Tap water, which comes from municipal supply systems, is subjected to rigorous treatment processes to ensure it is safe for drinking and general use. Despite being clean and safe, the water that flows from your faucet is not considered sterile. Sterility is an absolute condition that is not the goal of public water treatment, which instead focuses on making the water potable. Understanding the distinction between a safe-to-drink product and a sterile product is important for understanding the limits of the water utility system.
Defining Sterility Versus Disinfection
Sterilization and disinfection are two distinct concepts with different endpoints for microbial control. Sterilization is the complete destruction or removal of all forms of microbial life, including highly resilient bacterial spores. This absolute absence of living organisms is typically required for medical environments, such as surgical instruments or injectable fluids.
Disinfection, by contrast, is the process of eliminating or inactivating harmful microorganisms, or pathogens, to a level considered safe for public health. Public water systems are designed to achieve this state of high-level disinfection, making the water reliably safe for consumption, bathing, and cooking. Achieving true sterility for the massive volumes of municipal water would be prohibitively expensive and unnecessary for general use.
The Journey from Source to Tap: Ensuring Public Safety
The process of turning raw source water into safe drinking water involves multiple physical and chemical stages at the treatment plant. Initial steps include screening and straining to remove large debris like leaves and fish. Next, chemicals like aluminum sulfate are added in a process called coagulation, which neutralizes the negative charges on suspended particles.
This neutralization allows particles to clump together, forming larger, heavier masses called floc. These flocs settle out in large basins during the sedimentation stage, removing the majority of particulate matter. The clearer water then moves to filtration, passing through layers of sand and gravel to trap remaining fine particles.
The final step at the plant is primary disinfection, which kills or inactivates most waterborne pathogens. Chemical disinfectants like chlorine or ozone are introduced to destroy bacteria, viruses, and protozoa. This multi-barrier approach ensures that the water leaving the treatment facility meets strict government standards for safety.
Post-Treatment Contamination and Biofilms in Distribution Systems
Water highly disinfected at the treatment plant must travel through an extensive network of pipes, reservoirs, and storage tanks before reaching the consumer’s tap. It is within this distribution system that the water’s non-sterile nature becomes apparent. Microorganisms can survive disinfection or be reintroduced from the environment, readily attaching to the inner surfaces of the pipes.
These microbial communities form a slimy layer called a biofilm, a complex mixture of organisms embedded in a protective matrix. Biofilms are ubiquitous in water systems and shield microbes from residual disinfectants in the flowing water. Parts of the biofilm can detach and re-enter the water, leading to increased bacterial levels at the point of use.
To maintain water quality, utilities add a residual disinfectant, such as chloramine, which is more stable and longer-lasting than chlorine. This residual limits microbial regrowth and controls biofilm formation, but it cannot eradicate established colonies. Household plumbing, including water heaters and stagnant pipes, provides additional surfaces for these microbes to colonize before the water reaches the faucet.
Health Implications and Specific Use Cases
While tap water is considered safe for drinking by the general population, its non-sterile nature poses a risk in specific situations. Individuals who are immunocompromised, infants under six months, or the elderly are more susceptible to infections from the low levels of microbes present. Pathogens like Legionella and Pseudomonas aeruginosa can be found in tap water and are a concern when the water is aerosolized or used in sensitive procedures.
In sensitive scenarios, consumers must use water that is sterile or distilled because the eyes and nasal passages lack the protective stomach acid that kills ingested pathogens. Examples include using tap water in:
- Medical devices that create a mist, such as Continuous Positive Airway Pressure (CPAP) machines or humidifiers, which can allow microbes to be inhaled.
- Nasal rinsing devices.
- Contact lens care.
A simple and accessible method to achieve household sterility is to boil tap water for at least one minute and allow it to cool before use.