Dec 1994

The Urban Waste Water Treatment Directive Implementation Group (UWWTDIG) was established by the Department of the Environment in 1991 to determine how the Directive should be implemented in the UK. Existing regulatory practice in the UK. is to define compliance with Look-up Table (95%ile) and, where applicable, absolute (upper tier) limits in consents for sewage works effluents on the basis of spot samples collected at frequencies determined, in principle, by the size of the works. In contrast, the UWWTD prescribes 95%ile and absolute limits referred to a 24-hour composite effluent quality. As a result, there was an urgent need to establish sound relationships between spot-based and composite-based 95%ile values of effluent quality values for the relevant determinands to ensure a neutral (i.e. with no impact on quality of effluent or cost of treatment) translation of consents to the new basis. The translation factors are particularly important for those sewage treatment works whose existing consent limits require more stringent performance than those indicated by the UWWT Directive. Neutral translations were needed where relevant for upper-tier consent limits.

The Directive also allows the performance of sewage treatment works to be assessed by compliance with prescribed minimum percentage reductions of the influent loads. During the progress of this study, and to a large extent independently of it, the UWWTDIG agreed that percentage reduction in load may also be used as an alternative method of demonstrating compliance with the Directive.

The objectives of the National Consent Translation Project (NCTP) were to:

• establish a sound basis for the neutral translation of consent conditions to the new procedures to be introduced between 1994 and 1998;
• undertake a preliminary intensive survey of a limited number of discharges to obtain better information about the relationships between instantaneous (spot) and daily mean (composite) samples;
• develop a statistical model defining the relationships between the different quality measures;
• produce a programme of sampling covering a greater number of discharges as part of the main survey;
• undertake a full scale survey of discharges, measuring influent and effluent quality and flow;
• refine the statistical model produced through the preliminary survey using data from the main survey, and use it to extrapolate the results to other treatment works;
• report on the study and make recommendations on the implications for implementation for the neutral translation of consents.

An additional objective was introduced once the study had been started. This was to evaluate the 'percentage reduction' option in the UWWT Directive, although the study was not originally designed with this assessment as an aim.

1.3 Organisational arrangements and responsibilities

These activities can be summarised as follows:

The FWR was responsible for management of the overall programme through the appointment of a Programme Manager whose role was to liaise with all participants and undertake report production and information dissemination.

WRc was responsible for the design of the sampling programme and its technical management, collation and statistical interpretation of data, and recommendations on the neutral consent translations and their implementation.

The Regulators (comprising the National Rivers Authority (NRA), and the Scottish River Purification Boards and the Department of the Environment for Northern Ireland represented by SNIFFER (the Scotland and Northern Ireland Forum for Environmental Research)), funded the programme management by FWR and the work of the specialist sub-contractor, WRc.

The Operators (comprising the Water Service Companies of England and Wales, together with the Sewage Treatment Operators in Scotland and Northern Ireland) were responsible for undertaking, at their own expense, the sampling and analysis programme developed by WRc, providing access to quality control and other required information, and providing WRc with results for the programme to an agreed format. The Operators were required to sign an agreement confirming acceptance of these arrangements.

A Steering Group representing all participants in the Project was formed to oversee the study and approve outputs.

Copies of this report are available from the Foundation, price £150.00, less 20% to FWR Members