From Gas to Liquid: Large-Scale Chlorination System Conversion

A number of wastewater treatment facilities using gas disinfection have decided to convert to liquid disinfection for several reasons including safety, regulatory, security, and economic factors. An optimal technology plan should be developed to facilitate the conversion. This presentation provides a case study of a large chlorine disinfection system, and demonstrates key issues of a full-scale conversion. The San Jose/Santa Clara Water Pollution Control Plant (SJ/SC WPCP) currently uses chlorine and sulfur dioxide gases for several disinfection applications including final effluent (annual dry weather flow of 135mgd and peak wet weather flow of 350 mgd). There is a substantial safety concern regarding the storage and use of the chorine and sulfur dioxide because both gases are extremely toxic and the potential for an accidental release remains as long as these two chemicals are used for disinfection. Implementation of a Risk Management Plan (RMP) demands organizational resources and associated costs (~12 percent of the current annual O&M cost). Continued use of the existing gas disinfection facility requires improvements to meet more than just basic security needs. Provisions are required to provide high-level security against potential purposeful acts. An in-depth evaluation of disinfection alternatives has led to the decision that the optimal approach for the plant final effluent disinfection is "to convert gas to liquid". The goal of the conversion is to completely eliminate chlorine and sulfur dioxide gases from the SJ/SC WPCP and provide facilities for applications of liquid sodium hypochlorite and sodium bisulfite. A gas conversion implementation plan was developed to expedite the full-scale conversion. The plant consumes more than 4,100 lbs of chlorine and 1,250 gallons of 25% sodium bisulfite on average daily basis. During peak wet weather flow period, the chlorine consumption is as high as 70,000 lbs per day and sodium bisulfite 17,000 gallons per day.