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

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