A review of the effectiveness of different on-site wastewater treatment systems and their management to reduce phosphorus pollution

May, L.; Woods, H.. 2015 A review of the effectiveness of different on-site wastewater treatment systems and their management to reduce phosphorus pollution. York, Natural England, 17pp. (Natural England Commissioned Report NECR179, CEH Project no. C05045)

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Diffuse water pollution (DWP) is a major cause of SSSI waterbodies not meeting favourable condition and/or water quality objectives set under the EC Water Framework and Habitats Directives. Diffuse water pollution can come from a variety of sources, including agriculture, small un-consented point source discharges, and runoff from roads or urban areas. Natural England estimates that around 166 SSSIs are affected by DWP where pollution reduction plans are being developed for all or part of these sites; 40 of these plans cover Natura 2000 Protected Areas. Many of these DWP plans highlight small domestic discharges (SDDs), as potential sources of nutrients that are causing water quality problems, although often it is not known how significant they are. These include those from septic tanks (STs) and package treatment plants (PTPs). The most frequently highlighted problem associated with discharges from these systems is phosphorus (P) pollution, which causes eutrophication problems. However, it is often unclear how such problems should be resolved because there is little information on the efficacy of management advice that is currently being given to householders. Such advice includes the regular de-sludging of tanks, the use of P-free detergents, and the replacement of traditional STs with PTPs. This review assessed the information contained in publicly available literature with a view to enabling Natural England to improve the advice that they give on the correct management of SSDs in relation to reducing their P-related impacts on designated sites. The review found that there was very little knowledge and information available that could be used to develop a more evidence based approach to reducing P discharges from these systems. Based on the existing knowledge and information reviewed, it was concluded that the most effective options for reducing the levels of P discharged from on-site sewage treatment tanks to the environment are as follows: 1) Reducing P inputs to the tank, e.g. through the use of P free detergents, and 2) Using chemical precipitation to retain P within the tank and incorporate it into the sludge. It should be noted that the use of P free detergents is becoming increasingly common due to the introduction of recent European legislation (European Union, 2012). It is also important to note that, whilst chemical precipitation is an effective method of retaining P within tanks, issues of personal and environmental safety can be associated with this approach, and at this stage it is not appropriate for widespread use. In addition to the above, many other approaches to tank management also have the potential to effectively reduce P discharges from these systems. However, at present, there is insufficient data and information available for their level of efficacy to be determined. Other options that are often recommended include frequent de-sludging or replacing traditional STs with PTPs. It is unclear how and to what extent these other options are effective at reducing the P concentration in tank effluent or whether, in practice, these approaches could be making the situation even worse in some situations. In areas where the water table is high, ie <1.5 m below soil surface, impacts on nearby watercourses can probably be reduced by installing mounded soakaway systems. This review also identified key gaps in knowledge and recommends that further research be undertaken to answer the following questions: 1) To what extent do P concentrations of ST effluents differ from those of PTPs? 2) How does temporal variation in P output from STs and PTPs (from sub-daily to seasonal) affect the level of uncertainty associated with collecting and analysing a single effluent sample for monitoring/regulatory purposes? 3) How effective are STs and PTPs at breaking down waste if usage is intermittent or seasonal and what are the implications for levels of P in the discharged effluent? 4) Does de-sludging reduce effluent P concentrations? The results of this research would provide information on the key factors that affect effluent P concentrations from these tanks. This could then be incorporated into advice give to householders on how to reduce P outputs from these systems.

Item Type: Publication - Report
UKCEH and CEH Sections/Science Areas: Watt
ISBN: 9781783541782
Funders/Sponsors: Natural England
Additional Information. Not used in RCUK Gateway to Research.: Freely available via Official URL link.
Additional Keywords: small discharges, septic tanks, package treatment plants, phosphorus, freshwater SSSIs
NORA Subject Terms: Ecology and Environment
Date made live: 21 Dec 2015 17:28 +0 (UTC)

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