F OR many years before the institution of main sterilization practice, water works operators recognized the necessity for care in the laying of new mains and in the repair of distribution systems. The practices of flushing mains prior to their use and of employing special care in handling the pipe sections before their installation had been in use for a number of years before any thought was given to the additional precaution of sterilization. In fact, it was as late as 1925 before references to the chlorination of new mains began to appear in the literature of the field.At that time, in spite of the knowledge that, by meticulous care in main-laying operations, new water systems could be installed without danger of contamination, sterilization practice was initiated for the control of those features of the operations which were particularly difficult to control by any other means. Just as the dairy industry practices pasteurizationdespite the knowledge that near sterile milk can be produced without it, the water works field adopted sterilization as an additional safeguard. It should be pointed out here, however, that its use is no license for carelessness in other features of main-laying procedure. The desirability to control as many factors as possible, using sterilization to provide for the factors of ignorance and error, should be understood by every water works operator.After the first few years of experience with chlorine sterilization various problems of proper procedure became apparent. Among the early practices was that of placing dry calcium hypochlorite or chlorinated lime in each section of pipe as it was laid in the trench. This procedure was found undesirable primarily because it made impossible the flushing of the system prior to its disinfection. Also, as reported by Poole (1) in Indiana, many contractors were placing the chlorine compounds in the pipe weeks, or even months, before the water was admitted, with the natural result that sterilization was not at all effective. Still another objection, reported by ./t