CHAPTER 8WATER-QUALITY ASPECTS

OF CONSTRUCTION AND

OPERATION

Thomas M. WalskiPennsylvania American Water Co.

Wilkes-Barre, PA

8.1 INTRODUCTION

During construction and operation, numerous factors can have an impact on water qualityin the distribution system, including handling and disinfection of new mains; preventionand elimination of cross-connections; elimination of leaks and breaks; disinfection ofstorage tanks after construction; inspection, or maintenance; installation and operationof blowoffs; air releases, and flushing hydrants; implementation of a flushing program;proper break repair practices; covering and properly venting storage tanks; maintenanceof adequate separation from sewers; enforcement of applicable building plumbing codes;and, of course, maintenance of positive pressure at all times. These considerations aredescribed in the appropriate state water systems standards and in numerous references(AWWA, 1986a; Departments of Ak Force, Army and Navy, 1984; Great Lakes andUpper Mississippi River Board of State Public Health and Environmental Managers,1992). AWWA also produces training videos on some of these subjects.

Three of the more important aspects of distribution system water quality, new maindisinfection, tank disinfection, cross-connection control, and flushing are covered in moredetail in the following sections.

8.2 DISINFECTIONOFNEWWATERMAINS

Disinfection of water mains is addressed in AWWA Standard C651-92 (1992), althoughsome utilities may have their own variations on the AWWA Standard.

8.2.1 Need for Disinfection

Ideally, water mains would be delivered to construction sites in sterile condition and bekept that way during pipe installation. Unfortunately, there is no reasonable way to keeppipes sterile during shipping, storage, and installation. Pipes may be left outdoors formonths to years before they are installed, with contamination potentially caused by avariety of animal, plant, and microbial life entering the pipe. Pipes may become floodedbefore and during construction with water of varying quality. Jointing, packing, andsealing material may become contaminated. To the extent possible, pipes and jointingmaterials should be kept as clean as possible before placing the pipe in service, but onlydisinfection of the pipe once it is installed can reasonably ensure clean pipes.

8.2.2 Disinfection Chemicals

Numerous disinfection chemicals are available. The following three are used mostcommonly:

1. Liquid chlorine (Cl2), which is usually available in 100-lb (45.4-kg) or 150-lb (68-kg)pressurized containers. Liquid chlorine is inexpensive but highly toxic and should beused only by appropriately trained individuals with the proper chlorinators andejectors.

2. Sodium hypochlorite (NaOCl), which is a liquid stored in glass, rubber-lined, or plasticcontainers of varying sizes. It is more expensive and bulky than liquid chlorine, but ismuch safer to handle. It is usually 5-15 percent chlorine, but it has a finite shelf life.

3. Calcium hypochlorite (Ca(OCl)2), which is approximately 65 percent chlorine byweight. It is easy to handle in either tablets or granular form, but is relatively expensiveand must be kept dry to prevent degradation. An exothermic reaction yielding oxygenand chlorine can occur if the (Ca(OCl)2) is heated to 35O0F.

8.2.3 Disinfection Procedures

Disinfection involves contacting the pipe with a sufficiently high dosage of chlorine for asufficiently long period of time. The three methods presented in C651-92 are summarizedbelow.

8.2.3.1 The tablet method. This method can be used if mains have been kept clean anddry during construction and involves placing hypochlorite granules or tablets in the pipesduring installation at intervals no greater than every 500 ft (150 m). The number of 5-gtablets per length of pipe can be estimated from

N =0.0012 L D2

where W = number of 5-g tablets, L = length of pipe (ft), and D = diameter of pipe (in).Using this method, the average concentration of chlorine during the test should beapproximately 25 mg/L. The main must be filled with potable water at a velocity slowerthan 1 ft/s (0.3 m/s), making sure to eliminate all air pockets. If the temperature is higherthan 50C, the water must be kept in the pipe for at least 24 h; if the temperature is lowerthan 50C, it must be kept in the pipe for 48 h. The tablet method is acceptable for smallermains, mains without solvent welded, or threaded steel joints.

8.2.3.2 The continuous feed method. This method consists of, first, flushing the main,at a velocity of at least 0.76 m/s (2.5 ft/s) to remove any sediment and air pockets. Forlarger mains, where flushing may not be effective, brooming or swabbing can be used.Chlorine must then be fed at a rate that maintains a concentration of 25 mg/L for 24 h. Atthe end of that time, the free chlorine residual must be greater than 10 mg/L at all pointsin the pipe.

8.2.3.3 The slug method. This method consists of placing hypochlorite granules, as inthe tablet method, and flushing the main, as in the continuous feed method. Then a slugof highly chlorinated water with a concentration of at least 100 mg/L is passed throughthe main so that the slug is in contact with the pipe for at least 3 h, and the concentrationof the slug does not drop below 50 mg/L. Pigs or swabs can be used to separate the slugfrom the potable water in the pipe. The slug method is used most commonly for largermains, where the volume of water required for the continuous feed method is impractical.

Highly chlorinated water must be disposed of in an environmentally safe manner incompliance with all applicable water-quality regulations. This may necessitate the use ofa reducing chemical (e.g., sodium sulfite) at the downstream end of the pipe beingdisinfected to react with the excess chlorine in the water.

8.2.4 Testing New Mains

After disinfection and flushing, the new main is filled with potable water and is sampledfor coliform bacteria at least every 1200 ft (366 m) in accordance with Standard Methods(APHA, 1995). Other tests, such as heterotrophic plate count, may be required. If a samplefails the test, the main should be flushed and the sampling repeated. If flushing does notresult in an acceptable test, the main may need to be disinfected again.

8.2.5 Main Repairs

After water main breaks, the mains should be flushed to remove any water in the vicinityof the break and also may need to be disinfected if pressure was lost in the mains.However, because mains are needed to maintain fire protection and sanitation, they areusually placed back in service before the results of testing are available. If there is concernthat contamination has occurred during repair, a precautionary "boil water" notice shouldbe posted.

8.2.6 Disposal of Highly Chlorinated Water

Highly chlorinated water can be toxic to living things. Before such water is discharged tothe environment, an assessment should be made to determine if the water can bedischarged without causing any harm. Ideally, the water should be discharged to a nearbysanitary or combined sewer-collection system, provided the system has the capacity tohandle the discharge and the water utility has received permission for the discharge fromthe sewer utility. Depending on the circumstances, direct discharge into a water body orstorm sewer system may be a violation of the Clean Water Act or may require a permit.

If the water cannot be discharged without treatment, then the chlorine must beneutralized before the water is discharged. This is accomplished by adding a reducingagent that reacts with the chlorine. Table 8.1 gives the amount of several disinfection

TABLE 8.1 Dose of Reducing Agent Needed to Neutralize 1 MG ofWater with 1 mg/L Chlorine

Chemical Formula Dose (Ib/MG) Dose (kg/MG)

Sulfur dioxide SO2 8 3.6

Sodium bisulfite NaHSO3 12 5.4

Sodium sulfite Na2SO3 14 6.4

Sodium thiosulfate Na2S2O3SH2O 12 5.4

chemicals required to neutralized 1 MG (3.78 ML) of water with a residual chlorineconcentration of 1 mg/L based on 100 % purity and sufficient mixing and detention timeto complete the reaction. To determine the quantity needed for any volume of water, adjustthe quantity from Table 8.1 using

Dose = (V) (O (100/P)

where V = volume of water (Mg), C = chlorine concentration (mg/L), P = purity ofreducing agent (percent).

8.3 DISINFECTIONOFSTORAGETANKS

Disinfection of tanks is covered in AWWA Standard C652 (1992b). The procedures aresummarized below. In the case of water mains, once a main is disinfected, it may neverneed to be disinfected again. Tanks, however, are occasionally taken out of service forinspection, cleaning, repairs, and painting.

The chemicals used for disinfection of tanks are the same as those used for disinfectionof pipelines and are described in Sec. 8.2.2. All tools and equipment are removed from thetank, and the tank is washed, swept, or scrubbed to remove any debris or dirt.

8.3.1 Disinfection Procedures for Filling Tanks

AWWA Standard C652 recognizes three methods for disinfecting storage tanks when theyhave been taken out of service and drained. It also describes procedures for disinfectionduring underwater inspections.

8.3.1.1 Method 1. Liquid chlorine or sodium hypochlorite is added to the influentpipe while filling, or calcium hypochlorite is placed on the bottom of the tank beforefilling so that the chlorine concentration shall be at least 10 mg/L after the retentionperiod. A 6 h retention period is used when chlorine has been fed uniformly with theinfluent water, whereas a 24-h period is used when the chemicals are mixed into the tank.

8.3.1.2 Method 2. The tank is sprayed or brushed entirely with a solution of 200 mg/Lof available chlorine. Drain pipes are filled with a solution of 10 mg/L chlorine, as inmethod 1. The highly chlorinated solution is left on the surface for at least 30 min. Onlysurfaces that will be in contact with potable water need to be sprayed or brushed. Uponfilling and bacteriological testing, this method may produce water that can be delivered tothe distribution system rather than be discharged.

8.3.1.3 Method 3. Chlorine is added using the procedures in method 1, except that thetarget concentration when the tank is approximately 5 percent full is 50 mg/L. This wateris held for 6 h, then the tank is slowly brought to full level with potable water and thiswater is kept in the tank for an additional 24 h. Following bacteriological testing, this tankmay be placed in service, provided any drain lines have been purged of highly chlorinatedwater and the chlorine residual is at least 2 mg/L.

8.3.2 Underwater Inspection

Underwater inspection by divers or robotic cameras is becoming increasingly popular. Allequipment, clothing, and personnel entering the tank must be cleaned thoroughly, and itmust be certified that the equipment and clothing has been used only for potable waterinspections. Equipment can be cleaned by spraying, submersion, or sponging with achlorine solution of at least 200 mg/L.

If divers are used, two certified divers should be on site, and an additional diver shouldbe available outside the tank in case of an emergency. Before anyone or any equipment isallowed to enter the tank, the residual chlorine should be checked to determine that it isadequate. Air should be supplied to the divers from external air-supplying equipment.

When feasible, tanks should be valved offline during the inspection and should not bebrought back online until a satisfactory bacteriological test result has been received. Tanksshould be full during inspections.

BA CROSS-CONNECTIONCONTROL

8.4.1 Definitions

A cross-connection is any connection or potential connection between a potable watersystem and any source of contamination that can affect the quality of water in such a waythat the contamination could enter the potable system under certain circumstances.Backflow is the actual reversal of flow in such a way that contamination enters a watersystem through a cross-connection. Backflow will occur when the head on the potableside of the cross-connection drops below that on the contaminated side. It can occurthrough backsiphonage, a drop in the pressure on the potable side, or backpressure, anincrease in the pressure on the potentially contaminated side.

Backflow is undesirable because it can result in the introduction of contamination intothe potable system. Examples might include water from a neighboring utility, antifreezein fire-protection piping, toxic chemicals at factories, or etiologic agents from a hospital.Numerous cases of contamination of water systems from cross-connections have beendocumented (Angele, 1974; AWWA, 1990; Foundation for Cross-Connection, Controland Hydraulic Research; 1988; United States Environmental Protection Agency(USEPA), 1989).

8.4.2 Cross-Connection Control Programs

Cross-connection control consists of the implementation and enforcement of appropriateordinances and regulations to eliminate backflows. States and provinces require thatutilities (usually called water purveyors in cross-connection regulations) must implement

a cross-connection control program. These programs usually require the installationof assemblies that prevent backflows consistent with the level of hazard associated withthe user, at the user's expense. They also authorize appropriate officials to inspect theuser's plumbing at any reasonable time and to terminate service if a hazard has not beencorrected.

8.4.3 Backflow Prevention

Backflow preventers are the assemblies or other means used to "prohibit" backflow.Several types of backflow preventers are used, depending on the situation.

8.4.3.1 Air Gap. An air gap is a physical separation between the potable water systemand the source of contamination. The air gap between the outlet of the potable system andthe maximum level of any source of contamination must be at least twice as large as thediameter of the potable water outlet and never bigger than lin (25 mm). An air gap isconsidered to be the safest and simplest means of backflow prevention. However, an airgap results in a loss of any head and therefore is not used when the downstream pipingmust be pressurized from the water source.

8.4.3.2 Reduced-pressure backflow preventers and double-check valve assemblies.These are two devices used to prevent both backflow and backsiphonage. Both consist oftwo independently acting, tightly closing, resilient seated check valves in series with testports. The check valves are usually spring loaded (Figs. 8.1 and 8.2). The differencebetween the two is that the reduced pressure assembly also contains an independentlyacting pressure-relief valve between the two check valves and lower than the first checkvalve (Fig. 8.3). The reduced-pressure backflow preventer can be used in highly hazardoussituations.

8.4.3.3 Atmospheric and pressure vacuum breakers, and barometric loops. Thesedevices that can prevent backsiphonage, but not backpressure. In the vacuum breakers,water pressure keeps the valve open, as shown in Fig. 8.4. When water pressure stops(i.e., backsiphonage can occur), the internal float seats and siphonage cannot occur.

FIGURE 8.2 Double-checkvalve. (From USEPA, 1989.)

FIGURE 8.1 Reduced-pressurebackflow preventer. (FromUSEPA, 1989.)

A pressure vacuum breaker can be tested in line. A barometric loop is a continuous pieceof pipe that rises at least 35 ft (10.5 m), then returns back to the original level. Watercannot be siphoned over this loop.

8.4.3.4 Single and dual check valves. These types of valves are not recognized asbackflow prevention assemblies because they can allow a small amount of leakage andcannot be tested in place. However, they can provide an inexpensive level of protectionwhen the hazard is minimal and an approved backflow preventer is not required.

8.4.4 Application of Backflow Preventers

Although backflow preventers help reduce the risk of contamination, they introducesignificant headloss in the piping and additional cost, of installation and required annualtesting. These factors are especially troublesome when retrofitting existing services,particularly in cold climates, because the assembly cannot be located outdoors. This istroubling in situations where installation of backflow prevention can compromise fire

FIGURE 8.4 Vacuum breakervalve. (From USEPA, 1989.)

FIGURE 8.3 Reduced-pressure backflow assemblyapplication. (Photographby T. M. Walski)

% inch thru 2 inches

Gate valve

Test cock

1st check valve.

Test cock

Spring

Gate valve

protection. Duranceau et al. (1998) showed that water quality can seriously deteriorate infire sprinkler piping.

Hart et al. (1996) performed a risk analysis and concluded that "the risk of death andinjury associated with burning dwellings that lack a sprinkler system is higher than therisk of illness associated with unprotected sprinkler systems" and recommended the useof single check valves in residential sprinklers. Wood (1993) highlighted the need for ahydraulic analysis before retrofitting prevention of backflow in an existing fire protectionsystem. Residential sprinklers also can be interconnected with plumbing fixtures toprevent stagnant water. In such a system, whenever someone flushes a toilet or uses ashower, clean water is drawn into the combined sprinkler-domestic system to preventdeterioration of water quality.

8.5 FLUSHINGOFDISTRIBUTIONSYSTEMS __

8.5.1 Background

Distribution system flushing consists of opening up appropriate hydrants or blowoffs toimprove water quality and can be done to remove sediment, eliminate low chlorine, solvetaste, odor, and turbidity problems, and to remove biofilms from water mains. Flushingcan be accomplished as a systematic program or in response to customer complaints(Chaterton et al., 1992; Pattison, 1980). Rushing is usually done by opening fire hydrants.

Rushing has the obvious benefit of bringing water a high concentration of withdesinfectants and generally "fresh" water into a portion of the distribution system. Yohe andGittieman (1986) and Wajon et al. (1988) reported a reduction of taste and odor problems asa result of flushing. Wakeman et al. (1980) and Larson et al. (1983) reported reduced levelsof tetrachloroethylene (PCE) after flushing. Shindala and Chisolm (1970) and Lakshman(1981) listed numerous benefits of flushing.

8.5.2 Flushing Procedures

AWWA (1986b), Chadderton et al. (1992, 1993) and California-Nevada Section AWWA(1981) presented a summary of the mechanics of a flushing program. Good planning andpublic notification, if required, represent the start of successful flushing. In particular,hospitals and laundries should be warned about impending flushing. Crews should haveproper safety equipment, including lights and reflective gear if work is done at night.Hydrants should be flushed from the clean water source outward. A rule of thumb used bycrews is "always flush with clean water behind you." In general, a large main should notbe flushed from a smaller main, and valves and hydrants should be opened and closedslowly to prevent waterhammer. Care must be taken not to reduce pressure below 130 kPa(20 psi) at nearby customers and high local elevations. Row should be directed in a waythat minimizes damage and disruption of traffic. Diffuser outlets (Fig. 8.5) should be usedwhere needed. Rushing should not be conducted when the temperature is likely to dropbelow freezing before the water can run off roads and sidewalks. Rushing should beconducted at times when water is plentiful and reservoirs are full rather than duringdrought periods. Rushing is needed most when water consumption is relatively low.

Hydrants should be opened to generate a velocity suitable for scouring solids frompipes. The velocity should be at least 2 ft/s (0.61 m/s) to suspend sediment and should beno more than 10 ft/s (3.1 m/s) to minimize the potential of waterhammer in startup andshutdown. Walski and Draus (1996) presented data showing that most of the scouring is

FIGURE 8.5 Hydrant diffuser. (Photograph by T. M. Walski)

accomplished within the first seconds after maximum velocity is reached and that velocitycan then be decreased. Flushing should continue until the water clears up or disinfectantresiduals increase.

Flushing also provides maintenance personnel the opportunity to measure staticpressure and discharge from the hydrant and to assess the general condition of thehydrants. The amount of water used during flushing should be recorded so that an overallestimate of this nonmetered use can be made. Crews should maintain a record of flushing.

Walski (1991) and Walski and Draus (1996) discussed the importance of modeling theeffects of flushing and showed that models can be used to estimate the time it would takefor chlorine residual to increase during flushing.

8.5.3 Directional Flushing

Flushing can be enhanced by closing valves to maximize velocity in pipes being flushed.Oberoi (1994) showed that this type of directional flushing is desirable despite the additionallabor because it uses less water, creates higher velocity, and provides the opportunity to testdistribution valves. By restricting the direction in which water can flow to the open hydrant,it is possible to maximize the velocity in the mains and control the individual main beingflushed. Figure 8.6 shows how the velocity to a hydrant can be increased by valving off thedirection the water can take in reaching the flowed hydrant.

8.5.4 Alternating of Disinfectants

Some utilities find that switching disinfectants (e.g., from chloramines to free chlorine)during flushing results in more effective cleaning of mains. Such a change in disinfectants,coupled with the high velocities of flow during flushing, can be especially effective inattacking biofilms that have become adapted to a given disinfectant. Public notification isespecially important when switching disinfectants because this can have an impact onsuch items as tropical fish and dialysis machines.

(B) Directional flushing.

FIGURE 8.6 Effect of valving on directional flushing.

REFERENCES

Angele, G.J., Cross Connection and Backflow Prevention, American Water Works Association,(AWWA), Denver, CO, 1974.

APHA, 1995, Standard Methods for Examination of Water and Wastewater, American Water WorksAssociation, (AWWA), Denver, CO.

AWWA, 1986a, Principles and Practices of Water Supply Operation, Vol. 3, Water Distribution,Denver, CO.

AWWA, 1986b, Maintenance of Water Distribution System Water Quality, American Water WorksAssociation, Denver, CO.

AWWA, Disinfecting Water Mains, AWWA C651-92, Denver, CO, 1992a.AWWA, Disinfection of Water-Storage Facilities, AWWA C652-92, Denver, CO. 1992b.

(A) Standard flushing.

AWWA, 1990, Recommended Practice for Back/low Prevention and Cross-Connection Control,AWWA Manual M-14, Denver, CO.

California-Nevada AWWA, Distribution Main Flushing and Cleaning, California-Nevada SectionAWWA, 1981.

Chadderton, R. A., G. L. Christensen, and P. Henry-Unrath, Implementation and Optimization ofDistribution Flushing Programs, AWWA RF, Denver, CO, 1992.

Chadderton, R. A., G. L. Christensen, and P. Henry-Unrath, Planning a Distribution System FlushingProgram, Journal of the American Water Works Association, 85:(7)89., 1993.

Departments of Air Force, Army and Navy, 1984, Maintenance and Operation of Water Supply,Treatment and Distribution Systems, TM 5-660, Washington, DC, 1994.

Duranceau, S. J., J. V. Foster and J. Poole, Impact of Wet-Pipe Fire Sprinkler Systems on DrinkingWater Quality, AWWA RF, Denver, CO, 1998.

Foundation for Cross-Connection Control and Hydraulic Research, Manual of Cross ConnectionControl, University of Southern California, Los Angeles, 1988.

Great Lakes and Upper Mississippi River Board of State Public Health and Environmental Managers,"Recommended standards for water works" Albany, NY. 1992.

Hart, F. L., C. Nardini, R. Till and D. Bisson, Backflow Protection and Residential Fire Sprinklers,Journal of the American Water Works Association, 88:(10)60, 1996.

Lakshman, B. T, 1981, "Water Main Rushing," Water Engineering and Management, 128:(1)28,1981.

Larson, C. D., O. T. Love, and G. Reynolds, "Terachloroethylene Leached from Lined Asbestos-Cement Pipe Into Drinking Water," Journal of the American Water Works Association, 75: (4) 184,1983.

Oberoi, K., "Distribution Rushing Programs: The Benefits and Results," AWTM4 Annual Conference,New York, 1994.

Patison, P. L., "Conducting a Regular Main Rushing Program," Journal of the American Water WorksAssociation, 72:(2)88, 1980.

Shindala, A., and C. H. Chisolm, "Water Quality Changes in Distribution Systems," Water and WastesEngineering, 62:(1)35, 1970.

USEPA, Cross Connection Control Manual, EPA 570/9-89-007., 1989.Wajon, J. E., B. B. Kavanach, R. I. Kagi, R. S. Rosich and R. Alexander, "Controlling Swampy Odors

in Drinking Water," Journal of American Water Works Association 80:(6)77, 1988.Wakeman, S. G., et al., "Tetrachloroethylene Contamination of Drinking Water by Vinyl-Coated

Asbestos Cement Pipe," Bulletin of Environmental Contamination and Toxicology, 25:(4)639,1980.

Walski, TM., "Understanding Solids Transport in Water Distribution Systems," In Water QualityModeling in Distribution Systems, AWWA RF, 1991.

Walski, T. M. and S. J. Draus, "Predicting Water Quality Changes during Flushing," AWWA AnnualConvention, Toronto, Ont., 1996.

Wood, T. R., A Study of Backflow Prevention and Fire Sprinkler Systems, National Fire Academy.,1993.

Yohe, T. L. and T. S. Gittleman, "Tastes and Odors in Distribution Systems," in Water QualityConcerns in Distribution Systems, AWWA, Denver, CO, 1986.