Chlorine gas has been successfully used to disinfect drinking water for more than a century. When added to water in the appropriate amounts, chlorine forms hypochlorous acid and ions, which are active disinfectants. This disinfectant is used by more than 90% of the drinking water plants in the U.S., and more than 200 million Americans and Canadians receive chlorine-disinfected drinking water every day.
Chlorine gas is mainly produced at chlor-alkali plants and shipped to water treatment facilities as a liquefied gas in pressurized bulk containers. The main strengths of chlorine gas disinfection include:

• Destroys a broad range of microorganisms including viruses, bacteria and some protozoa;
• Controls many taste, odor and color problems in raw water; and
• Remains as chlorine residual in water distri- bution systems to protect against the regrowth of algae or microorganisms.

This broad range of capabilities makes chlorine gas disinfection very cost-effective. However, many municipalities have become concerned about the hazards it presents in transportation and storage, the possible creation of harmful disinfection byproducts (DBPs), and its weakness in inactivating Cryptosporidium. Despite the safety concerns, however, gas chlorination still has the best-documented safety record when compared with the alternative methods of chlorine disinfection.

Measuring chlorine

The addition of chlorine to a water supply readily combines with chemicals dissolved in water, microorganisms, and plant material. “Chlorine demand” of a treatment system is the chlorine that is consumed by these components. “Free residual chlorine” is the disinfectant that has not combined with these components in the water. Sufficient chlorine dosage must be added to a water supply to meet the chlorine demand and provide a level of residual.
Real-time, online water quality monitors, analyzers and controllers are used to measure levels of aluminum, chlorine (free and total), chlorine dioxide, DO, fluoride, nitrate, nitrite, ORP, ozone, pH, temperature and others in water and wastewater.
Amperometric measuring cell technology is widely used in water quality analyzers that measure free and total chlorine in the water and wastewater treatment systems because of the accurate results and process efficiencies it affords a user across varying applications. However, all amperometric measuring cell technology is not alike. There are two distinct forms of amperometric cells; a mechanical (conventional) method and a progressive method. The major difference between these two technologies is the matter in which abrasives are used to clean the surface of the electrodes within the measuring cell.
The conventional amperometric measuring cell technology uses 100-200 tiny cleaning spheres circulated across the electrode surface by a mechanical sweep or scoop attached to the cell’s motor shaft. This continuous movement of the cleaning spheres across the electrodes removes residue that can potentially collect on the surface from routine operation.
The progressive method replaces the use of cleaning spheres with corundum sand, a highly abrasive material. The design of this measuring cell allows the incoming water flow to create a vortex, which in turn, continuously circulates the sand centrifugally across the electrode surfaces to scour and remove residue.
There are numerous operational and maintenance benefits to the amperometric measuring cell design which uses abrasive grit scouring.

Preventive maintenance

Installation of an amperometric-measuring cell that uses abrasive grit scouring is simple. The unit comes equipped with 100 grams of corundum sand and a spoon; one spoonful must be placed into the cell when commissioned. The cell should be flushed with clean water and a new spoonful of sand added to the cell monthly.
To perform this maintenance an operator needs to do the following:

• Step 1: Turn off the water flow to the measuring cell;
• Step 2: Remove the top and bottom cap that frames the electrode cell. No tools are required;
• Step 3: Once the top and bottom caps are removed, the body of the cell must be rinsed/flushed with water to remove any accumulated particulate and the corundum sand. This is easily done by pouring 500ml of water into the top opening, which will flow through the cell and discharge from the bottom opening;
• Step 4: Replace the bottom cap. No fastening tools are required because this is an O-ring fit;
• Step 5: Insert a spoonful of the corundum sand into the cell and replace the top cap; and
• Step 6: Restart the flow of water to the
  system and note your calendar to perform the maintenance one month from this
  completion point.

In addition to being an extremely efficient system, because corundum sand is more abrasive and has a greater cleaning power than cleaning spheres, the operation of this unit is simple to monitor. An operator can look into the top or bottom of the cell and see the circulation of the abrasive grit across the electrode surface. This is an effective way to monitor and maintain a visual indication of adequate flow levels.
When it becomes difficult to calibrate the system after a monthly preventive maintenance procedure, it is indicative of the electrodes losing sensitivity. Therefore, it is suggested that an operator follow Steps 1 and 2 from above.
Once the caps are removed from the cell, a Scotch Brite pad (coarse yet not too abrasive) should be used to manually clean the surface of the copper electrode. The spiral gold electrode does not require manual cleaning because its small surface area is not easily susceptible to residue build up during the course of normal operating procedures.
If manually scouring the system does not enable an accurate calibration of the system, the copper electrode should be replaced. Manufacturers typically recommend annual replacement of the copper electrode.