Three continuous, laboratory scale, high rate biological wastewater treatment systems operating in a similar mode were evaluated. The feed was a synthetic sewage with a COD and BOD of approximately 300 mg/l and 200 mg/l, respectively. The total hydraulic retention time in each system was 2-1/4 hours - 40 minutes aeration, 95 minutes sedimentation. The applied food to microorganism ratios (F/M) were equal to or greater than one. Systems I and II employed a single aerobic reactor whereas System III employed two aerobic reactors in series each with a hydraulic retention time of 20 minutes. Sludge was returned to the second reactor only, thus establishing dispersed growth in the first reactor and a mixed microbial population in the second. System I was strictly a bacterial system isolated from atmospheric contamination. System II employed a mixed population of ciliated protozoa and bacteria.

Tests performed included soluble COD reduction, effluent, mixed liquor and return sludge suspended solids, SVI, pH, polysacharides, bacteria count, and protozoan count. Comparison of System I (bacterial) and System II (mixed population) revealed an improvement in effluent quality (SS, Total COD) as a result of the presence of ciliated protozoa. The higher effluent SS in System I was a result of the poor settleability of dispersed growth.

Based on the knowledge of the effects of protozoa on dispersed growth, System III (bacterial reactor and mixed population reactor in series) was established to determine if such a series arrangement would further improve treatment efficiency. Several different two stage reactor system configurations were investigated including sedimentation after the first reactor with the effluent passing to the second reactor. Such an arrangement proved impractical due to the poor settleability of the dispersed growth associated with a predominantly bacterial system. System III resulted in a vast improvement in sludge settleability while maintaining an effluent quality equal to that of System II. This improvement in sludge settleability indicates improved system stability and reduced handling costs.

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