On Line Control Strategies for Disinfection Systems: Success and Failure

There are many levels of automation for control of chlorination systems. To select the best control approach for a facility, both control strategies and chlorination chemistry must be understood. Permit conditions including organism concentrations and operating data must also be fully identified. Many States require compliance with organism concentrations, as well as residual chlorine and disinfection byproduct concentrations.

Breakpoint chlorination chemistry will play a role in most disinfection systems. This paper discusses combining on-line monitoring and automated control with chlorine chemistry to develop several levels of process control that will result in effective disinfection. Another factor must be considered while developing a process control strategy is disinfection byproduct formation. Disinfection byproducts (DBPs) in wastewater effluent are being regulated and the limits being imposed are much stricter than SDWA standards. Data from a recent disinfection study is presented to examine DBP formation and other selected chemical properties.

Many attempts have been made to automate chlorine-based disinfection systems. Two basic problems are always encountered when controlling chlorination systems: the lag time inherent in the feedback control of plug flow systems and interpreting the impacts of chlorine chemistry on process control. Until these two issues are properly addressed in the control system logic, the control system will have serious limitations and may be more trouble to operate than the  money spent to install the equipment.

Strict nutrient limits are forcing many chlorine disinfection systems to vacillate between monochloramines and free chlorine. Both will effectively disinfect a wastewater, but each has its own characteristics. Along with strict nitrogen limits, some geographical regions in the United States are implementing end-of-pipe limits for trihalomethane (THM) compounds for control of DBPs. Drinking water standards regulate THMs as an aggregate parameter; however, the standards that govern water quality standards specify individual limits for each THM compound.

Many States are setting low organism limits for reuse applications and for effluent receiving streams that are used for recreation. Some States are going so far as to set non-detect limits. To meet these stringent limits, chlorine addition must be carefully controlled. Overdosing is no longer a viable alternative because it would lead to THM formation and because of the increasing costs of chlorination and dechlorination.

Effective control of the chlorination process will achieve the following results:

  • Meet either permit limits or target levels for organisms.
  • Minimize chlorine use.
  • Lower the chemical costs.
  • Reduce the use of dechlorination chemicals.
  • Minimize THM formation.

This paper will describe various levels of chlorination system control, from manual control through complete automation, and will discuss the benefits and limitations of each approach.

In order to develop an effective control strategy, the chemistry of chlorination must be thoroughly understood. Wastewater treatment plants use chlorination to kill pathogenic microorganisms and viruses. The NPDES permits issued to utilities throughout the U.S. define many different water quality goals. Disinfection limits are usually expressed as concentrations of groups of organisms or specific types of organisms. “Total coliforms” is a generic term for a large group of organisms, whereas “fecal coliforms” are still a group of organisms but a more specific group of organisms known to be present in wastes from warm-blooded organisms. E. coli and enterococci are examples of specific organisms found in wastewater that are being regulated in some parts of the U.S.

Chlorine is a strong oxidant that will react with a wide range of organic materials. As chlorine oxidizes the organic material in cell walls or in other cell components, it kills the target organism. The effectiveness of chlorine as a disinfectant depends on its concentration and on contact time. Increasing the chlorine dose or the contact time will result in killing more organisms. A common NPDES permit limit for discharge to many watersheds is a 30-day geometric mean of 200 CFU/100 mL (CFU = Colony Forming Unit). NPDES permits for environmentally sensitive areas such as watersheds used for water supplies and reuse systems will have lower limits that can range from a 30-day geometric mean of 22 Total Coliforms/100 mL, to nondetect for fecal coliforms. In order to design a successful disinfection system, the designer must clearly understand chlorine chemistry to ensure that the system dispenses enough disinfectant to produce an effluent that meets the NPDES permit limits.

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