1
Edited b y NORMAN V. STEERE,
1
~ i n n e a ~ o lMinn., ~ s , 55455
Supplies
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P a u l H. Woodruff, Roy F. Weston, Inc., Environment01 Science a n d Engineering Consvltants, Newtown Square, Pennsylvania 19073
Water Supply System A review of the essential elements in a typical wat,er snpply system is helpful in appreciating the opportunit,ies for contamins1,ion t,o occur. A water supply gystem is made up of
several components. First, s. source of water which may he a ground water snpply or surface water srrpply. The next link is u s d l y conduits, whirh bring the raw water to a pumping station. The raw water is ~ l s r d l ypassed bhrough treatment processes before use. The water treatment may be very simple or quite complicated depending on t,he end use of the water and the condition of the raw water. Following treatment, high pressure pumps will move the water into the distribution syslem and perhaps treated water storage. As can be imagined, contamination of the water supply can occur at any point in the system. The effect of contamination is usnally more serious if it, occurs after t,he water is treated. At t,he point water enters the distribution system, nothing lies between ihe consumer and his use of contaminated water. What are the catrses of eontaminstion'? The apparent causes are as many as t h e sources. Basically, however, contamination rexdts from two conditions: A cross-connection between a patahle system and a non-potahle system, and Pressure conditions such that tho nonpotable material is forced into the potable water supply. Cross Connections Figwe 1 illustrates schematically the simplest and most obvions t,ype of crossconnection, a potable supply separated from a non-potable supply only by a valve. This is t,he type of cross-cnnnect i m that is easiest to locate. Table 1 lists some typical types of plumbing fixtures where contaminatinn hy cross-connections of the direct type is possible. It should he noted that familiar
NON-POTABL
Ld
Figure 1. Valved connection water ond nor~-~otable fluid.
Ld
teafure
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Labwatmy Equipment, Aspirator, Laboratory Aspirator, hledicnl Attt,orlave and Sterilizer Air Line Pump, Water Operated Ejertor Cooling System Dishwasher Fire Standpipe or Sprinkler System Air Conditioning, Air Washer Air Conditioning, Chilled Water Air Conditioning, Condenser \Vi~ter Aspirator, Wecdicide and Fertilizer Sprayer Anxilinry System, Indnstrial Auxiliary System, Stirface Water Anxiliary System, Unapproved Well Sopply Buller System Chemical Feeder, Pot-Type Chlorinator Callee Urn Foi~ntnin,Ornamental Hydraolio Equipment Luhricatiun, Primp Bearings Photostat Equipment Pwnp, Pneurnntia Ejector Pumo. Prime Line Sewer, Storm Swimming Pool equipment such as air conditioning apparatus, st,erilisers, etc., could rontribnte contamination by cros7-connection to the potahle supply. The second type of cross-connection exists in s. mnch more insidious form. Table 2 lists t);pical types of plumbing fixtures that indirectly connect potxhle water with a non-potable system. These h t u r e s have in common one essential Table 2. Partial List of Plumbing Hozordr (Fixtures with Submerged Inlets) Laboratory Sink, Serrated Nozzle Photo Laboratory Sink Drinking Fountam l e e Maker Bathtub Bedpan Washer, Flushing Rim Brine Tank Cooling Tower Floor Urain, Floshing Rim Garbage Can Washer Laundry Marhine Lavoratory Lawn Sprinkler System Sewer Flushing llanhale Slop Sink, Flushing Rim Slop Sink, Threaded Supply Steam Table Urinal, Siphon J e t Blowout Water Closet, Flush Tank, Ball Cnck Water Closet, Flush Thlve, Siphon J e t
M POTABLE
Presented a t the llt,h National Conference on Campus Safety a t Rutgers University, New Bronswick, h'ew Jersey, on July 14, 1964. I
1
Portiol List o f Plumbing Table I. Hazards (Fixtures with Direct Connection)
XVI. Prevention of Contamination of Drinking Water
Safety is something we are all, in general, eonee~nedaho>,t, hut t h e safety of otlr tap w a k is taken for granted. This article is meant to advise of the ever present danger of cross-connections in our p o t a h l ~water systems and the resulting health hazard. Although it is l r w that our water supplies w e safe lor use some 99.99% of the time, still the freqnency of "accidental eontamination" of nur drinking water is too p e ; ~ t . Tile literature is replete with report,ed inslnnces of proved contamination of water s~cpplies causing sickness and death. T o illustrate the point, several rases can be qtloied. I n 1942, in a Pittshurgh casting plant employing some 500 people, a new water connoctiun war heirrg made from t h e building's pltmhing to the city water supply. T h e hidlding's water was shut off for a short period tu make the connection. Employees nmtnwhile continued to use water in the building on the 10wer floors. I n this way x pnrbisl vncnnm was drawn in the huilding's plumbing, and toilets were siphoned back into the drinking water. Several hundred employees came down with intestinal disturbances vithin a few hours. As a res,dt, many manhaws of work wore lusl. Fartmately, no one became seriously ill. Anot.her inst,ance orcrrred in 1947 a t a srhotrl in Rlilford, Nebraska. There had been a small fire in the school and the school's fire system used a sewage polluted river as a water alpply for fire light,iug. Inadvertently, someone had left a valve open which connected the lire water snpply wit,h the drinking water system in the school. Contaminated river water was pumped int,o the school's phmbing and 50 students came dawn with intestinal dismiers. These situations can, should, and must he avoided.
I
between potable
(Conlhued on page AS96)
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A395
Figure 2.
Backsiphonage
factor and that is the ability of the inlet fixture to he flooded with non-potable liquid that in many cases will eventllally he drained away to waste. Pressure which allows the non-potable liquid to reach 8. potable water supply can be caused by many conditions. However, these pressure conditions can be broken into two general classes, backsiphonage and backflow. In backsiphonage, the driving farce is atmospheric pressure, meaning the potable water will be a t some pressure less than atmospheric (under a partial vacuum). Backflow ocours when a non-potable system is under greater pressure than the potable system.
Backsiphonage Following are several exmnples of contamination occurring by cross-connections and bbaoksiphonege. Figure 2 shows a sterilizer which is connected t,o the city water system. The sterilizer is allowed t o cool after use without opening the air e a. result a vacuum is created in vent; r the sterilizer and suction is created on the inlet to the sterilizer. In the next room 8. hospital attendant is washing out bed pam. A hose is connected to thesink with the discharge end left under wrtter in the basin. Suction from the sterilizer pulls contents of the sink over into the sterilizer. Meanwhile, people on the lower floors flush toilets, creating a demand on the plumbing system greater than the undersieed main will supply and thus contaminated contents of the sterilizer are drawn hackinto thepl~mbin~systern in the h d d ing. A correction for this situation would be vacuum breakers on both the sink and t,he sterilieer. Figure 3 illustrates another set of eircumstances where the villain is a rubber (Continued on page AS981
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hose in n sink. In this rase, the came of reversed flow is differenl. Two neighbnr-
ing multi-storied buildings are served hp the same water main which often lacks adeql~atepressure. A booal.erptrmp in the adjoining hirilding supplies additional ~i-eswre when needed. However, the
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Main
Figure 4.
BOOSTER
1 Figure 3.
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Journal o f Chemical Educafion
Backsiphonoge
pump a t times will pull more water than the main can supply, thereby rreatillg n partial vacuum on the surtiun side of the pump. Then the circnmstsnce are right for backsiphonage. Correction for this condition would be s. vnrntim breaker on the sink and a vac\l\lrn citl-oil switch on t,he homter pnmp. Figure 4 illustrstes another common condition which can cause a contaminated
(Continued on pnge 4 4 0 0 )
Safety
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water supply. I n this case, a dishwasher is installed with solid connections t o bobh the waste line and t,he hot and cold water supply. The fimt two floors of this building are supplied by city pres-
Figure 5.
Becksiphonage
sure hnt because of relatively low prcssure in the city main, the upper floors are supplied by s. boaster pump drawing water from t,he main. I h r i n y periods of exceptionally low pressure in the city main, the booster p,lmp creates a partial vseunm on the suet,ion side of the pump. This, in t,nrn, will siphon contents of t h e dish water and waste line down to the pump snet,ion. To correct this situation, the dishwasher piping should be modified to instre both supply and discharge Lhroilgh
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an air gap. I n addition, a low pressure switch should be installed on the booster pump. A fairly common sitnation occurring in indrastr,y is shown in Figure 5, where s submerged inlet is used for a processing tank. The submerged inlet is probably intended to provide mixing. A fire e l s e where in the building activates the fire
sprinkler system. The fire pimp pulls diredly from the city main which is n o t sufliriently large to supply wat,er for fire fighting and water t o feed the process tank. A vamwn is created on the snclion side of the fire anmp and the contents of the processing tank will be siphoned hack into the bnilding's plumbing. An air gap is needed in the feed line to the process tank. (Continued on page 2140s)
Safety
Figure 7.
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Backflow
Backflow Circwmtances in which contamination oerurs becmse of hackflow are fairly
common in oreurrence. Figure G shows a boiler that is cross-connected to the eit,v stq~plyfor purposes of filling the s,yst,em and for boiler wnter make-up. The bailer water is chemieall,v l.renl.ed for corrosion and scale cont,rol. I h e to some onforeseen circtmst,mces, the city pressme is redneed below the boiler water pressure. This muld happen if t,here were a large fire in n neiehborhood. The valve a q arating the lwilw water from the rily wnter leaks, or has perhaps been left partially open. The chemically treated waier will br pltmped out intr, the city mains. In this mse, :to air gap or r ~ d u c e d pressure p k r i p a l I~aekHow pvevent,er slm\~ldhe wed l,o pwvent mntnminntion of thr city walw. A priv:&lcwell is shown in Figitre 7 being wed 11, irrip,te a lawn. A cesspool lacatod ndjarpnt t o t,he well is polluting the well. A mws-ronnertion has beem made flam I he vity main t,o the well, prabably for ~ I I T P O S P S of priming the pump. If and wbm the valve sepamling t,he two systems is lefl open, or leaks, palluted water from the well will be pumped into t,he In~ilrling'owat,er s ~ p p l ys,ystem. The connertion betwren the well nnd the city main should be hrokon t o prevent contamination. Fire sysl.ems are often improperly installed. Figure 8 shows n sprinkler water syslcm for lire pmteebion snpplied by a high presswe pump from a sownge polluted river. For "safel.y," the fire system has .2k0 been connected lo the cit,y supply. Again, we have only a single valve sepxaling the two syst,ems. If t,his valve
Figure
8.
Bocktlow
shordd he left open, or leak, the river water would be pnmped i n h the potable water supply. This installation should be pratected by a n sir gap, or red~lcedpressure principal hack How preventer.
Protective Measures Safl.ey practices which are up-to-date with regard to protection of potnhle wsler supplies should include: An adequate plumbing code. (Check with ,your L x d or State Health Departmenl.) Insistence on all work heing done by licensed phlmbers. Periodic reminders to all personnel of the h a ~ s r d s of improper operation 01 eq)~iprnentwhich is at,tached t o the potable wnter supply. ( I t should be stressed that vneunm breakers m,t be removed from fawets by laboratory personnel.) Periodic inspectiuns of plumbing and phrnbing fiturea to insure all eqllipmrnt is properly installed and maintained. However, s t the heart of prevenl.ing conlaminat,ion of wat,er snpplies are two fundnrnentd rrnles: 1. j\'evcr, if it can possibly be avoided, directly connect a non-potable system to n pot,xhle water system. (Contina~don page A404)
Safety
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2. If s. direct croas-connection is aba,lutely necessary (rare, if ever) then the use of properly installed backflow preventers and vacuum breakers are neeessary. Several common types of devices in use to prevent backsiphonage and bacmow are illustrated in Figures R, 10, and 11. Figure 9 shows a met,hod that is not recommended, bat is commonly used. The use of a swing joint is too prone to human failure. The vacuum breakers shown in Figures 10 and 11 are only useful fur preventing eontl~mination caused by backsiphonsje and, uf course, must he
maintained and checked periodically if they are to serve their purpose.
A better methadof eliminating the possibility of contarninat,ion by cross-connection is to install an air gap. Figure 12 illustrates the techniqne for sopplying water
\-..... >....
AUXILIARY SUPPLY
Figure 9.
.''.. '.\..
Swing connection.
NORMAL
FLOW
........ ........ ........ ........ ........ ........ ........ ........ ........ ........ ........
........ ........ ........ ........ ........ ........
VACUUM CONDITION Figure
10. Operation of a vocuum
breoker.
DlSC IN NORMAL FLOW POSITION
DlSC IN VACUUM BREAKING POSITION Figure 1 1 . Pressure-type vacuum breoker (not applicable where bock-pressure may OCCU~.
for a non-~otitblepurpose through a surge tank and boaster pump. Figure 13 illustratetes a dual water supply system for fire protection, again utilizing an air gap. With an air gap, of course, neither
(Continued on page A406)
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backflow or backsiphonage is possible. Figure 14 is a schematic illr~stration of the proper sir gap on a lavatory installation. Figures 15 and 16 show a recommended method of discharging into an indirect waste s,ystem through an air gap, again inwring that backflow is impossible even if the sewer should be nnder pressore. In conclmion, do not take t h e safety
NON -POTABLE
Figure 12.
SUPPLY
Surge tank and booster pump,
POTABLE
SUPPLY
FLOAT VALUES
NON- POTABLE STORAGE FOR Figure 13.
+-
L
FIRE SYSTEM
Fire system make-up lank for a dual water system.
(Continued o n page A408)
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of potable water for granted. Do not be double-crossed by a cross-connection!
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Figure 15. pressure.
PUMP
Air gap to ,ewer mbjsct to bock.
+I-
INDIRECT WASTE
yJE!L CHECK
Figure 14.
Air gop on lavatory.
Editor's Note If reduced pressure principal lmckflow preventom are used n, striel maintenance schedule must be set up. These unit,s must be tested each year by a certified plumbing inspection and dismantled for repair each five years.
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The nulhar wishes to express his ap-
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GRAVITY
Figure 16. pressure.
WASTE L I N E
Air gop to sewer wbject to back-
preeiation to the United States Publio Health Servire for permission t o use illostrations from U.S.P.H.S. publication No. 967, "Water Supply and Plumbing Cross-Connections."
