Water Sanitation Part 3: Oxidizers Used for Water Sanitation
In part 2, we discussed the importance of “preparing” irrigation water for sanitation. This involved the removal of total suspended solids, reduction of biofilm in pipes and tanks and consideration of water pH and electrical conductivity. All these factors influence the application rate and efficacy of a sanitizing agent. Now in this final part, we will discuss different types of sanitizing agents, how they work and some of the advantages and disadvantages of each. As many have found, there is not one system that works best for all growers. This article will focus on oxidizing agents used to sanitize greenhouse irrigation water and the second half, which will be in the next Grower Services Newsletter, will focus on other methods to sanitize water.
Oxidizing Agents
Oxidizing agents are added to irrigation water to kill pathogens by direct exposure to the agent for a specified length of time. The length of exposure required to kill pathogens depends on the pathogen itself, its inoculum structure, the type and concentration of the oxidizing agent and biological load in the water.
An advantage of most oxidizing agents is that there is residual product that persists in the water. Therefore, if the ‘sanitized’ irrigation water becomes re-contaminated with a pathogen before reaching the crop, it can control the pathogen. Oxidizing agents work well but are often ‘used up’ in organic media components, such as peat, bark, compost and coir as well as algae, plant debris, biofilm and of course, pathogens. Water must be filtered and cleaned for an oxidizing agent to effectively control pathogens in water. Also consider that oxidizers react and bind with iron, manganese, boron and other metals found in the water, also reducing their efficacy.
The most common oxidizing agents used to clean irrigation water will be discussed in the following chart below. These are chlorine gas, chlorine dioxide, calcium hypochlorite, sodium hypochlorite, ozone and activated peroxygen.
| Sanitazer (Active Ingredient) | Delivery into water | Rate | Comments |
|---|---|---|---|
| Chlorine gas (CI2) | Chlorine gas is bubbled into the water where it combines with water to form hypochlorus acid(HOCI) or hydrochloric acid (HCI). | 0.5-2.0 ppm free chlorine. 2 ppm controls zoospores of many Pythium and Phytophtora species. | Lowest cost form of chlorine per ppm. Chlorine gas is stored in a cylinder, so no pre-mixing is required. Hazardous gas requires special equipment, ventilation and handling. Exceeding 2 ppm free chlorine in the water can be phytotoxic to plants. Ideal water pH 6.0-7.5 (below 4 it gases off, above 7.5 it is less effective). Metals such as Fe, Mn, B, Cu, etc. And organic matter can tie up free chlorine. |
| Chlorine Dioxide (CIO2) | Gas injected into water forming a concentred stock solution of CIO2, then it is injected. | 0.25 ppm or less CIO2 residual (1 ppm CIO2 may be required for some bacteria). | Chlorine dioxide moves freely within water making it effective at controlling biofilm. Effective at apH of 4-10. Not as sensitive to organic matter as other chlorine sources. Often requires equipment to make chlorine dioxide on the premises. Should use stock solution within 15 days to minimize loss from volatilization. can be phytotoxic at high rates. Forms precipitates with metals (Fe, Mn, B, Cu, etc.). |
| Calcium Hypochlorite (Ca(OCI)2) | Granules or tablets are weathered with a small amount of irrigation water that is bypassed through the chlorinator. A gate controls the ppm CI2 rate. | 0.5-2.0 ppm free chlorine. 2 ppm controls zoospores of many Pythium and Phytophtora species. | Safer than chlorine gas and sodium hypochlorite. Easier to store and less corrosive than other chlorine forms. Granular and tablets dissolve at slower rates as water temperature drops. Exceeding 2 ppm free chlorine can be phytotoxic to plants. Ideal water pH 6.0-7.5 (below 4 it gases off, above 7.5 it is less effective). Metals and organic materials in water can tie up free chlorine. Can produce cloudiness and sediment if free chlorine concentrate exceeds 200 ppm CI2. |
| Sodium Hypochlorite (NaOCI) | Liquid sodium hypochlorite (5-15% chlorine) is injected directly into water. Also known as bleach. | 0.5-2.0 ppm free chlorine. 2 ppm controls zoospores of many Pythium and Phytophtora species. | Easy to apply through injection. Exceeding 2 ppm free chlorine can be phytotoxic to plants. Ideal water pH 6.0-7.5 (below 4 it gases off, above 7.5 it is less effective). Metals and organic matter can tie up free chlorine. Requires a corrosion-resistant injector. Has limited shelf life - use within 1-2 months - warm temperatures and sunlight speed up degradation. |
| Ozone (O3) | Generated on-site with an electric arc and then bubbled through water. | Injection rate <1 ppm O3, residual activity of 0.01-0.2 ppm O3. Residual at or above 1 ppm O3 may be phytotoxic. | No raw materials are needed and no waste is created. High initial set-up cost, but low operating cost. Has to be professionally designed to prevent hazardous levels of ozone from escaping from water. Best to inject into water in an enclosed tank, ozone should be in contact with water for 20-60 min. 0.2 ppm O3 can destroy biofilm if held for 30 minutes. Short-lived residual activity in water, ideal pH = 4 and cold water temperature provide the best shelf life decreases. Will oxidize metals in water creating precipitates. Filter water to remove organic materials as they reduce efficacy. Difficult to monitor as ozone degrades quickly. |
| Hydrogen Peroxide/Hydrogen dioxide (H2O2) and Peroxyacetic acid/Peracetic acid (CH3COO-OH) | Both types are injected directly into water. | Depends on the manufacturer but the range is 27-540 ppm H2O2. | No toxic waste is produced- only carbon dioxide, hydrogen and oxygen- and is environmentally friendly. Easy to use, but corrosive, so special injectors are needed. Peroxyacetic acid/ peracetic acid are more effective biocides and more stable in water than hydrogen peroxide/hydrogen dioxide. Organic matter greatly reduces efficacy, forms precipitates with metals and the ideal water pH is below 7. More effective at warmer temperatures. UV light will degrade these products. |
These oxidizing agents all have their advantages and disadvantages. Whether you consider cost, residual effects, environmental hazards, worker safety, etc., not one system meets all these criteria. In the next edition of the Grower Services Newsletter, we will continue presenting the remaining types of water sanitation methods that are currently offered today to sanitize irrigation water for plants.
References:
- Konjoian, P. 2011. Chlorine Dioxide in Horticulture: A Technology Review. Greenhouse Grower March 17, 2011
- Powell, C.C. 2001. "Are Your Plants Drinking Dirty Water?" Grower Talks 79(7)
- http://pnwhandbooks.org/plantdisease/pesticide-articles/treating-irrigation-water-eliminate-water-molds
- Newman, S.E. 2004. "Disinfecting Irrigation Water for Disease Management." 20th Annual Conference on Pest Management on Ornamentals – Society of American Florist Meeting (San Jose, CA).
