Water Quality - Urban Waste Water Treatment Directive
Sewage Treatment in the UK: Implementation of the
EC Urban Waste Water Treatment Directive
Glossary
(References to "the Directive" are to the Urban Waste Water Treatment Directive)
POPULATION EQUIVALENT
The unit of measure used to describe the size of a waste water discharge.
Population equivalent is the biodegradable load (matter) in waste water
having a 5-day biochemical oxygen demand (BOD) of 60g of oxygen per day.
Population equivalent doesn't necessarily reflect the actual population
of a community (or Agglomeration).
AGGLOMERATION
An agglomeration is a community of homes, shops hospitals and certain
industries which are sufficiently concentrated for the waste water to
be collected for treatment at a sewage treatment works. With a very few
exceptions, an agglomeration is the community served by a single sewerage
collection network and served by a single treatment works, its catchment.
APPROPRIATE TREATMENT
Term used in the Directive to refer to treatment of waste water discharges
below the size thresholds in the Directive. It can be any process and/or
disposal system* that ensures receiving waters meet water quality objectives
set by pollution control authorities and can therefore be from no treatment
to tertiary treatment *eg septic tank, rotating biological contactor,
trickling filter, activated sludge process, reed beds etc.
BATHING WATER DIRECTIVE
Introduced in 1975 to protect the environment and public health. Sets
microbiological and physico-chemical quality requirements for all identified
bathing waters.
BIOCHEMICAL OXYGEN DEMAND (BOD)
A widely used measure of polluting potential - BOD is a measure of oxygen
use, or demand, by bacteria breaking down the biodegradable load in sewage
treatment plants or environmental waters. BOD is the basis for deriving
the Population Equivalent of a catchment of a sewage
works.
DISCHARGE CONSENT/REGISTERED STANDARDS
An authorisation issued by an environmental regulator to allow discharge
of treated water. Consents are set to minimise pollution of receiving
waters and may include the requirements of Directives, national legislation
or local water quality objectives. Consents specify the discharge location,
total volume of treated water permitted and the quality conditions of
the discharged water, such as the biochemical oxygen demand, limits for
suspended solids, ammonia, metals and toxic substances.
EUTROPHICATION
The process where excessive nutrients, especially nitrogen and/or phosphorus
compounds, cause an accelerated growth of algae and higher forms of plant
life. The result of eutrophication is an undesirable disturbance to the
balance of organisms present in the water and to the quality of the water
concerned. This can result in low biodiversity with reduction and changes
in the range of species of water invertebrates and fish species present
in eutrophic waters.
GUIDELINE STANDARDS
Under the Bathing Water Directive: The more stringent water quality standards
Member States should endeavour to observe. See also Mandatory standards.
LESS SENSITIVE AREA
Estuarine or coastal water designated under the Directive as having characteristics
of good water and oxygen exchange, not subject to eutrophication or oxygen
depletion and that will not be adversely affected by primary treatment
discharges. Discharge of primary treated effluent into a Less Sensitive
Area is permitted if a study shows the discharge would not adversely affect
the LSA.
MANDATORY STANDARDS
Under the Bathing Water Directive: The minimum water quality standards
Member States are required to observe and against which the performance
of Member States is principally assessed.
MORE STRINGENT TREATMENT
Term used in the Directive to describe tertiary treatment of discharges
affecting Sensitive Areas (SAs). More stringent treatment means:
- Removal of phosphorus and/or nitrates for SAs designated under the (a) "eutrophication" criterion;
- Means removal of nitrates for SAs designated under the (b) "drinking water source" criterion
- Whatever treatment is needed for SAs designated under the (c) "other Directives" criterion - to meet the standards of other Directives, such as the Bathing Water Directive, Shellfish Waters Directive.
NORMAL WATERS
Waters that are not either designated as Less Sensitive
Areas or Sensitive Areas. They can be inland, estuarine
or coastal waters. Large discharges into normal waters receive secondary
treatment.
PRELIMINARY TREATMENT
Simplest treatment that waste water can have. It can involve screening
to remove rags and other similar large solids and/or maceration to reduce
the size of solids. It can also involve grit removal.
PRIMARY TREATMENT
Primary treatment involves a physical and/or chemically-enhanced settlement
of suspended solids that is not removed by preliminary treatment. The
Directive sets percentage reduction figures for biochemical oxygen demand
of the influent by at least 20% and a reduction in total suspended solids
in the influent by at least 50% before discharge to receiving waters.
SECONDARY TREATMENT
The treatment provided after preliminary and primary treatment. Secondary
treatment involves "biological" treatment - using bacteria to break down
the biodegradable matter in waste water. After secondary treatment there
is further settlement. Secondary treatment should comply with the standards
of the Directive.
SENSITIVE AREAS
An area designated under the Directive according to three criteria: (a)
waters that are, or have the potential to become, eutrophic if no protective
action is taken. (b) drinking water sources that contain or could contain
more than 50mg/l of nitrate if no protective action is taken. (c) waters
in need of protective action to meet the requirements of other Directives.
Waste water discharges over 10,000 PE that pollute Sensitive Areas need
treatment that relates to the designation criterion or criteria. See More
Stringent Treatment.
TERTIARY TREATMENT
Treatment provided after Preliminary, Primary and Secondary Treatment.
Tertiary treatment can involve disinfection (to reduce pathogenic bacterial
and viral organisms), eg by treating waste water with ultra violet light
(UV treatment) and/or nutrient removal to help prevent eutrophication
and/or removal of specific toxic substances. See More Stringent
Treatment.
Page published 25 April
2002;
Page last modified
29 September, 2002