SEPA is responsible for the regulation of discharges to controlled waters under Part II of the Control of Pollution Act 1974 (as amended) and for the granting of consents and service of instruments to discharge under that Act. It inherited these functions under the Environment Act 1995 from the river purification authorities.

Since its inception, SEPA has been formulating policies to discharge these responsibilities, including :

• Consenting policy for discharges to controlled waters (SEPA Policy No. 3, Version 1, July 1996) (draft);
• Microbiological standards in marine waters (excluding shellfish waters) in relation to design criteria for discharges (SEPA Policy No. 27, Version 1, September 1998);
• Initial dilution and mixing zones for discharges from coastal and estuarine outfalls (SEPA Policy No. 28, Version 1, September 1998).

Point source discharges for which consents are required include sewage and industrial discharges from non-prescribed processes (trade effluent). Prescribed processes require an IPC authorisation which is also issued by SEPA. Discharges from marine fish farm installations are considered to be trade effluent and require a discharge consent.

The procedure for the application for a discharge consent and the form of consent conditions (95 percentiles, upper tiers and absolute limits) is similar to that in England and Wales.

There is a requirement for all existing discharges to meet the following statutory requirements:

• EC Bathing Waters Directive and UK Regulations (appropriate mandatory standards must be met at designated bathing waters);
• EC Urban Waste Water Treatment Directive and UK Regulations (the appropriate level of treatment should be applied to the sewage, based on the population equivalent of the agglomeration served by the sewerage system);
• EC Dangerous Substances Directive and UK Regulations (EQSs for List I and II substances in receiving waters must be met, including standstill provisions);
• EC Shellfish Directive and UK Regulations (appropriate standards must be met for designated shellfish waters).
• North Sea Conference and OSPAR commitments (reduction in loads of toxic substances in discharges to the marine environment must be demonstrated).

SEPA=s policy on initial dilution and mixing zones (SEPA Policy No 28) sets out requirements to be met in the design of new or modified discharges.

For discharges with greater than 100 population equivalent, outfalls should be designed and constructed to provide the following minimum initial dilution to reduce both the visibility of density slicks and the occurrence of smell nuisance to acceptable levels:

• minimum initial dilution of 100 times (95 percentile) for primary treated effluent;
• minimum initial dilution of 50 times (95 percentile) for secondary treated effluent, including septic effluent;
• minimum initial dilution of 50 times (95 percentile) for significant new or modified industrial discharges (to be judged on an individual basis).

Modelling studies are required to determine the best location and design of the outfall and to demonstrate achievement of the minimum initial dilution requirements. SEPA specifies the requirements for modelling studies and recommends the use of one of 3 models: ELSID, PLUMES or CORMIX (see SEPA Policy No. 28).

Requirements for mixing zones are also specified to inform the design of an outfall and the consent conditions to be set to achieve compliance with statutory requirements and to protect the environment. These requirements include:

• a limitation on the size of a mixing zone to 100 m in any direction;
• UK or SEPA Environmental Quality Standards (EQSs) should not be breached outwith the mixing zone;
• where toxicity-based criteria are used, there should be no residual toxicity outwith the mixing zone;
• neighbouring mixing zones should not merge and ideally should be at least 100 m apart;
• no mixing zone should impinge on the Mean Low Water Springs (MLWS) shoreline;
• no mixing zone should plug an estuary, sea loch or small bay;
• the mixing zone will not be allowed to jeopardise the integrity of any European marine site;
• the mixing zone should not give rise to significant slicks or other aesthetic problems;
• accumulation of solids on the sea bed must not threaten the achievement of standstill clauses for List I substances outwith the mixing zone and not cause acute toxic effects to sediment-dwelling organisms within the mixing zone.

SEPA has developed a policy for microbiological standards in marine waters (except shellfish waters) in relation to design criteria for discharges (SEPA policy No 27). This policy has some influence on all marine waters in Scotland.

For all marine waters, no new or modified discharges will be allowed to result in deterioration of the class which is currently achieved under the coastal classification scheme nor threaten progress in improving class C and D marine waters identified in SEPA's corporate plans.

For identified bathing waters, existing discharges must enable mandatory standards to be achieved and, for new or modified discharges, the outfalls must be designed so as to achieve guideline standards.

SEPA may also designate >recreational waters= where significant water contact activities are practised outwith identified bathing waters. SEPA will require mandatory microbiological standards to be achieved at relevant times of year and promote the achievement of guideline standards where appropriate.

SEPA will also adopt a strong presumption that mandatory microbiological standards are achieved at 'shoreline waters' (i.e. those visited by the public).

As in England and Wales, the primary driver for the derivation of consent conditions is compliance with statutory requirements, in particular bathing water, Urban Waste Water Treatment and shellfish water standards. SEPA has supplemented these requirements in Scotland to some extent with their policies. These policies enable Scottish Natural Heritage (SNH) to verify that consent conditions for new or modified discharges in or close to European marine sites have been set according to the procedures outlined in these documents.

The approach to the regulation and monitoring of cage fish farming in Scotland has been comprehensively laid out in a manual of procedures (SEPA 1998). This manual provides details of the application process, the assessment of the application, setting consent limits, process for granting and refusing a consent, monitoring, data management and use of information and review of consents. Conservation agency staff should have access to this document to ensure that all cage fish farming consents have been set according to the process.

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