Water Filters and Giardia
During the past 10 years, scientific knowledge about what is required tokill or remove Giardia cysts from a contaminated water supply hasincreased considerably. For example, it is known that cysts can survive incold water (4 deg C) for at least 2 month s and that they are killedinstantaneously by boiling water (100 deg C) (23,24). It is not known howlong the cysts will remain viable at other water temperatures (e.g., at 0deg C or in a canteen at 15-20 deg C), nor is it known how long theparasite wil l survive on various environment surfaces, e.g., under a pinetree, in the sun, on a diaper-changing table, or in carpets in a day-carecenter.
The effect of chemical disinfection, such as chlorine, on the viability ofGiardia cysts is an even more complex issue. It is clear from the numberof waterborne outbreaks of Giardia that have occurred in communities wherechlorine was employed as a disin fectant that the amount of chlorine usedroutinely for municipal water treatment is not effective against Giardiacysts. These observations have been confirmed in the laboratory underexperimental conditions (25,26,27). This does not mean, however, that chlorine does not work at all. It does work under certain favorableconditions. Without getting too technical, one can gain some appreciationof the problem by understanding a few of the variables that influence theefficacy of chlorine as a disinfectant.
1) Water pH: at pH values above 7.5, the disinfectant capability ofchlorine is greatly reduced. 2) Water temperature: the warmer the water,the higher the efficacy. Thus, chlorine does not work well in ice-coldwater from mountain streams. 3) Organic content of the water: mud,decayed vegetation, or other suspended organic debris in water chemicallycombines with chlorine making it unavailable as a disinfectant. 4)Chlorine contact time: the longer Giardia cysts are exposed to chlorine ,the more likely it is that the chemical will kill them. 5) Chlorineconcentration: the higher the chlorine concentration, the more likelychlorine will kill Giardia cysts. Most water treatment facilities try toadd enough chlorine to give a free ( unbound) chlorine residual at thecustomer tap of 0.5 mg per liter of water.
The five variables above are so closely interrelated that an unfavorableoccurrence in one can often be compensated for by improving another. Forexample, if chlorine efficacy is expected to be low because water isobtained from an icy stream, either the chlorine contact time or chlorineconcentration, or both could be increased. In the case ofGiardia-contaminated water, it might be possible to produce safe drinkingwater with a chlorine concentration of 1 mg per liter and a contact timeas short as 10 minutes if all the other variables were optimal (i.e., pHof 7.0, water temperature of 25 deg C, and a total organic content of thewater close to zero). On the other hand, if all of these variables wereunfavorable (i.e., pH of 7.9, water temperature of 5 deg C, and high organic content), chlorine concentrations in excess of 8mg per liter with several hours of contact time may not be consistentlyeffective. Because water conditions and water treatment plant operations(especially those related to water r etention time and, therefore, tochlorine contact time) vary considerably in different parts of the UnitedStates, neither the U.S. Environmental Protection Agency nor the CDC hasbeen able to identify a chlorine concentration that would be safe yeteffec tive against Giardia cysts under all water conditions. Therefore,the use of chlorine as a preventive measure against waterborne giardiasisgenerally has been used under outbreak conditions when the amount ofchlorine and contact time have been tailored t o fit specific waterconditions and the existing operational design of the water utility.
In an outbreak, for example, the local health department and water utilitymay issue an advisory to boil water, may increase the chlorine residual atthe consumer's tap from 0.5 mg per liter to 1 or 2 mg per liter, and, ifthe physical layout and operatio n of the water treatment facility permit,increase the chlorine contact time. These are emergency proceduresintended to reduce the risk of transmission until a filtration device canbe installed or repaired or until an alternative source of safe water, such as a well, can be made operational.
The long-term solution to the problem of municipal waterborne outbreaks ofgiardiasis will involve improvements in and more widespread use of filtersin the municipal water treatment process. The sand filters most commonlyused in municipal water treatmen t today cost millions of dollars toinstall, which makes them unattractive for many small communities.Moreover, the pore sizes in these filters are not sufficiently small toremove a Giardia (6 to 9 micrometers x 8 to 12 micrometers). For the sandfilter to remove Giardia cysts from the water effectively, the water mustreceive some additional treatment before it reaches the filter. Inaddition, the flow of water through the filter bed must be carefullyregulated.
An ideal prefilter treatment for muddy water would include sedimentation(a holding pond where the large suspended particles are allowed to settleout by the action of gravity) followed by flocculation or coagulation (theaddition of chemicals such as alu m or ammonium to cause microscopicparticles to clump together). The large particles resulting from theflocculation/coagulation process, including Giardia cysts bound to othermicroparticulates, are easily removed by the sand filter. Chlorine is thenadd ed to kill the bacteria and viruses that may escape the filtrationprocess. If the water comes from a relatively clear source, chlorine maybe added to the water before it reaches the filter. The point here is thatsuccessful operation of a complete water treatment facility is a complex process that requires considerabletraining. Troubleshooting breakdowns or recognizing potential problems inthe system before they occur often requires the skills of an engineer.Unfortunately, most small water utilities that have a water treatmentfacility that includes filtration cannot afford the services of afull-time engineer. Filter operation or maintenance problems in suchsystems may not be detected until a Giardia outbreak is recognized in thecommunity. The bottom line is that although, in reference to municipalsystems, water filtration is the best that water treatment technology hasto offer against waterborne giardiasis, it is not infallible. Formunicipal water filtration facilities to work properly, they must beproperly constructed, operated, and maintained.
Details mentioned in this article were accurate at the time of publication