We use several different treatment processes to turn raw water into safe and wholesome drinking water.
We get our water from two types of sources. About three quarters of our supplies are from rivers and the rest is taken from groundwater sources.
The water then goes through a rigorous set of processes at our water treatment works before being pumped into the network to your home or business.
The treatment process varies according to the type and quality of the source of water.
Groundwater treatment works
These works abstract water from below ground sources such as aquifers and springs.
Groundwater tends to be relatively clean in comparison to surface water, 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.
The treatment process
The information below will tell you more about the different stages of the water treatment process.
Abstraction of raw water
Water used for drinking water supplies comes from either surface sources such as rivers, or underground sources such as springs.
Raw water is pumped from these sites to the treatment plant for processing.
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 for some water quality improvements:
- Debris and solid contaminants settle-out
- Sunlight breaks down organic material
- Some bacteria die off
This process is mainly used for surface waters, and removes floating objects such as leaves from the water by passage through metal grills and screens.
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.
There are several different types of filter available for use in the water industry.
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 of 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.
This process can be used to remove, or reduce the level of or oxidise:
- Unwanted compounds such as hydrogen sulphide and carbon dioxide from water.
- 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.
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.
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 various reasons, to:
- Benefit downstream treatment processes
- Have an immediate anti-bacterial effect
- 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.
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.
- Each treatment works employs several processes to produce safe and wholesome drinking water.
- Each treatment works is tailored according to its incoming raw water.
- Effective water treatment is essential for protecting public health.