Anaerobic sewage treatment systems, notably the upflow anaerobic sludge blanket reactor, have been applied successfully in Brazil in dozens of large scale units. In this paper a small reactor is proposed for use in single family houses, without a connection to a sewerage network. The proposed reactor is much smaller than the traditional septic tank and yet the treatment efficiency is much higher. It comprises a lower cylindrical digestion zone and an upper settling zone, connected by a transition zone. Several configurations with different digestion and settling zone diameters were tested to establish their influence on reactor performance. A high degree of organic matter removal was obtained with 250 L reactors with retention times of 6 to 12 hours. A large sludge mass was retained in the digestion zone and high treatment capacity was maintained throughout the study. Tests showed that sludge settled better in the reactor than it did after discharge with the effluent. In contrast the, methanogenic activity of the sludge in the reactor was the same as that of the sludge expelled.

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