Topic: Aeroponic System Best Practices - Cleaning Overview
When not properly maintained, recirculating aeroponic systems become ideal breeding grounds for pathogens. Once pathogens become established, transmission and proliferation occur soon after, making removal more difficult. In all cases, the most effective treatment is prevention.
Effective Pathogen Prevention in Aeroponics
Roots in aeroponic systems are more vulnerable to pathogens than traditional substrate-based system due to the lack of a rootzone buffer. For this reason, preventing pathogens from establishing themselves is critical. Preventing the establishment of pathogens necessitates certain thresholds of cleanliness to be maintained throughout a grow. For growers familiar with traditional hydroponic and soil-based systems, adopting this preventative mindset necessitates a paradigm shift with regards to what is required for long term success.
Maintaining Cleanliness Through Disinfection
Long term success in aeroponics requires a certain degree of cleanliness to be maintained at all times. Unless operating under sterile conditions, hydroponic systems will be exposed to contaminants overtime. Therefore, cleanliness can only be maintained through regular disinfection. Different disinfection techniques exist - filtration, pasteurization, irradiation, and chemical treatments are most common. Chemical disinfection is the most dynamic method available for most hydroponic systems as it can be applied to most systems with minimal footprint or disruption. We have found that chemical oxidation is one of the most practical and affordable forms of chemical disinfection.
Chemical oxidation is a method of disinfection that utilizes specific chemically reactive oxidizing agents. Chemical oxidizing agents react with organic matter in a process known as chemical oxidation. When the organic matter being reacted with is particularly small, as is the case with fugal and bacteria cells – the reaction damages the structural integrity of the cell walls, killing or disabling the organisms.
Chemical Oxidizing Agents
Several chemical oxidizing agents exist - the most common of which include hydrogen peroxide (H2O2) and hypochlorous acid (HOCl). Despite both being oxidizing agents, the process by which each reacts with the organic matter in solution is different. More importantly, the effective concentrations at which each oxidizer effectively controls various pathogens differ substantially.
As a strong oxidizer, H2O2 can be applied to prevent the growth of plant pathogens within fertigation water. When added to water, hydrogen peroxide reacts to form H2O and a wide variety of free radicals and other reactive species capable of reacting with and decomposing organic material, including pathogens. Regardless of the intermediate reaction steps, the final by-products of H2O2 are oxygen and water. Commercially available forms of H2O2 are sold in a variety of different concentrations. Typically, hydrogen peroxide is available for purchase in concentrations of 3%, 26%, and 35%. Typical concentrations of H2O2 in solution for different applications can range between 25-150 PPM depending on exposure time, the concentration of organic matter in solution, plant species, and plant age. In appropriate contexts, higher concentrations can be advisable, but doing so can risk phytotoxicity. See Informed Dosing Decisions below for an explanation of the relationship between concentration percentages and PPM.
Hypochlorous acid is a special form of chlorine that is highly effective at disinfecting. Hypochlorous acid’s strong oxidizing potential, combined with its neutral charge, grant it exceptional bactericidal, fungicidal, and viricidal properties. When added to a hydroponic system, the free hypochlorous acid in solution reacts rapidly with organic matter and ammonium containing compounds in fertigation water. Commercially available forms of hypochlorous acid are typically available for purchase in concentrations of .028%, .04%, .05%, or .15%. Hypochlorous acid can also be generated for use on site using a commercially available electrolyzed water generator. At low concentrations in solution (.3-.8 PPM), hypochlorous acid works effectively as a mineral descaler and irrigation line cleaner. At higher concentrations in solution (.8-2.5 PPM), hypochlorous acid can also work as an effective oxidative cleaning agent with minimal risk of phytotoxicity. Despite its general utility, the instability of hypochlorous acid in solution and the creation of disinfection by-products can cause issues if not properly addressed.
Measuring Chemical Oxidizer Activity using an Oxidation-Reduction Potential (ORP) Sensor
The activity of a chemical oxidizer in solution can be gauged using an ORP sensor. ORP (Oxidation-Reduction Potential) is a measurement, in millivolts (mV), of a solution's capacity for electron transfer (oxidation or reduction). This capacity for electron transfer can indicate the relative cleaning capacity of the solution being measured. In general, when an oxidizing agent is added to a solution, a measurable change in ORP is observed. As the oxidizer in solution works to disinfect, the oxidizing agent is consumed and the ORP tends to decrease.
The use of an ORP sensor in concert with a well-planned chemical oxidizer dosing regimen can provide additional information about the potential of the chemical cleaning agent in solution. However, due to the large variety of factors that can influence ORP readings, ORP values are abstract without context and should not alone be used to inform chemical dosing decisions. An ORP sensor is unable to discriminate the activity of one oxidizing agent from another and different oxidizing agents cause different ORP value responses. Additionally, many external factors can also influence ORP; if the effect of these factors are not considered, a misleading conclusion will be drawn – resulting in under or over overdosing solution, leading to poor pathogen control, nutrient lockouts, or phytotoxicity. In any IPM program, it is imperative that the grower understands the active and cumulative concentration of the chemical being applied.
Hydrogen Peroxide and ORP
The oxidizing potential of H2O2 is greater than that of hypochlorous acid. Despite H2O2 being a powerful oxidizer, ORP is not a practical method for monitoring the antimicrobial potential of water treated with hydrogen peroxide. There is no relationship between the applied concentration of H2O2 and the measured ORP. As such, the antimicrobial potential of water should be extrapolated by monitoring active concentration of H2O2 in solution using test strips or other colorimetric methods.
Hypochlorous Acid and ORP
ORP can be used to monitor the activity of hypochlorous acid in solution. A measurable change in ORP following an application of hypochlorous acid is typically observed. In testing, the relationship between the applied concentration of hypochlorous acid and the change in ORP did not correlate well at levels below 15 PPM; which, is well beyond the recommended concentration for plant safety. Therefore, the disinfection potential of water treated with hypochlorous acid should not be extrapolated by monitoring the ORP. Instead, the disinfection potential of the water should be extrapolated by monitoring the active concentration of free chlorine in solution by using free chlorine test strips or other colorimetric methods. Despite ORP not being a practical method for informing dosing decisions at the typically applied concentrations used for disinfecting water in hydroponic systems, it can still be utilized as a secondary reference point when developing dosing regimens using hypochlorous acid products over long periods of time.
Informed Dosing Decisions
When adding any chemical to a hydroponic system, it is critical to understand the dosing rate, dosing schedule, and acceptable applied concentration in solution. A proper understanding of these concepts is necessary to effectively control pathogens through chemical means and avoid chemically induced phytotoxicity. The concentration of any chemical in a solution is best described by determining the number of “parts per million” (PPM) of that chemical in the solution.
All commercially available chemical products are required to list the active ingredient concentrations in a Safety Data Sheet (SDS). Typically, active ingredients are listed by percent weight - that is, the percent weight of the active ingredient relative to the total weight of the solution. For example, commercially available forms of hydrogen peroxide (H2O2) can be purchased in stock solution concentrations between 3% and 35%. The PPM value of a solution with a known percent weight value can be found by multiplying the percent weight by 10,000 – for example, a 3% (H2O2) solution has an equivalent PPM concentration of (3 *10,000) = 30,000 PPM. Using this knowledge, individuals can calculate the strength of their stock solution in PPM, then determine the ideal dilution ratio to achieve a desired final PPM concentration of active ingredient for their specific application.
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