THE ROLE OF SLOW FILTRATION FOR DISEASE CONTROL IN CLOSED HYDROPONIC SYSTEMS
Department of Phytomedicine, State Research Institute, D-65366 Geisenheim, Germany
Closed hydroponic cultivation systems basically possess a relatively high risk of spreading root infecting diseases via nutrient solutions. Several means of disinfecting nutrient solutions are available to prevent rapid disease development . Most of them need high investment and operating costs (e.g. heat treatment, ozonation, UV-irradiation) and are therefore economically feasible only for cultivation areas of at least 1 ha.
In contrast, the very old slow filtration technique uses cheap material (e.g. sand) and needs only minor running costs. Most important, it can be fitted for every size of cultivation area. Slow filtration is a surface filtration technique with capacities mostly in the range of 100 to 300 l/m2h. Despite the long history of slow filtration the working mechanisms are not completely understood. Mechanical straining, sedimentation, adsorption, and chemical and biological activity are suggested as the most important processes .
Slow filtration as a means of disinfecting nutrient solutions in closed cultivation systems was the first time mentioned by Wohanka  and primarily used for eliminating zoosporic fungi like Phytophthora. Further research revealed that microconidia of Fusarium, viruses and bacteria are eliminated at high efficiency rates, however, these pathogens may pass through the filter bed to some extent. Nevertheless, in practical use and at several trials slow filtration has been successful in preventing widespread dispersal of various root infecting phytopathogenic fungi or bacteria via nutrient solutions. The few studies with nematodes or vectorless transmitted viruses indicate that these pathogens are not sufficiently eliminated by standard slow sand filtration.
With respect to the special conditions of hydroponic culture a further optimisation of slow filtration is necessary. Particular attention should be paid to the filter material (e.g. porosity, alternatives to sand), the depth of the filter bed and the flow rate. Furthermore, biological activity and the influence of temperature, pH and other factors has to be investigated.
Recent studies in Germany and the Netherlands revealed that granulated rock wool or glass wool may be good alternatives to standard filter sand. The efficiency rates for these materials are somewhat higher than that for sand or similar filter media and they have some handling advantages. Independent of the kind of filter material the filter bed should have a thickness of at least 60 cm and the filtration rate should be as low as possible (preferably 100 L/m2h).
Recent studies  indicate that slow filtration as a "passive" disinfection technique may be useful for developing sustainable cultivation systems with optimised microbial suppression of root infecting pathogens.