IMPACT OF URBAN RUNOFF ON WASTEWATER TREATMENT PLANTS AND RECEIVING WATERS Ninth European "Junior" Scientist Workshop - LATEST DEVELOPMENTS IN URBAN WASTEWATER MANAGEMENT
Report No FR/WW001

Nov 1996

SUMMARY

This article summarises the proceedings of the Ninth European 'Junior' Scientist Workshop: Impacts of Urban Runoff on Wastewater Treatment Plants and Receiving Waters. Twenty-one scientists from all across Europe met in Kilve, UK in April to present results of current research projects, to develop communication networks, and to discuss the importance of their work m view of future developments in the water industry. The workshop was sponsored by the Smisson Foundation, organised by the Department of Civil Engineering, Imperial College London and chaired by Dr. Atal Eralp of the United States' Environmental Protection Agency.

Integrated urban wastewater management and its role in the sustainable use of water resources

An integrated approach to urban wastewater management considers all aspects of urban wastewater systems and their impacts upon receiving water courses, groundwater reserves, and the effects of sludge disposal practices. With regard to urban storm runoff management, we are specifically concerned with the integrated system encompassing the sewer network, treatment plant and the local recipient water course. This holistic approach is necessary in order to evaluate problems and develop solutions which cannot be assessed when focusing on one part of the system only.

It is clear that research into aspects of sustainable development in the field of wastewater resources management is currently still in its infancy, but experiences from the Czech Republic show how the theoretical principles of sustainable development may be applied and tested at a regional level by documenting examples of non-sustainable processes. It was recognised by the workshop that sustainable wastewater management practices can only be implemented within an administrative framework incorporating a combination of economic, legal, and social incentives.

Lack of integrated planning was clearly demonstrated by a case-study from Sweden in which a failure to take into account the impact of infrastructural improvements to a sewerage system has led to problems at the treatment plant and increased volumes of contaminated runoff from storm overflows during the thaw of winter snow. On the other hand, an example of a successful exercise in integrated planning was presented from the city of Kolin in the Czech Republic in which close co-operation between designer and numerical modeller in the working team was a key factor in the formulation of the optimal design strategy.

Advances in Integrated Computer Modelling Techniques

In order to ensure compliance with increasingly strict environmental standards, validated mathematical models are required to quantitatively assess the performance of different water quality management options. A wide variety of computer software packages are currently available, but generally these models tend to focus on one particular element of the wastewater system and there is also a lack of compatibility between models. There is therefore an urgent need to develop overall simulation tools incorporating mutually compatible modules for the representation of water quantity and quality processes in sewer, wastewater treatment and receiving water systems. There is currently considerable progress in this field and a number of papas presented at the workshop reflected the most recent developments.

One of the most obvious benefits of an integrated approach to computer simulation is the ability to carry out long-term simulations which enable the acute as well as the accumulative effects of transient rainfall and pollution events to be modelled. A recent example of a successful application of this methodology was presented from a study in Brussels to evaluate different strategies for the control of pollution from combined sewer overflows. Hydrological response times in rivers are far greater than those associated with urban drainage systems and the effects of accumulative pollutant loads are neglected if work is concentrated upon the acute effects of single events only. Consequently, it should be noted that conventional definitions of 'storm event' and 'return frequency' become more problematic when considering the urban wastewater system as a whole.

Simulation of the integrated system presents distinct advantages, but there are a number of issues of concern expressed by participants at the workshop. Besides the necessity for increased computational power there are inherent difficulties in defining the boundary conditions between the component parts of the interacting systems. There is also the need for an increasing amount of computational simulation time needed for long-term dynamic simulations of integrated systems. Furthermore, the need for in creasing amounts of data for model calibration and verification was stressed at the workshop. A recent investigation indicates that at present there seems to be no data, even on a European-wide basis, that is easily available for a complete case-study of sewer system, treatment plant and receiving water.

Problems associated with model complexity may be reduced by a suitable simplification of the processes involved. For example, a conceptual model with rainfall as input data and internal sewer flow as output data was formulated in the Netherlands. In this instance, the adoption of a simplified 'multi-linear' reservoir model for the calculation of total seasonal emissions from combined sewer overflows gave good results in comparison to a more complex dynamic model.

Uncertainty prediction and risk evaluation

Traditional urban drainage modelling attempts to explicitly describe all processes involved in the urban wastewater system. This approach has tended towards the development of complex, potentially over-parameterised models in which there is an inherent uncertainty in the accuracy of any output data arising from errors in the measured input data combined with the errors arising from mathematical simplifications and scientific assumptions built into the models. Many of the less dominating processes can often be collectively described by fewer stochastic terms resulting in models whose complexity and structure can be justified by observation.

It is also becoming increasingly apparent that to predict the impact of pollution from urban runoff using computer simulation models, a methodology to assess the precision of the model output is necessary. Sound practice requires information on the statistical properties of model input data as an integral part of model development. Stochastic models, as well as having the advantages of simplification, also incorporate an element of uncertainty prediction. Work at Leuven University in Belgium shows promising results through the application of probabilistic design of sewer systems based upon a stochastic relationship between rainfall input and sewer flow output.

Infrequent transient events such as accidental spills of pollutants can severely effect the performance of treatment and this can have considerable implications on the quality of receiving waters. Risk assessment is becoming more and more important in waste management and a useful categorisation of risk analysis methods applied to urban hydrology was outlined at the workshop. The four categories of risk analysis presented were direct fitting, moments transformation, long-term simulation, and analytical or numerical integration. Another paper at the workshop evaluated three methods based upon Monte Carlo simulation and stochastic analysis for estimating the uncertainty in prediction of bacterial growth in the activated sludge biological treatment system.

Discussions at the workshop indicated that there is a pressing need for a clarification of vital modelling terminology and, following a more recent workshop in Denmark, a task group for the definition of a consistent terminology is to be set up. Also, engineers working with mathematical models need to appreciate that the notions of stochastic and deterministic models are independent of the nature of the model, whether it is based upon physical, chemical, or biological theoretical concepts. Finally, it was pointed out that historic rainfall series constitute only one sample of a stochastic process and that they therefore will not necessarily be representative of future rain series. This presents major problems regarding the uncertainty of predictions of future events due to changes in rainfall patterns as a result of climate change.

Solids in sewers and associated pollutant attachment

There has been a considerable amount of research recently that has focused upon the physical, chemical, and biological transformations that occur in wastewater as it travels through sewers. Work from Denmark shows that the rate of hydrolysis of organic matter in sewage into readily biodegradable compounds is highly dependent upon the nature of the waste and its attachment to sediments. Measurement of the oxygen utilisation rate of sediments therefore gives further information about the impact of combined sewer overflows discharges on the resultant dissolved oxygen depletion in receiving waters.

As pollutants adsorb onto the surfaces of particulate matter in sewage, the resulting fate of pollutants is highly attributable to the deposition and transportation of suspended solids. Research at Aston University in the UK shows that the degree of chemical oxygen demand is highly related to certain size fractions of particulate matter. The extent of this association is likely to vary for different wastewaters, but if the attachment of pollutants to sediment particles was applied to the design of separation devices such as vortex separators, removal efficiencies for the identified pollutants associated with these solids would be improved.

Micro-organisms attach themselves to sediment particles and microbiological contamination of recreational waters is therefore affected by combined sewage discharges from overflows. It was emphasised at the workshop that the use of E.coli bacteria as an indicator of recreational water quality is not an appropriate measure for safeguarding the health of people bathing in sea waters.

Real-time control

Besides precautionary planning and design strategies (static control) to minimise the adverse impacts of urban drainage systems on receiving waters during rainfall events, there is currently much interest in the adoption of real-time control measures. Using telemetry to monitor the status of transient rainfall events, the urban wastewater system and receiving water, control decisions are taken during the actual time of the rainfall event. Overflow volumes may be reduced by maximum utilisation of the storage capacity of the existing system, therefore potentially negating the need to provide additional and costly storage facilities.

In addition to flow control in which the hydrodynamic performance of the system is optimised, a great potential lies in exploring real-time control options for pollution abatement. A practical application of this approach is currently underway in the Emscher region in Germany where mathematical optimisation combined with a knowledge-based system is being used to make computer-based decisions used to protect the ecology of the river systems from acute and chronic effects of storm runoff

Conclusions

Urban populations impose an enormous demand on natural water resources. In many instances inappropriate and inefficient wastewater management procedures contribute to the catalogue of anthropogenic activities deemed to be unsustainable. Urban runof contributes to many of these problems by distorting the natural hydrological cycle and to a deterioration in water quality with concurrent changes to ecosystems. It is evident that in order to address problems of both chronic as well as acute impacts of urban runoff pollution there are clear advantages of an integrated methodology to modelling of the sewer-treatment plant-river system. A simplified approach to modelling can be used by practising engineers to give a quick analysis of the system before a detailed simulation is considered to be necessary and the application of real-time control measures might result in considerable achievements within integrated stormwater management within an integrated strategy of stormwater management.

The workshop highlighted that international co-operation between researchers is of fundamental importance to progress in the field of urban wastewater management. The workshop concluded that it is necessary to improve our knowledge of the fundamental nature of in-sewer processes and the detailed composition of wastewater. Additionally, further research in refining integrated modelling approaches and tools as well as their application in planning, design and operation of urban wastewater systems is needed. Both practitioners and researchers of different disciplines should recognise that mutual cooperation and communication is required to achieve these laudable goals.

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