Phosphorus removal characteristics of a combined AS-biofilm process cultured by different COD/TP ratios

The COD/TP ratio of influent is an important parameter for the phosphorus removal in a biological nutrient removal (BNR) process. In this study, we investigated the phosphorus removal and denitrification characteristics of a combined activated sludge-biofilm process, as cultured by different influent COD/TP ratios ranged from 12 to 120. Experimental results indicated that, when COD/TP ratios exceeded 30, the removal efficiencies of COD, TN and TP were 98%, 76% and 100%, respectively. However, when the COD/TP ratios were less than 30, the COD removal efficiency still surpassed 98%, but the TP removal efficiency decreased to 41% and 31.8% when COD/TP ratios were 20 and 12. Moreover, the weight percentage of phosphorus in the sludge attained a stable value when the COD/TP ratios were below 30. The maximum weight percentage of phosphorus in the sludge was found to be about 6% and, under this condition, the critical COD/TP ratio of influent was 30 of the process under a sludge retention time of 12 days. Although the amount of accumulated PHAs per mg phosphate released (gamma PHA/PO4) remained stable between a COD/TP ratio of 30 to 120, it increased with a decreased of the COD/TP ratio when the COD/TP ratio was less than 30.