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A detailed study was conducted on biological removal of phosphorus by an activated sludge process composed of an anaerobic stage followed by an aerobic stage in order to clarify the role of the anaerobic stage of the process. Two distinctive sludges, one contained approximately 10 percent phosphorus, most of which existed in the form of polyphosphates, and the other contained 1.9 percent phosphorus, were obtained by changing BOD concentration of influent and hydraulic detention time of the process. Although the polyphosphate-containing sludge released phosphorus in proportion to the absorbed BOD at anaerobic conditions, the sludge which did not contain polyphosphates also absorbed an equal amount of BOD at the identical rate without any external sources of oxygen. Both sludges accumulated identical levels of poly-β-hydroxybutyrate within the sludges as the BOD sources were absorbed. These results suggest that the polyphosphate-containing sludges have no advantage over the sludges with no polyphosphates when they uptake BOD in the anaerobic stage.

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A pilot plant study on biological removal of phosphorus was conducted over fifteen months to obtain optimum operational conditions and evaluate process performance. The pilot plant was composed of an anaerobic zone followed by an aerobic zone, and had a capacity of treating 100 m3 municipal wastewater per day. In the anaerobic zone, BOD loading rates and BOD to MLSS ratios were found to be important factors. In the aerobic zone, on the other hand, the hydraulic retention time had a significant influence on the phosphorus removal efficiency. For successful removal of phosphorus, a shorter hydraulic retention time in the aerobic zone along with appropriate parameters in the anaerobic zone (BOD loading rate ≦ 2 kg-BOD/kg-MLSS day, BOD to MLSS ratio ≦ 0.1 kg-BOD/kg-MLSS) are essential.

Penetration of three kinds of biocides into biofilm formed on the surface of PVC tubes was examined. Penetration depth curves reached a plateau in 2 hours or less. Penetration depth at the steady-state was dependent on external concentrations of the biocides in the fluids. Penetration depth was limited by decomposition of the biocides by biofilm, and the decomposing abilities were maintained even in the killed bacteria.