A review of the effectiveness of different on-site wastewater treatment systems and their management to reduce phosphorus pollution (NECR179)

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Nutrient enrichment from diffuse sources is a major issue for freshwater SSSI sites not meeting favourable condition and for water bodies not meeting good ecological status under the Water Framework Directive (WFD). Therefore, failure to tackle diffuse water pollution effectively presents a significant risk to the delivery of Biodiversity2020 and the WFD.

There is growing evidence that small sewage discharges (SSDs) may pose a significant environmental risk to freshwater habitats under certain circumstances. However, the extent of this risk and its potential impact across the freshwater SSSIs are not well understood. Linked to this, it is often difficult to confidently judge where they can be safely located in terms of eutrophication from phosphorus and what type of system will pose the lowest risk to sites.

To improve our advice on the eutrophication risks posed by different types of SSD (eg package treatment plants, septic tanks and cesspits), and options for risk management, a full literature review was undertaken for Natural England by the Centre for Ecology & Hydrology (CEH), with contributions from the Environment Agency

The main aims of this work were to:

  • Characterise the different SSD systems available, highlighting key differences between them, and the relative risks they pose to the environment in terms of eutrophication from Phosphorus
  • Review the options for reducing the phosphorus pollution risk from these systems and their applicability under different environmental conditions
  • Identify the key knowledge gaps in this area highlighting priorities for further research.

The findings contained within this report have allowed Natural England to refine the advice that it provides on the risk of different types of SSDs and their management with respect to potentially vulnerable freshwater SSSIs. It is hoped that the findings will also help steer further applied research in this area within the wider scientific community.

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