CONTROL OF WASTEWATER CHLORINATION AND DECHLORINATION: STRATEGY, OPERATION, AND MAINTENANCE

Chlorination is perhaps the most commonly used process for wastewater effluent disinfection. Usually a chlorine solution is mixed with the effluent and held for a minimum of 20 to 30 minutes before discharge. Depending on the discharge permit, facilities must demonstrate a minimum residual chlorine or a maximum fecal coliform count or both. Most facilities are also required to remove all excess chlorine (dechlorination) before ultimate discharge. Thus, chlorination control is doubly important since there is both a cost for adding chlorine as well as for removing the excess chlorine.Successful chlorination and dechlorination systems depend on implementing the right control strategy, selecting the correct chlorine analyzer technology, operating the system correctly, and implementing an effective maintenance program. The elements of a successful control strategy will be presented along with several potential mistakes to avoid. The characteristics, advantages, and disadvantages of three different chlorine analyzer technologies will be discussed with recommendations on where to use each. The role of operations and maintenance will be presented with an emphasis on quality control.A case study of the upgrade of the chlorination control system at the City of Houston's 400 MGD 69th Street treatment plant will be presented. This system included facilities for good mixing, chlorine residual measurement, and automatic control. The cost of chlorination was reduced from 600,000 to 400,000 per year while increasing maintenance costs by less than 5,000 per year. Additional benefits included improved compliance, less effluent variability, less bisulfite usage, and less operator attention.