Drinking Water Treatment

Ground Water Treatment Works: These Works abstract water from below ground sources (e.g. aquifers and springs). Groundwaters tend to be relatively clean in comparison to surface waters, and fewer treatment steps are required.

Surface Water Treatment Works: These Works take their water from above ground sources such as rivers and reservoirs. Raw water from these sources is open to direct environmental input, and multiple treatment steps are required to clean the water.

Individual process steps are used in varying combinations to clean and disinfect the abstracted water. Below is a brief description of the main types of treatment steps that Thames Water employs.

Abstraction of Raw Water
Water used for drinking water supplies comes from either surface sources (e.g. rivers, reservoirs) or underground sources (e.g. aquifers, springs). Raw water is pumped from these sites to the treatment plant for processing.

Storage Reservoir
Water abstracted from surface sources (such as rivers) can be temporarily held in large, open storage reservoirs. Storage reservoirs allow the blending of abstracted water with water already held in the reservoir, which permits dilution of incoming contaminants. Water can be held in storage reservoirs for hundreds of days before it is pumped to the works for treatment. The long storage time allows some water quality improvements e.g. debris and solid contaminants settle-out, sunlight breaks down organic material, and some bacteria die off.

Screening
This process is mainly used for surface waters, and removes floating objects (such as branches, leaves and other debris) from the water by passage through metal grills and screens.

Clarification
A chemical coagulant is dosed into the water, and acts to bind together fine suspended material such as silt and mud particles. The coagulant forms a ‘floc’, which traps the clumps of material, and is then removed from the water by settlement or flotation.

Filtration
There are several different types of filter available for use in the water industry (e.g. slow sand filters and rapid gravity filters). However, all function in a similar way by sieving suspended material from the water. Slow sand filters also have a biological action, as the top sand layers trap organic material, and support the biological growth e.g. bacteria and other micro-organisms. These micro-organisms breakdown organic compounds held within the water as it trickles through the filter, thereby providing an additional cleaning action.

Aeration
This process can be used to remove, or reduce the level of, unwanted compounds (such as hydrogen sulphide and carbon dioxide) from water, or oxidise dissolved metals such as iron in order to ease subsequent removal. Various techniques are used to achieve this.

Granular Activated Carbon (GAC)
This process is an advanced system used to remove pesticides, organic compounds and unpleasant tastes and odours. Water is directed into GAC vessels, which contain highly porous carbon particles. These particles are able to adsorb organic compounds from the water, thus improving its quality.

Ozone Dosing
This is an advanced water treatment process, which involves injecting ozone into the water to breakdown pesticides and organic material. Ozone is a very reactive molecule that attacks these compounds and destroys them. Ozone also has an anti-bacterial action.

Disinfection
Chlorine is used as a disinfectant and can be added to water as either a liquid or a gas. It can be dosed immediately into the incoming raw water for a variety of reasons e.g. to benefit downstream treatment processes, to have an immediate anti-bacterial effect, to oxidise unwanted chemicals in the water. Chlorine is dosed at the end of treatment, before the water is sent into supply to destroy harmful micro-organisms. The disinfection process is performed in specially designed contact tanks, which contain a series of baffles. The tanks ensure that the chlorine remains in good contact with the water for a set amount of time, to ensure effective disinfection, before it is sent into the distribution network.

Ammoniation
Ammonia can be dosed into the water following final chlorination to form a longer lasting disinfectant. Ammonia reacts with chlorine to form chloramines, which decay at a slower rate compared to free chlorine. This can be useful if the water has to travel large distances to reach customer properties. However, chloramines are less effective disinfectants compared to chlorine, so it is important to disinfect the water with free chlorine first, before converting it into chloramines.

Source:
http://www.thameswater.co.uk/UK/region/en_gb/content/
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