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WATER STORAGE AND DISTRIBUTION
Once water has been abstracted, treated and disinfected, it is ready to be distributed to our customers. To provide an unrestricted and economical supply of water we utilise pumps and pipelines to deliver water to our customers' doorstep. Because demand for water constantly varies throughout the day together with seasonal variations created, for example, by garden watering, we utilise service reservoirs to store water avoiding the need for constant variations in pumping rates. Service reservoirs are constructed and sited on top of hills. Water is normally pumped from the Treatment Works at night to the reservoir thereby making use of cheap electricity. At all times of the day water gravitates to households and businesses through our distribution network regardless of whether or not pumping stations and Treatment Works are operating. It is common for water to be abstracted from a source and pumped to a number of reservoirs strategically placed to supply the local population. In rural areas or areas of exception demand, we commonly boost or transfer water to secondary reservoirs at a higher ground level.
Enclosed pipelines are utilised as the most economical method of transferring treated water in bulk from our Treatment Works to service reservoirs and onwards to our customers. Portsmouth Water has approximately 3,000 kilometres of pipe ranging in size from 50mm to 1200mm, enough pipe to stretch to Kuwait. The pipes must be capable of transferring the required flow, be capable of withstanding internal pressure created by pumping without bursting and when laid under roads be capable of withstanding the crushing loadings imparted by vehicle traffic. They must also be suitable to withstand accidental damage caused by mechanical and manual digging, a common problem in urban areas. Metal pipes must be protected against corrosive action by groundwater and soils. Supply pipelines are designed to last in excess of 75 years and they range in size from 200mm in diameter up to 1000mm. The selection of pipe size and materials needs to take into consideration many factors. Some of Portsmouth Water's earliest pipes date from the 1800s and because of careful selection, construction and maintenance, still remain serviceable to this day. Cast iron pipes were commonly used for over 100 years, whilst today supply pipelines are usually of ductile iron or steel construction.
Treatment Works are invariably located at low levels whilst service reservoirs are sited on hilltops to enable water to gravitate into supply. In order to physically move water from Treatment Works it is necessary to pump water uphill to reservoirs. The outline design and the specification of pumping plant is undertaken by qualified engineers. The performance of pumping plant must be 'matched' to the current network system, but also take into consideration any future requirements. Pumps are manufactured by specialist contractors and installed in conjunction with our engineers. Booster pumping is used where development has taken place in a local high area which cannot be satisfactorily supplied from an existing reservoir. The booster pumps are automatically controlled and as the need for boosting is frequently restricted to the hours of peak demand, the pumps may stand idle for some hours each day. It is normal for pumping times to be scheduled and programmed in advance, or they may be controlled from a centrally manned Operations Centre such as ours located at Havant. A monitoring 'telemetry system' which sends information to the 24 hours Operations Centre enables us to ensure that all the pumping plant is operating satisfactorily.
Water which has been treated and suitable for drinking is stored in service reservoirs. Service reservoirs therefore need to be totally enclosed and protected from outside contamination by animals, vegetation or ground and surface waters. They should not be confused with surface water storage reservoirs commonly seen in other parts of the country and used for fishing and other waterborne activities. Service reservoirs are used to fulfil the following functions: 1. to provide a reserve of treated water so as to minimise the possibility of supply interruptions due to failure of mains, pumping or treatment plant. 2. to enable a fluctuating demand within the distribution system to be met. 3. to enable pumps to operate at a constant output and to make economical use of power tariffs. 4. to provide a reserve of water for fire fighting. 5. to provide stable and adequate mains pressure in the distribution network to minimise bursting. Policies for storage capacities of reservoirs will often vary between different companies. At Portsmouth we try to ensure that a minimum of two days' supply is available. They range in size from around 1 million litres serving small rural communities to the largest on Portsdown Hill holding almost 45 million litres. Portsmouth Water has constructed its supply network (the Treatment Works pipelines, reservoirs and distribution mains) in such a manner as to enable the running of pumping plant during the cheaper night periods. This enables the reservoirs to fill during the night and gravitate into supply during the day. The reservoir levels therefore fluctuate between pre-set limits which are monitored by our Operations Centre in Havant. Our reservoirs are generally constructed from reinforced concrete for strength and security. We have a routine inspection and maintenance programme carried out by our qualified engineers and scientists. The planned maintenance programme ensures reservoirs remain watertight and contamination risks are eliminated. Whenever any work is completed, the reservoirs are thoroughly cleaned, sterilised, watertightness checked and microbiologically tested before returning to service.
Portsmouth Water supplies over 285,000 domestic and commercial properties. To transport water from our sources to customers' properties necessitates a vast network of over 3,000 km of water mains and more than 250,000 individual service pipes which connect each property to the water mains usually in public highways. Pipes may be sized to ensure excessive energy losses do not cause unacceptably low pressure. Calculations may be done by hand, however sophisticated computer programmes are available to 'model' the pipe network. The model may be used to evaluate the impact of increasing water demands and the need for new pipelines. Once a pipe size and theoretical route has been evaluated a suitable pipe material must be chosen. The most common currently used materials today for new mains are ductile iron, polyethylene, unPlasticised Polyvinyl Chloride (uPVC) and steel. The process of design and construction is undertaken by a combination of Portsmouth Water engineers and competent contractors. The key project objective is to achieve a well engineered, secure, reliable asset that will perform throughout its design life at an economical cost with minimal impact upon the local environment. We currently lay over 10 km of new mains and renew approximately 25 km of the network each year. Inevitably the process of laying new pipes causes some disturbance but we aim to minimise inconvenience to householders and road users by completing the work as efficiently as possible. The water mains network consists of a vast array of pipes and fittings in different sizes and materials, some of which date back as far as 1809! Inevitably leakage can occur from time to time, but we try to minimise the amount, setting targets each year, which are monitored by OFWAT, one of our Regulators. Surprisingly considerable volumes of water can escape below ground without being seen and so we need a continuous programme of leakage monitoring and detection. Using new technology we can monitor the flow into our mains network all the time with 53 strategically located flowmeters throughout the Company's area of supply. At night-time, when demand is at its lowest, measurements from these flowmeters give an indication of the level of leakage that can be occurring, although we have to take into account manufacturing processes which work around the clock. Having identified a suspect area of the network, our skilled Technicians carry out further investigations initially by monitoring waste zones within the strategic meter areas. Having identified a small zone with unexpected flows they then deploy a number of "leak loggers", relatively new devices for "listening" to the noise emitted by the leaks; alternatively, they systematically close down mains supplies in the middle of the night to identify the leaking pipes. (You may occasionally have noticed an unexpected interruption to your supplies during the small hours). Having identified the particular pipes affected, leak noise correlators then enable the Technicians to accurately pinpoint the location of the leak before repairs can begin. Once the leak location has been pinpointed, the necessary repair must be planned in order to minimise the interruption to water supplies as well as to road users. We employ around 90 staff whose daily duty involves excavating and repairing our vast mains and service pipe network at all times and in all weathers! When burst mains occur emergency staff are called out to effect repairs and at this time emergency supply interruptions sometimes are needed. Our Mains Renewal Programme prioritises over 25 km of the network which needs to be renewed each year. This enables us to control leakage levels as well as to minimise future supply interruptions by replacing old valves, fire hydrants, stopcocks and service pipes in conjunction with renewal of the water main.
The mains renewals process can sometimes appear to be longwinded but it is essential for us to be able to maintain supplies whilst the replacement process proceeds. There are several stages: Stage 1 - Wherever possible a new main is laid underground close to the old pipeline to be renewed. To avoid leakage and contamination, the new pipeline has to be swabbed, pressure tested, chlorinated and sampled and this can take some time after the new main has been laid. Stage 2 - After the main has been 'passed' as satisfactory for use, service laying teams transfer the existing properties from the old main to the new often relaying old service pipes at the same time. Stage 3 - When all the properties have been transferred to the new main the old main is decommissioned. Occasionally unknown connections are discovered and a property has to be reconnected on an emergency basis. Stage 4 - All the refilled trenches, materials storage areas and surfaces have to be permanently reinstated to highway standards.
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