like rainfall intensity and the quality parameters of the wastewater treatment plant are available (NH 4 -N and turbidity in the effluent). The application of a sewerage management system should be realized stepwise, also for involving and training the operators. 7 Conclusion The operation of a sewerage system has significant influence to the discharge and to the water quality in the receiving river. By using a management system the outflow from the storage tanks the effects of non equal rain distributions and the changing capacity of the wastewater treatment plant can be used. Required for this management tool are continuous information about rainfall, filling of the storage tanks and the performance of the wastewater treatment plant, characterized by NH 4 -N and turbidity in the effluent. In a case study (Hildesheim/ Germany, 240,000 PE) a reduction of the direct discharge of 25 % could be achieved by using this tool. Dr.-Ing. Peter Hartwig aqua consult Ingenieur GmbH Mengendamm 16 D Hannover Tel.: +49 (0) 5 11/ Fax: +49 (0) 5 11/ [email protected]