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Water
Supply and Treatment Public perception is often that water supplies should be free! After all, the Company is only intercepting water that exists in the natural water cycle and supplying it to its customers! In fact the abstraction of water from the environment and its treatment to ensure that it is safe to drink, is a technologically advanced process. It carries on for 24 hours a day, 365 days a year, so as to ensure that safe drinking water is always on tap. In this section we outline the facilities which are used for abstracting raw water and the treatment processes which are used to convert it into drinking water. Following the amalgamation of predecessor companies dating back to the early 1800s, Portsmouth Water has been supplying water to Portsmouth and the surrounding area since 1857. Its principal sources are natural springs at Havant and Bedhampton, wells and boreholes throughout its area and a river abstraction at Gaters Mill on the River Itchen near West End, Southampton. All the Company's abstractions are licensed by the Environment Agency who strictly monitor the quantities of water abstracted to ensure that environmental damage is minimised. Havant and Bedhampton Springs It was in 1860 that the Company first made use of the springs at Havant and Bedhampton, springs which are derived from the chalk of the South Downs. The cross-section shown demonstrates how rain falling on the South Downs seeps into the porous chalk through cracks and fissures whilst flowing underground by gravity towards the sea. After travelling under the clays of the Hampshire Basin around Waterlooville, Cowplain and Rowlands Castle, the chalk comes close to ground level at Havant and Bedhampton where a unique set of natural springs appear. These springs are reputed to be the largest group used for public water supplies in Europe and can produce anything between 53 and 170 million litres of water every day. The spring water is treated at the Company's Farlington Filtration Works before supplying Portsmouth, Havant and Hayling Island.
Wells and Boreholes As the Company's area expanded in the 1950s and 1960s it took over wells and boreholes, many of which had been used to serve local populations since the late 1800s/early 1900s. As demand increased from a rapidly growing population, so further underground boreholes were drilled to tap into the chalk aquifers of the South Downs. The boreholes principally consist of a deep vertical shaft which intercepts water flowing through the cracks and fissures of the chalk aquifer. Pumps are installed deep below ground to lift the water to the surface for treatment before it is pumped to underground storage reservoirs. Today, the Company has 18 well and borehole sites strategically situated throughout its supply area. River Itchen In response to the continuing rise in demand for water in the 1960s, the Company constructed its only surface water abstraction facility at Gaters Mill on the River Itchen. The abstraction point is just 1 km above the tidal limit at Woodmill and was constructed in this location to minimise the impact on river flows. Although a high quality salmon and trout river, the basic river quality still requires a sophisticated treatment process to convert the river water into drinking water. As the Works is outside the Company's area, it requires a 14 km pipeline from Gaters Mill to Wickham to enable the treated water to supply the Company's customers in Fareham, Gosport and Waterlooville.
Ensuring that your water is perfectly safe to drink is a very important issue for the Company. Very strict standards are laid down in the UK legislation for drinking water and these are covered in more detail on the Water Quality pages of the website. To meet these standards we conduct a great deal of monitoring of our treatment processes to ensure that the quality of water leaving our Treatment Works is strictly controlled. All sites are monitored on a continuous basis by telemetry systems connected to our remote Operations Centre which is manned 24 hours a day, 365 days a year. All our sources are designed to automatically shut down if the water quality does not meet specified criteria. Wells and Boreholes At many of our well and borehole sites the quality of the water is of such a high standard that the only treatment required is the addition of a small quantity of chlorine which ensures that the supply is microbiologically safe. Occasionally some customers with particularly sensitive palates will notice the presence of chlorine due to its 'swimming pool' odour. At certain sites the raw water has to be given a stronger dose of chlorine, usually referred to as superchlorination. After a contact period to ensure adequate disinfection, the chlorine level known as the 'residual' is reduced to an acceptable level for public consumption. Havant and Bedhampton Springs For the majority of the time the spring water abstracted at Havant and Bedhampton is of high quality and is very similar to our borehole/well supplies. However, due to the nature of the water catchment and the chalk aquifer, the spring supplies can become affected by turbidity (cloudiness). This is as a result of a large number of swallow holes in the upper part of the catchment which receive large quantities of surface water during and after rainfall. These natural geological features are in direct contact with the underground aquifer and when they 'swallow' the incoming surface water, that poorer quality surface water affects the composition of the natural springs at Havant and Bedhampton. The Company has been aware of this phenomenon for many years and it was for this reason that a Filtration Works was constructed on Portsdown Hill at Farlington in the early 1900s. Since 1910 the water supplied to the residents of Portsmouth has been subject to a process of filtration, initially through slow sand filters, but more recently by utilising rapid gravity filters. The Treatment Process The treatment process at Farlington Water Treatment Works is outlined in the diagram below:
Stage
1 Stage
2 Stage
3 Stage
4 Stage
5 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. Stage
6 Chlorine disinfects the water by killing bacteria and viruses. Orthophosphoric acid reacts with any lead pipes in customers' individual service connections and coats the inside of the pipe with lead phosphate. This inhibits the dissolution of lead pipe and reduces the lead content of drinking water at the customer's tap. Stage
7
After some time the filters start to become blocked by the small particles they remove from the spring water. They are cleaned by a special 'backwashing' process which can either be started automatically by sensors in the filter itself, or on a manual/timed basis from the Treatment Works Main Control facilities. Filter cleaning is carried out by temporarily isolating the filter and using air and clean water to remove the particles as outlined below:
Backwashing Stage 1 - Compressed air is bubbled upwards through the filter bed for approximately five minutes. Backwashing Stage 2 - Washwater is passed upwards via the filtered water channel, through the filter bed for approximately three minutes. Backwashing Stage 3 - The backwash water is drained away by opening the washwater outlet valve for a short period. The inlet valve is then re-opened, the filter refills and then after stabilisation and checking the outlet valve to the filtered water channel opened to allow the filter to return to service.
Membrane Microfiltration Process The microfiltration process consists of 6 cells designed to treat up to 84 million litres per day turbidity. Each cell contains 396 microfiltration modules containing hollow fibre membranes with a pore size of 0.2 microns. 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 accumulates on the outer surface of the membranes.
Modules within the cells units are cleaned as follows: 1.
Low pressure air assisted backwash, to maintain the filtrate flowrates.
Filtered water is used to 'backwash' the fibres, through the membrane
wall, removing any accumulated suspended solids from the membranes outer
surface. The Dirty wash water is discharged under gravity to the Wastewater
Holding Sump and subsequently discharged to the drainage system. After each chemical cleaning process the membranes are thoroughly rinsed with filtrate water before the Cell is put back into service. 3. All chemical waste and rinse water is directed to a Neutralisation Tank. Once a chemical cleaning process is complete, the tanks contents are recirculated by the duty Neutralisation Pump, before being neutralised with sodium hydroxide or sodium bisulphite for discharge to the existing drainage system via the Wastewater Holding Tank. 4. The membrane modules within each cell are subject to a Pressure Decay Test, which is a means of testing the integrity of the modules. The test is automatically carried out on each cell, once every 24 hours. The test pressurises the filtrate side of the module with compressed air to a set value, and then monitors the rate of decay of that pressure over a set time period. 5. If the pressure decay test on a particular Cell is out of limits then this Cell will be shut down to allow for maintenance repair of the broken fibres within a module. 6. Once these fibres are repaired and the pressure decay test has proven that the test for the Cell is within limits, then the Cell is put back into service.
At Gaters Mill the Company has its most complex treatment process. Although river water quality is good enough to support salmon and trout, it is not of a high enough standard to meet the stringent requirements needed for drinking water. The location of the Company's intake in the lower reaches of the River Itchen catchment results in wide variations in river water quality, sometimes as a result of surface run-off from fields and other water sources, but also from industrial and commercial discharges. As a result the Company has constructed Highwood Raw Water Storage Reservoir within the Itchen Valley Country Park. Here up to 135 million litres (about 3 or 4 days supply) can be stored. The reservoir enables the Company to shut down its intake for short periods if the river water becomes excessively turbid (cloudy) or, alternatively, if pollution has occurred. Storage in the reservoir begins the treatment process by ensuring that many bacteria and viruses die out; it also smoothes out variations in quality thereby avoiding frequent changes to the treatment process at the Works. The Treatment Process A diagrammatic layout of the process is shown below with explanations of each stage of the process following.
Stage
2 Stage
3 Stage
4 Stage
5 Stage
6 Stage
7 At
this stage a number of monitoring checks are conducted to ensure that
the supply meets the necessary legislative standards.
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