What's In Your Water?

Problems with water treatment systems designed to control algae, biofilm, or waterborne pathogens are sometimes not identified until green and slippery algae grows on greenhouse floors, emitters are clogged or plant diseases occur. That is not the best monitoring method.

Monitoring water
All water treatment systems require regular maintenance and checking. For example, copper systems rely on controlled electrolysis to form soluble copper ions, often resulting in corroded connections and plates or rods that require maintenance. If an injector is not functioning for a product such as chlorine dioxide or activated peroxygen, an incorrect level can be dosed of a sanitizing agent. Too high a dosage is a worker safety, plant phytotoxicity, and environmental risk; too little dosage leads to ineffective treatment.

Click image above to see full Table.

On-site tests enable the grower to check the concentration of sanitizing agents. These tests, with examples in Table 1, include manual colorimetric tests, handheld meters or inline controls for continual dosage systems. Because sanitizing products are generally short-lived in water, onsite tests are the best option for reliable measurement. On-site tests have the advantage of being low cost and rapid, allowing repeat measurements and the tracking of trends over time (just as one can track water pH or EC).

Measurements of the concentration of sanitizing agents can be easily performed on site using a colorimetric test kit. These colorimetric test kits can be categorized into visual tests, such as test strips and test ampoules; titrimetric test (such as the droplet test kit where drops of a reagent is added until a color change occurs, and instruments that measure the color of solutions or test strips. Visual tests tend to be more subjective than instruments but are generally adequate for horticultural use.

We have evaluated water treatment systems in several greenhouse operations. More often than not, the technologies have either not been recently checked or the sensors are out of calibration. If the manufacturer provides or recommends a test kit, make sure the kit is used regularly – at least once a month – and has not exceeded its shelf life. Because colorimetric tests – and especially test strips – are inherently subjective, train one person to do the tests. With in-line control systems, train staff to check that sensors are calibrated and ensure they have a technical and common-sense understanding of the system.

The Oxidation Reduction Potential (ORP) can be read with a meter similar to a pH meter. ORP is a measurement of the sanitizing (oxidation) power of oxidizers including chlorine, chlorine dioxide and ozone. ORP sensors are often placed in line for controlling dosage of these oxidizers, and hand held meters are also available. The units of ORP are in millivolts (mV) of oxidation or reduction and high values (650 to 800 mV) indicate increasing sanitizing strength. ORP is not useful for measuring concentrations of activated peroxygens (such as ZeroTol or X3) or hydrogen dioxide, or for non-oxidizing technologies such as copper or UV light.

Monitoring water quantity, quality and drainage is best done preventatively, rather than after problems occur. Photo by Paul Fisher

The value of ORP in a sanitizing solution is affected by the specific sanitizing chemical, the concentration of active ingredient, and the solution pH. For example, the ORP value in a 2 ppm free chlorine solution increases as the pH decreases because the strong sanitizing and oxidizing form (hypochlorous acid) of free chlorine predominates at low pH whereas hypochlorite, a weak sanitizer, predominates at high pH. At constant pH, ORP value increases as the residual chlorine concentration increases.

One practical challenge to measuring ORP is that the water should flow over the sensor, and several minutes may be required before the ORP value stabilizes. For example, you can place a hose with solution into a bucket or other container, and place the end of the sensor into that solution. The sensor should be calibrated with a standard solution before use. Clean the sensor after each measurement, and store according to manufacturing instructions. If the water treatment system is injecting a sanitizing agent according to a targeted ORP value (for example, automated dosing of chlorine or ozone), then regular maintenance and calibration of the ORP sensor is an essential part of dependable control.

For plant pathogens, research has shown rapid control of Pythium zoospores with a chlorine solution when the ORP was over 700 mV. At 2 mg/L of chlorine, and pH around 7, the ORP is approximately equal to 700 mV. Research isn't yet available on the relationship of ORP and control of plant pathogens for chlorine dioxide or ozone. For control of human pathogens in wash water of fruits and vegetables, a target ORP of 650 to 700 mV is often used in packing house solutions and so is a good starting point for horticultural crops.
 

Sanitizing agents can be monitored using test strips. The test strips above are used to measure (from left to right) free chlorine, chlorine dioxide, copper, peracetic acid and hydrogen peroxide. Photo by  Jinsheng Huang

Sanitizing agent demand
Sanitizing agent "demand" affects how much "residual" active ingredient remains that is available to control target organisms such as plant pathogens after a chemical is dosed into irrigation water. For example, 2 mg/L (ppm) of residual "free chlorine" is recommended by plant pathologists for control of Pythium and Phytophthora zoospores. The residual free chlorine represents the combined hypochlorous acid and hypochlorite that has not reacted with organic matter or other contaminants after dosing into the water sample.

For example, a grower might find that they need to inject 5 mg/L of chlorine to pond water, in order to have 2 mg/L of residual free chlorine at the irrigation emitter furthest away from the pond intake. In that case, the suspended solids or other contaminants would create a demand of 3 mg/L (5 mg/L applied minus 2 mg/L residual). That means that 5 mg/L of total chlorine would need to be applied at the source to have the 2 mg/L of residual free chlorine available for control of pathogens present in the irrigation system at the furthest outlet.

Demand will usually change during the year and can even change during a crop production cycle. For example, the density of microbes tends to increase in pond water as the temperatures increase in the summer. Increased demand will reduce the residual free chlorine. Consequently, the residual concentration of free chlorine may be too high in the winter (causing phytotoxicity and wasted chemical), or too low in the summer to control the target pathogens. This change over time means that regular testing of dosed and residual active ingredient is advised. The same concept can be applied to sanitizing agents other than chlorine.

Click image above to see full Table.

Other ways to measure the demand of a water sample is the biological oxygen demand (BOD, Table 2) and chemical oxygen demand (COD). These variables are mainly used in drinking water supplies, although the EPA has recommended levels for irrigation use. The BOD represents the amount of oxygen used up by microorganisms to decompose organic waste matter in water. To measure BOD, the dissolved oxygen is measured in the water, the container is closed and incubated at a constant temperature for a set period (for example, five days), and then dissolved oxygen is again measured. The measured drop in oxygen is the BOD. The COD is similar, but tests how much of a strong oxidizing chemical is needed to completely oxidize the contaminants (particles, ions, and microbes). Although BOD and COD are related to the amount of sanitizing agent required in horticulture irrigation water, this relationship is not direct and requires further research.

Dissolved oxygen (DO) levels are important for root health in hydroponic production. Researchers have found that when root zone oxygen level dropped to below 3 ppm in hydroponics, tomato plant roots were much more susceptible to Pythium infection, and growth of tomato and cucumber was reduced. Aeration of pond water reduces algal growth, and 5 mg/L is recommended to avoid fish kills. Aerating subirrigation tanks is likely to increase efficacy of oxidizers such as chlorine, however clear recommendations are not yet available.

Jinsheng Huang is research scientist, Paul R. Fisher is associate professor and extension specialist and Dustin Meador is Ph.D. candidate, University of Florida, pfisher@ufl.edu. William R. Argo is technical manager, Blackmore Co., bargo@blackmoreco.com.

The authors thank sponsors of the Water Education Alliance for Horticulture (watereducationalliance.org) for supporting this series, including AquaPulse Systems, BioSafe Systems, Blackmore Co., Chem Fresh, Chlorinators Incorporated, DRAMMwater, Fafard, Greencare Fertilizers, Griffin Greenhouse and Nursery Supplies, Hanna Instruments, Phyton Corporation, Pindstrup, Premier Horticulture, Pulse Instruments, Quality Analytical Laboratories, Sun Gro Horticulture and Trueleaf Technologies. The authors also thank the USDA-ARS Floriculture and Nursery Research Initiative and the Young Plant Research Center (floriculturealliance.org) for supporting the underlying research.