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 suitability of different types of SSD (package treatment plants and septic tanks), sampling of package treatment plant systems 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 package treatment plant SSD systems available and the different manufacturer designs, detailing how they treat phosphorous in particular
- Sample on a monthly basis 6 package treatment plants for soluble reactive phosphorous, total phosphorous and total dissolved phosphorous
- Compare this to recent research on the effluent quality of septic tanks to understand if there is a difference
The findings contained within this report have allowed Natural England to further understand the risk of SSDs related to the different types of systems. It is hoped that the findings will also help steer further applied research in this area within the wider scientific community.