Maximum Yield USA December 2020 / January 2021 | Page 43

Slow-Sand Filtration or Biological Disinfection
Slow-sand filtration or biofiltration is a method of water / nutrient disinfection that works by passing solution slowly through different layers of aggregate where large numbers of beneficial microbial populations reside . The sand / substrate screens have organic material in the upper layers , while the microbial species in the lower layers provide biological filtration that assists with pathogen removal . This system has the advantage that the nutrient solution discharged from the filter is enriched with beneficial microbial species that can further assist with the reduction of pathogens in the hydroponic system . To be effective the flow rates of solution through the biofiltration system need to be correct and slow enough for disinfection to occur .
Heat
Heat disinfection ( pasteurization ) of nutrient solutions is one of the most reliable methods of eliminating all types of plant pathogens and is used commercially by greenhouse growers . Most heat treatment systems first pre-filter the solution to remove organic material , then heat to a temperature of 203 ° F for a holding time of 30 seconds which has been found effective in the control of root rot pathogens such as Phytophthora and Pythium . While heat treatment is effective and crop safe , the drawbacks are the high energy requirement to heat solutions up to temperature and the time required to cool the solution back to ambient before reintroduction to the hydroponic system .
Chemical Disinfection
The two main chemical disinfection agents that may be used to treat water supplies and hydroponic nutrient solutions are hydrogen peroxide and chlorine . While both are effective in killing pathogens if the correct rate is applied , both have the drawback of potentially damaging crops if overused . The effectiveness of chlorine as a disinfection agent is dependent on dose , temperature , organic loading , and microbial content of the solution being treated . Also , pathogen species and the life stage of the pathogen being controlled are also factors in the effectiveness of chlorine treatment . Studies have reported that zoospores of Pythium were controlled by exposure to 2-2.5mg / l , however , a higher
“ From time to time , root pathogen outbreaks may occur , and prevention of such issues is generally far more effective than any type of control measure .”
level of free chlorine of 14mg / l was required to control other pathogen species — this level was also phytotoxic to many common nursery species . Chlorination toxicity symptoms include necrotic mottling , stunting , and premature leaf drop . Hydrogen peroxide ( H 2O 2 ) is an unstable oxidizing agent that reacts to form H 2O and an O-radical that reacts with any type of organic material including pathogens . The by-product of using hydrogen peroxide is the release of oxygen into the nutrient solution . The overuse of H 2O 2 in recirculating nutrient solution carries the risk of damaging root systems . Young sensitive plants are particularly prone to damage from H 2O 2 dosing with rates as low as 8-12ppm found to reduce the growth of hydroponic lettuce , while concentrations of 50ppm were required to kill Pythium and 100ppm to control Fusarium . Both chlorine and hydrogen peroxide react with organic matter in the nutrient solution , thus rates need to be carefully considered and based on the organic loading of the system to prevent the risk of plant damage from occurring .
Surfactants
Application of surfactants to the recirculating nutrient solution is another method used for the control of pathogens . Non-ionic surfactants have the ability to rapidly lyse mobile zoospores of pathogens such as Pythium and Phytophthora within one minute of exposure . While surfactants can destroy large numbers of mobile zoospores in the nutrient solution , the application has no residual effect on plants already infected when the disease is already contained inside plant tissue . Thus , the use of non-ionic surfactants as a nutrient solution additive is more of a preventative action than a curative one .
The use of any nutrient solution disinfection method should be made carefully as many methods not only destroy plant pathogenic microbes , but also those species that are beneficial in hydroponic systems . Disinfection treatments can have potential downsides and incorrect use of some can even be phytotoxic to plants , particularly young sensitive crops , so knowing the pros and cons of each method is an important step before implementation .
Maximum Yield 43