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 350mgd). 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. Challenges and solutions of this large-scale gas to liquid conversion include: Chemical Delivery and Storage - Bulk hypochlorite and sodium bisulfite can be delivered by either railcar or tanker truck. Based on SJ/SC WPCP average daily chlorine use, about one 90-ton railcar every three days or 1.2 tanker trucks daily will be needed to deliver bulk sodium hypochlorite to the plant. The conversion plan provided extensive evaluation of the following viable options: 1) Railcar delivery and storage; 2) Railcar delivery and tank farm storage; 3) Tanker truck delivery and tank farm storage; 4) “Just-in-time” delivery with 1-ton cylinders. The conversion plan evaluates the delivery and storage options and suggests abandoning the railcars and using tanker truck delivery and tank farm storage. Use of the Existing Facilities - In order to maximize the use of the existing facilities, the plan recommends converting the entire existing backup facility to a bisulfite storage and feed facility. Then decommissioning the sulfur dioxide storage and converting that location to sodium hypochlorite storage (reusing the newly constructed awning and placing the facility close to the
demand). Conversion as Quick as Possible vs. as Quick as Practical - The conversion plan recommended three initial steps to a successful conversion: 1) identify equipment, suppliers and lead times; 2) modify existing backup system after decision to convert; 3) identify potential chemical suppliers and develop RFPs. For the purposes of the implementation, general procedures for construction (i.e. design-bid-construct) were compared against a design/build approach. Design/build approach reduces the project duration, but not the overall project cost. Liquid Chlorination Impacts on Plant Operation - There are several operational considerations associated with the use of sodium hypochlorite and sodium bisulfite: 1) Decomposition of sodium hypochlorite; 2) Gas accumulation from sodium hypochlorite solution; 3) Scaling at sodium hypochlorite dilution point; 4) Crystallization of sodium bisulfite. In addition, sodium hypochlorite and sodium bisulfite represent changes of pH and an increase in salt and the total dissolved solids (TDS) in the treatment plant effluent. Special design requirements were provided in the conversion plan to address these impacts. The
implementation plans for the gas conversion at the SJ/SC WPCP is divided into five phases: 1) Decision to convert; 2) Chemical supply contract; 3) Permitting; 4) Facility design; and 5) Construction and operation. The estimated conversion cost is approximately $4.0 million. The conversion project is currently going through facility design phase.

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