 Household water storage tanks, or "tinácos", are common on almost all household rooftops and are used to store water when water pressure in the system is inadequate.In many instances, the tanks are open and not cleaned regularly, permitting the residual chlorine to dissipate and encouraging the growth of microorganisms.The standard levels of chlorine (0.2 milligrams/liters) maintained in the distribution system as it reaches the customer's tap are not sufficient to inactivate microorganisms that may have entered the pipelines.