Aeration and heating improve slow sand filtration as a disinfection system for closed hydroponic systems

Abstract

Aeration and heating were used to improve the disinfection activity of slow sand filtration of recirculating nutrient solutions in a closed hydroponic system. Filter performance was evaluated under different running conditions using E. coli elimination efficiency as an indicator of filter performance. Aeration with or without heating during filter 'ripening' increased filter performance so much as to obviate differences in performance associated with filtration rate. The filter reduced influent E. coli concentrations by nearly 3 orders of magnitude with aeration. Effluent E. coli concentrations from, filters pre-circulated with aeration and heating were over 4 orders of magnitude lower than influent concentrations. Vertical distribution of viable E. coli in an effective slow sand filter which reduced E. coli concentrations from 10 6 cfu ml-11 to less than 102 cfu ml -1 was mostly in the Schmutzdecke (106-107 cfu g-1) and first 10 cm of the surface of the sand layer. High-erformance filters did not eliminate Ralstonia solanacearum as effectively as E. coll or the fungal pathogens Fusarium oxysporum and Pythium helicoides.