Impact of Substrate Feed Patterns on Solids Reduction by the Cannibal™ Process

A laboratory study of a biological solids reduction process, the Cannibal™ process, that used a sidestream anaerobic reactor was undertaken to determine why variations in solids reduction occur in field installations. Two issues were thought to be improtant, the iron concentration in the feed and the activated sludge process configuration. Based on field observations, it appeared that processes that had a single stage reactor and a high sludge age would frequently not provide a low organic solids yield. Iron oxidation/reduction cycling was thought to influence the bioavailability of organic matter in activated sludge flocs. Laboratory sequencing batch reactors were operated—one of which was operated as a conventional activated slduge system and the others included a sidestream anaerobic reactor (Cannibal system). Initially, the iron content in the feed was varied. Later, the feeding cycles were varied. The “fast feed” system used a 5 minute feed period and the “slow feed” had a 4 hour feed period. The objective was to mimic a plug flow system with fast feed and a complete mix system with slow feed. These systems may also be considered to be high substrate pressure (fast feed) and low substrate pressure (slow feed). It was found that the Cannibal process operated as a fast feed system generated up to 80% less solids than the conventional activated sludge system without any negative effect on the effluent quality or the settling characteristics of the activated sludge. For the system operated under low substrate pressure (slow feed), solids production was lower than the conventional activated sludge system but remained higher than for the fast feed system.