This Maintenance Study presents reported instrumentation, control, and automation (IC & A) data received from surveyed water, wastewater, and industrial treatment facilities. In particular, this publication focuses on maintenance practices reported for the treatment process areas of disinfection and effluent. Online total and free chlorine residual analyzer technologies, calibration and maintenance practices, and reported effectiveness of performance and accuracy are analyzed. In addition, typical online total and free chlorine residual analyzer technological specifications; applications; and recommended calibration, maintenance and installation practices are discussed.