Hong Kong meets 20 to 30 percent of its water demand from local sources, making the control of leaks in the distribution system vital.
Surface runoff and impoundments provide the locally sourced water while the remainder is supplied via pipelines from the Dong River in adjacent Guangdong Province. The city’s Water Supplies Department (WSD) distributes water to seven million people through a network of 8,000 kilometers (5,000 miles) of mains mostly fed by gravity from service reservoirs at high points. A great many water mains are thus subject to relatively high pressure.
Excessively high pressure could lead to high leakage rates, particularly in areas with aging pipes, and could constitute a key contributing factor to the system’s overall water loss. Water loss is defined as the volume of water that passes through the treatment plant but never makes it to the customer’s meter.
Research has shown that the quantity of water lost through leakage is proportional to the operational pressure of the system. Therefore, reducing system pressure, particularly during periods of low demand, should significantly alleviate leakage.
The WSD has been implementing active leakage control strategies with a target of cutting water loss rates to 15 percent by 2015. The strategies include the replacement and repair of aging mains, of course, but the WSD has also embarked on a number of continuous monitoring and pressure management programs. Since 2005, Black & Veatch has been working with the WSD on continuous monitoring and pressure management assignments for 10 of the 17 major supply zones.
MONITORING SMALL AREAS
There is an adage “if you cannot measure it, you cannot manage it.” WSD recognizes that the details and data derived from monitoring the flow at the service reservoir supply zones are not sufficient for developing a leakage control strategy. Therefore, the WSD has been dividing its water supply system into smaller units called district meter areas. About 375 areas established thus far are discrete service areas. They are used to monitor minimum night flow and discrepancies between input volumes and metered consumption. Electromagnetic flowmeters are installed in the water mains and are connected with data loggers to wirelessly transmit data on the water flow back to WSD’s control center. The data is continuously monitored at the control center.
Overlaying the district meter areas are pressure management areas, which can comprise a group of district meter areas, a single area, or even a partial area. So far, about 106 such pressure management areas have been established to reduce excessive pressure in the system, particularly during periods of low demand, mainly at night.
Reducing pressure can cut down leakage by lessening the stress on pipes and fittings. Area pressure is monitored at critical pressure points, and pressure is adjusted through pressure-reducing valves that also wirelessly report data back to the control center.
The amount of water saved by pressure management can be determined based on the nighttime flow. The WSD compares the night flow before and after the pressure reduction. Any reduction of flow that is measured will be the amount of the water saved.
USING HYDRAULIC MODELS
Hydraulic models representing system performance were developed to gain a better understanding and to achieve optimum designs of the district meter and pressure management areas. In developing the models, WSD compiled a database using two consecutive years of consumption information from its customer billing system. Individual usage calculations are updated as the billing data are monitored.
Improving the leakage rates in Hong Kong’s water supply system has considerable benefits, in addition to conservation of a critical resource. It will:
- Reduce the treatment and distribution costs, primarily those of energy and chemical usage;
- Reduce the frequency of burst water mains, thus reducing the disruption of water supply services, and maintenance and repair costs along with the nuisance of road closures;
- Defer infrastructure expenditures and delay the need to develop new water supplies through more efficient use of existing resources;
- Contribute to sustainable development and a better environment.
Reducing excessive pressure in the distribution system – and reducing flows – will help reduce incidents of pipe failures, extend asset life, save costs for pipe repairs and maximize water savings by minimizing demand.
SUBJECT MATTER EXPERT
Stephanus Shou: firstname.lastname@example.org
This article has been republished with permission by Black & Veatch.