Removing Giardia Cysts from Drinking Water
How did the Giardia cysts enter drinking water supplies?
Studies showed that the outbreaks occurred in communities with inadequate chlorination systems, improperly operated and maintained filtration equipment, and even unfiltered water supplies. Since water supplies are not regularly tested for Giardia, the contamination was not detected until it was too late and entire communities had been exposed to contaminated drinking water.
From a health standpoint, Giardia has no long-term health effects. However, EPA studies conducted in Pennsylvania communities reveal that even a single Giardia outbreak may require investment in high cost filtration systems.
For residents receiving water from private wells, the presence of Giardia cysts from unfiltered water, although unlikely, may indicate that other contaminants are making it to the tap, as well. In either case, everyone will agree that the illness-causing cysts must be removed.
What is Giardiasis?
Giardiasis is a gastrointestinal illness caused by the introduction of Giardia cysts from human or animal wastes into water supplies. The microscopic cysts are capable of surviving in cold water for extended periods of time. Mammals, such as beavers, that spend time in or near water have been implicated in some Giardia outbreaks, but many animals as well as humans are known to carry the disease.
Ingested by the host animal, the cyst develops into the adult protozoan life stage and attaches itself to the wall of the small intestine at the outlet of the stomach. There it reproduces cysts which can develop and infect other hosts. One gram of feces from an infected animal may contain as many as two million cysts. Diarrhea, abdominal cramps, and gas are among the common symptoms that may appear within 1-4 weeks after cyst ingestion.
Giardiasis affects persons of all ages. Medication will eliminate the organism from the intestines, but reoccurrence of the symptoms occurs in some individuals.
Detection of Giardia cysts is difficult and often doesn’t occur until an individual becomes infected. Testing water for Giardia requires filtering several hundred gallons of water and having a trained analyst inspect the filter with a microscope. If no cysts are present, there is still no guarantee that none exist in the water supply.
The Department of Environmental Protection (DEP) currently samples public water supplies for Giardia and more stringently enforces regulations concerning proper operation and maintenance of water treatment plants to insure removal of the cysts. If you suspect that Giardia contamination has occurred, you should contact your local community water company or the DEP.
For individuals drawing water from private wells, the safeguarding of the water supply rests on the individual. Where deep (greater than 80') groundwater sources are used for water supply, Giardia cysts are usually not a problem. However, if surface water seeps directly into a well, Giardia can contaminate even the deepest well. A well casing that extends above the surface of the ground and is properly grouted will keep surface water out of the well.
Springs and shallow wells have high contamination probabilities because there is inadequate soil filtration of the water flowing into these systems.
Removing Giardia Cysts
Removing Giardia cysts before they reach the tap usually involves disinfection to inactivate the cysts and filtration through a fine media to physically remove the cysts from the water. Because Giardia cysts are resistant to normal disinfection, filtration is usually required. Cysts are large in comparison to bacteria and viruses (ranging from 7-10 microns in diameter); consequently, they are more easily removed by filtration.
Both disinfection and filtration processes are used in combination at municipal plants to protect water supplies and assure customers of high quality drinking water. Home water treatment techniques and devices are also available to assist water users in effectively removing Giardia. Techniques such as superchlorination, boiling, and manufactured treatment devices are available to the homeowner.
Because normal chlorine levels used to kill bacteria in water supplies will not inactivate Giardia cysts, super chlorination is required. Super chlorination requires a higher concentration of chlorine and increased contact time with the water.
Figure 1. Comparison between normal and super chlorination for disinfection.
Super chlorination can effectively eliminate one-time contamination events. The strong smell and taste of chlorine at this level will make the water unpalatable for a short period of time.
An alternative to chlorination is boiling. Boiling is a simple, effective means of killing Giardia cysts. It is also a method that can be used away from home on camping and hiking trips. To inactivate the cysts, bring water to a full boil for 1 minute on a stove or in a microwave. The heat from a hot water tank is not sufficient to inactivate Giardia cysts.
Boiling water is a viable, temporary solution to giardia contamination. Since boiling often leaves water with a bland taste, you may wish to purchase bottled water in lieu of boiling drinking water (See F 142 Bottled Water fact sheet).
Manufactured treatment devices
Manufactured treatment devices also remove Giardia cysts. Various filters are probably the most common devices used. They are constructed of very fine media that traps small particles like bacteria, cysts and sediment. Sand, diatomaceous earth, spiral wound fiber, ceramic and activated carbon are five common media used for filtration.
Figure 2. An activated carbon filter with a partially spent filter.
This illustration shows how the filtration process works to trap Giardia and other water constituents. The most widely available filter for point-of-use/point-of-entry treatment is the activated carbon filter (ACF). Many types of ACFs are currently on the market. Tap mounted filters attach to the the faucet and pour through models work like automatic drip coffee makers. High volume models are installed under the sink.
Although many people believe that filters are a fail-safe treatment, filters are ineffective unless properly maintained and operated. Where undisinfected water is used, filters are susceptible to bacterial growths which plug and coat the filters, reduce the filtering capacity, and create a source of bacterial contamination. For this reason, only disinfected water should be filtered.
Filters must also be cleaned regularly and replaced. Filters such as the tap mounted type must be changed at least every six months. Read the manufacturers instructions to make sure that you are properly maintaining your filter.
Other Disinfection Methods
A disinfection process widely used in European water treatment plants, is now emerging as a home treatment technology in the United States.
Ozone, a strong oxidizing gas, is injected into water and kills bacteria and Giardia cysts with less contact time than other disinfection processes. Unlike chlorination, ozone leaves no after taste or residues and has been found to be more effective and more costly at Giardia deactivation than chlorine.
Some manufacturers have also marketed Reverse Osmosis (RO) and ultraviolet light equipment for Giardia treatment. Reverse Osmosis devices are compact units designed to force pressurized water through a semi-permeable membrane. Contaminants are left on one side of the membrane while clean water flows out the other side. Tests conducted on RO have shown that in the long run, the membrane becomes less and less effective at removing Giardia cysts.
UV light, a process that passes water through a chamber housing a quartz mercury lamp, has also been shown to kill cysts. These processes have not proven to be consistently effective at Giardia removal; consequently, they are not recommended for this use.
Prepared by Paul D. Robillard, Associate Professor, Agricultural Engineering, William E. Sharpe, Professor, Forest Hydrology, and Bryan R. Swistock, Extension Associate