Water Disinfection by Chlorine

Many of the water sources used for irrigation require preliminary treatment before they can be considered safe to use. Water disinfection is one of the treatments that may be needed.

Disinfection is important in order to prevent spread of diseases originating in the source water, and also to prevent bacteria and fungi growth in the irrigation system.

The Importance of Irrigation Water Disinfection

Deciding whether to disinfect your irrigation water depends both on your water source as well as on the susceptibility of the crop to pathogens that can be spread by the water.

Surface water might contain plant pathogens and infect plants with diseases.

Bore-hole water might contain bacteria, such as iron bacteria, sulfur bacteria etc. These bacteria can grow in irrigation lines and may cause severe clogging of irrigation systems and drip lines.

Water recirculation and re-use are especially dangerous because it may cause rapid spread of plant diseases.

Various methods are available for water disinfection, including chlorine, UV, slow sand filtration, Ozone, heat etc. This article reviews chlorine disinfection, which is one of the most economic and the most effective methods.

Advantages of Chlorine As a Disinfectant

Efficiency - chlorine eliminates most microorganisms, including most bacteria, viruses and fungi.

Cost - both set-up and maintenance costs are relatively low compared to other water disinfection methods.

Prolonged protection - when chlorine disinfection is done correctly, residual free chlorine remains in water, protecting it against re-growth of micro-organisms. Residual free chlorine also protects irrigation lines against clogging as a result of growth of slime and algae. Other methods leave water exposed to re-infection.

Basic Principles of Water Disinfection by Chlorine

Combined chlorine versus Free (residual) chlorine - Chlorine binds to organic matter in water, resulting in compounds with very low disinfection efficiency.

Free chlorine is the concentration of the residual chlorine in water, which has high disinfection efficiency. This is why free chlorine measurements are most frequently used in order to evaluate the efficiency of the disinfection.

Total chlorine is the sum of the combined chlorine and the residual chlorine. Measuring the total chlorine will not necessarily give a good indication of the disinfection efficiency.

Factors Affecting Disinfection Efficiency

Organic Matter - organic matter load in the water affects chlorination efficiency.

Higher organic matter levels consume more chlorine and therefore, in order to reach a particular residual chlorine concentration, addition of more chlorine is needed.

Preliminary water filtration, especially in water recirculation systems, may substantially decrease organic matter load, thus reducing the amount of chlorine needed for effective disinfection.

Contact Time and free chlorine concentration - the duration of time allowed for contact and reaction between chlorine and the micro-organisms is extremely important.

Free chlorine level must be related to the contact time. At longer contact times lower concentrations of free chlorine can be used and vice versa.

Water pH - free chlorine in water exists in 3 forms: Cl2 (dissolved gas), HOCl (Hypochlorous acid), OCl^- (hypochlorite). HOCl is 100 times more effective than OCl^-.

The relative proportions of these three forms are determined by the water pH.

The chart below shows that in pH above 6.7 the relative proportion of HOCl significantly decreases, and consequently the efficiency of disinfection is markedly decreased. At pH of 7.4, only 50% of chlorine is in the form of HOCl.

Temperature - disinfection is more effective in higher temperature, although too high temperature actually reduces the efficiency of disinfection. As a rule of thumb, a decrease of 10 degrees Celsius reduces efficiency of disinfection by 50-60%.

Turbidity - in turbid water, that contain many suspended particles, bacteria can "hide" inside and in between the particles, thus "escaping" contact with chlorine. Therefore, in many cases it is necessary to filter the water prior to disinfection.