2002 Volume 71 Issue 1 Pages 107-113
Slow sand filtration (SSF) is one method for disinfecting nutrient solutions in a closed soilless culture system. Microbe removal efficiency of such filtration was evaluated by determining the definite inoculant microbe density of influents and filtrates. The well water was inoculated with a suspension culture of tomato bacterial wilt pathogen (Pseudomonas solanacearum) to a density of 1.2×103 cfu·ml-1 as an influent. When the inflow of this water to the sand filters kept at 30°C lasted for 32 hr, the bacterial density of filtrate began to increase 16 hr after the inflow started (HAS) and reached the maximum value of 2.2×102 cfu·ml-1 at 32 HAS. The time course of the density of filtrate was observed to have a plateau between 24 and 36 HAS. The mean value of 1.7×102 cfu·ml-1 during the period was employed as the definite density of filtrate. The filtrate/influent ratios revealed that the removal efficiency was 86% whereas it was 99% when the temperature of the sand filter was uncontrolled. The performance of SSF against the spread of tomato bacterial wilt was investigated in a NFT tomato growing system. When the system did not include SSF, the disease spread from two previously inoculated tomato plants to the rest of the 18 plants in the same bed within 25 days. The pathogen density of the nutrient solution kept increasing up to 107 cfu·ml-1 by the end of the culture. When SSF was installed into the NFT system, no wilting damage was observed unless plants were located downstream of the previously inoculated ones. The pathogen density of the solution, which passed through the rhizosphere of infected plants, increased up to 104 cfu·ml-1 and did not decrease throughout the experiment, although no pathogen was detected in the filtrates.
