From Volume 20, Issue 7 – July 1997
Safety a factor in choosing between gas, liquid and solid methods.
by: Gerard F. Dooley
While chlorination is an accepted process for treating drinking water today, the best method of chlorination is still being analyzed and debated by water treatment experts.
Three methods of water chlorination are most frequently used: gaseous chlorine, liquid sodium hypochlorite and solid calcium hypochlorite.
The advantages of using chlorine gas are its relative inexpense and that it produces no byproducts such as chlorite or chlorate ions. However, chlorine gas is toxic and corrosive. Cylinders containing the compound must be kept in vented rooms, and personnel need to be specially trained before servicing the cylinders. Unfortunately, special procedures for safely shipping, storing and handling the chlorine cylinders increase production costs. Two people are required to handle a chlorine cylinder, and it’s recommended that a self-contained breathing apparatus be worn to protect them from chlorine gas exposure. Chlorine gas also may corrode supply line fittings, which can create leaks. Special equipment is needed to ensure the safety and accuracy of adding chlorine gas to water. The equipment also requires a great deal of maintenance, making the process even more costly. The U.S. Occupational Safety and Health Administration (OSHA) has imposed regulations on record- keeping and reporting for chlorine cylinders weighing more than 2,500 pounds.
Liquid sodium hypochlorite commonly referred to as chlorine bleach is much less toxic than gaseous chlorine. It can be an easily monitored and administered product for water chlorination systems. But liquid sodium hypochlorite also has several disadvantages. The product breaks down over time, losing efficacy and forming byproducts. The breakdown of sodium hypochlorite depends on storage temperature and the presence of impurities in the concentrated product. Product suppliers can inform you of procedures to minimize contaminants. Sodium chlorate is one byproduct of sodium hypochlorite. For every one percent of hypochlorite loss, 0.8 percent sodium chlorate is formed. If an 8 percent active hypochlorite solution is used to provide 6 ppm of available chlorine in water, it also will deliver 1.5 ppm sodium chlorate. If it is not properly maintained, sodium hypochlorite can corrode metering pumps and feed lines. Personnel should wear proper protective clothing when handling concentrated sodium hypochlorite, and the product should not be stored near acid products, since the reaction between acid and sodium hypochlorite will produce chlorine gas.
The U.S. Environmental Protection Agency (EPA) is considering whether it should regulate the level of chlorate ions in drinking water.
Calcium hypochlorite is a dry form of chlorine available in powder or pellets. The solid product is easier to handle and is less toxic than gaseous chlorine. Calcium hypochlorite is also significantly more stable than concentrated liquid sodium hypochlorite, so sodium chlorate is not formed. Additionally, water chlorination systems using calcium hypochlorite are easier to clean and maintain and have long-term reliability. Dry chlorine does not cause corrosion and also eliminates the need for metering pumps and gas regulators. Using dry hypochlorite instead of liquid sodium hypochlorite reduces the levels of both sodium and chlorate ions. For example, the addition of 2 ppm chlorine from calcium hypochlorite will add less than 0.2 ppm sodium ions and 0.04 ppm chlorate ions.
Solid chlorination delivery systems provide consistent, automatic dosing of hypochlorite. The chlorinators are activated by water flow, so they deliver a predetermined amount of active chlorine based on water volume requirements. An in-line flow meter is used to accurately dose the required amount of water. Several industries, including the beverage industry, rely on solid chlorination. For breweries, solutions enable them to prevent bacterial growth and assure the purity and fresh taste of their products on a continuing basis. Solid chlorination is effective as a general sanitizer, because of its dissolving action on beer stone, proteins, slime, yeast and other matter commonly found in brewery lines, tanks and hoses. It’s also effective in controlling mold growth in humid malt house conditions.
For residential wells, solid chlorine is just as effective. Feeding chlorine pellets directly into the intake line of the well pump is the best use of the compound. Even if it’s not possible to locate a feed at the intake line, well owners can feed pellets at any point in the pump discharge line.
Gerald F. Dooley is manager of sales and marketing for PPG Industries’ calcium hypochlorite chemicals group in Pittsburgh, PA.