The River Itchen Treatment Process
A diagrammatic layout of the process is shown below with explanations of each stage of the process following.
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.
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.
A coagulant Poly Aluminum 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'.
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.
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.
To provide an effective barrier against Cryptosporidium, the filtered water is then passed through a membrane filtration system.
The membrane system consists of submerged microfiltration modules designed to treat 84 Ml/d.
The microfiltration plant consists of 6 cells, each incorporating 396 microfiltration modules containing PVDF (Polyvinylidene Fluoride) hollow fibre membranes with a pore size of 0.2 microns (1 micron = one thousandth of a millimetre). Feed water passes through the walls of the hollow fibres to the centre of the fibres producing a filtrate virtually free of suspended solids which accumulate on the outer surface of the membranes.
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. Chlorine disinfects the water by killing bacteria and viruses.
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.