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Mutton Island Waste Water Treatment Plant

The Mutton Island site is located 1 km offshore of South Park in Galway City and was officially opened in May 2004.

The Wastewater Treatment Works has been designed to meet the requirement of the EU Urban Waterwater Treatment Directive (1991) and to ensure that the water quality at the designated bathing beaches of Silverstrand , Salthill and Ballyloughan are not compromised.

A Public Awareness File for the plant is available for inspection by members of the public. This file can be accessed during normal working hours at City Hall

How Does it Work

The Wastewater arrives at the inlet to the treatment works via a number of trunk mains from the east and west sides of Galway City.

JPG file icon   Mutton Island - Process Flow Diagram (JPG, 65 kb)

Steps Involved

Step One:
The Wastewater passes through coarse screens (100mm) to remove large objects followed by fine screens (6mm) to remove paper, plastics and large solids.

Grit (that is washed in from the streets by surface water) is removed in two circular grit traps.

Step Two:
Four Primary Settlement Tanks allow 60% of the pollutants (primary sludge) in the wastewater to settle.

Fats, oils and grease are removed from the surface of these tanks.

Step Three:
The settled wastewater (“primary effluent”) is pumped to four aeration lanes where air is injected via 3 Air Blowers.

Aerobic Microorganisms developed in the process are used to breakdown organic matter from the wastewater along with ammonia and nitrogen.

Step Four:
The organic matter produced in the above secondary treatment process is allowed to settle in the final Settlement Tanks, as secondary sludge.

Step Five:
The treated effluent is discharged to Galway Bay via the sea outfall, 420 metres long and 900mm diameter.

Discharge occurs 24 hours per day under all tidal conditions.

Step Six:
Sludge is a by-product of the treated wastewater.

The Sludge is thickened and pasteurised to 60 degrees for 1 hour to kill pathogens and digested to reduce volatile solids contents.

The Sludge digestion process also produces biogas as a by-product, which is in turn used to fuel the boilers and heat the Sludge.

Step Seven:
The Final Product, i.e. the dried sludge, is dewatered and transported off site where it is spread on land as an organic fertiliser.

Environmental Monitoring

In accordance with the requirements of the Environmental Impact Assessment throughout the project ongoing monitoring was carried out of:

Results confirm that the works during and post-construction have not had any notable adverse impact.

Water quality monitoring results indicate significant improvements in Galway Bay since the diversion of sewage flows to the Works.

Blue Flag status was achieved for Sliverstrand Beach in 2005.

Maintenance of the Plant

A short-term Operation and Maintenance contract was awarded to Earth Tech (Ireland) Limited in 2003 pending appointment of a long-term operator. 

It is intended to appoint a contractor who will operate the plant under a 20-year operation and maintenance contract.

Galway City Council, City Hall, College Road, Galway.
Open: 9am - 4pm. Phone +353 91 536400 Fax: +353 91 567493 Email: