Drinking water quality standards are a critical basis for evaluating the safety of drinking water. Currently, there are three internationally authoritative and representative drinking water quality standards: the World Health Organization (WHO) Guidelines for Drinking-water Quality, the European Union Drinking Water Directive, and the United States National Primary Drinking Water Regulations. Among these, the WHO Guidelines for Drinking-water Quality serves as the foundation for many countries to establish their own drinking water standards. Numerous countries, including Japan, Brazil, Argentina, and the Philippines, use the WHO standards as a key reference for developing their national standards. The WHO guidelines are updated regularly and provide comprehensive water quality indicators. Below are the seven key drinking water standards published by the WHO, along with their limits and interpretations:
Microbiological Indicators
Total Bacterial Count: The total bacterial count in water should not exceed 100 CFU/mL (Colony Forming Units per milliliter). For example, if a water sample exceeds this limit, it may indicate the presence of intestinal pathogens, posing risks of gastrointestinal diseases such as diarrhea and vomiting.
Escherichia coli (E. coli): E. coli must not be detected in any 100 mL sample. E. coli is considered an indicator of fecal contamination, and its presence suggests that the water source may be contaminated with human or animal waste, potentially carrying other pathogenic microorganisms such as Salmonella or Shigella, which pose serious health risks.
Thermotolerant Coliforms: Similarly, thermotolerant coliforms must not be detected in any 100 mL sample. These bacteria are more heat-resistant than regular E. coli and can survive at higher temperatures, making them a key indicator of recent fecal contamination. Their detection indicates poor sanitary conditions of the water source.
Chemical Indicators
pH Value: The pH of drinking water should be between 6.5 and 8.5. Human body fluids maintain a stable pH range, and drinking water with a near-neutral pH helps maintain acid-base balance. If the pH is too low (acidic), it may corrode metal pipes, increasing the levels of heavy metals such as iron, manganese, copper, and zinc in the water. If the pH is too high (alkaline), calcium and magnesium ions may precipitate, forming scale and causing issues such as reduced soap lathering, affecting daily use and health.
Total Dissolved Solids (TDS): TDS should not exceed 1000 mg/L. TDS reflects the total amount of dissolved minerals, salts, and organic matter in water. High TDS levels indicate the presence of excessive impurities, potentially including harmful chemicals such as nitrates, sulfates, and heavy metal ions, which can damage the nervous system and kidneys over time. Conversely, very low TDS levels may indicate overly "pure" water lacking essential minerals, which is not conducive to nutrient absorption.
Toxicological Indicators
Heavy Metals: For example, lead levels should not exceed 0.01 mg/L, mercury levels should not exceed 0.001 mg/L, and cadmium levels should not exceed 0.005 mg/L. Lead can accumulate in bones and blood, damaging the nervous and hematopoietic systems, particularly affecting children's cognitive development. Mercury can bioaccumulate through the food chain, causing kidney and nervous system damage. Cadmium tends to accumulate in the kidneys and liver, leading to osteoporosis and kidney failure.
Fluoride: The optimal fluoride concentration is 0.5–1.0 mg/L. While fluoride helps prevent dental caries, excessive intake (e.g., from water with fluoride levels above 1.5 mg/L) can cause dental fluorosis, characterized by white or brown spots on teeth, and in severe cases, skeletal fluorosis, leading to joint pain and bone deformities.
Nitrate: Nitrate levels should not exceed 50 mg/L (measured as nitrogen). Elevated nitrate levels, often caused by agricultural fertilizer runoff or sewage contamination, can be reduced to nitrite in the human body. Nitrite can bind to hemoglobin, forming methemoglobin and reducing its oxygen-carrying capacity, leading to methemoglobinemia, which is particularly dangerous for infants and can be life-threatening.
Sensory Indicators
Color: The color of water should not exceed 15 degrees on the platinum-cobalt scale. Excessive color may indicate the presence of humic substances, metal ions, or industrial wastewater contamination, giving the water a turbid and unclean appearance, which can affect consumer confidence and suggest the presence of other harmful impurities.
Turbidity: Turbidity should normally not exceed 1 NTU (Nephelometric Turbidity Unit), with a maximum allowable limit of 5 NTU under special circumstances. High turbidity indicates the presence of suspended particles, which not only make the water visually unappealing but also provide a breeding ground for microorganisms, increasing health risks. Additionally, high turbidity can lead to sediment buildup in water supply systems, affecting stability.
Odor and Taste: Drinking water should be free of any unusual odors or tastes. Abnormal smells such as musty, fishy, or chemical odors may indicate contamination by algae, industrial pollutants, or microbial metabolites. Even if harmful substances are within limits, unpleasant odors can make the water unappealing and reduce consumption.
Radiological Indicators
Total Alpha Radioactivity: Should not exceed 0.5 Bq/L (Becquerels per liter). Water sources in areas with high natural radioactivity or those affected by uranium mining or improper nuclear waste disposal may contain alpha-emitting radionuclides. Prolonged exposure to water with excessive alpha radioactivity can damage cells, increasing cancer risks, particularly affecting the hematopoietic, digestive, and reproductive systems.
Total Beta Radioactivity: Should not exceed 1 Bq/L. Beta-emitting radionuclides can also originate from natural or anthropogenic sources. Long-term consumption of water with excessive beta radioactivity can cause continuous low-dose radiation exposure, damaging DNA and leading to genetic mutations, chromosomal aberrations, and various cancers or hereditary diseases.
Disinfectant Residue Indicators
Free Chlorine Residual: After 30 minutes of contact with water, free chlorine levels should remain between 0.2 and 4 mg/L. Chlorine is a common disinfectant used to kill bacteria and viruses in water. Insufficient chlorine may fail to maintain disinfection efficacy, allowing microbial regrowth, while excessive chlorine can produce a strong chlorine taste and react with organic matter to form disinfection byproducts such as trihalomethanes, some of which are potentially carcinogenic and harmful to health.
Biodegradability Indicators
Biochemical Oxygen Demand (BOD5): Should generally not exceed 3 mg/L. BOD5 measures the amount of oxygen required by microorganisms to decompose organic matter in water. High BOD5 levels indicate the presence of biodegradable organic pollutants, which can deplete dissolved oxygen, harm aquatic life, and produce harmful byproducts such as ammonia and hydrogen sulfide during decomposition, degrading water quality and posing health risks.
These standards comprehensively ensure the safety of drinking water, guaranteeing that the water people consume daily meets health requirements. They serve as an essential reference for countries worldwide in formulating drinking water regulations.
