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WARFSA/WaterNet Symposium: Sustainable Use of Water Resources, Maputo, 1-2 November 2000

Estimation of Potable Water Loss in High Density Municipal Areas 1

Estimation of Potable Water Loss in High Density Municipal Areas

R. HRANOVA

University of Zimbabwe, Department of Civil Engineering, P O Box MP 167, Mount Pleasant,Harare, Zimbabwe. E-mail: [email protected] or [email protected]

Key words: water demand management, water reticulation system, water loss, and water leakage.

1. INTRODUCTION

The management of potable water supply systems in urban areas is aiming at the provision andtransport to the households of water with specified quality characteristics at an acceptableconsumer price. A successful management would result in improvement of public health care andwould contribute in rising the social status and the standard of live in the area. Lately, the trendsof population growth in Zimbabwe show sharp increase in the population of urban areas, whichreflects in considerable pressure on the existing engineering structures (Ndamba. J., et al, 1999 ).

Thus a proper management practice to reduce water loss, due to leakage in water supplysystems is essential in order to achieve the main goals and objectives at an optimal cost.

In the past, due to inadequate information, the adoption of leakage control policies has beenbased on subjective assessments of the benefits. And one of the factors that have prevented theuniversal adoption of positive leakage control measures has been the absence of an acceptablemethod of assessing the benefits and weighing them against the costs. This has resulted in oneor more of the following in some areas (Standing Committee report, 1980) :I. The implementation of a leakage control method, other than the most appropriateII. An incorrect level of effort being applied to leakage control method adoptedIII. Positive leakage control measures not being adopted where it would be economic to do

so.

Factors, affecting water loss trough leakage, are:• Pressure - it affects the rate of leakage through broken pipes or faulty fittings. It also affects

the frequency of bursts as an increase of pressure within a system can result in a fairly largenumber of bursts occurring within a relatively short period; while reducing the pressure mayreduce the frequency at which future bursts occur. Higher pressures will increase rate of lossof water from an individual leak and this may cause that leak to appear sooner and hencemakes it easier to locate using sounding methods. The effects of pressure surges can causethe main pipe or the service pipe to fracture and fittings to move out of place. Cycling thepressure between high and low value within the design pressure causes fatigue. This occurswhen a pump set or a booster is switching on and off or when pressure reducing valves arebadly maintained.

• Movement of the soil - it may cause pipeline to break, joints to move, or may result in localstress concentrations within pipe or fitting which eventually leads to its failure.

• Soil characteristics - they affect the running time of individual leaks, for example, in somesoils water from underground leaks may show on the surface fairly quickly whilst in othersleaks can run indefinitely without showing.

• Deterioration of water mains and pipes - the common forms of failure of pipes due todeterioration of their quality are hole formation and transverse or longitudinal fracture of thepipe ( Wiessman W jr. et al, 1998). Concrete or asbestos cement pipes can also corrode dueto presence of high levels of sulfates in the water or soil. Poor quality pipes, fittings and



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workmanship can promote leakage. Age also comes into play, as older pipelines tend todeteriorate more than pipelines, which have just been laid.

This paper revises and discusses methods of estimation of water loss and analyses their applicability in conditions of high-density urban population areas, within bulk water supplysystems, which are managed under conditions of financial restrictions and lack of specificallytrained personnel.

2. METHODS FOR WATER LOSS ESTIMATION AND LEAKAGE DETECTION

Estimation and detection of the loss in a water supply system can be applied only if the followingbasic requirements are in place:• thorough understanding of the structure and functioning of the system;• availability of adequate measuring devices for flow volumes/ flow rates;• Availability of engineering drawings, instruction and operation manuals and/or GIS systems, if

available,The analysis and comparison of the methods, discussed in this paper, is based on a generalwater supply system, shown on Figure 1.

The configuration of a water supply system may vary in wide limits, depending on water source,topography and the engineering solutions adopted. The generalized scheme, shown on Figure 1,is based on the assumption that water is abstracted from a surface impoundment.

2.1 TYPES OF WATER LOSSES ACCORDING THEIR LOCATION

Water losses along trunk mains – these losses are due to leakage from trunk mains, includingthe main from the source to the treatment plant, from the plant to the reservoir(s) and from thereservoir to the first deviation within the reticulation system. Due to the bulk amounts transported,and in most cases high pressures, the volumes of water losses might be considerable, but they

are relatively easy to detect by visual inspection and low in numbers. Trunk mains are usuallydesigned and constructed at high standards of reliability, because of their importance, andleakage along them is an indication of aging and the need of replacement.

Water losses within the treatment plant – these are losses due to the treatment process itself,their amount can be decreased by improving the operation of the plant or introducing treatmentmethods for reclamation of the sludge generated. To estimate these losses and to include theminto the water balance of the system water flows measurements are imperative (points M1 andM2 on Figure 1).

Water losses within the distribution system - these losses are usually smaller in volumes butnumerous in numbers. Their identification and location is difficult, because of the large area

covered by the reticulation system. Measures for leakage control and detection are usuallydedicated to the reduction of this type of water losses. This need is also supported by the fact,that water supply projects executed in conditions of financial restrictions are resulting in lessreliable solutions, correspondingly the expected number of breakdowns is higher, especially inhigh pressure zones.



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Water losses within consumer’s premises – these might be very little in volume but their bignumbers can lead to a considerable percentage of the total water loss of the system. If ameasurement device (M5) is installed within each household, the amount of water consumed,including losses will be recorded.

Provided that water consumed is paid on the basis of metered volumes, water losses within thesystem, but excluding consumer premises, are unaccounted and correspondingly not recovered

financially. Under this context, it is the responsibility of local authorities to reduce as much aspossible water losses within the system, excluding consumer’s premises, while consumers areresponsible for the reduction of water losses within their premises. The methods given below arerelated to water lost within the reticulation system.

M1

M2

M3

M4A M4B M4N

Figure 1. General scheme of a water supply system M1 – M5 – water flow/volume measurement location

WATER SOURCE

WATER

TREATMENT

SERVICE

RESERVOIR

RETICULATION

SYSTEM

DA DB D N

Dn1 Dn

2 DnnD b

nD b2D b

1Da1 Da

2 Dan

CONSUMERS M5



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2.2 METHODS FOR ESTIMATION OF WATER LOSSES

A major objective of the management of water supply systems is to account of water volumeswithin the system. Thus monitoring of the system’s water balance is very important in order toestimate demand trends, expenditure, as well as, to account for water losses. This can be doneonly on the basis of adequate number and adequate accuracy of water discharge measurementdevices, located at characteristic points in the system. Preferably, these points should be

identified during the design stage, but experience gained during the exploitation period canindicate the need of additional measurement points. Regular data collection, recording andleakage history is essential (Burstall T., 1997).

District meteringSeparately defined areas typically containing 2000 to 5000 properties are metered continuously(measurement points M4), and total quantity of water entering the district is recorded. The metersare read regularly and if supply is inexplicably high, inspectors are sent into that district to locateleaks.

This method has the advantage that inspectors are concentrating their efforts in those districts of high leakage occurrence. It also has the added advantage that information regarding flows anduse of water within the network is obtained which can be useful for the day to day running of thenetwork and for the planning and design of future extensions. However the method is notsensitive to changes in leakage and does not determine the position of leaks.

Waste meteringThe distribution system is sub divided into subdistricts (Da

1,…Dan) containing 200 to 3000

properties. These areas are isolated and fed through a single meter, capable of measuring andrecording the low rate of flow that occur during the early hours of the morning, also known asnight flows . The flows are recorded at regular intervals, so that inspectors are directed to thedistricts with higher flow records, indicating leakage.(Wikely:,1964).The flow meter, which is used, is one that is capable of measuring low rates of flow and isnormally, referred to as a waste meter . The waste meter may be permanently installed on a by-pass or carried on a mobile trailer and connected temporarily into the system via hydrants.There is a type of waste metering technique that is referred to as step testing or valve inspection.This technique involves closing the valves within the district, so that the metered area issuccessively reduced. The resultant reduction in flow rate following the closure of a particular valve indicates the total leakage plus legitimate night consumption in that section of thedistribution system. There are a number of ways in which the step test can be undertaken:• -Isolation method: Starting furthest from the waste meter, valves are successively closed so

that the different subdistricts are isolated sequentially and less and less of the district issupplied via the meter. The sequence of closing valves is followed right up to the last valvewhereupon the flow should drop to zero.

• -Close and open method: This entails closing valves on each step, noting the resultant dropon the flow meter and then re-opening again.

• -Backfeed method : The method consists of closing valves in the same sequence as that withthe isolation method but each time a valve is closed a corresponding valve is opened behindit, starting with the boundary valves, thus allowing water to backfeed from some other part of the distribution system.

The success of both waste metering and step testing depends to a large extent upon the ability toisolate completely the waste meter district from the rest of the system and this depends on valvesshutting down tight. The method is sensitive to small leaks and also establishes the position of that leak between valves within the subdistrict. But time is spent in monitoring districts where noleakage has occurred and hence no benefits would be obtained (Grisham.A., et al.1989).



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Combined district and waste metering:This method consists of both district metering and waste metering. When increases in supply areindicated on the district meter, the waste meters downstream of it are read in order to subdividethe district into more manageable units and therefore guide inspectors to the areas containingmost leaks.

2.3 METHODS FOR LOCATION OF LEAKAGESPassive leakage controlIn this method only those leaks that become self-evident are located and repaired. A leak may beself-evident because water shows on the surface or may become so upon investigation followingconsumer complaints such as poor pressure or noise in the plumbing system. Leakage isidentified by visual inspection or based on consumer complaints. This method is widely appliedand requires regular inspection by the managing authority. No special professional skills areneeded. It is a low cost measure and is effective in areas where pipelines are laid at lower depthand soil conditions are such that leaks quickly come to the surface.

Regular sounding (proactive leak detection )

This method involves teams of inspectors seeking to locate leaks by systematic direct soundingon all stopcocks, hydrants and valves through distribution system and listening for thecharacteristic noise of leaking water. As water under pressure exits a crack or a small hole, the pipe wall and the surrounding soil emitsound waves in the audible range. Water impacting the soil and circulating in a cavity createslower frequency waves that have limited transmission through the ground. Through the use of surface microphones, leaks can be located with greater precision.(Hammer M., M.Hammer jnr.,1996).The leak noise detected will depend upon the position at which a sounding is made.

Pressure controlPressure control does not directly involve leakage detection, but sudden drop in pressure mayindicate to a possible leak. In general, reduction in pressures leads to reduced rate of escapethrough each leak and may also affect the outbreak of leaks. Pressure reduction is relativelycheap and can be quickly effected, but lower pressure may also increase the leak population bymaking them less detectable. Pressure reduction can be achieved in a number of ways such asreducing pumping heads, installing break pressure tanks and using pressure-reducing valves.The control of pressure surges and cycling is likely to reduce the numbers of bursts and leaks thatoccur, especially in plastic pipes.

2.4 DISCUSSION

Comparing the methods described above, it could be mentioned that:1. District metering allows the detection and estimation of water losses within the district, but ishighly dependable on the range of measurement and accuracy of the water meter.2. Waste metering is more accurate and locates water losses within subdistricts. It requires theapplication of waste meters, with higher level of accuracy, in addition to the already installeddistrict meters. These could be installed permanently in identified subdistricts of high leakoccurrence or could be installed temporarily for specified periods in order to identify zones of highleaks. The need to use waste meters in addition to district meters might be overcome if the rangeof measurement of the district meter is chosen so that it can detect the night flows in thesubdistricts. Combination of two meters, one of them installed on a bypass, is also a possiblesolution.3. The close and open step testing procedure has the advantage to be easy for application, lesstime consuming, because can be performed in one single night. The procedure of closing and



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opening of the valves gives the possibility to double-check the results and to improve their reliability.4. Proactive leak detection requires special sounding equipment, specifically trained personneland considerable practical experience in order to be applied successfully. Regular proactive leakdetection could be recommended to areas where leaks could cause severe damage of surrounding structures or along parts of the system of highst importance.5. Pressure control is a necessary tool for the technical management of the system and combined

with any other method of water loss estimation could give very useful information in order toidentify the causes of water lost trough leakage.

3. PRACTICAL EXERCISE – WATER LOSS ESTIMATION AND DETECTION

3.1 THE STUDY AREA

There are a total of nine hundred and forty three (943) properties in the area and approximately9500 people living in it. These properties all receive their water supply from the town council of Chitungwiza and they pay a fee of $7.90 per cubic metre of water consumed. The water consumed is recorded on meters that the town council has installed at every one of theseproperties. The meter readings are taken once every month and the consumer is charged for water consumed for each month.

The outline of the water distribution network is as shown in Figure 2. The distribution network isapproximately twenty years old. The network was originally made up of A.C pressure pipes butover the years, PVC pipes have been used in some parts of the area as replacements for burstpipes. The main line connecting the properties to the 300mm-distribution line is a 200mm-diameter class 12 A.C pressure pipeline. This pipeline is reduced to 150mm and 100mm, 75mmand 50mm diameter, class 12 A.C, pressure pipelines as the network branches into individualproperties. The water mains are placed 1.0m to the high side of the mid-stand or road reserveboundaries in most parts of the area. The area is relatively flat and it slopes gently from thehighest point of 1442m to the lowest point of 1428m above sea level.

3.2 METHOD

The exercise included the application of district metering for estimation of water loss during theperiod 14 February to 3 April 2000. One leaking valve was identified by visual inspection andconsumers reported 3 burst pipe leaks. All leaks were repaired. For the application of thismethod the whole district was isolated during two nights, between 12 and 3 a.m., and allowed tobe fed only by the pipeline passing trough the district meter chamber. Water measurements weredone by a Kent Helix 2000 water meter, 200mm diameter, installed permanently in the chamber,and a Spectralog Data Logger equipment for detection and record of the flow data, which wasused only for the period of the exercise. Also, the close and open step testing procedure for waste metering was applied, on a separateoccasion, which consists of consequent isolation of specified subdistricts, as shown on Figure 2,by closing the corresponding valves, and reopening them. The measurement equipment,described above, was used, and no special waste meter was applied, the objective of theexercise being to assess the applicability of this procedure to the concrete site conditions withoutthe use of a special waste meter, relying on records of the district meter only.



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3.3 RESULTS

District metering

Before leaking valve was located and repaired (14 February 2000)Minimum night flow = 4.3 l/ secTotal night volume = 46.440 m3

After leaking valve was repaired (3 April 2000)Minimum night flow = 2.7 l/secTotal night volume = 29.160 m3

Table 1 Costs of district metering

LABOUR REQUIRED ACTIVITYDescription Hours Rate Amount

Recordminimum night

flow (1night)

- Plumber (1no.)

- Driver (1no.)

3

3

$38.56 \hr

$47.72 \hr

$115.68

$143.16

TOTAL $258.84TOTAL PER ANNUM(for frequency of 6 recordings per year) $1553.04

Cost of repairs for the period of measurement (materials +labor): Z$ 792

Cost of detection: Z$ 258

Total: Z$ 1051

Estimated leakage per day for the district (before repairs): 46.44m

3

Estimated leakage per day for the district (after repairs): 29.16 m3

Water loss per day due to identified leak: 17.28 m3

Water loss during the period 14 February to 30 March: 795 m3

Cost of water lost during the period 14 February to 30 March: Z$6280

Note: The exchange rate at the time of study was 38.5Z$ =1US$



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Key

Distribution water meter chamber

Valve

Water Supply pipeline

Subdistricts Boundaries

Figure 2. Map of the study area



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Waste metering – the step testing procedure

Table 2 Estimates of leakage by step testing (20 April 2000)

Period of detection, h

Area Supplied Subdistrict(Number of stands)

Average flow,l/s

Volumesuppliedto the

area, l

Volume per subdistrict,l

Volumeper stand, l

0.00-0.15 D1+D2+D3+D4 5.2 4680D1 (265) 180 0.68

0.30-0.45 D2+D3+D4 5.0 4500D2 (345) 270 0.78

1.00-1.15 D3+D4 4.7 4230D3 (172) 3420 19.88

1.30-1.45 D4 0.9 810D4 (161) 810 5.03

2.00-2.15 NONE 0 0D4 (161) 270 1.68

2.30-2.45 D4 0.3 270D3 (172) 2250 13.08

3.00-3.15 D4+D3 2.8 2520D2 (345) 1980 5.74

3.30-3.45 D2+D3+D4 5.0 4500D1 (265) 270 0.98

4.00-4.15 D1+D2+D3+D4 5.3 4770

Analysis of data in table 2 indicates that the district meter can be successfully used for wastemetering as it records flow rates changes within the subdistricts. Results obtained indicateconsiderable water loss in subdistrict 3 during both the closing and the opening procedure. The

discrepancies in the water lost values for subdistricts D2 and D4 could be attributed to thedetection level of the meter.

Table 3 Costs of step testing

LABOUR REQUIRED ACTIVITYDescription Hours per

nightRate Amount

Step test (for 1night)

-Plumber (2no.)-driver (1no)

4

4

$38.56 \hr

$47.72 \hr

$308.48

$190.88

TOTAL $499.36TOTAL PER ANNUM(for frequency of 4 recordings per year ) $1997.44



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4. CONCLUSIONS AND RECOMMENDATIONS

Estimation of water loss in high-density population areas

1. The district metering method is recommended as a reliable and cost effective tool for water loss estimation. The period for night flow measurements should be selected between 0.00 and3.00 h, which represents correctly water losses, given the consumption pattern of the suburb.

2. The procedure of water loss estimation should be incorporated in the regular water balance of the system, which should be done at least once per month.3. It is recommended that regular pressure control should complement the water balance

exercise.

Location of leaks in high-density population areas

1. Passive leakage control based on regular visual inspection and consumer reports isrecommended as more cost effective method, compared to sounding. However, costeffectiveness depends heavily on the expedience of leaks repair, after identification.

2. The waste metering (step testing procedure - closing and opening valves) could be appliedsuccessfully to complement district metering, in order to locate the subdistricts with higher leak occurrences. It is recommended that the size of subdistricts should be not less than 300properties, and the number of subdistricts identified per night, should be not more than three. Also, the successful application of the method relays heavily on a precise time coordination of the exercise.

3. It is recommended that the waste metering procedure should be applied once per quarter.

Acknowledgements: This study was funded by WARFSA. The author would like to expressgratitude to the sponsors for the financial support; to the research assistants - Ms A. Danha andMs. J. Magaya - for the invaluable help and assistance throughout the study; and to theauthorities of the Municipality of Chitunwiza - for allowing access and support during theexecution of the exercise.

References

Burstall T., (1997) Bulk Water Pipelines, Tomas Telford, UKGrisham.A.Fleming.W: (1989) Long Term Options for Municipal Water Conservation, AWWA, Vol

81,no.3Hammer M. and Hammer M.jnr (1996) Water and Wastewater Technology ,3rd edition,Prentice

Hall INC, USANdamba. J., Sakupwanya J.S., Makadho J., and Manamike P., (1999) Water demand

management practices in southern africa :the Zimbabwe experience, IWSD,IUCN and SIDAreport, Harare, Zimbabwe

Standing Committees Report No 26 (1980) Leakage control and practice, Department of Environment and the National Water Council, USA

Wiessman W jr. and Hammer M. (1998) Water Supply and Pollution Control , Addison WesleyLongman Inc.,USA

Wikeley .J..B., (1964) Municipal Engineering Law and Administration, C&R Books Ltd., UK

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