Effective Environmental Management of Wastewater
By Chris Chubb, Water Quality Policy Manager, Environment Agency
People have used rivers, estuaries and coastal waters to wash away effluents from human activities for thousands of years, relying on natural purification processes to mitigate the potentially harmful impact of the wastewater.
Discharges from sewage treatment works and other industries may contain a range of pollutants. These discharges need to be carefully controlled to prevent harm to the aquatic environment. Pollutant loads from consented discharges have decreased since 1990.
Under the pollution legislation it is an offence to cause or knowingly permit poisonous, noxious or polluting matter; solid matter; trade or sewage effluent to enter controlled waters in England and Wales. Discharges into controlled waters of any possible pollutants can only be allowed if the discharger has (and is compliant with) discharge consent issued by the Environment Agency.
‘Controlled waters’ include virtually all freshwater, groundwater and tidal waters to a distance of three nautical miles out to sea. Exceptions include small ponds and reservoirs that do not supply water to other watercourses (although public water supply reservoirs are controlled waters).
There are about 100,000 water quality discharge consents in force. There are about 2000 IPC authorisations, which will become PPC authorisations as the IPPC Directive is implemented by 2007, and a further 3000 installations will fall under IPPC by that time. In 2001, there were 13,500 ‘significant’ consented discharges from sewage treatment works or trade sites. ‘Significant’ discharges tend to be those with a flow greater than 5m³/day.
The remainder of the 100,000 discharge consents belong to smaller sewage or trade discharges (less than 5m³/day) and other discharges such as storm overflows, emergency overflows and septic tanks for isolated dwellings.
Most sewage treatment works discharges are small with only around 700 serving a combined trade effluent and population equivalent load greater than 10,000 persons.
A sewage treatment works’ numeric consent can include concentration limits for a range of substances, but the three principal consented limits are for suspended solids, biochemical oxygen demand and ammonia. The pollutant loads from sewage treatment works for these three key substances, and phosphate, have reduced since 1990 due to substantial investment by water companies. Further reductions in pollutant loads are expected due to the continued investment programme.
There are different types of discharge consent:
Numeric consents are for discharges that have the greatest potential to affect the quality of the receiving water. They specify flow limits and have numeric concentration limits. These limits may apply to an individual substance or groups of substances, and include limits that are necessary to ensure compliance with a number of EC Directives and with commitments made for various international conventions. The limits are generally derived from calculation of the mass balance between the effluent and the receiving water to calculate the maximum allowable concentration in the receiving water after discharge of the effluent. The limit may be expressed as a statistic applicable to the number of samples taken, or as a maximum permissible value.
There are about 40,000 numeric limited consents in England and Wales, of which the Environment Agency routinely monitors 10,000 each year.
Descriptive consents apply to many small discharges to the aquatic environment. They have a low potential to adversely affect the receiving water because of the nature and low volume of the discharge. These are often difficult to control by means of specific numeric quality values. Descriptive consents specify flow limits, define the nature of the effluent treatment plant and require that the plant be correctly operated and adequately maintained.
There are currently around 50,000 discharges that are successfully controlled by descriptive consents without the need for any numeric concentration limits.
Discharges with descriptive consents can be assessed by means of inspections rather than by the chemical sampling that is required for numeric consents. The Environment Agency routinely inspects approximately 2,000 discharges with descriptive consents.