On-site Chlorine Generation Feasibility Study

Gaseous chlorine has been effectively used for disinfection in drinking water and wastewater treatment for decades. While various regulatory programs have placed numerous restrictions on the uses of this chemical due to its hazardous nature, recent focus on the possibility of a terrorist attack at a drinking water or wastewater facility, has underscored the potential hazard this chemical presents. On-site chlorine or sodium hypochlorite generation is a feasible alternative compared to purchased and delivered chlorine. The technology is based on the electrolysis of brine (sodium chloride) solution to produce either variable concentrations of sodium hypochlorite or pure chlorine gas. Three best available technologies that should be considered for this application include: 1) direct electrolysis, producing 0.8% liquid sodium hypochlorite by weight; 2) membrane cell electrolysis, producing 12.5% liquid sodium hypochlorite by weight; and 3) membrane cell electrolysis, producing gaseous chlorine and liquid caustic soda. A side-by-side comparison of advantages, disadvantages, and costs of each technology was conducted to help utilities determine if the existing gas system should be replaced with an alternative system. An economic evaluation was performed to assess the cost effectiveness of the various chlorine generation alternatives under consideration. Although purchased and delivered gaseous chlorine is still the most prevalent form of chlorination in water treatment systems in the United States, the use of aqueous sodium hypochlorite is increasing due to safety concerns associated with the use, storage, and transportation of chlorine gas. Reduced permitting requirements are also an advantage, particularly for low concentration hypochlorite systems. While safety and permitting considerations are attractive aspects of on-site sodium hypochlorite generation, there are also some unattractive aspects, as well. One consideration is the footprints required to store large amounts of solution. Another is the impact on water quality, which can result in higher total dissolved solids and bromate concentrations in the treated water.