Effects of slow sand filtration on mineral and inoculum concentration of nutrient solution in a NFT system

Abstract

The applicability of slow sand filtration as a disinfection system for the recirculating nutrient solution in a Nutrient Film Technique (NFT) growing tomato was evaluated. The recirculating nutrient solution was filtered through a slow sand filter installed between the nutrient solution tank and the culture bed; the filtrate was supplied to the hydroponic system. The pH of the filtrate was maintained at around 7 by the buffer action of the filter sand. The dissolved oxygen concentration in the nutrient solution decreased by 10-30% through filtration; but the tomato plants did not show any O2 deficiency symptoms. The influence of sand filtration on electrical conductivity (EC) value was small. Nitrate-N and Mg concentrations in the nutrient solution declined slightly by filtration; part of K in the solution was seemingly exchanged by Ca, previously held by the sand. NH4-N, P, Fe and Mn concentrations in the filtrate did not change quickly after their addition to the water layer. Mn in the filtrate was undetected. Young leaves of tomato plants grown in NFT with this slow sand filtration system became yellow, indicating that some micronutrient, probably Mn, was deficient. The slow sand filtration system removed most fungi and bacteria added to the NFT tank. The maximum concentration of Fusarium oxysporum detected in the filtrate was about 0.1% of that in the tank; 0.6% for Pseudomonas solanacearum. Escherichia coli was not detected in spite of the initial concentration of 2.0 x 106 cfu · ml-1 in the tank. The top layer of about 1cm thick from the sand layer was scraped bimonthly to re-establish the original filtration rate.