On-Site Generation: Design, Construction, Cost and Long Term Operation

Authors:

  • Kate Keenan, Alana Loughlin and Michael Wang – Hazen and Sawyer
  • Lindsey Olson – American Water

Disinfection using sodium hypochlorite has become attractive as an alternative to chlorine gas because of the hazards associated with handling chlorine and regulations concerning the use of chlorine. Bulk sodium hypochlorite storage and feed systems are being used very successfully at many water treatment plants; however, plants using bulk hypochlorite must deal with issues such as degradation, corrosivity, and chemical cost of bulk hypochlorite. Many utilities are instead implementing on-site hypochlorite generation facilities. This paper will delve into design of on-site generation (OSG) facilities, the cost of construction and operation of these facilities, and current and anticipated regulations affecting hypochlorite use.

The process of generating sodium hypochlorite on-site will be described. The paper will also provide design considerations and details for individual system components, including brine tanks and pumps, electrolytic cells, power rectifiers, hypochlorite storage tanks, and metering pumps, as well as requirements for salt and water for hypochlorite production. In addition, lessons learned during installation and start-up of OSG systems that can be applied to future designs will be discussed.

An economic evaluation of sodium hypochlorite purchased in bulk versus hypochlorite generated on-site will be provided. This life cycle cost analysis will take into consideration capital costs of bulk and OSG systems as well as operating and maintenance costs of the two types of systems. Although OSG systems generally have higher capital costs than bulk systems, on-site generation often provides a lower present worth cost over the life of the equipment.

This paper will also address regulatory concerns about compounds that can occur in both on-site generated sodium hypochlorite and purchased bulk hypochlorite solutions. Bromate can be formed from salts containing concentrations of bromide, and other compounds of concern can be formed in hypochlorite solutions. US EPA has recently given notice to the Federal Register indicating its decision to regulate perchlorate in drinking waters at levels as low as 2 ?g/L, and chlorate has been added to the 3rd Unregulated Contaminant Monitoring Rule (UCMR3) program. Many utilities are justifiably concerned about the selection and use of any disinfectant in their treatment process and how such use may impact contaminant concentrations in finished waters.

For more information, please contact the author at ckeenan@hazenandsawyer.com.

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