Principal Application: Urban Waste Water Treatment Directive - Planet WGSW-24010 User Manual

2.3 Pr incipal application: Ur ban Waste Water Tr eatment Dir ective
This is the principal application for which the Mean Trophic Rank has been developed and
tested. The survey methodology and calculation of MTR are described in detail in Chapters 3–5
of this procedural manual and guidance given in Chapter 6 on interpretation of results. Before
commencing surveys, Environment Agency staff should also refer to current internal guidance
on information gathering for future reviews of SA(E)s for the rationale behind UWWTD
monitoring (Environment Agency 1998b: supersedes NRA 1994b).
In brief, the MTR is used to provide evidence of eutrophication impact on riverine macrophyte
communities, in order to support designation of identified reaches as SA(E)s under the Directive
and to provide evidence of the specific impact of 'qualifying' discharges on such reaches.
2.3.1 Designation of Sensitive Ar eas
Sensitive Areas (Eutrophic) (SA(E)s) are water bodies which are considered eutrophic, or
which in the near future may become eutrophic if protective action is not taken. They are
identified using the criteria listed in Annex II of the Directive, with the definition of
eutrophication as given in Article 2(11) (see 2.1.1 above). The size of discharges and type of
receiving water are taken into account. Once SA(E)s are designated, discharge requirements can
be set for 'qualifying discharges' (QDs) in terms of nutrient levels or a percentage reduction in
nutrients. 'Qualifying discharges' are those with a loading of greater than 10,000 population
equivalent (pe): they may discharge either directly into the SA(E), or indirectly into the
relevant upstream catchment areas of SA(E)s, contributing to the pollution/eutrophication
of these areas. H owever, no action needs to be taken (ie consents do not need to be
determined) if it can be demonstrated that nutrient-removal will have no significant effect
upon the level of eutrophication. In the case of most freshwaters, nutrient removal would
usually be of phosphorus, the principal limiting nutrient in freshwaters.
A Government consultative paper was published in March 1992 (DoE et al 1992), proposing
criteria for identifying SA(E)s and subsequent procedures. This guidance was finalised in
March 1993, in Annex B of the paper published on the methodology for identifying SA(E)s
(DoE et al 1993). Under this methodology, waters are only identified if affected by QDs. For
rivers, the upstream limit of a SA(E) is either a QD or the point at which the symptoms of
eutrophication become manifest. The downstream limit is where the effects are reduced to
'typical'. For riverine environments, the criteria relate to orthophosphate, chlorophyll a, algal
biomass, water retention time, dissolved oxygen, fauna (fish/invertebrates), macroflora and
microflora. The MTR provides information on the macroflora by providing an estimate of the
degradation to aquatic macrophyte communities in these areas.
2.3.2 Assessment of eutr ophication and the impact of qualifying dischar ges
The main pollutant causing eutrophication arising from QDs will usually be phosphorus. This
phosphorus will usually be in the form of soluble reactive phosphate (SRP) and therefore be
available immediately to submerged macrophytes downstream of the discharge. Given the
relationship between MTR and phosphate concentration (Dawson et al 1999b), the MTR can be
used to answer the two key questions required for evidence in support of SA(E) designation:
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