Treatment of Wastewater

The primary responsibility for groundwater protection rests with any person who is carrying on an activity that poses a threat to groundwater. Groundwater in Ireland is protected under European Community and national legislation. Local authorities and the Environmental Protection Agency (EPA) have responsibility for enforcing this legislation. The Geological Survey of Ireland (GSI), in conjunction with the Department of Environment and Local Government (DELG) and the EPA, have issued guidelines on the preparation of groundwater protection schemes to assist the statutory authorities and others to meet their responsibility to protect groundwater (DELG/EPA/GSI, 1999). A groundwater protection scheme incorporates land surface zoning and groundwater protection responses.

In Ireland, wastewater from approximately 400,000 dwellings is treated by on site systems. On-site systems can be subdivided into two broad categories: septic tank systems and mechanical aeration systems.

A conventional septic tank system consists of a septic tank followed by a soil percolation area. As an alternative to a conventional percolation area the effluent from a septic tank can be treated by filter systems such as:

• a soil percolation system in the form of a mound;
• an intermittent sand filter followed by a polishing filter;
• an intermittent peat filter followed by a polishing filter;
• an intermittent plastic or other media filter followed by a polishing filter; or
• a constructed wetland or reed bed, followed by a polishing filter.

Mechanical aeration systems include: biofilm aerated (BAF) systems; rotating biological contactor (RBC) systems; and sequencing batch reactor (SBR) systems. The effluent from a mechanical aeration system should be treated by a polishing filter to reduce micro-organisms, phosphorus and nitrate nitrogen. On-site systems are the primary method used for the treatment and disposal of domestic wastewater in rural areas. These systems are also used in urban areas, which are not connected to public sewer systems. On-site systems are often located close to private or public wells. When choosing the location and type of on-site system, developers should have regard to any nearby groundwater source, the groundwater as a resource and the vulnerability of the underlying groundwater. The groundwater protection responses in this guidance combine these factors to produce a response matrix. The objectives of these groundwater protection responses are:

• to reduce the risk of pollutants reaching drinking water supplies;
• to reduce the risk of pollution of aquifers;
• to minimise pollution of domestic wells; and
• to provide advice where it is proposed to locate domestic wells in the vicinity of existing wastewater treatment systems and vice versa.

The risk from on-site wastewater treatment systems is mainly influenced by:

• its proximity to a groundwater source;
• the groundwater vulnerability;
• the value of the groundwater resource;
• the height of the water table;
• the groundwater flow direction; and
• the type of on-site system and the quality of the final effluent.

The use of these groundwater protection responses allows decisions to be made on the acceptability or otherwise of on-site wastewater treatment systems from a hydrogeological point of view.