Since 1988 when I started my career as a consulting engineer to the water supply industry, my views and understanding of water supply have changed. My first project was a leakage reduction project and used computer based hydraulic models of a water supply network in southeast England to design a district metering scheme, much like those now being set up in cities in India.
District metering allows the water supply operator to continuously record the volumes of water delivered to small, hydraulically discrete parts of the distribution system referred to as districts, typically between 1,000 and 3,000 households in size, and so to monitor the leakage levels in each district. This means that new leakage can be spotted a day or two after it occurs and leak detection and repair teams sent out. Leaks only run for days before they are repaired instead of months and years. This saves a lot of water.
The volume of water lost through a leak also depends on the pipe pressure. Higher pressures mean larger volumes of water are lost through leaks. Higher pressures also make it more likely that new leakage will occur. Therefore, another part of my first project was to assess the scope for pressure reduction in each district and to estimate whether the costs of the pressure reducing valves and associated chambers would be outweighed by the likely savings in leakage.
When our water supply systems are new they typically have a very low level of leakage. It is therefore common for water supply operators in charge of new systems not to have a leakage detection department. Instead, the leakage control is passive and only visible leaks that are reported by the public are repaired. But since our water pipework is buried in the ground, many leaks are invisible. As the system ages and leakage increases the water supply operator may decide to actively look for leaks, perhaps using traditional listening sticks or by using more recent inventions such as noise loggers. But the invisible leakage could be anywhere and so the leak detection team need to cover the whole water supply network including areas that may have no leakage.
When the leak detection team detect a leak, it must then be accurately located, perhaps using a leak noise correlator to pinpoint the location. Then the excavation team will dig down and see what they find. If the leak location is inaccurate, they will find a dry hole.
District metering allows the leak detection effort to be prioritised and focused on the particular parts of the distribution system known to have the highest leakage. District metering not only helps low leakage levels be achieved but helps them be maintained. A technique known as step testing can also be used within each district metering area or DMA to subdivide the DMA further into step test areas for a short, temporary, period usually at night. This allows the leakage levels in each step test area to be estimated. Leak detection efforts can then be focused on the individual step test areas, typically between 100 to 500 households in size, with known leakage.
In the past in many locations, leakage control has been seen primarily as a cost issue. Every litre or gallon of water put into the distribution system is first treated and pumped and both treatment and pumping costs money. In the UK, leakage was later introduced as a performance measure with poor performance decreasing the price that water supply operators were permitted to charge customers. Leakage control is also seen as a tool for deferment of capital expenditure. High volumes of leakage mean the water supply operator will need to expand the water treatment plant or build a new one sooner rather than later. The same applies to transmission mains.
Consequently, economic levels of leakage or, more recently, sustainable economic levels of leakage, SELL, have been estimated by water supply operators to justify leakage control expenditure. Reducing leakage beyond the SELL is not seen as justifiable.
But now, increasingly, leakage control is viewed as an essential water saving measure. This is important because leakage control costs can be high. In London, where digging up a road to repair a leak can cause huge inconvenience and associated expense, and where many of the water mains were between one and two hundred years old, it was concluded that leak repairs alone were not a viable means of leakage control. Instead, for DMAs with high leakage levels, all old cast iron pipework is being replaced on a DMA by DMA basis. This not only reduces leakage but also removes the water quality issues that old, unlined iron pipes can cause. Any lead service pipes are also being replaced. Before construction work starts, computer based hydraulic models are used to understand the existing system and to design the replacement system. During construction, if any design changes are deemed necessary because of unforeseen conditions underground, the models are used to revise the designs.
Water supply networks are complicated with typical flows and pressures changing every minute of every day and from month to month with the seasons and year to year with changes in residential, commercial, institutional and industrial demands. Computer based hydraulic models allow all this to be simulated. When calibrated against field measurements, a precise understanding of the existing system can be gained. This calibrated model can then be used as a basis for designing minor and extensive operational and infrastructure improvements to the distribution system with confidence. In experienced hands, the model ensures that designs are fit for purpose and will meet all functional requirements of flow and pressure delivery to customers and firefighters for minimum cost.
Most of the water supply systems I have worked on are 24 hour systems which deliver supposed drinking quality water to customers. Indeed, continuous 24 hour supply is considered essential to delivering drinking quality water. If supply is interrupted and pressure is lost, contaminated ground water can enter the pipe at underground locations where the pipe is leaking. Instead of clean water leaking out, contaminated ground water may leak in.
For complex leak repairs, the section of pipe surrounding the leak may need to be shut down and disinfection may be necessary. The extent of the pipework that will need to be shut down and the numbers of customers that will be affected and the duration of the shut down will depend, in part, on the locations and serviceability of the line valves. Good design will increase operational flexibility and ensure that line valves are appropriately placed.
Some water supply operators have in house teams of water modellers, others use consultants or have a combination of both. Either way, the design of water distribution systems costs money. However, design costs are small compared to the costs of operating and constructing a new or rehabilitated system and controlling leakage. Good design can reduce construction, operation and leakage control costs significantly while remaining fit for purpose. Since water supply systems are expected to last many decades, competent assessments of future demands are essential to good design.
But how can a water supply operator select a suitable design consultant? Selection on cost alone is likely to lead to higher construction, operation and leakage control costs or a design that is not fit for purpose. A selection based on both cost and experience would be an improvement, but good design is an art and not every architect is Gaudi, Le Corbusier or Norman Foster, no matter how experienced they are. Worse, good water distribution system design is more of a dark art. Unless the water supply operator is an accomplished practitioner, they are unlikely to be able to recognise a sub optimum design when they see it, let alone a sub optimum design consultant.
A recent approach to overcome this dilemma has been for the water operator to contract responsibility for design, construction, leakage control and even customer complaints to a consultant and contractor joint venture, or JV, capable of undertaking all of the various types of work necessary to these activities. Such contacts are large and complex and are awarded through a tendering process. The larger scale of the contracts can be good for stabilising water sector employment for those involved. Close coordination between the operator and JV is essential. I have worked as part of such JVs in Bangalore, Bangkok and London.
All of the above is based on the conventional western approach to water supply, which is centuries old: centralised water treatment with a single supply to each customer delivering all of a customers water needs, all designed independently of any sewerage collection system. Where water scarcity is a current or future issue, decentralised water treatment of our grey water from sinks, showers, baths and washing machines has potential to reduce our water usage dramatically as would a waterless design of toilet. But such options are not yet widely available or cheap. And there is little to no incentive to bear associated costs at a local or household level when other users in the watershed may not follow suit.
Much of the water scarcity problem isn’t really about water scarcity, it is about clean water scarcity caused by pollution and the current high cost of water treatment and transport. It is about our antiquated household water use arrangements and lack of decentralised recycling. It is about the volumes of water used by some types of power generation and the inefficiency of some irrigation systems.
Where ground water usage exceeds the recharge rate of the aquifer and water tables are decreasing, water scarcity results from a combination of inefficient water use and a lack of surface water infrastructure and storage.
Water supply operators can help combat water scarcity by reducing leakage but a far greater and wider ranging effort is needed by other organisations and sectors of society. Since our society runs on money, the speed at which these efforts occur and their effectiveness will depend on the money made available to fund them. Unfortunately there are many indications that our money systems simply are not up to the job and that the scale of effort necessary will not be achieved.
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