1 State Research Institute Geisenheim, Department of Phytomedicine, 65366 Geisenheim, Germany; 2Technical University of Munich, Chair of Phytopathology, 85350 Freising-Weihenstephan, Germany

Backround and objectives
Slow filtration is a well tried method for elimination of pathogens from water and nutrient solutions. Despite the long history of slow filtration the mode of operation is not completely understood. Amongst physical and chemical processes (mechanical straining, sedimentation, adsorption) the biological activity is suggested as most important [1]. However there is no conclusive evidence for the importance of biological activity. Therefore the efficiency rate of a sterile and an nonsterile slow filter should be compared.

Materials and methods

A steam sterilizable slow filter was constructed.The slow filter unit consisted of a tube of high-graded steel with an inner diameter of 9 cm, coming to an effective filter surface of 63.62 cm2, filled with fine sand (grain size 0-2 mm). By means of a flow meter the flow rate was set to 20 ml/min, which is equivalent to a filtration rate of 190 L/m2h.

To investigate the efficiency of a sterile slow filter the whole system was steamed three times at daily intervals (at least 100C for 30 min). The filter was fed with sterile NaCl-solution (0.85%). The same filter was integrated into a closed growing system connected to an ebb/flow bench where Dracaena marginata was grown hydroponically to investigate the efficiency of a biological activated slow filter. After 4 weeks of biological activation the experiments were started.

For inoculation colonies of a Rifamycin resistant strain of Xanthomonas campestris pv. pelargonii (XcpR) were suspended in sterile 0.85% NaCl to an OD590nm of 0.70-0.85. 10 ml of inoculum were mixed with 5 L sterile 0.85% NaCl achieving an average density of 1.72x106 cfu/ml for sterile slow filtration. For biological activated slow filtration 10 ml of inoculum were mixed with 10 L of nutrient solution (4.64x105 cfu/ml in average). This solution was piped into the slow filter for 4 hours. To detect XcpR in the effluent samples were taken hourly for 10 hours and plated on Tryptic Soy Broth Agar (sterile slow filter) or on a modified selective medium according to Woelk and Sakar [2] (biological activated slow filter).

Efficiency rates were estimated on the basis of pathogen concentrations in the solution before filtration in relation to the highest cfu-value in the effluent. The experiments were replicated 6 times both sterile and biological activated.

Results and conclusions
Besides XcpR there were no detectable microorganisms in the effluent of the steamed slow filter. Therefore it is assumed that the biological activity was eliminated during experiments. Without biological activity the average efficiency rate was 80.38%. The average effiency rate of a biological activated filter was 98.47%. From the high efficiency of a sterilized slow filter it is concluded that physical and chemical factors - presumably sedimentation and the effort of bacteria to settle on surfaces - make up the main part of the efficiency. However, with support of biological activity much better efficacy of slow filtration is achieved.

1. Ellis KV, 1985. CRC Critical Reviews in Environmental Control 15 (4), 315-354
2. Woelk M and Sakar S, 1994. Anzeiger fuer Schaedlingskunde, Pflanzenschutz, Umweltschutz 67 (4), 69-71.