Introuction

 

Mutton Island Wastewater Treatment Plant
(WWTP) is located on Mutton Island in the middle of Galway Bay. The plant officially
opened in 2003, and first took flow in September of the same year. This came
after some 15 years of objections. Locals were opposed to the sewage treatment
plant as they believed it would damage the scenery of the bay, as the island is
also home to the Mutton Island lighthouse and a keeper’s cottage, which were
built in 1815. Others objected to the plant over fears for the various birds
and wildlife that inhabited the island, which is located in the Galway Bay
Special Area of Conservation. The primary concern relating to the plant was its
environmental impact. In October 1995, approval for EU funding for the plant was
withdrawn due to the same fears of a significantly negative impact on the
surrounding environment. However, Irish government officials decided to proceed
with the plans, instead using State funding. Construction began, and a 1.2km
long causeway was built to connect the island to the mainland. The first plans
designed the plant for a population equivalent (PE) of 12,000. However, due to
the several years of delays, when construction finally began on the plant, it had
to be increased to facilitate a PE of 91,000. In 2016, to cope with further
expansion of Galway city’s population, the capacity of the plant was upgraded
to 170,000 PE. The upgrade was funded by a €5 million investment, and the plant
currently averages out at around 150,000 PE. Mutton Island WWTP is responsible
for the treatment of wastewater from the towns of Barna to Oranmore.

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Figure 1. Map showing location of Mutton
Island WWTP and the area it serves

 

Treatment Processes

 

The wastewater treated by Mutton Island
WWTP comes from five pumping stations located throughout the designated catchment
area. The raw sewage is pumped collectively into South Park, an area approximately
900m north of Mutton Island. From South Park, the sewage flows by gravity to
the plant through a steel mains sewer. It is then pumped into the site where it
undergoes a number of treatment processes.

 

Preliminary Treatment

 

The first stage of treatment is the
preliminary stage. The raw effluent is preliminarily treated, meaning it is
screened to remove any inorganic materials, such as rags and plastics. Mutton
Island WWTP uses bar or rack screens, inclined at 45 degrees, to screen for
particles that are larger than 6mm, such as pebbles, stones, golf balls, etc. This
step is vital in preventing damage to any equipment and machinery further down
the process due to clogging. Bar screens are composed of a number of parallel bars,
usually between Screens with openings of 6mm or greater are defined as coarse
screens, whereas fine screens have openings of between 1.5 and 6mm. Bar screens
can be cleaned manually or mechanically to remove trapped particles. The plant
uses mechanical screens, which use a raking action to remove the solids from
the screens

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