of the legislative requirements and
responsibilities relating to on-site
treatment systems and their impact on
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:
- Review of legislative requirements and responsibilities and
identification of best practice;
- Literature review of the impacts of on-site wastewater treatment
systems, including cumulative loadings and impacts on water quality;
- Identify methods used to estimate / quantify the nutrient
loadings in discharges from small wastewater treatment works on water
quality and provide recommendations on a suitable methodology for use
by the Environment Agencies
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
- Limited information on the number, location and level of
maintenance of OSWTSs
- Limited information on the impacts of associated discharges on
water quality and ecology.
- Inadequate communication and lack of a ‘joined-up’ approach
between responsible authorities.
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
- Establish location of existing systems and incorporate into a
Geographic Information System
- Ensure OSWTSs undergo annual maintenance
recommendations above could
be achieved by making desludging an
annual requirement and free for the first year, as
done in Scotland; this would encourage owners to get desludging carried
out more regularly, and would create a record of all OSWTSs
- Prioritise evaluation of existing systems based on location,
waterbody characterisation, soil depth etc.
- Coordinate more effectively the responsibilities that are divided
between planning and pollution control; in particular, the involvement
of environmental regulators early in the planning process is seen as a
- Place responsibility for the preparation of applications on the
applicant, as done in the Republic of Ireland
- House the results of site assessments and associated decisions,
including details of sites deemed unsuitable, in a georeferenced
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.
- In Scotland and Northern Ireland, microbiological
contamination of groundwater supplies is of concern due to the
relatively high contribution from OSWTSs compared to other sources*.
- Phosphorus (discharged in the form Soluble Reactive
Phosphorus) from OSWTSs is estimated to contribute over 28% of the
total diffuse phosphorus load to surface waters in the Republic of
Ireland, 10% in Northern Ireland, and 4% in Scotland.
- The contribution of OSWTSs to diffuse nitrate
pollution, when compared to other sources, is relatively small.
Recommendations on a suitable
methodology to estimate nutrient loadings
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
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report is available for download from the SNIFFER