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Water Treatment

The process of taking rainfall and turning it into potable (drinkable) water that flows from your tap appears deceptively simple. When treating stream water supplies and stored water intended for human consumption, it is absolutely vital to ensure that the final product is safe, wholesome and conforms to the latest United Kingdom drinking water standards.

Rainwater may appear to be clean, but by the time it has absorbed or dissolved a host of compounds, dirt and dust from the air, run off buildings, streets and across muddy fields, it is far from pure. The following describes the basic principles of water treatment, which takes you through the steps of collecting rainfall, storing it, treating it, and distributing it to our customers.


Step1 Step 1

Rainfall is collected from the island Water Catchment Area and either flows into streams or percolates into the ground. It is important that we collect as much of this 'untreated' raw water as possible to fill up the islands storage reservoirs, the majority of which comprise ex granite quarries.

Our Water Quality Staff work closely with landowners, businesses and the Office of Environmental Health to make sure that the island's water supplies are protected from all manner of potentially polluting effluents, chemicals and pollution incidents.


step2 Step 2

Raw stream water is collected and pumped across the Island to one of the many reservoirs, where it is stored prior to treatment. Storage is an important first step in the treatment process where blending of different surface water sources can take place, natural nitrate and bacterial reductions also occur in these reservoirs, improving the quality of water supplied.

Many of our storage reservoirs are aerated to improve mixing and water quality which is especially beneficial during warmer summer months when algal blooms can dominate these water bodies.


step3 Step 3

There are two types of treatment process currently in operation in Guernsey, "Conventional Coagulation and Filtration" and "Ultrafiltration".

Conventional treatment involves the settlement of raw water solids in large clarification tanks, with additional dual media filtration (sand/anthracite) of this clarified water.
Ultrafiltration involves the pre filtering of raw water supplies in large sieves followed by ultrafiltration using a membrane treatment system.

Both systems use small volumes of chemical coagulants dosed to remove as much of the suspended solids from raw water supplies as possible.


step4 Step 4

The final stage of water treatment involves the dosing of a small amount of chlorine solution to the filtered water in a sealed 'contact tank' to provide thorough sterilisation of any remaining micro-organisms, ensuring acceptable quality as the water travels through the distribution system to customer taps.



step5 Step 5

Some of the treated water is pumped to 'service reservoirs', large sealed tanks usually on high ground. Service reservoirs are important for balancing the peaks and troughs of water demand throughout the day. This water is mixed and further treated with chlorine solution to ensure that it remains free of harmful microorganisms before being pumped to customers' taps.



step6 Step 6

When required, the water travels through over 400kms of distribution pipework to our customers' houses. Customers now have access to clean, safe potable water, available 24 hours a day, 365 days a year.

Online monitoring and daily checks together with island wide testing of customer premises ensures that our treatment works provide excellent water quality year round.


Responsibility to our customers and the environment

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