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

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 thro ugh 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 t he filter sand. The dissolved oxygen concentration in the nutrient solution decreased by 10 similar to 30% through filtration; but the tomato plants d id not show any O-2 deficiency symptoms. The influence of sand filtration o n electrical conductivity (EC) value was small. Nitrate- N and Mg concentra tions in the nutrient solution declined slightly by filtration; part of K i n the solution was seemingly exchanged by Ca, previously held by the sand. NN4- 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 s ystem became yellow, indicating that some micronutrient, probably Mn, was d eficient. The slow sand filtration system removed most fungi and bacteria added to th e 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 solanace arum. Escherichia coli was not detected in spite of the initial concentrati on of 2.0 x 10(6) 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.