Effect of polysaccharides in slow filters integrated into closed hydroponic greenhouse systems

Abstract

SummarySlow filtration is a widespread and environmentally sound method for the disinfestation of nutrient solutions in closed hydroponic systems. The efficacy of slow filters can be explained in part by straining and adsorption of pathogens onto filter material surfaces. Both mechanisms are influenced by choice of filter material, flow rate of the nutrient solution, biomass and extent of extracellular polymers on the filter skin. In a 2-year study, the total polysaccharide content and enzyme activity of the filter skin, and filter efficacy were monitored. Closed hydroponic systems using the nutrient film technique connected to slow filters were run with or without weekly inoculation with a fungal cell wall preparation (10.5 g m–2 of filter surface). This organic matter amendment increased the amount of total polysaccharide and enzyme activity within the filter skin. The fungal cell wall preparation did not accumulate, but was degraded rapidly. The total polysaccharide content (tPS; in mg g–1) of the filter skin increased linearly with time at 34.50 mg g–1 + 0.26 mg g–1 d–1 × days (d) (R2 = 0.556; P = 0.02) for untreated filters, and at 43.40 mg g–1 + 0.70 mg g–1 d–1 × days (d) (R2 = 0.826; P = 0.000) for treated filters. The extracellular polysaccharide content in the filter skin, determined after 16 and 20 weeks of slow filter operating time, ranged from 87.9 – 109.7 mg g–1 mineral wool (rockwool) sampled. The amount of extracellular polysaccharide was not influenced by the total polysaccharide content of the filter skin, by enzyme activity, or by addition of the fungal cell wall preparation. There was a trend towards higher filter efficacies when fungal cell wall preparations were added to the filter skin (treated: 98.3% ± 0.96; untreated: 97.9% ± 1.8); however, on the basis of the present data, no correlation between the assessed parameters and filter efficacies were observed.