Irrigation systems can become partially or completely clogged from biological growths of bacteria, fungi, or algae which are often present in surface and ground water. Bacteria, fungi, and algae use chemical elements such as nitrogen, phosphorus, sulfur, or iron as nutrient sources to grow and develop. Generally, filtration alone cannot effectively remove these micro-organisms. Chlorination can be used to minimize the growth of micro-organisms within the pipes and other components of irrigation systems.

If water is not properly treated, clogging of pipes, fittings, and emission devices (sprinklers, drippers, spray jets, etc.) can occur, resulting in decreased crop growth and development because of reduced water application amounts, uniformity, and efficiency. Information is provided on sources of chlorine and the amounts required for treating irrigation water and systems to control the growth of micro-organisms.

Sources of Chlorine

Chlorine is available in gas, liquid, and solid (granular or tablet) forms. However, only the liquid form (liquid sodium hypochlorite) has an Environmental Protection Agency (EPA) special local need (SLN) label for use as a pesticide in irrigation systems in Florida. All pesticides or other chemicals should be injected into irrigation systems only as directed on the chemical labels. Also, all label directions must be followed to ensure that the chemicals will be safely applied.

Each of the three (3) different chlorine forms reacts differently with irrigation water, depending on the other chemicals or elements in the water. Reactions may be changes in the pH of the water, or precipitation of some element which could result in clogging of micro-irrigation components.

Effects of Chlorine

Hypochlorous acid (HOCL) is the effective agent that controls bacterial growths. The amount of HOCL that will be present in solution, and thus active, will be larger at lower pH levels  (more acidic conditions). At pH 8, only about 22% of the chlorine injected will be in the active HOCL form, at pH 7, about 73% will be in the HOCL form, and at pH 6 , about 96% will be in the HOCL form (Nakayama and Bucks, 1986). Thus, if the irrigation water pH is high (as often is the case when pumping from the Floridian aquifer), the effectiveness of chlorine may be enhanced by injecting an acid to reduce the pH of the water before injecting chlorine (Kidder and Hanlon, 1985). In addition to increasing the effectiveness of chlorine, acid injection can also prevent the precipitation of minerals which may plug micro-irrigation systems. However, it is normally only necessary to reduce the pH one or two units to achieve these desirable benefits. Procedures for calculating the amount and form of acid to inject were given by Kidder and Hanlon (1985).

At extremely low pH levels (or high acidity) chlorine gas (CL.2) will form. Therefore, for safety, it is very important to store chlorine and acid sources separately. Also, storage and use areas should be well ventilated so that gasses cannot concentrate and become a hazard in a building or other enclosed area.

Chlorine may react with some metal and plastic components of irrigation systems. Therefore, always check with the manufacturer, or supplier of system components to identify any potential problems before beginning a chlorine injection program.

Hypochlorous acid will react with iron in solution to oxidize the ferrous iron to the ferric form. The ferric iron then becomes the insoluble ferric hydroxide as a precipitate. Chlorine should be injected before (upstream from) the filters so that these precipitates may be trapped in the filters.

Chlorine will also react with hydrogen sulfide to form elemental sulfur. Because some of the chlorine is used up by reacting with the sulfide or ferrous ions, additional chlorine must be provided for these reactions to occur. Enough residual chlorine must be injected to control sulfur or iron bacteria, or algae, which can clog micro-irrigation systems.

Most micro organisms will be inactivate and controlled at free residual chlorine concentrations of 1 ppm. However, higher concentrations must be injected due to the inherent chlorine demand of different water sources. As a start, use 2 ppm of chlorine for each ppm of hydrogen sulfide, plus 0.6 ppm of chlorine for each ppm of ferrous iron. A chemical water test can be used to determine the levels of hydrogen sulfide or ferrous iro0n present in solution. Water from surface sources  such as lakes, ponds, or canals should be treated with approximately 5 to 10 ppm of chlorine. Higher levels may be needed for water with high amounts of microbial activity such a may occur during warmer months of the year.

The chlorine injection rate should be checked by testing the water at the most distant part of the irrigation system using a test kit designed to measure 'free' residual chlorine. Residual concentrations of 1 to 2 ppm at this location indicate that active chlorine still exists after the water and system parts have been appropriately treated.

Test for active chlorine using a D.P.D colour indicating test kit that measures 'free' residual chlorine. Do not use a test kit that only measure total chlorine. While levels of total chlorine may appear to be adequate, the active 'free' residual form may not be adequate. Therefore, ask for D.P.D. test kit from either a swimming pool or irrigation supply company.

Chlorine Application Amounts

After determining the desired chlorine concentration, the proper amount to be injected must be determined. The amount of chlorine to apply per gallon of irrigation water will depend on the desired concentration in the irrigation system and the concentration or strength of the chlorine source.

Liquid sodium hypochlorite is the most convenient and generally safest form of chlorine available to inject into irrigation systems. Stock solutions can be bought with concentrations of 5.25, 10 or 15 percent available chlorine. Equations 1 to 3 may be used to determine the chlorine solution injection rate in gallons per hour (gph) for different desired ppm injection levels and irrigation system flow rates. Equations 1 to 3 are specific for liquid chlorine injection and are designed for stock solution chlorine concentrations of 5.25, 10 and 15 percent, respectively.

For a 5.25% available stock chlorine solution: For a 10% available stock chlorine solution: For a 15% available stock chlorine solution: For example, an irrigation system has a flow rate  of 500gpm and the water is to be treated with 8ppm of available chlorine using a stock solution with 10% available chlorine. Using equation 2, the injection rate of the stock solution should be : (8ppm)(500gpm)/1850 = 2.2gph Or for a treatment level of 8ppm and a 10% available chlorine concentration, read an injection rate of 0.43gph. Note that this is the required injection rate for each 100gpm. Thus, for 500gpm, the injection rate would be five times as large, or 2.2gph.

If the stock solution concentrate was 5.25% available chlorine, then the injection rate should be :(8ppm)(500gpm)/971 = 4.1gph. Or, for a treatment level of 8ppm and a 5.25% available chlorine concentration, read an injection rate of 0.82gph per 100gpm of irrigation flow rate. Then for 500gpm, the injection rate would be five times as large or 4.1gph.

If the injection rate calculated is too small for the injection pump to be used, the chlorine stock solution can be diluted with irrigation water. Thus, if the 10% stock solution was diluted with 1 part water and 1 part 10% chlorine solution, the new stock solution would be diluted by 1/2. It would have 5% available chlorine, assuming that the water added did not tie up any of the available chlorine. Likewise, if the 10% stock solution was diluted with 4 parts water and 1 part 10% chlorine solution, the new stock solution would be diluted by 115 and it would have 2% available chlorine.

Summary

The sources of chlorine used to treat water for micro-organisms include chlorine gas, powder, or tablets of calcium hypochlorite (pool bleach), and liquid sodium hypochlorite (laundry bleach). However, only liquid sodium hypochlorite is labeled for use in irrigation systems in Florida. The concentration of available chlorine ranges from 5.25 - 1517c in liquid sodium hypochlorite. Therefore, the amounts of these products to be injected will depend on the stock solution concentration used. The user should check with the chlorine supplier to ensure that material is labeled for injection into irrigation systems. In addition, safety and proper backflow prevention are always required when injecting chemicals into an irrigation system.

Safety

Chlorine is a powerful oxidizing agent and it must be handled carefully. A fresh water source should be available at the field site where liquid sodium hypochlorite is being used so that nay contact or spills can be immediately washed off. Protective clothing should be worn while handling this chemical and injectors. Goggles should be worn to protect eyes against splashes.

Chlorine is a respiratory irritant which affects the mucous membranes. It can be fatal after a few breaths at 1000ppm. Therefore, users of chlorine gas must exercise extreme caution to ensure that it is safely injected. Maximum air concentrations should not exceed 1ppm for prolonged exposure. Chlorine gas should only be used in well ventilated areas so that nay leaking gas cannot concentrate. This form of chlorine is commonly used in municipal water treatment systems. It should only be used by experienced or licensed users. For safety, only vacuum type injectors should be used.