Urban Wastewater Management

Water Waste Treatment Process

Typical Treatment Process
© Department of Environment, Community and Local Government

Typical Treatment Process
© Department of Environment, Heritage and Local Government

Step 1: Sanitary Sewer                                    Step 4: Aeration Tanks

Step 2: Grit Chamber                                      Step 5: Secondary Treatment Tank

Step 3: Primary Treatment                          

The image above shows the layout of a typical treatment works. Ideally, wastewater treatment in a municipal treatment works involves four stages: preliminary, primary, secondary and tertiary treatment. There are two end products from the treatment process; sludge solids and liquid effluent. The treatment process reduces the effluent so that it will not adversely affect the quality of the receiving waters.

Preliminary treatment takes large solids and floating debris from the raw wastewater.

Primary treatment separates the smaller solids.

Secondary treatment uses micro-organisms to remove the biodegradable or organic waste.

Tertiary treatment includes nutrient removal and filtration. 

Urban Waste Water Treatment Directive

Urban wastewater can be described as a mixture of domestic and industrial wastewater and run-off rain water. The level of treatment wastewaters receive depends on the size of population served. In urban areas wastewater from homes and industry is carried off by a network of pipes and pump stations to a municipal treatment plant. This treatment of the sewage (the contaminated wastewater) involves primary, secondary and tertiary treatment:

1. solids are separated from the wastewater

2. dissolved biological matter is converted to solid mass using micro-organisms

3. solids are then neutralised and reused or discarded

4. treated wastewater is discharged to receiving waters

The proportion of waste water subject to secondary treatment has increased significantly from 26%  between 1998-1999 to 78% in 2009 (EPA, 2012). This is mainly due to the new waste water treatment plants at Ringsend (Dublin), Cork City, Limerick City, Galway City and Dundalk. Furthermore, because of major investment in recent years, construction of secondary treatment facilities at many locations around the country is at an advanced stage. This is expected to deliver significant improvements in the quality of urban waste water discharges.

That said, eleven large urban areas (
>2,000 p.e.) do not meet the UWWTD requirement to have secondary treatment in place. In the case of Bray and Ringaskiddy the provision of treatment is now over 10 years late (EPA, 2012). More information on urban waste water discharges from the EPA can be found here.

The Urban Waste Water Treatment Directive is designed to ensure that sewage collection systems are established for domestic and industrial waste waters and that this waste water receives appropriate treatment to reduce its environmental impact before being released into our waterways. In addition, the 7th Implementation Report on the Urban Waste Water Treatment Directive has been adopted. Read it here.