compounds could include iron, manganese, nitrite; nitrogen-based compounds, such as ammonia may also be found due to decaying plant material.
An understanding of chlorine demand is just as important as the pH of the water source. As chlorine is added to irrigation water, initially, a portion of the added chlorine is consumed as it reacts with minerals and organic compounds, including bacteria. It is desirable to add sufficient chlorine so that there is a residual, thus overloading chlorine (to a small extent). This extra chlorine provides downstream protection and can help prevent or reduce the likelihood of biofilms that may form in the water distribution system.
Figure 3 - Comparison of the different forms of Chlorine (Source: Faith Critzer, Washington State University).
The total residual chlorine is the remaining chlorine that has disinfectant properties. The residual chlorine is divided into two groups: 1) combined residual chlorine; and 2) free residual chlorine (Figure 3). Combined residual chlorine is chlorine that has reacted with ammonia to form monochloramine, dichloramine, and trichloramine. These chloramines are poor disinfectants because they take an extremely long time to kill microorganisms (over 30 minutes). Free residual chlorine or free available chlorine is the sum of HOCl and - OCl. Free residual chlorine is available downstream of the injection point, after the initial binding of chlorine molecules to organic, inorganic, and ammonium compounds. The chlorine dose will include the demand as well as the desired residual. An additional decision that needs to be made is the quantity of the residual. Depending on the situation, a residual of 10 ppm may be desirable. Too high of a chlorine residual may damage the soil health and the crop.
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