Review of the legislative requirements and
responsibilities relating to on-site wastewater
treatment systems and their impact on water
Work Package B
(Review of impacts)

Objectives of the project

This project aimed to improve knowledge of the impacts of a dispersed population pattern on water quality and to inform future legislation, policies and procedures to address pollution related to wastewater treatment provision in rural areas. It was a desk-based research carried out for the Environmental Regulators within Northern Ireland, Republic of Ireland and Scotland.

The key outputs from the project are:

On-site wastewater treatment systems (OSWTSs) have been identified as a potential source of pollution to surface and groundwaters in areas where the dispersed nature of the population makes connection to a mains sewer impossible or overly expensive.

The impacts of OSWTSs arise from three main contaminants – nitrates, phosphates and microbiological pollutants. Nitrates and phosphates are a concern because of their effects on surface water ecology; in particular, the release of nutrients in sensitive waterbodies can lead to eutrophication and failure to comply with quality standards established for the Water Framework Directive (WFD). Microbiological contaminants can have serious consequences for human health if allowed to enter drinking water supplies.

However, there can be ineffective control of OSWTSs due to:
Key Findings and Recommendations

Review of legislative requirements and responsibilities and identification of best practice

Recommendations are made to address the management of OSWTSs, both in the light of current problems and in order to respond to the developing needs of the WFD.
The recommendations include:

Review of the impacts of OSWTSs

A significant number of households rely upon OSWTSs (>400,000 properties in the Republic of Ireland, ~120,000 in Northern Ireland, >100,000 in Scotland). In theory, loadings of contaminants from OSWTSs should not be of concern to water quality; however, if sited incorrectly or poorly maintained, effluent quality will be reduced.

The project reviewed previous studies to estimate the contribution from OSWTSs to diffuse pollution loadings:
Impacts of OSWTSs may be more significant at a local scale. The status of receiving watercourses is also important, as systems discharging into already sensitive waters could give rise to status deterioration with only small increases in contaminant concentration.

Recommendations on a suitable methodology to estimate nutrient loadings in discharges

To estimate nutrient loadings from small wastewater treatment works, this project recommends a methodology based on a “pressure-pathway-receptor” model. The methodologycould be used proactively during the consenting stage to prohibit the use of OSWTSs in areas most at risk. Alternatively, the tool could be used to assess the risk of systems already in operation, and the impact of poorly-maintained OSWTSs on water quality.

If the methodology is to be taken forward, it will require significant further development to ensure the assumptions used are appropriate for a particular region, and to make it into a user-friendly tool.

*Comparable data was not available for the Republic of Ireland

Copies of this report are  available from the Foundation  in electronic format on CDRom at 20.00 + VAT or hard copy at 35.00, less 20% to FWR members.

N.B. The report is available for download from the SNIFFER Website