2017 ASHS Annual Conference

Non-contained greenhouse irrigation water may soon be required to be sanitized before it can be released into sewer lines, or recycled back for reuse. A 2015 USDA Farm Bill grant was converted into a cooperative agreement between Colorado State University and USDA-Animal Plant Health Inspection Service, Center for Plant Health Science and Technology lab to conduct an irrigation water study. The objective of the water study was to test the ability of ozonated water and ultra-violet light to sanitize recycled irrigation water in greenhouses. Bacillus subtilis spores were used as a hard-to-kill surrogate for a wide range of plant pathogens that may bio-contaminate irrigation water. A private microbiology lab prepared the spore suspension and analyzed the treated water samples. The study involved four factors: 1) first water treatment – ozonated water or ultraviolet light (UV-C) radiation, 2) second, follow up water treatment – none, ozonated water, and UV-C radiation, 3) organic challenge - humic acid at 0 and 0.1% v/v, and 4) water treatment exposure time – 15 and 30 minutes. Ozonated water was generated on-site with a custom-made ozone machine, and the UV-C lamp dosage was 23 µw-s/cm² for one 9 watt lamp. Oxidation Reduction Potential (ORP) of ozonated water averaged 933 mV when it was added as a water treatment. Humic acid was added to water samples inoculated with a spore suspension to: 1) act as an organic challenge to ozone and to decrease water quality and 2) reduce UV-C light transmission by increasing water turbidity. The water treatment with the highest B. subtilisspore efficacy was UV-C light for both first (30 minute) and second stage (30 minute) treatment, which resulted in a 2.55 log10 reduction and 99.7% reduction of viable spores when compared to the controls. When humic acid was added as an organic challenge/water quality factor it had no effect on the spore efficacy results. Future studies are planned to evaluate commercial water treatment systems that generate a water solution with hydroxyl radicals and ozone which should increase efficacy. In addition, higher intensity UV-C lamps (55 watts) will be also be tested for enhancing water sanitation efficacy.