Water Cycle Study - East Devon District Council

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Ref EWCS April 2010 Exeter and East Devon Water Cycle Study longer to plan and deliver than new strategic systems through greenfield land. However, new strategic solutions can be significantly more costly. 6.3 Wastewater treatment works 6.3.1 The Wastewater Treatment Works (WwTWs) that have been identified by the study as having potential to receive significant additional sewage associated with housing growth (above typical windfall development rates) are listed in Table 5.1 in Section 5, alongside the current Water Framework Directive classification status for each water body that receives discharges from those WwTWs. 6.3.2 The main wastewater treatment works (WwTW) serving the study area is Exeter Countess Wear WwTW which serves the Exeter urban extent. Many of the smaller settlements in East Devon have their own smaller WwTW. A new WwTW is already planned by SWW to serve the proposed new community of Cranbrook. SWW have obtained planning permission for the Cranbrook WwTW and have applied for approval for ring-fenced funding for its provision in their Asset Management Plan for 2010 – 2015 (AMP5). 6.3.3 The future expansion potential of a wastewater treatment works with respect to water quality is determined by assessing the discharge consent, set by the Environment Agency. This consent is designed to protect the water quality objectives of the receiving watercourse and specifies a flow and quality that the WwTW has to achieve to protect those objectives. 6.3.4 As the population connected to a sewage treatment works increases, the amount of treated wastewater (or effluent) being discharged to the receiving water generally increases in proportion to the population increase. If the population increase is forecasted to cause the treatment works to exceed the consented discharge volume allowed by the EA consent, improvements are likely to be required to improve the standard of treatment and to ensure river quality does not deteriorate. 6.3.5 The quantity of treated effluent discharged from each treatment works and its quality is specified by the legal discharge consent, issued by the Environment Agency under the Water Resources Act 1992. 6.3.6 The consent is normally based upon the dry weather flow (DWF) of the treated effluent, and stipulates limits for the concentration of biochemical oxygen demand (BOD), total suspended solids (TSS) and ammoniacal nitrogen (NH3). Compliance is determined by means of statistical analysis of effluent quality data. Those works which discharge to sensitive watercourses also have limits relating to phosphorus and total nitrogen, consented under the Urban Waste Water Treatment Directive (UWWTD) or the Habitats Directive. Compliance with these standards is measured on an annual average basis. In addition discharges to or close to designated bathing waters are required to treat to a higher standard to prevent bacterial and viral matter causing a failure of bathing water standards. 6.3.7 In the foreseeable future, consent limits will be set with a view to meeting the requirements of the Water Framework Directive (WFD) whose aim is to ensure that good river quality standards are met throughout each waterbody. The current consent practice derives consent limits that are based upon the volume and quality of the receiving watercourse, along with the effluent quality and volume, at the point of discharge 54
Exeter and East Devon Water Cycle Study 6.4 Consented capacity at WwTWs 6.4.1 New development increases the amount of sewage needing to be treated by the wastewater treatment works. Most wastewater treatment works have a certain amount of headroom between the current population connected to the works, and the population at which the discharge consent will be breached. If forecast future flows will exceed the consented capacity of the WwTW a variation of the consent will be needed from the Environment Agency, normally resulting in a tightening of the consented quality parameters to ensure river quality does not deteriorate. 6.4.2 This assessment has not been able to look at the actual hydraulic or process capacity at the sewage treatment works because of a lack of available data. It is the responsibility of the water company to effectually drain developments, and to provide sewage treatment capacity under the Water Industry Act 1991. As long as there is environmental capacity available and a new consent can be granted, additional sewage treatment capacity can always be provided, and should not be seen as a showstopper to development. When there is greater certainty surrounding the timing and location of development, South West Water will carry out more detailed assessments of infrastructure capacity to ensure any additional sewage treatment or conveyance infrastructure is provided where and when it is needed. 6.4.3 Having identified which WwTWs within the study area will be affected by development and which development growth areas these WwTWs would be likely to drain (see Table 5.1), this study has assessed the maximum number of additional dwellings that could be connected to the WwTW without causing a breach of discharge consent. Table 6.1 shows the results of this analysis, which have been colour coded, with red showing those sites that have no current capacity, amber showing where the level of growth tested will cause the wastewater treatment works to breach its consent, and green where the planned level growth is within consented capacity. 6.4.4 Table 6.2 shows the water company planning period in which our assessment forecasts consented capacity will be exceeded based on a linear projection of forecast housing completions. 55 Ref EWCS April 2010
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Water Cycle Study - East Devon District Council

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