Wastewater Discharge Licencing: Key Assessment Criteria – Receiving Water
A licencing authority will grant a discharge licence where the applicant can demonstrate to the licencing authority that a proposed or existing discharge can comply with the requirements of the Water Framework Directive and the objective of “Good” status receiving water.
- Where inadequate information has been presented the application may be refused or the licence may impose more strict discharge limits which are difficult and/or more expensive to achieve.
All applicants for a licence to discharge effluent to surface water are required to provide information on the flow in the receiving water body. Flow data may be obtained from published historical data for the receiving water, measured flow or estimated flows.
- If the point of discharge is close to an existing hydrometric station, the information from that station may provide adequate information on the flow of the receiving water.
- However, published guidance procedures advise applicants to check the confidence and quality of the published data relating to the location of the proposed discharge.
It may not be suitable in all cases to use flow data from a hydrometric station, and in such situations flow data must be obtained by an alternative means.
- A computer model “Flow Estimating Tool” has been developed for the Estimation of Flow Duration Curve for Ungauged Catchment.
- The “Flow Estimating Tool” can be used to provide estimated 95%ile flows for ungauged river catchments based on recorded flows from the nearest appropriate gauged site.
- The data presented is not appropriate for use in all circumstances and a number of disclaimers are presented in the reports generated for the “Flow Estimating Tool”.
Where flow data is not available or where the flow data from existing monitoring programmes is considered to be of poor quality or of low confidence, applicants are normally required to carry out on-site flow measurements to provide information which would allow an estimate to be made of the assimilative capacity of the receiving waters under low flow conditions.
- The flow monitoring data must have a good level of confidence associated with it and must be representative of the stretch of the river to which it is proposed to discharge i.e. flow measurement should not be carried out in flood conditions.
- Any such flow measurement should be correlated with rainfall data from the closest rainfall station and/or on site rainfall measurement for the periods preceding and during the flow measurements.
Once suitable data has been obtained for the receiving water, it can be incorporated into the key assessment calculations which will be used to determine key discharge licence limits eg. Assimilative Capacity, Mass Balance and Headroom.
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