Key Operating Strategies for Chlorine Disinfection Operating Systems

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Courtesy of Water Environment Federation (WEF)

ABSTRACT
Chlorine has been used as a standard for disinfecting wastewater for many years. While being the standard There are many levels of automation for control of disinfection. To select the best control approach for your facility both control strategies and chlorination chemistry must be understood. Permit conditions must also be fully identified; organism concentrations and operational data. Many States require compliance with both organism concentrations and residual chlorine concentrations.

Breakpoint chlorination chemistry plays a role in most chlorine disinfection systems. This paper discusses blending on-line monitoring and automated control with chlorine chemistry to develop several levels of process control that will work. Disinfection byproduct formation must also be considered while developing a process control strategy. Trihalomethanes are being regulated in wastewater effluent and the water quality based limits are much stricter than SDWA drinking water standards.

This presentation will highlight the key operating factors for maintaining chlorine gas disinfection systems. Check lists developed from working with successful operations staff will be presented that will allow users the most update information on the operation of chlorine gas disinfection systems.

Introduction to Chlorination
Chlorine is available for use as a disinfectant or oxidant. This means that it can be used to remove pathogens and other undesirable organisms as well as oxidize such materials as odor causing sulfides, iron and similar metals as well as some organics. It is this multiple capability that makes chlorine such a desirable chemical for disinfection and oxidation in the treatment of wastewater effluents. Whether a wastewater treatment plant is using chlorine for disinfection or not, it is likely that chlorine is used somewhere else in the treatment or collection system process. In addition to chlorine’s use in the effluent for disinfection, it is also used in the collection system to maintain wastewater freshness and to control or eliminate sulfide odors. Chlorine is used to control midge flies and odor control applications at locations such as the preliminary treatment section of a facility. Chlorine is also used in the return activated sludge (RAS) process and for  algae control at weirs and filters.

Chlorine is available from suppliers in three forms, or it can be generated on-site. On-site generation is covered in the WEF Disinfection Training Manual. It should be noted, the term chlorination applies equally to the process regardless of the form of the chemical.

Physical and Chemical Properties
Chlorine is available to wastewater facilities in three basic forms:

  • Chlorine Gas
  • Liquid Sodium Hypochlorite, and
  • Solid Calcium Hypochlorite

Chlorine gas, sometimes referred to as elemental chlorine, is 2.5 times heavier than air, has a greenish color, and a characteristic pungent odor that is noticeable at levels of 0.2 mg/L (0.2 ppm). Chlorine gas is corrosive in the presence of moisture. Steel, stainless steel, iron and copper will readily corrode in the presence of moist chlorine gas. Chlorine gas is available as a pressured liquid in steel containers.

Sodium hypochlorite, sometimes referred to as liquid chlorine or liquid bleach, is a solution of anywhere from 5 to 15% available chlorine. Occasionally the term “javelle water” is used to refer to hypochlorite solution. Solutions of sodium hypochlorite are unstable and will start to decompose at the time of manufacture. Decomposition will continue until it is used. Exposure to heat, light, and metals (e.g. iron, copper) will enhance decomposition. Higher concentrations have more rapid decomposition rates than lower concentrations. Mixing with materials such as alkalis, acids, alum, and ferric chloride will release chlorine gas. Liquid sodium hypochlorite is available in bulk shipments, plastic carboys or lined tanks, or generated on-site.

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