Keep it clean

Chlorination is by far the most cost-effective water treatment today but its efficacy is subjected to water pH. In 2002, our research team reported an improved chlorination protocol recommending 2.0 ppm free chlorine at sprinklers for irrigation water treatment. This protocol has since been widely used in the ornamental horticulture industry across the country.

This article is intended to help growers ensure their chlorination systems work as hard as they could be in removing pathogens from irrigation water. We will revisit the chlorine chemistry and pathogen-killing power under different water pH conditions; present the latest research data on water pH in recycling irrigation ponds and their implications for chlorine performance, and; discuss ways to manage recycled water pH for the best performance and economic return of this water treatment.
 

Chlorine chemistry and killing power

There are three chlorine species: combined chlorine, residual chlorine or free chlorine, and total chlorine. Free chlorine is the molecule that does the hard work killing pathogens and oxidizing contaminants. When free chlorine molecules oxidize contaminants such as ammonia, other nitrogen-containing organic compounds, they are tied up and are no longer available for killing pathogens. We call these tied-up chlorine molecules combined chlorine. Total chlorine is the sum of combined chlorine and free chlorine. There are numerous chemicals in irrigation water that could tie up chlorine molecules. Thus, among the three chlorine species, only free chlorine is a good indicator of potential pathogen-killing power.

How much this potential pathogen-killing power of free chlorine may be realized at production facilities depends largely on their water pH level. This is due to chlorine chemistry. Chlorine in water could be in three forms: dissolved chlorine gas (Cl2), hypochlorous acid (HOCl), and hypochlorite ion (OCl-); and they have very different pathogen-killing powers. For example, hypochlorous acid was estimated to be 20 to 80 times more effective in controlling Escherichia coli than hypochlorites. These three forms of chlorine co-exist in equilibrium, water pH dependent (Figure 1). At pH 4.2 to 5.3, nearly 100 percent of chlorine is hypochlorous acid but this percentage drops sharply with increasing water basicity. This means that the same level of free chlorine could have rather different pathogen-killing powers, depending upon the pH level in waters to be treated.
 

It is impractical to treat water at pH 5.0 or below due to severe corrosion. Thus, the water pH range for the best chlorine performance is between 5.0 and 5.3. Chlorine is most efficacious at this narrow pH range and its pathogen-killing power decreases with decreasing percentage of hypochorous acid in increasingly basic water environments. The estimated loss of chlorine pathogen killing power is approximately 4 percent at water pH 6.0, 25 percent at pH 7.0, 84 percent at pH 8.0, 90 percent at pH 9.0 and 94 percent at pH 10.0 (Table 1).
 

Water pH in recycling ponds

Water in irrigation ponds directly receiving runoff from production areas is alkaline for the most time of year as illustrated in Figure 2. Water pH readings in this pond of eastern Virginia were taken hourly for a 6-year period from 2006 to 2012. Water pH fluctuated from 5.0 to over 12.0 during this monitoring period. Overall, only 5 percent of the hourly pH readings fell between 5.00 and 6.99 while 95 percent were at 7.00 and above. This pH range and water basicity was totally unexpected in the first place, but later we found out through a project supported by the USDA National Institute of Food and Agriculture -Specialty Crop Research Initiative (SCRI) that this range of water pH fluctuation and basicity is actually not uncommon for runoff water containment ponds.

Another surprise finding was the extent of diurnal water pH fluctuations in the same ponds. Water pH always read the lowest in the early morning from 4 to 8 a.m. and the highest in the evening from 5 to 9 p.m.

Water pH is closely related to photosynthesis activity in the ponds. When the sun rises, algae and other photosynthetically-active agents remove carbon dioxide, a weak acid, from water to make carbohydrate. Consequently, water pH goes up. This process is expedited with rising temperature. This was particularly obvious from 10 a.m. to noon when water pH rises sharply. The greatest diurnal pH fluctuation observed during this project ranged from 6.5 in the morning to 10.0 in the evening, mounting a 3.5-unit difference from the lowest to highest point of the day.

These seasonal and diurnal water pH fluctuations have tremendous impact on chlorine chemistry and its pathogen-killing power.
 

Improving chlorine performance

As discussed above, recycled water is mostly alkaline but chlorine is most efficacious at pH 5.0 to 5.3. Thus, any strategies and practices that lower the pH level in waters to be treated will improve chlorine performance and economic return. Based on the latest water quality data from the SCRI project, we recommend the following:

Irrigating plants in early morning when water pH is at its lowest point of the diurnal cycle. This recommendation should work for all production facilities that are set to turn chlorination on whenever irrigation cuts on. This practice alone could greatly increase chlorine performance compared to irrigating plants in the evening when water pH could be 3.5 units higher than that in early morning. This practice will work particularly well for pond water with most pH readings at 9.0 and below. An additional benefit of this practice is reduced lengths of wetness and drier plant foliage that is suppressive for pathogen growth and disease development.

Taking water pH measurements regularly and acidifying water as needed prior to chlorination. This is essential to ensuring the best chlorine performance and the greatest economic return. As illustrated in Table 1, chlorine dollar is watered down to 10 cents or by 90 percent if alkaline water at pH 9.0 is chlorinated without pre-acidification. Please note that a water pH reading must be taken at the time of irrigation as it may change substantially at different hours of the day.

Whether and how water quality, in particular of water pH, in irrigation ponds may be further manipulated to improve chlorine performance is currently under investigation by the SCRI project team and collaborating growers. As these studies progress, new results will be reported in the future articles of this Recycled Water Quality series.

Chuan Hong is a professor at the Hampton Roads Agricultural Research and Extension Center, Virginia Tech; chhong2@vt.edu.