Sludge production and settleability in biofilm-activated sludge process

The sludge production and settleability have been estimated experimentally in a completely mixed biofilm-activated sludge reactor (hybrid reactor). A steady-state hybrid reactor was run at different stages of suspended biomass concentration (X) under constant values of influent substrate concentration (S-o) and hydraulic retention time (HRT). The values of X were gradually decreased in these stages until the system completely washed out of the suspended biomass and converted to pure biofilm reactor. As a result, the role of biofilm in the treatment gradually increased with an increase in the effluent substrate concentration (S). The experiment was supported by a mathematical expression for describing the sludge yield in the system under the previous conditions. The experimental and theoretical studies in the present work reveal that there is a critical phase of the hybrid system at which the system produces a high rate of excess sludge. That critical phase is found at a specific ratio between the suspended and the attached growth. Avoiding that critical phase enables the sludge production in the hybrid reactor to be reduced and optimized. Further, the minimum sludge production was found when the biofilm is theoretically inactive for chemical oxygen demand (COD) removal (S < S-min). At this stage, biomass production was reduced by about 48% compared to the conventional activated sludge process. The observed sludge yield (Y-obs) in the hybrid process was varied from 0.148 to 0.25 gm volatile suspended solids (VSS)/gm COD removed at the minimum and critical stage respectively, while the values of Y-obs were 0.285 and 0.185 gm, VSS/gm COD removed in the pure suspended and pure attached growth, respectively.