Potable water is defined as water that is safe for humans to consume and use for domestic purposes, such as drinking, cooking, and personal hygiene. This safety assurance is paramount because water is drawn from sources like rivers, lakes, and groundwater, which naturally contain various impurities. Making water potable involves treating it to remove harmful substances and disease-causing agents, thereby protecting public health from waterborne illnesses.
Establishing the Standards for Safe Water
The determination of whether water is safe for consumption is governed by a regulatory framework that sets measurable quality standards. In the United States, the Environmental Protection Agency (EPA) establishes these standards under the Safe Drinking Water Act (SDWA). Globally, the World Health Organization (WHO) provides guidelines that inform many national standards.
The EPA divides its water quality mandates into Primary and Secondary Drinking Water Standards. Primary standards are legally enforceable rules focusing on contaminants that pose a direct threat to human health. For these contaminants, the EPA sets a Maximum Contaminant Level (MCL), which is the highest concentration allowed in public drinking water.
Secondary standards are non-enforceable guidelines that address aesthetic qualities of the water, such as taste, odor, color, and corrosivity. While public water systems must comply with Primary Standards, Secondary Standards are recommendations aimed at ensuring a high quality of consumer experience.
Categories of Contaminants
Contaminants are broadly classified into categories based on their nature and the threat they pose to human health.
Microbiological Contaminants
Microbiological contaminants represent the most immediate risk, consisting of living organisms like bacteria, viruses, and parasites. Bacteria such as Escherichia coli (E. coli) are often found in water contaminated with fecal matter and can cause severe gastrointestinal illnesses. Viruses, including norovirus and rotavirus, are highly contagious and can lead to symptoms like vomiting and diarrhea.
Chemical Contaminants
Chemical contaminants are either inorganic or organic compounds that can have long-term health consequences. Inorganic chemicals include heavy metals like lead and arsenic, which can leach into water from natural deposits or corroded infrastructure. Exposure to these heavy metals is linked to neurological disorders, developmental issues, and various forms of cancer. Organic contaminants are carbon-based molecules, such as pesticides, industrial solvents, and pharmaceuticals, that enter the water from agricultural and manufacturing runoff.
Physical Contaminants
Physical contaminants impact the water’s appearance and clarity. Turbidity, or cloudiness, is caused by suspended solids like sediment and organic material. High turbidity is a concern because these particles can shield harmful microorganisms from disinfection treatments. Radiological contaminants, which are unstable atoms that emit radiation, also represent a category of concern.
Essential Treatment Processes
The process of transforming raw source water into potable water involves a sequence of mechanical and chemical steps. The initial stage begins with coagulation and flocculation, where chemicals like aluminum sulfate are added to the water. These coagulants neutralize the charge on microscopic particles, causing them to stick together and form larger clumps called “floc.”
Following floc formation, the water moves through sedimentation and clarification, allowing the heavy clumps to settle out by gravity. This settling process removes a significant portion of the suspended solids, improving the water’s clarity. The clarified water then undergoes filtration, where it is passed through beds of materials such as sand, gravel, and activated charcoal. This step physically strains out smaller particles that did not settle during sedimentation.
The final step is disinfection, which is necessary to kill any remaining disease-causing pathogens, including bacteria and viruses. Chlorine is the most common disinfectant used because it is highly effective and leaves a residual level in the water. This residual chlorine protects the water from microbial growth as it travels through the distribution system. Sometimes, ultraviolet (UV) light or ozone is used as an alternative or supplementary disinfectant.