For almost 100 years now, chlorine gas has been the primary disinfectant used in treating municipal water supplies worldwide. Using chlorine gas for disinfection is a logical choice in that it is very effective and economical, and easy to operate and maintain. Many will also argue that chlorine gas is actually the safest disinfection method due to the fact that is it now almost always fed under vacuum, greatly reducing any chance of leaks. Although chlorine gas is a hazardous substance, most people do not know the facts and safety statistics of its use in water disinfection. Without this information many tend to overestimate the hazards of chlorine gas when comparing it to alternative disinfection methods such as sodium hypochlorite and calcium hypochlorite.
The selection of a gas chlorinator is dependent upon the flow rate of water to be treated, in Gallons Per Minute (GPM); and the chlorine demand of the water, in Parts Per Million (PPM). The feed rate of the chlorinator is expressed in Pounds Per Day (PPD).
Chlorine demand is the tendency of the water you are treating to consume the chlorine that you mix into the water. Chlorine demand in your water is effected by many different things. Some of them may be: algae, bacteria, iron, manganese, hydrogen sulfide, water temperature, and agitation, or aeration, of the water. While it is possible to test for chlorine demand, experience has shown the following values to be very reliable for estimating chlorine demand:
- Well Water - 1 to 2 PPM
- Surface water - 4 to 5 PPM
- Waste water - 8 to 10 PPM
- Chlorine gas is supplied in steel cylinders, as a compressed gas in a liquid state. The pressure in the cylinder is dependent upon the temperature of the cylinder. Cylinder pressures will range from 28 PSI at 20° F to 191 PSI at 120° F. As a safety feature, the cylinders are fitted with fusible plugs that will melt at about 165° F. This is done to prevent the cylinder from exploding if it is in a fire situation.
- For smaller water systems, chlorine gas is typically supplied in "150 pound cylinders," while larger systems will use "ton ( 2000 pound ) containers". The weight refers to the weight of chlorine that is being supplied, not the weight of the full cylinder. Full 150 pound cylinders will weigh from 235 ponds to 290 pounds. Full ton containers will weigh from 3,300 pounds to 3,650 pounds.
- Chlorine gas is sold "by the pound". In addition to the price of the chlorine, a deposit will be charged for each cylinder. A typical deposit for a 150# cylinder is $200.00, while a typical ton container deposit is $1,500.00. The deposit on the 150# cylinder allows use for 180 days (six months), and the ton container deposit allows use for 30 - 90 days (depending on the supplier). If the cylinder is not returned within the designated time, a penalty will be assessed for each day over the allowed time.