Sludge production characteristics were investigated in many on-site small-scale wastewater treatment facilities used to treat domestic wastewater of about 1.25m3/day. Two popular types have been used in experiments for several years; one was an anaerobic bed reactor with constant flow-rate control system followed by aerobic bio-filtration reactor CFR type facility), and the other was an aerobic bed reactor with plug-flow stream followed by aerobic biofilm reactor (PFS type facility). Circulation ratio was set at 0 and 4. From the results obtained, lower sludge production rates were observed in CFR type facilities. Furthermore, it was suggested that a promotion of sludge decrease occurred in facilities with high denitrification activity at circulation ratio of 4. The results from experiments with different SS loading rates in laboratory-scale reactors suggested that at higher loading rates, the produced sludge was greater on account of the decrease of nitrification activity even on a circulating operation.

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