This manual is part of a series of guides devised by the Oxfam Public Health Engineering Team to help provide a reliable water supply for populations affected by conflict or natural disaster. The object of water treatment is to provide potable water, i.e. pathogen-free and chemically safe, which is low in physical impurities and is also aesthetically acceptable to the consumer. However, the greatest health risks in the overwhelming majority of situations where disasters occur are due to the presence of pathogens (microbiological contamination), whereas chemical contamination is rarely an immediate health impact. Therefore these guidelines reflect this and focus accordingly.