In a local town like Yonezawa city, a large city type sewerage system has not yet been spread. Most houses use septic tanks treating waste water from both toilet and kitchen/bath. The treated water from them is discharged directly into surface water of side ditches, etc., which produces environmental problems such as water pollution and eutrophication. Enhancement of purification effects was studied by putting walnut charcoal and bacilli into the aeration tank circulating air in the septic tank to secure bacteria and improve work of them in the aeration tank. The treated water is further reused as drinking water using peat layer. Walnut charcoal can be a nest of bacteria. By this, it became possible to cope also with environmental changes such as water quality and temperature in the septic tank. It is possible to always keep water quality in the purifying tank and quality of the treated water in a stabilized condition. Moreover, existence of bacteria can be confirmed even inside the pores of walnut charcoal can be confirmed. Porosity of walnut charcoal is made use of, and it is useful to use walnut charcoal as a nest of bacteria in the septic tank. 5 refs., 12 figs.