Bridging the GAPs: Approaches to Treating Water On Farms

Figure 2 - pH impact on HOCl concentrations (Adapted from:

As for irrigation water, it’s important to understand that the source of the water can directly impact its pH. For example, the geology from which the water passed through or passed over influences pH as ions from the rocks and soil dissolve into the water. Geology high in carbonates tends to produce waters with higher pH; geology high in iron and/or sulfur tends to produce waters with lower pH. In the water treatment industry, it is very common to adjust the pH before adding chlorine. With an understanding of the water source and its pH, growers can determine the amount of additives needed. It is best to take water samples (of a known volume) and add the pH adjustment in small increments. With each increment, measure (and write down) how that addition changed the pH. For high pH values, acid may be added such as sodium bisulfate (NaHSO4), muriatic acid (HCl), or citric acid, for example. For low pH water sources, a base (high pH) solution can be added, such as sodium carbonate (soda ash – Na2CO3) or sodium hydroxide (lye – NaOH). For pH control, it is important to note chlorine and acids/bases are not compatible; addition of the acid or base should be injected separately from the chlorine.

Based on Figure 2, at what pH is chlorine most effective? Are there any drawbacks at this pH?

Determining Chlorine Addition: Chlorine Demand and Chlorine Residual

Chlorine is a strong oxidizer and will react with both organic and inorganic matter. The demand by organic and inorganic matter is known as chlorine demand. Chlorine demand is defined as the difference between the amount of chlorine added and the residual chlorine available (Figure 3). Examples of inorganic


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