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Predictable Wastewater & Potable Water Disinfection Control

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Abstract:

The Clean Water Act has set guidelines for federal, state and local regulators to tighten the disinfection and discharge limits for thousands of municipal and industrial permit holders. The discharges limits for chlorine, ammonia, and other nutrients continue to tighten.

In order to ensure compliance in disinfection many plants are overfeeding chlorine or other oxidants used in the disinfection process thereby causing the need for overfeeding of dechlorination agents. This practice causes chemical costs to skyrocket and added risk of discharge violations. Traditional residual monitoring and testing procedures are woefully inadequate for determining the efficacy of the treatment at the front end of the process or determining low levels required in the dechlorination process.

The tightening regulations for disinfection requirements also exposed the inadequacies with present methods for determine proper levels of disinfection. This presentation deals with the factors that severely impact these processes and testing procedures. It offers enough understanding of disinfection chemistry and role of other compounds with respect to their direct effect on the disinfection process and the typical methods of instrumentation used for their control.

As the requirements for overall discharges tighten better process control is also required to meet these standards. The effect of uncontrolled nitrification and denitrification has severe impact on the disinfection process. Many plants transition in and out of nitrification inadvertently and never know the true cause of poor disinfection for a given hour, day, or longer. Data is examined to determine what process changes would produce satisfactory disinfection results and still maintain compliance relative to process parameters. In many cases there can be changes made in process that can lead to considerable savings in energy and chemicals, if and when that window to the process can be reliable identified.

The ability to identify changes relative to the disinfection process and how to maintain the proper level of disinfection, also allows one to determine the impacts from the upstream process. This information is used in controlling the disinfection process and it can also be used to signal other changes required in upstream plants processes.

The goal of the project was to develop a disinfection control strategy that was able to deal with the dynamic conditions found in the disinfection process in both wastewater and potable water disinfection. The algorithms would need to account for any of the process changes and have the ability to react in a manner to maintain proper levels of disinfection and discharge permit compliance.

The transmitters on the left or at the head of the contact tank monitor the system several minutes after chlorine is injected into system. In this instance it is used to control oxidant feed.

The second transmitter assembly looks at the process well after all reactions and chemical compositions are formed or consumed in the process. The set point desired ranges for the readings on this sensor can be used to automatically adjust the set point of the sensor at the head of the tank. This is critical in the case of water reuse or a plant that goes in and out of nitrification.

The third transmitter in this instance is controlling the feed of the dechlor agent used to meet discharge permit.

Document Type: Research Article

DOI: https://doi.org/10.2175/193864705783978078

Publication date: 2005-01-01

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