The purpose of this investigation was to define and analyze some operational parameters of the FAS process. Specifically: the settleability of filamentous sludge produced by the treatment system, the yield coefficient of the biological population, and the carbon to phosphorus ratio which limits organic removal efficiency. Findings of this study indicate that the sludges produced by the FAS process have settling characteristics and settleability remarkably similar to the solids of conventional activated sludge. While being tested, the sludges displayed a single clearly defined sludge-supernatant interface, good compaction, and cohesion between particles. The effect of varied diameter cylinders revealed that the true settleability of the sludge could not be determined in cylinders with a diameter less than 10 cm because smaller diameter settling tubes accentuated the "wall effect." The SVI and suspended solids appeared to be directly related because as the solids concentration decreased the SVI increased. No significant relationship could be established between the suspended solids concentration and the interface settling velocity because factors other than solids concentration influenced the value of this parameter. The yield coefficient of 0.23 derived from this investigation is approximately one-half the value associated with activated sludge but comparable to some coefficients reported for the FAS process. Filamentous organisms are capable of utilizing twice as much carbon per unit of phosphorus as the organisms of conventional activated sludge. A carbon to phosphorus ratio of 307:1, based on C.O.D., was found to be the limiting ratio for efficient organic removal. The amount of carbon used by the system per mg/l of phosphorus uptake averaged 230 mg/l as C.O.D. both before and after breakpoint. The dominant organisms in the biological unit throughout this investigation were fungi. / Master of Science