The impact of sewer system overflows on receiving waters as defined by mathematical modelling Wolfgang F. Geiger

Transcription

1 The impact of sewer system overflows on receiving waters as defined by mathematical modelling Wolfgang F. Geiger Abstract. Storm runoff causes the same magnitude of pollution loads entering receiving waters as secondary treatment effluent. The impact of these loadings is intermittent and cannot be defined with reasonable costs by measurements only. Computerized mathematical simulation models were recognized to aid the assessment of pollution loads and decision making in selection of overflow abatement alternatives. But, as the processes involved in urban runoff have statistically the properties of random events, it follows that meaningful and realistic conclusions on urban runoff and its pollution can only be derived from the investigation of long precipitation records. For a simulation model, sufficient detail is necessary to represent the rapidly varying runoff processes. In addition to monthly and yearly totals, statistical analyses of continuous simulation results provide frequencies and durations of overflows and overflowing pollution loads. These results assess the impact of sewer system overflows on receiving waters. Predicting the impact of future conditions, a technical basis for cost-benefit considerations is provided supporting storm water management decisions. Conséquences des déversements d'un système d'égoûts sur l'eau des rivières, étudiées au moyen d'une modèle mathématique Résumé. L'écoulement des eaux d'orage provoque dans les rivières une pollution du même ordre que celle d'autres effluents. L'arrivée de ces charges polluantes est intermittente et elles ne peuvent pas être appréhendées à des prix raisonnables uniquement par des mesures. Les modèles mathématiques de simulation aident à estimer les charges polluantes et facilitent la décision du choix entre différentes solutions pour réduire les déversements. Cependant, comme les processus mis en oeuvre dans le ruissellement urbain ont statistiquement des propriétés d'événements aléatoires, on ne peut tirer des conclusions logiques et réalistes concernant le ruissellement urbain et sa pollution qu'à partir de recherches sur des relevés de précipitations de longue durée. Encore, pour une simulation, est-il nécessaire d'avoir des données suffisamment détaillées pour représenter l'allure rapidement variée du ruissellement. En plus des totaux mensuels et annuels, l'analyse statistique des résultats d'une simulation continue fournit les fréquences et les durées des déversements et de leurs charges polluantes. Ces résultats permettent de saisir les conséquences des déversements de ces égoûts sur les eaux des fleuves. Une prévision des conséquences des conditions futures fournit une base technique pour l'analyse coût-bénéfice, ce qui est de nature à faciliter les prises de décision en matière d'aménagement pour l'utilisation des eaux d'orage. THE OVERFLOW POLLUTION PROBLEM Storm drainage design philosophy in the past was generally to size pipes, culverts and channels for rapid removal of storm runoff to the receiving waters. Storm water was presumed clean and there was no concern about pollution from separate storm sewers. However, within the last two decades changes in land use and the developments in the modem environment have drastically increased the sources of pollution. The result is that today essentially every metropolitan area subject to rainfall has a storm water pollution problem, whether drained by combined or separate sewer systems. A study of an 11 ha residential area in Cincinnati, Ohio, USA, compared the pollution loadings from storm water with loadings from sanitary sewage on a yearly basis (Weibel, 1964). Suspended solids contributions from storm water runoff, for instance, were found to be 40 per cent larger than that from raw sewage. For Metropolitan Toronto, by 1980 the average BOD5 loadings on Lake Ontario from sewer system overflows were estimated to reach 544 tons per month, almost half of the anticipated total loading (Dunbar and Henry, 1966). Urban runoff measurements show that concentrations of suspended solids in surface runoff often exceed those recorded for sanitary sewage, while BOD5 levels are, on the average, similar to those in secondary treatment effluent. In combined overflows, peak inorganic and organic pollutant levels higher than in raw sewage have been recorded. Consequently, the effects of storm runoff must be considered in assessing or controlling the impact of urban runoff pollution on receiving waters. In Roanoke, Virginia, USA, although domestic waste load removal was upgraded 303

2 304 Wolfgang F. Geiger from 86 to 93 per cent, there was only a reduction of 3 per cent in the receiving water BOD5 load. If Durham, North Carolina, USA, provided 100 per cent removal of organics and suspended solids from the raw municipal waste, the total reduction of pollutants discharged to the receiving water would be only 59 per cent of the ultimate BOD 5 and 5 per cent of the suspended solids (Field etal., 1976). These examples illustrate that there is little sense in seeking higher levels of treatment efficiency to improve receiving water quality unless storm water runoff is efficiently controlled. Many techniques are available for abatement of urban runoff and its pollution, some of which are listed as follows: (1) downstream controls such as in-line storage, surface and underground retention basins, flow regulators, storage in treatment units, combined sewer separation, sewer cleaning and upgrading of treatment processes; (2) upstream controls such as roof and parking lot retention, increasing infiltration to the ground, erosion control, improving street sweeping and employing land-use restrictions. The complexity of these methods requires comprehensive analysis of the total system. Although the following discussion is primarily technical, the nontechnical aspects such as legislative, administrative, and fiscal considerations are recognized to be equally important considerations. ASSESSMENT AND CONTROL OF OVERFLOW POLLUTION Sewer system overflows are a result of randomly occurring storm events. To define their impact on receiving waters, one not only should investigate peak and average overflowing loads of individual events or yearly loadings, but must also consider overflow duration and frequencies as well as the coincidence of these loadings with corresponding receiving water flows and the background pollution. For aquatic life in flowing waters, a considerable total yearly loading may be acceptable if discharged continuously at low concentrations, whereas intermittent loadings at higher concentrations may prove fatal. Conversely, for a lake or embayment, total annual loadings might prove to be a more important consideration than intermittent intensive loadings. Naturally, the most complete assessment and control of overflow pollution would be if all overflow events could be registered, flows measured, quality samples taken and the data statistically analysed and correlated. Since overflow processes of urban systems are rapidly varying, flows would have to be recorded and samples taken every 5 10 min. To apply this method of study to all intermittent loads from the immense number of sewer system overflows would be both technically and financially unreasonable. Even if such a data base were available, only existing conditions could be assessed. Still, judgment of the reasons for the overflow occurrence, the improvement of overflow abatement measures, or the impact of projected land use on receiving waters would not be possible. Mathematical simulation models were recognized as an aid for assessing overflow pollution and a means for determining the impact of projected conditions. REQUIREMENTS FOR MATHEMATICAL MODELS Urban runoff studies of recent years lead to the identification of three broad objectives: planning, design and operation. Each objective has triggered the development of a considerable number of simulation models. Planning models are usually intended to provide an overall assessment of the urban runoff problem. Data requirements are kept to a minimum and the mathematical description of the runoff process is simplified frequently. Design and analysis models are orientated towards detailed simulation of

3 The impact of sewer system overflows on receiving waters 305 individual storm events. They illustrate the manner in which different abatement procedures or design options affect flow quantity and quality at critical locations. Operational models are used to aid control decisions during storm events. Rainfall is entered automatically from remote stations and the model is used to predict system responses a short time in the future. These common types of mathematical urban runoff models provide, of course, some indication of the impact of sewer system overflows on receiving waters, but do not define it. Some of the basic requirements and reasons for specific technical features of a simulation tool to define this impact are given below : (1) As storm events have a stochastic nature and runoff is governed by numerous variables, the resulting overflows are random processes. This requires the investigation of many overflow events. One could assume that consideration of storm events causing overflows would be sufficient. But, as the overflow process is not governed solely by the storm pattern, an individual storm cannot be recognized as the only cause for an overflow. Also, storms in between overflow-causing events change the overflowinfluencing characteristics (i.e. surface depression capacities, infiltration capacities and available surface and network storage) significantly. This requires simulation of all events in order to maintain quantity and quality balances and to arrive at appropriate random conditions for the overflow-causing storm simulations. Further, for a given storm event the respective frequencies of rainfall, runoff, overflow and quality constituents may all differ, making it impossible to fabricate one or a small set of design storms for application of a single event simulation model. Therefore, a model for sewer system overflow definition should allow for continuous simulation of precipitation records of several years and subsequent statistical analyses of overflow properties such as overflow durations, total overflow volumes and BOD 5 or total suspended solids loads. From continuous simulation results one cannot only derive peaks, averages or totals for any desired time basis; but also a more meaningfull definition of the impact on receiving waters can be achieved by investigating overflow duration and frequencies as well as the coincidence of the loadings with corresponding receiving water flows and background pollution. Also frequencies and durations of violation of standards can be checked. (2) Urban runoff and overflows usually take place in a matter of minutes. Half hourly or longer calculation time steps would have an equalizing effect, providing no indication of peak impacts and tending to underestimate total overflowing volumes and pollution. Not only must retention be considered but also backwater effects throughout the network, and under some circumstances, even flow reversal. Network features such as branchings, overflow structures, retention basins, pumping stations or flow regulators, should be simulated close to reality. Therefore, a model for sewer system overflow definition must be unsteady and capable of simulating an event in short time steps. (3) The migration of storms over an area causes an entirely different loading of the sewer network and quite different overflows than when rainfall uniformity is assumed. Therefore, a model for sewer system overflow definition must allow for consideration of the spatial variation in precipitation and runoff. (4) Considering a waste water management system in total only assesses the overall dependencies of individual system components with respect to overflows properly and allows for cost effective and optimized designs. Some continuous simulation models available are: the US Corps of Engineers Storage, Treatment, Overflow and Runoff Model - STORM, the Chicago Flow Simulation Program - FSP, the CH2M-Hill model - SAM, the Hydrocomp model - HSP, and the Quantity-Quality-Simulation model of Dorsch Consult QQS. However, models based on rational methods or their derivatives fail to perform the necessary time depen-

4 306 Wolfgang F. Geiger dent calculations, but they may still be used for the planning phase as a first cut. The QQS model is an example of a model fulfilling most of the necessary requirements. This model allows for continuous simulation of up to three precipitation records of up to 20 years each in 5-min calculation intervals. Runoff and its pollution from catchment areas are calculated by a unit hydrograph method which is modified for the calculation of water quality. Flow routing through the network is based on kinematic wave equations while pollutant routing is handled as plug flow with partial mixing between successive time steps, considering backwater effects in the main and trunk sewers of a network. Statistical analysis of continuous simulation results provides annual and monthly frequency and duration curves for flows and pollutants at retention basins, pumping stations, overflows or in receiving waters. Single event simulation is possible, providing pollutographs and hydrographs at any network node (Geiger, 1975). The computer program is sized to handle, as a whole, metropolitan areas such as Munich, Germany. The working of this method provides that it is not necessary to adopt simplified relationships between storage and treatment or to neglect water transport in order to perform continuous simulations. APPLICATION OF MATHEMATICAL MODELS The major phases of mathematical model application are obviously, after defining study objective and selecting the model, data preparation, calibration, verification, simulations of the schemes investigated, interpretation and utilization of simulation results. The study objectives will influence not only the model selection but also sophistication of application and data preparation effort. If for instance the objective is to establish or control standards, to define the most critical impact on receiving waters (the most critical impact on receiving waters does not necessarily occur under low receiving flow conditions), to reduce flooding, to define the impact of an overflow abatement alternative, or to safeguard fish and other aquatic life, figures on durations and frequencies of overflowing quantities and pollution loads, total and peak loadings are of importance. As examples masterplan studies for the City of Rochester, New York, USA, Augsburg and Munich, Germany, are named. If the objective is recreational use with full body contact and keeping bathing beaches open for a maximum number of days in a season, because of bacterial contamination, the occurrence or non-occurrence of overflows is more important than, for instance, duration. This is the main objective of an application of the QQS model to an area of the city of Vancouver, British Columbia, Canada, facing the shores of English Bay and False Creek. The choice of pollutants to be investigated is dependent on the study objective as well. Although toxic parameters, heavy metals, phosphorus, nitrogen, mercury or oil are also of major concern, field data normally available mainly allow for reasonable calibration of input data and model verification for BOD5, total suspended solids, settleable solids, faecal coliforms and COD. Above all any simulation model can only produce results pertinent to the input data provided. Therefore along with data preparation the reliability of all inputs should be investigated and remembered when utilizing model results. Sensitive input data, i.e. ratio of imperviousness, depression and infiltration losses must be calibrated with local measurements. As a simulation of an overall system usually has to be based on an approximation of complicated structures and detailed processes as storage, distribution and regulation under different flow conditions, the working of a mathematical model itself should be verified not only for parts as surface or catchment runoff but also for its overall behaviour throughout a network. For these comparisons local field rainfallquantitative and qualitative runoff measurements should be used. Besides direct comparison with measurements, plausibility checks help to increase the trust in a model.

5 The impact of sewer system overflows on receiving waters 307 o C E a o "- a o «w es D O E

6 308 Wolfgang F. Geiger For instance, continuous simulations for the combined sewer systems of Augsburg, Germany, and Rochester, New York, USA, indicated extremely high overflowing frequencies at some points and no overflows at others. For both cities, this compared quite well with reality, the one overflowing if dry weather flow quantities were slightly exceeded, the others never overflowing due to unrealistically designed overflow structures. The data calibration and model verification effort is often underestimated technically and financially in model applications. Dependent on the study objectives, present and projected land-use conditions may be simulated for existing or alternative network layouts. The following examples may illustrate how simulation results can be used to define the impact of sewer system overflows on receiving waters and how this definition can serve as a basis for improvement strategies and costs involved. Augsburg, Germany, with a projected population of and an area covering approximately 9000 ha is situated at the junction of the Lech and Wertach rivers. The major study objective was to define an overflow abatement scheme minimizing the impact on receiving waters. However, criteria for allowable overflow loads, frequencies or durations were not available, except for a general guideline that required an average of 90 per cent of annual BOD5 and settleable solids loads from the runoff during storm events to be led to the treatment plant for processing. Also, the receiving water background load was not clearly definable. The overall intent was to obtain a water quality classification 'II' in the Lech River, which among other criteria allows for a BOD5 level of2-6mg/l. Continuous simulations representing a 15-year rainfall record were done for future conditipns using the QQS model. Input data for quantitive, BOD 5 and settleable solids catchment runoff were calibrated and the model was verified on field measurements from a 70-ha test area. Some of the statistical properties investigated were overflow duration, overflowing totals and averages of flows, overflowing and receiving water BOD5 and settleable solids concentrations. Figure 1 is a computer output example of the statistical analysis of continuous simulation results for average BOD5 levels in the Wertach River. The example shows that approximately three times a year a BOD5 level of 6 mg/1. will be exceeded. The uncertainty in criteria was handled by presenting the statistical results dependent on average receiving water background BOD 5 levels. For this the target classification 'IF with an allowable BOD5 level of 6 mg/1. was used. Figure 2 shows the total impact of the city's overflows on receiving waters with and without treatment plant bypasses. As an example one may read from the graph that assuming a background BOD5 level of 2 mg/1. the target receiving water classification is contravened 6.5 times per year when treatment plant bypasses are taken into consideration, but approximately only once a year when treatment plant bypasses are excluded. The level of treatment was a given boundary condition. The combined sewer system of the city of Rochester, New York, USA, (population ) discharges overflows into the Gennessee River, which results in a significant depression of the dissolved oxygen level and an increase in faecal coliform indicator organisms. Within the Rochester Pure Waters District Combined Sewer Overflow Pollution Abatement Program, application of mathematical models helped to assess the present condition and to define an abatement scheme which complies with the New York State Department of Environmental Conservation classification 'B', allowing for the best usage for resource, contact recreational use and all other uses except as a source of potable water. One of the models applied was QQS. The input data were calibrated and the model was verified on field measurements of runoff quantity, BOD5, total suspended solids and faecal coliform loads at two sites in the network and three test catchments comprising areas of ha. The overflowing loads simulated with the QQS model were combined with recorded

7 AUGSBURG, GERMANY The impact of sewer system overflows on receiving waters 309 I! -! I 8 z 7 LEGEND average over 6 h peak overflow period (with treatment plant bypasses.) average over total overflow duration (with treatment plant bypasses.) average over total overflow duration ( without treatment plant bypasses.) assumed receiving water background level objective to achieve classification 'JL'. 0 mg/l FIGURE 2. Total BOD 5 impact on Lech River, Augsburg, Germany, predicted by QQS simulations. ROCHESTER, NY, USA Increased Potlutional împoct Feed Coiiform 4-2- Cumulative probability curve derived from continuous simuiatio Single event of present and changed conditions. iults of present conditions FIGURE 3. Impact of changed land-use conditions on the Gennessee River, Rochester, New York, USA, at an individual overflow point. river flows. An average annual number of contraventions of the classification 'B' were derived for each storage and treatment combination. The model output was also used to determine the total BOD5 loads presently overflowing to the Gennessee River on an average annual basis and the reductions of annual BOD5 loads to be expected from storage facilities sized for different storage volumes. Also total suspended solids and faecal coiiform loads were investigated. A cost effective system size with a storage volume in the order of cbm was found to comply with the stream standards. The impact of changed land-use conditions in Rochester on Gennessee River pollutional loadings is illustrated in Fig.3 using a combination, of single event and continuous simulations. The altered conditions increased BOD5, total suspended solids and faecal coiiform loads at the overflow point considered. The procedure applied was suggested as an engineering approach to approximate the more costly statistical results of many continuous simulations for altered conditions by referencing single event results to the findings of one continuous simulation (Geiger et al., 1976). A study done for the city of Des Moines, Iowa, USA, illustrates clearly the overwhelming effect of urban runoff pollution on critical DO concentrations (Field et al., 1976). Using a simplified receiving, water model, four control alternatives were compared, considering cost and true effectiveness in terms of frequency of DO standard violations in the Des Moines River (Table 1 ). This study showed that secondary treatment of DWF and 25 per cent BOD5

8 310 Wolfgang F. Geiger TABLE 1 Des Moines: control costs versus violations of DO standard (4 mg/1.) Options 1. DWF tertiary treatment 2. WWF 25% BOD removal 3. WWF 75% BOD removal 4. DWF secondary treatment only Total annual cost [S/year] 6.3 M 1.6 M 10.8 M 0 % of precipitation events violating standard removal of WWF result in slightly less events violating standards than tertiary treatment of DWF and no control of urban runoff. The annual cost for tertiary treatment is fourfold the cost of the first alternative. Of course, secondary and tertiary treatment only have in the case of WWF similar effects on the number of precipitation events violating standards. A significant reduction of DO levels in the Des Moines River could be obtained by 75 per cent BOD 5 removal of WWF. However, the cost for this alternative is significantly higher. The cost-effectiveness of various treatment levels could be determined only by a continuous analysis of the frequency of water quality violations. One cannot expect mathematical models to provide solutions to problems, but the solutions should be devised from modelling results. These results, however, may be presented in many different and also misleading ways. Therefore, one must always remember when evaluating results, how valid the data base and how reliable or simplified the simulation tool was. CONCLUSIONS The most complicated measurement scheme for monitoring sewer system overflows and receiving waters does not provide all the answers needed to define the impact of these overflows on receiving waters. Mathematical models can help to more thoroughly assess present conditions and predict future loadings. Thus they provide a technical basis for cost-benefit evaluations and assist in making storm water management decisions. To meet this target the modelling tool should allow for continuous, nonsteady simulations of sufficient detail. However, even the best of today's models can only be an approximation of the real system. The simulation results must be judged on the data used, the modelling tool applied and last but not least, on the user himself. It is believed that the insight into system behaviour and sensitivity gained by mathematical modelling is worth the effort. REFERENCES Dunbar, D. D. and Henry, J. G. F. (1966) Pollution control measures for stormwater and combined sewer overflows. J. Water Poll. Control Fed, 38, 9. Field, R., Tafuri, A. N. and Masters, H. E. (1976) Urban runoff pollution control program overview: FY'76. Environmental Protection Technology Series, Cincinnati, Ohio, USA, EPA-600/ Geiger, W. F. (1975) Urban runoff pollution derived from long-time simulation. National Symposium on Urban Hydrology and Sediment Control, University of Kentucky, Lexington, Kentucky, USA. Geiger, W. F., la Bella, S. A. and McDonald, G. C. (1976) Overflow abatement alternatives selected 'by combining continuous and single event simulations. National Symposium on Urban Hydrology, Hydraulics and Sediment Control, University of Kentucky, Lexington, Kentucky, Weibel S R. (1964) Urban land runoff as a factor in stream pollution. J. Water Poll. Control 36,914. Fed.