River Itchen Treatment Works

 

The River Itchen Waterworks is owned and operated by Portsmouth Water and is the only Treatment Works outside the Company's area of supply. It treats water from the River Itchen, has a maximum treatment capacity of 45million litres per day and on average produces 15% of the water supplied by the Company.

 

 

 

Water Quality Assurance

Highwood Reservoir stores up to 135 million litres of raw water before treatment. This improves the water quality in several ways. Many bacteria and viruses die during the storage period, and variations in ammonia content are smoothed out, making the water simpler to treat. River abstraction can be suspended if the water in the river becomes polluted or excessively cloudy or dirty. The works' staff carry out daily tests to check the pH, turbidity, chemical and bacterial content of the water - ensuring a safe supply.

 

The Treatment Process

A diagrammatic layout of the process is shown below with explanations of each stage of the process following.

Stage 1
Water leaves the River Itchen through six intakes, with surface debris removed by a floating boom. It then passes through a mechanically raked bar-screen before entering the Low Lift Pumping Station, where it is strained through fine mesh rotating screens before being pumped to the Highwood Reservoir, 1.8 km away.

Stage 2
Water from the Low Lift Pumping Station enters the first compartment of the reservoir where 90% of the solids sink to the reservoir floor. It then passes to the larger second compartment where it remains for three days. During this time many of the bacteria and viruses die off before the water gravitates to the Treatment Works where a preliminary chlorine dose is added to ensure the start of the disinfection process.

Stage 3
A coagulant Poly Aluminium Chloride (PAC) is added at the Flash Mixer to bind any small particles. Sulphuric Acid is also added to control the pH (acidity/alkalinity) and a Polyelectrolyte to improve the coagulation process. The 'dosed water' is now retained for a short period to enable the 'binding process' to start before the water passes to the 'Clarification Stage'.

Stage 4
The water dosed with coagulant is discharged into the bottom of 16 'upward flow clarifiers' and as the water flows upwards so the particles bound together by the PAC form a sludge blanket just below the surface of the water. The sludge blanket traps more particles as the water flows through it to the outlet channels which span the clarifier at the water surface. From time to time some of the sludge blanket is 'drawn off' and discharged to the sludge processing plant.

Stage 5
The 'clarified water' is then divided equally between six rapid gravity filters, each containing a gravel base and a bed of granular activated carbon which removes any remaining fine particles. The granular activated carbon is also extremely useful in removing organic compounds which can cause taste problems in the supply.

Stage 6
Following filtration the 'filtered water' is further dosed with chlorine to ensure adequate disinfection. It remains in contact with a high dose of chlorine for a minimum of six hours in a covered contact tank.

Stage 7
After leaving the contact tank, the final water is dosed with Sulphur Dioxide to reduce the chlorine residual to its set point before being pumped by High Lift Pumps to Wickham, near Fareham, for distribution to customers in Fareham, Gosport and Waterlooville.

At this stage a number of monitoring checks are conducted to ensure that the supply meets the necessary legislative standards.

 

Itchen History

In the 1960s it became clear that the Portsmouth Water Company needed an additional source to supplement its wells, springs and boreholes. Negotiations began in 1967, and in 1970 the company was authorised to take 45 million litres per day at Gater's Mill on the River Itchen. Abstraction here does not significantly affect the flow of the river through an important fishing area. The River Itchen Treatment Works, which came into operation in 1973, is highly-automated and was completed at a cost of £4 million.