Influence of ripening state of filters on microbe removal efficiency of slow sand filtration used to disinfect a closed soilless culture system

Abstract

The property of slow sand filtration to disinfect a closed soilless culture system was examined in terms of stable and efficient estimation of microbe removal efficiency using a small-scale experimental apparatus. Escherichia coli was chosen as a microbe to be removed. With the freshly-prepared filter, the microbe density in the filtrate reached a plateau level after 3 to 6 hr of operation when that in the influent was kept constant for longer than 3 hr. The height of a plateau level varied with the microbe density in the influent. Duration of a plateau extended as the inoculative period was longer. The microbe removal efficiency of the filtration system increased when the runoff solution from the rockwool culture of rose plants was circulated through the filter for 5 weeks, but no increase occurred when water was used. However, the once-increased efficiency of the filtration system was lost when the runoff solution was changed to water. The microbe removal efficiency of the filtration system fluctuated according to the microbe density in the influent, but the efficiency was affected more by the ripening state of the filter system.