Variables Limiting Efficacy of Slow Filters Integrated into Closed Hydroponic Growing Systems

For sustainable production of greenhouse vegetables and ornamentals, losses of water and nutrients must be avoided. Closed hydroponic systems are highly efficient in their use of water and nutrients, produce high yields and minimise the needs for soil sterilization. However, they carry a risk of dispersal of root-borne pathogens, mainly zoosporic oomycetes, which can cause major crop losses. To prevent this, nutrient solution is of disinfested before reuse by pasteurisation, UV-light treatment, oxidation with ozone or peroxides, or filtration. Slow filtration is a low-cost technique with high efficacy against oomycetes and bacterial pathogens. However, tools for easy monitoring of slow filter efficacy are currently unavailable. Abiotic and biotic factors are critical for slow filter performance. Enzyme activity in the filter skin is a key factor and is a possible predictor of filter efficacy, explaining 50% of the filter efficacy. A number of state variables, e.g. polysaccharides, total organic carbon and oxygen content, were examined in this study in attempts to explain the remaining 50% of efficacy. Abiotic and biotic state variables affecting filter efficacy were monitored for filters subjected to sub-optimal factors such as high electric conductivity or at massive pathogen bombardment. Supporting studies were performed in commercial greenhouses. The main conclusions were that electric conductivity of the nutrient solution affects the efficacy of slow filters, whereas bombardment of the filter skin with pathogen mycelia had minor importance on filter function. The state variables total polysaccharides, biofilm polysaccharides, total organic carbon and oxygen content did not support prediction of filter efficacy. Interactions were found between enzyme activity and the microbial colonisation of the filter skin.