Tap-water has mainly been produced by rapid sand filtration of modern mechanized faclities,alongside highly sophisticated ozone and reverse osmosis("RO")membrane filtration updates.Only 5% of total tap-water is currently produced in Japan by its traditional counterpart,i.e.,slow sand filtration.The former depends on physico-chemical processes with coagulants and the relevant expert systems,whilst the latter is based on the activities of ubiquitously-occurring microbial communities.Naturally,there are big differences between both choices in terms of cost-effectiveness,quality of the end-products,and environmental sustainability as a whole.This article has unveiled the microbial profiles of slow sand filtration beds which are somewhat similar to those of activated sludge for sewage treatment.The biofilm on the sand granules was also effective to reduce coliform bacterial counts,eventually fulfilling the bio-safety needs.