91ST Avenue Wastewater Treatment Plant Chlorine Conversion Project

A chlorine system conversion project at the 91st Avenue Wastewater Treatment Plant was initiated to convert the bulk chlorine system which consisted of four 20-ton permanent storage tanks and two 17-ton mobile truck trailer mounted system to a multiple one-ton cylinder system. The project also included the replacement of plastic pipe used for the conveyance of both chlorine gas (vacuum) and chlorine solution.In recent years, municipal, industrial, and private utility users of chlorine disinfection systems have encountered a condition known as “embrittlement” which is characterized by fractures in the pipe walls that could eventually result in failures in PVC/CPVC (thermoplastics) piping systems. Because of this problem and the potential for premature aging, the chlorine industry as a whole has expressed the opinion that thermoplastic pipes used for the conveyance of chlorine gas or chlorine in solution may require more frequent replacement; perhaps as often as every 10 years.The City of Phoenix as the facility operations agency, having safety as a high priority, commissioned CSA Engineering to study the problem and provide a design for a plant-wide chlorine pipe replacement project. With the prospect of a disinfection piping system having a limited life cycle and high piping replacement costs, CSA proposed a study to determine degradation effects of thermoplastic material in an aggressive elemental chlorine environment.