Hach Disinfection Series - Step 1

1. Chlorine Applications in Drinking Water Treatment

Chlorine is a common disinfectant and oxidant used in drinking water treatment for a variety of purposes. Chlorine is economical, relatively easy to apply, and convenient to measure using both laboratory and online instrumentation.

Some common chlorine applications and monitoring points include:

• Raw Water Intake — Chlorine may be added to a raw water intake for zebra mussel and Asiatic clam control and to prevent biological growth.
• Rapid Mixer — Chlorine added to the rapid mix prior to sedimentation often is used for disinfection, oxidation of iron and manganese, taste and odor, control and oxidation of hydrogen sulfide.
• Chlorination Prior to Filtration — A low concentration dose of chlorine prior to the filters helps prevent biological growth on the filter media and improve the filtration process.
• Purchased Water — (often later blended with local source water) may be monitored at the valve to the distribution system or near the inlet to the plant to help operators assess incoming chlorine (typically total chlorine measurement) and chloramine levels.
• Clearwell — Chlorine is added and monitored in the clearwell for disinfection purposes and to determinte CT values.
• Chloramination Control — Monochloramine is a parameter of interest for chloraminating facilities. Some facilities will choose to monitor total chlorine and free chlorine and then calculate monochloramine by difference (not recommended). The better solution is to monitor monochloramine and free ammonia to optimize the chloramination process, to keep free ammonia to a minimum, and prevent nitrification in the distribution systems.
• Distribution Lines — In distribution systems, the Total Coliform Rule requires facilities to maintain a detectable total chlorine residual throughout the distribution system in order to minimize microbial risk. Chlorine concentrations are routinely measured by utility personnel using manual colorimetric techniques.
• Booster Stations — Chlorine (and sometimes free ammonia and monochloramine) is typically measured at the inlet and outlet of a booster station. The inlet analyzer measures the chlorine residual as it enters the booster station, and can be used to control the chlorine injection rate in order to achieve the desired residual value downstream of the booster station. The chlorine analyzer used at the outlet of the booster station is used to monitor and ensure that the desired residual value has been achieved. Chloramination has both complicated and increased the need to monitor at booster stations. Several different approaches are used that are based on the individual situation.
• Storage Tanks — Residual chlorine concentrations in storage tanks are used by plant personnel to determine nitrification or contamination occurrences and to ensure that proper disinfection levels are maintained throughout the distribution system. These nitrification occurrences can happen quickly and the residual chlorine levels will decrease suddenly. On-line analyzers are the best choice to use in the monitoring of these storage tanks, especially in some of the older tanks where dead zones and extended water age often occur due to poor mixing when water is added or withdrawn from the tank.
• Line Repairs — After construction, line repairs, or installation of a new distribution line, the line must be disinfected, using a high concentration of chlorine with a long contact time, and flushed before it is used. This concentration should be monitored to ensure compliance.
• Hydrant Flushing — Although the flushing of hydrants may be controlled according to visual parameters, such as color and turbidity, control of flushing based on chlorine residual may lead to better process control and less waste.
• Hypochlorite Concentration — Bulk hypochlorite solutions (bleach solutions) will lose strength during shipment and storage. The percent chlorine of aqueous sodium hypochlorite solution can be measured for verification purpose and for setting accurate pump feed rates.

In any of these applications, chlorine concentration must be monitored both for purposes of compliance and for process control. Chlorine can be measured continuously with an online analyzer or manually as a grab sample. Continuous monitoring, as its name implies, provides real time and continuous information and is best used to control chlorine feed rates and monitor chlorine residuals. Measurements of grab samples (typically with lab or portable instrumentation) provide a snap shot at a particular time in the process and are often used for field testing and to validate process instrumentation.