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Weighing the risks of microbial versus chemical contamination, the WHO Guidelines for drinking water quality recommends that, where local circumstances require that a choice must be made between meeting either microbiological guidelines or guidelines for disinfectant by-products, the microbiological quality must always take precedence and, where necessary, a chemical guideline value can be adopted corresponding to a higher level of risk. Efficient disinfection must never be compromised. World Health Organization. Guidelines for Drinking- Water Quality, 2nd edn, Volume I-Recommendations. WHO, Geneva (1993).