In this manuscript the on-site generation of sodium hypochorite for water and wastewater disinfection will be examined. The objective of this paper is to familiarize the reader with the equipment and operational requirements necessary for on-site sodium hypochlorite generation systems. This manuscript will review the following topics relative to on-site generation systems:
- Understand the basic equipment requirements necessary for on-site generation of sodium hypochorite,
- Understand the basic design considerations for in sodium hypochorite systems, and
- Understand the operation and maintenance requirements of on-site generation systems.
Introduction/Origins of On-Site Sodium Hypochlorite Generation
On-site generation of sodium hypochlorite can be accomplished using electrolyzer systems. In these systems, crystallized salt is dissolved and used for electrolysis. The electrolysis cells are designed for very low brine feed flow rates, narrow electrode gaps, and produce sodium hypochlorite concentrations approaching one percent. The following narrative discusses the principles of operation associated with on-site sodium hypochlorite generation systems.
General Description - On-site sodium hypochlorite generation systems can be used for any application requiring chorine or chloramines as a part of the disinfection regimen. A typical system schematic is shown in Figure 1. These systems are designed to provide 1 to 3 days of stored sodium hypochlorite. They also are designed with excess product storage to assure that disinfection capacity is always available to the end user. To accommodate these requirements systems are generally configured with the following components and operate in the manner described below:
- Water softener: Essential for removal of calcium and magnesium from the feed water.
- Salt dissolver: Provides the required salt solution for electrolysis.
- Electrolyzer cell or cells: Electrolyzes the dilute brine solution.
- DC power rectifier: Provides the Direct Current for electrolysis.
- Storage tanks: Product storage to meet dosing requirements as well as any excess capacity essential to assure continuous dosing capabilities.
- Hydrogen dilution blowers: Provided to dilute the byproduct hydrogen produced during the electrolysis process.
- Dosing pumps with dosing controls: Provides the needed disinfection dose based upon the chlorine residual or flow rate of a receiving stream.
- Cell cleaning system: Used to remove the calcarious material deposited on the cell cathodes during the production process.
- Central control panel: Performs the system production control function.
A simple on-site generation system operation begins with the client’s domestic water supply flowing through the water softener where the water hardness is reduced. A portion of the softened water is added to the salt dissolver to make a concentrated salt brine solution of approximately 300 grams/liter. The concentrated salt brine is then mixed with the main stream of softened water to produce a final brine concentration of approximately 3% (30 grams/liter) salt concentration. This final brine solution is then pumped through the electrolyzer cell.