Biological Excess-Sludge Reduction and Sludge Settleability Improvement Using a New Modified IFAS-OSA Process by Adding an Anoxic Sludge-Holding Tank to the Return Activated Sludge Line of the System

Integrated fixed film activated sludge-oxic settling anoxic system (IFAS-OSA) is a new, modified system for excess-sludge reduction. In this research, the efficiency of the IFAS-OSA system in biological excess-sludge reduction with the addition of an anoxic sludge-holding tank to the return line of the system was studied. The results showed that the average oxidation reduction potentials (ORPs) in the ${\mathrm{IFAS}}_{\mathrm{Aerobic}}$, ${\mathrm{IFAS}}_{\mathrm{RAS}}$, $\mathrm{IFAS}\text{-}{\mathrm{OSA}}_{2\mathrm{h}}$, and $\mathrm{IFAS}\text{-}{\mathrm{OSA}}_{4\mathrm{h}}$ processes were $+80\pm 15$, $-62\pm 22,–102\pm 8$, and $-140\pm 13\mathrm{mV}$, respectively. The average values of $Y_{obs}$ in the IFAS, $\mathrm{IFAS}\text{-}{\mathrm{OSA}}_{2\mathrm{h}}$, and $\mathrm{IFAS}\text{-}{\mathrm{OSA}}_{4\mathrm{h}}$ processes were $0.43\pm 0.01$, $0.31\pm 0.02$, and $0.23\pm 0.01$ g MLSS/g COD, respectively. Accordingly, in the $\mathrm{IFAS}\text{-}{\mathrm{OSA}}_{2\mathrm{h}}$ and $\mathrm{IFAS}\text{-}{\mathrm{OSA}}_{4\mathrm{h}}$ processes, the sludge production was reduced by 27.9% and 46.5%, and the excess sludge was reduced by 13.65% and 22%, respectively, compared with the control system. Furthermore, as the average ${\mathrm{SVI}}_{\mathrm{s}}$ reached $95\pm 12,63\pm 6$, and $49\pm 4$ $\mathrm{mL}/\mathrm{g}$, the TSS values in the effluents of the IFAS, $\mathrm{IFAS}\text{-}{\mathrm{OSA}}_{2\mathrm{h}}$, and $\mathrm{IFAS}\text{-}{\mathrm{OSA}}_{4\mathrm{h}}$ processes reached $16.5\pm 3$, $7.2\pm 1.5$, and $5.5\pm 1.2$ $\mathrm{mg}/\mathrm{L}$, respectively. The average percentages of total nitrogen (TN) removal efficiency in the IFAS, $\mathrm{IFAS}\text{-}{\mathrm{OSA}}_{2\mathrm{h}}$, and $\mathrm{IFAS}\text{-}{\mathrm{OSA}}_{4\mathrm{h}}$ processes were $78.56\pm 2.46$, $83.60\pm 0.92$, and $85.03\pm 1.69$, respectively, whereas the percentages of phosphorus removal efficiency in these processes were $32.69\pm 8.25$, $36.35\pm 6.73$, and $39.87\pm 3.61$, respectively. This study revealed that the modified IFAS-OSA system had a more efficient performance than the IFAS system.