Sewage discharges

Industrial discharges

The Environment Agency's approach to discharge consents largely follows that explained in Discharge consents and compliance. The NRA's approach to control of discharges to water produced by the NRA (NRA 1994). The Environment Agency has produced a discharge consents manual which sets out detailed policies and technical information for the setting of all discharge consents. Many recent consents and all new authorisations will be set with reference to this manual. For discharges from sewage treatment works which have to comply with the requirements of the UWWTD, recent guidance from DETR provides information on the setting of appropriate consents (DETR 1997).

Point source discharges to the marine environment comprise sewage and industrial effluent from non-prescribed processes. Occasionally, discharges from industrial processes (trade effluent) are made directly to sewer. This is licensed by the appropriate water company and conditions are set in relation to the discharge consent for the combined effluent when it is eventually discharged to a receiving water. Marine cage fish farm installations are also considered point source discharges and are controlled by the consents process. Marine cage fish farming occurs mainly in Scotland and is considered with respect to the duties of SEPA and will not be considered further here.

The procedure for obtaining a discharge consent from the Environment Agency is described in Schedule 10 of the Water Resources Act 1991 for England and Wales. Application for a consent must contain all information related to the discharge required by the authority including the location of the discharge, and the composition and quantity of effluent. The Agency is required to advertise the consent application in the local newspaper and the London Gazette and to inform the appropriate local authorities. Representations and objections to the application must be received within six weeks of advertising. The Agency will subsequently decide whether a discharge consent should be given. However, before a formal discharge consent can be given, the Agency must notify all those who have made representations or an objection to their decision. 21 days are then allowed, during which the Secretary of State may be asked to review the application.

If a consent is issued, it may be subject to conditions, in particular relating to the:

• location of the discharge;
• design and construction of the outlet;
• composition and quantity of effluent;
• sampling requirements;
• flow metering requirements;
• records which must be kept.

The discharger can appeal to the Secretary of State regarding the conditions placed on the consent. The Agency can grant a consent without publishing or advertising an application if it considers that the proposed discharge will have not appreciable effect on the receiving water. The Agency can also grant an unconditional consent.

It is the duty of the Agency to review consents periodically (every two years). This can lead to:

• revocation of the consent;
• modification of the consent; and
• amendment of an unconditional consent to one containing conditions.

The consents may also be modified:

• to allow the UK Government to implement EU legislation and international agreements;
• to protect public health or fauna and flora dependent on the aquatic environment;
• in response to any representation or objection made to the Secretary of State or otherwise.

However, each consent must state the period for which the consent is valid.

The Agency must keep a register of all data relevant to the discharge and the receiving water. The information must contain details of any water quality objectives, the discharge consent and the monitoring data for the discharge and the receiving water. The register must also be readily accessible to the public.

Sewage effluent can vary in nature depending on the degree to which the sewage has been treated (see Box 1). Discharges of sewage effluent can arise from a number of different sources and be continuous or intermittent in nature:

• treated effluent from urban sewage treatment plants (continuous);
• storm discharges from urban sewage treatment plants (intermittent);
• effluent from 'package' sewage treatment plants serving small populations (continuous);
• combined sewer and emergency overflows from sewerage systems (intermittent);
• septic tanks (intermittent);
• crude sewage discharges at some estuarine and coastal locations (continuous).

Treatment of sewage ranges from:

• none at all (crude sewage);
• preliminary (screening and/or maceration to remove/disguise solid matter;
• primary (settling to remove suspended solids as sewage sludge). Typically removes 40% of BOD, 60% of suspended solids; 17% of nitrogen and 20% of phosphorus from the untreated sewage;
• secondary (settling and biological treatment to reduce the organic matter content). Typically removes 95% of BOD, 95% of suspended solids, 29% of nitrogen and 35% of phosphorus from the untreated sewage. Nutrient removal steps can be incorporated into secondary treatment which can reduce ammonia - N down to 5 mg/l and phosphorus to 2mg/l.
• tertiary (settling, biological treatment and an effluent polishing step which may involve a reed bed (unlikely for a coastal works) or a treatment to reduce the load of micro-organisms in the effluent). Typically removes 100% of BOD, 100% of suspended solids, 33% of nitrogen and 38% of phosphorus from the untreated sewage.

The form of the consent for these discharges depends both on the type of effluent discharged and their periodicity. In general, discharges of continuous sewage effluent (i.e. effluent discharged at all times) have consents with numeric conditions for substances or groups of substances among other conditions on the consent. For intermittent discharges (usually of sewage effluent or storm sewage), consents are descriptive and relate to the number of discharge events in a specified time period (termed the spill frequency).

Numeric consents for sewage effluents are generally expressed in terms of a 95 percentile, which means that samples of the effluent must not exceed the numeric condition for that substance on more than 95 percent of occasions, and an upper tier, which means that exceedance must not occur on more than 99.5% of occasions. Look-up tables are used by the regulator to enable this condition to be translated into the number of samples that are allowed to exceed the conditions. The use of the 95 percentile reflects the fact that dischargers of sewage have little control on the variability in composition of sewage entering treatment works such that, occasionally, the treatment process will not be able to deliver the typical reductions in the main components of the sewage. The use of the upper tier prevents flagrant abuse of the 5% of occasions when 95 percentile consent limits can be exceeded.

The numeric limits in discharge consents are set such that the quality of the receiving water is maintained for all existing uses and to comply with relevant statutory requirements. The process of relating concentrations of a substance in the receiving water to numeric limits in a discharge consent can be complex, especially in tidal waters. The concept of the mixing zone is applied to allow consent conditions to be related to environmental concentrations of polluting substances. The mixing zone is an area of receiving water around the discharge point within which EQSs can be exceeded. The choice of the size of a mixing zone is somewhat arbritary and a diameter of 100 m is commonly used. For the majority of situations, mass-balance or modelling approaches are adopted. Mass-balance calculations essentially involve defining the effective volume of the receiving water and calculating the amount of a substance that can be discharged into that volume to achieve a desired concentration. More complicated modelling approaches can be undertaken but these are usually reserved for particularly sensitive situations, e.g. where more than one discharge is under consideration, where discharges are close to bathing waters and when compliance with appropriate standards is likely to be borderline.

Decisions on appropriate consent conditions include consideration of the uses to which the receiving waters are put and the application of a policy of 'no deterioration' under the Shellfish Waters Directive.

For tidal waters, the following receiving water uses are considered:

• basic amenity and conservation of the general ecosystem;
• passage of migratory fish;
• commercial fisheries for fish, molluscs and crustacea for public consumption;
• bathing and other water contact based recreation;
• other recognised uses, such as industrial abstractions and harvesting of edible seaweed.

In addition, the requirements of the following legislation and commitments must be considered:

• EC Bathing Waters Directive and UK Regulations (appropriate mandatory or indicative standards must be met at designated bathing waters in the vicinity of the discharge);
• 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 and on the nature of the receiving water (HNDA or a sensitive area);
• 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 in the vicinity of the discharge);
• North Sea Conference and OSPAR commitments (commitments on the reduction in loads of toxic substances in discharges to the marine environment must not be compromised).

The policy of >no deterioration= as it applies to estuaries and coastal waters encompasses statutory requirements and includes provisions not to allow an increase in consented loads in existing discharges (particularly for ammonia and BOD in estuaries) and for all discharges not to allow more than a 10% change in receiving water quality unless there is insignificant environmental change as a consequence (particularly for bacteriological parameters in coastal waters in relevant circumstances).

For intermittent discharges of sewage from Combined Sewer Overflows (CSOs), the spill frequency is based on the uses of the receiving waters. Where discharges are likely to affect bathing waters, the spill frequency on the discharge consent will vary between 3 spills per bathing season where the discharge is below Mean Low Water Springs (MLWS) to 1 spill every 5 years for discharges above Mean High Water Springs (MHWS). For the majority of CSOs, the consent conditions will only relate to bathing seasons, that is, at a time when rainfall is usually lowest and when spill frequencies are likely to be less frequent. Control of CSO discharges in other waters is less well defined but should be sufficient to avoid nuisance and protect existing uses.

Industrial discharges are regulated by the Environment Agency under Integrated Pollution Control (IPC) if the process is 'prescribed'; under UWWT Regulations if the effluent is similar in nature to sewage; or under the provisions of the Water Resources Act 1991.

In practice, the approach to granting consents or authorisations is similar to that for sewage discharges. The main difference being that it is recognised that there is a greater degree of control on the composition of the effluent arising from industrial processes and, therefore, the numeric limits on the consents are set as absolute limits which must not be exceeded under any circumstances. Industrial effluents also tend to contain more toxic substances and numeric limits in consents and authorisations are set to ensure compliance with EQSs, where they exist, in receiving waters. There are many substances for which there is no EQS. In such cases, dischargers are required to provide as much information as possible on concentrations and loads in the effluent, and ecotoxicological effects in the environment, to support the consent application on which basis the Environment Agency will set appropriate consent conditions.

Some industrial discharges contain complex and varying mixtures of toxic substances, making the approach of setting consent conditions for each component problematic. The difficulties include:

• Cost and potential difficulties of chemical determination of all constituents;
• Frequent lack of ecotoxicological data for identified substances; and
• Difficulties in predicting the interaction between chemicals and how they combine to affect the environment.

In these circumstances, an alternative approach to consent setting is used, namely direct assessment and control of the toxicity of the whole effluent - Direct Toxicity Assessment (DTA). Samples are taken of an effluent and toxicity tests performed to assess the potential impact of the effluent. If toxicity is detected but considered acceptable, a toxicity based consent can be derived and applied. If the toxicity of the effluent is not acceptable, remedial action may be needed (i.e. eliminate, substitute, treat) and then a toxicity based consent can be applied (or a chemical consent if more appropriate).

References