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6.2.2 Priority issue: stormwater overflow

The high proportion of combined sewers reported above is part of the historical development of sewerage in Europe, and is the major reason for a high degree of water pollution originating from combined sewer overflow (CSO). At times of rain, stormwater overflow form combined systems contributes up to 50% of the organic load reaching rivers in Europe, especially now that a high degree of wastewater treatment has been achieved at dry weather flow.

Rainfall in Northern Europe is high though peak rains are the exception rather than the rule. Thus, combined overflows run for long periods of time compared with the tropical countries and countries in the South of Europe and, consequently, the amount of pollution discharged into the environment via overflows is relatively great.

In the past, combined overflows have been tolerated because water pollution was generally high. Today, a high degree of sewage treatment has been achieved in Europe at dry weather flow. It follows that the organic load carried into receiving waters at time of rain may be as high as 50% of the total according to some estimate. Half of this amount stems from combined overflows, the other half form other sources e.g. drainage from roads, industrial and residential areas which must be added to the pollution discharged via the overflows. These discharges are often thought of being clean but in reality contain pollutants originating from many types of surfaces, streets, yards, parking places, especially oil, organic matter, fallout from atmospheric pollution, and toxic metals, and debris deposited in pipes at dry weather. Separate stormwater drains also carry the organic and non-organic load discharged into them via illegal connections. A survey of the causes of poor river water quality in Scotland in 1955 found that 20% of the pollution load reaching rivers resulted from these types of runoff from urban areas (SEPA 1997). New ways of thinking about wastewater and stormwater in urban areas are emerging. They aim at an integrated approach comprising all different sources, the collection and transfer systems, treatment plants and the receiving body of water itself and addressing both water quantity and quality and the use of simulation techniques (A.H. Dirkzwager 1997 & S.P. de Jong et al. 1998).

Three basically different approaches are used to control water pollution from CSO:

• Source control: Retention at the source by temporary storage of rainwater and gradual release; infiltration or seepage of the rainwater into the ground via tanks or channels, or porous pavements of parkings etc.; the reuse of rainwater; ponding in artificial wetlands; or reuse in urban beautification (e.g. recreational ponds etc.). In contrast to the measures listed in the subsequent paragraph, those referred to here are often ignored by the organizations responsible for the management of the systems, partly because they may be cumbersome to plan, implement and manage and, thus, do not lend themselves to a centralized approach.
• Improved management: Better operation and maintenance of inlets, pipes and all other structures of the sewerage system; the prevention of solids entering; control of illegal connections; the redesign and/or improved operation of intended overflow structures and unintended other overflows with particular attention to frequency and volume of the overflows; the storage of peak flows, and, last but not least; the in-line retention of peak flow. Books have been written about most of the measures referred to here, especially the provision of stormwater retention tanks and their planning, design and operation.
• Separation: Theoretically at least, separation of combined systems into sanitary sewers and stormwater drains would appear to be an ideal solution. However, this option is of limited use (see below).

In its report of 1995, The European Waste Water Group concluded that the key factors crucial for the design and management of collecting systems, consistent with meeting both environmental and economic requirements, are:

• Clearly defined long-term quality objectives for the receiving water which must include parameters of particular relevance to CSO.
• Reliable and detailed information regarding the wastewater collection and treatment system and a good understanding of stormwater and CSO impact in the individual receiving water.
• A planed phased programme for improvements which cannot be designed without appropriate analytical procedures and modeling tolls for the assessment of the improvement options consistent with meeting the needs of the receiving water.

From the analysis of the situation in EU Member countries the Group recognizes that advantages and disadvantages are associated with both combined and separate systems. The latter are still generally preferred for new developments in Europe, more particularly residential areas although they are expensive. Obviously, nutrient discharges from separate systems are generally lower than those from combined systems but they often have higher loads of metals and hydrocarbons. Also, separate systems always involve the risk of cross connections and illegal connections many of which are difficult to locate and control. But in spite of the current preference, logic commands that the separation of existing combined systems in Europe is very costly and generally impractical. Often, a separation offers but limited benefits. This implies that in Europe combined systems are there to stay. Thus, the improvements measures indicated above must be adopted and their feasibility, timing and introduction investigated. In this process, some of the traditional approaches have undergone change:

• Stormwater overflows have traditionally been based on a dilution ratio expressed as a multiple of dry weather flow (between 2 to 10 with a common value of between 3 and 8).
• Design criteria have been based on the needs of the collection system with mainly economic reasons in mind. Now the needs of the receiving waters come to the fore and, thus, surface water quality criteria will be the decisive parameters such as those included the forthcoming EU Directive on Waster Policy (see Section 6.6.1).
• Many of the overflow structures and other network features have been designed by criteria longer applicable. New criteria are being tested.
• As sewage treatment processes become more advanced, both the hydraulic and pollution load imposed at times of rain will be more important than in the past.

The financial implications of CSO improvements are very large. Careful planning of the remedial measures and long-term solutions are given a great deal of attention. An integrated approach to panning and management of catchment reas is adopted. A project is underway funded by the EU´s Innovation Programme which combines some of the existing computer programmes (viz.: MOUSE, MIKE and HYSTEM-EXTRAM) into modeling tools for urban catchment covering rainfall, sewers and sewage treatment works, and the receiving water. The project also aims at the development of common European standards for integrated catchment management. 16 partners from six EU countries, the Water Research center of the UK, the Danish Hydraulic Institute, and the Swedish Water and Wastewater Association participate in the research (R. Crabtree et.al 1998). An example is exhibited in Case Study 1 (Section 6.11.1).

The Planning Guide for Urban Pollution Management (UPM) of the Foundation of Water Research (FWR) of the UK is another tool (FWR 1998). It is now in its second edition and available on CD ROM. The information presented in the following paragraphs is excerpted from UPM and outlines the four phases of the Guide´s Planning Procedure (see also Figure 6.2):

Figure 6.2: The UPM procedure (lager image)

• Phase A: The best possible understanding of the current performance of the system is obtained on the basis of existing information. At the end of this phase, agreement is reached, including on the standards applicable, viz for aquatic life, bathing or general amenities. The full course of the investigation is planned and judgements made on the relative importance of different discharges and water quality interactions.
• Phase B: The data and models to be used in the investigation are assembled and the models are adjusted by appropriate processes of calibration and verification. The work involved can vary enormously depending on the form of the study. Modeling may extend to rainfall, sewer flow and quality, STP quality, river impact, estuary impact, marine impact, and, finally, integrated urban pollution.
• Phase C: The models developed during phase B are used to study the performance of the existing system vis-a-vis the standards. Proposed upgrading solutions can be incorporated and the degree of improvement identified. This may involve iteration.
• Phase D: Phase D covers discharge consent and engineering design. The latter deal with CSO structures, detention tanks, source control, sewer system transport capacity, and STP performance. Post project appraisal will be undertaken and the measures needed to maintain the model and database for future use are identified.

There are extensive appendices and a software SIMPOL developed to facilitate implementation of the UPM procedure, and there is also an Annex with worked examples. The important concept of the Manual is to consider the system comprising the sewers, the treatment plant and the receiving water as a single entity which is underpinned by environmental standards. The Guide is applicable throughout Europe and written in a generic manner without reference to specific software in recognition of the wide range of software packages now available to users.

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