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

Published online
18 Jan 2021
Published by
Natural England
Content type

May, L. & Woods, H.

Publication language
UK & England


This review assessess 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 small domestic discharges in relation to reducing their phosphorus (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, (2) using chemical precipitation to retain P within the tank and incorporate it into the sludge. 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 septic tank (ST) effluents differ from those of package treatment plants (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.

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