Tap water is generally considered a safe source of drinking water due to comprehensive regulation and treatment. However, waterborne parasites, such as the protozoa Giardia and Cryptosporidium, are frequently present in source water like lakes and rivers. These microscopic organisms can cause gastrointestinal illness if ingested. Public water systems use a continuous, multi-step defense to eliminate these threats and uphold the safety of the water that flows from the tap.
Parasites Commonly Found in Water Sources
The two protozoan parasites of greatest concern to public water supplies are Giardia lamblia and Cryptosporidium (Crypto). They exist as hardy, dormant forms: cysts for Giardia and oocysts for Cryptosporidium. These parasites enter water sources primarily through fecal contamination from humans and animals, including livestock and wildlife.
Their protective outer shell makes them challenging for treatment plants to neutralize, allowing them to survive for extended periods and resist conventional chemical disinfectants. Standard chlorine disinfection, while effective against most bacteria and viruses, often struggles to inactivate the thick-walled Cryptosporidium oocyst. Giardia cysts also require high concentrations of chlorine and long contact times to be reliably killed.
The Municipal Water Treatment Process
Public water systems use a multi-barrier approach to eliminate resistant protozoa before the water reaches consumers. This process is governed by regulations like the EPA’s Surface Water Treatment Rule. The first step is coagulation and flocculation, where chemicals are added to bind small particles and protozoan cysts into larger clumps.
Next, sedimentation and filtration steps act as the primary physical barriers. The large, clumped cysts are physically trapped and removed as the water passes through granular media filters like sand and charcoal. Filtration is effective because cysts and oocysts are relatively large compared to viruses.
The final barrier is disinfection, which addresses the chlorine resistance of Cryptosporidium. Many modern facilities incorporate advanced methods like ultraviolet (UV) light or ozonation alongside traditional chlorination. UV light disrupts the parasite’s DNA, inactivating both Giardia and Cryptosporidium so they cannot reproduce and cause illness.
Causes of Contamination in Public Water Systems
Contamination occurs when the multi-barrier defenses are breached, often due to system failures or environmental events. Infrastructure failure, such as a water main break or significant loss of pressure, is a common cause. A drop in pressure can allow contaminated groundwater or soil to be drawn into the distribution pipes.
Environmental factors, particularly heavy rainfall or flooding, can overwhelm treatment capacity. Excessive runoff or sewer overflows introduce high concentrations of parasites into the source water, potentially exceeding the plant’s ability to meet safety standards. Operational errors, such as improper chemical dosing or filtration failure, can also compromise water safety.
When a potential breach occurs, public health officials issue a “Boil Water Advisory.” This advisory alerts the public to the potential presence of microbial contamination until the water system verifies the issue is resolved and the water is safe to drink.
Protecting Drinking Water at Home
Consumers can take specific actions to ensure drinking water safety, especially during public advisories or if they use a private well. The most reliable method to neutralize parasites during an advisory is to bring water to a rolling boil for one full minute. At altitudes above 6,500 feet, boil the water for three minutes, as the lower boiling point requires more time to kill organisms.
Home filtration systems certified by organizations like NSF International offer another defense. To remove protozoan cysts, a filter must be rated for “cyst reduction,” typically using a 0.5-micron nominal filtration level. This fine pore size physically blocks the larger Giardia and Cryptosporidium organisms.
Private well owners are responsible for their water quality and should consider regular testing and installing a UV disinfection system. A UV system is effective against both Giardia and Cryptosporidium and provides a robust barrier against microbial contamination. Regular maintenance, including inspecting the wellhead, is also essential to prevent surface contaminants from entering the supply.