This Standard W 556 aims to restore the hygienically safe operation of a drinking water installation by ensuring that drinking water installations are used and operated in accordance with regulations. It targets everyone involved in the rehabilitation of a drinking water installation including, in addition to designers and contractors, the operators of drinking water installations as well as hygienists and the competent supervisory authorities. The standard offers guidance on how to detect, assess and eliminate hygiene-related irregularities in existing drinking water installations that may have been caused by improper execution, improper operation, a lack of inspection or service activities or the negligent execution of necessary repair measures. Chemical and/or physical changes will be discussed to the extent to which they may favour the occurrence of pathogens in the drinking water. When executing measures designed to decontaminate drinking water installations, care shall always be taken to adequately protect the health of the involved specialist staff and of the consumers.