Reclaimed Water Chlorination: Reducing Chemical Cost and Improving Disinfection Efficiency

Irvine Ranch Water District's (IRWD's) Michelson Water Reclamation Plant (MWRP) produces 14 MGD of reclaimed water for unrestricted landscape irrigation. The reclaimed water distribution system includes two seasonal storage reservoirs that are considered waters of the State. Nitrogen removal at the plant and dechlorination prior to the reservoirs are therefore required. The MWRP fully nitrifies and denitrifies (N/dN) and subsequently filters and disinfects with gaseous chlorine in a 2 MG, 12 ft deep, open chlorine contact tank (CCT). The four-pass CCT is effectively operated as an extended wet well for both the reclaimed water distribution and filter backwash pumping stations. Filter effluent has an average turbidity of 0.5 NTU and less than 2 mg/L of suspended solids. CCT effluent total chlorine (Cl2 ) residual must be maintained as high as 12 mg/L to meet the 2.2 MPN/100 mL total coliform limit required by Title 22. This Cl2 residual multiplied by the effective contact time results in CT values that are two to three times higher than the Title 22 required 450 mg-min/L. Lower chlorine residuals typically result in higher coliform counts. Chlorine doses as high as 25 mg/L are required to maintain this Cl2 residual. High residuals increase distribution system maintenance and chlorination/dechlorination costs. This paper presents the results of a recent study on the effect of water quality, contact time, sunlight, and CCT fluid dynamics on chlorine consumption and disinfection efficiency. The study sought to answer two questions: • Why is such a high CT value required to meet effluent coliform limits, and how can it be lowered while maintaining equal disinfection efficiency? • What causes high chlorine depletion and what can be done to reduce chlorine demand?