Effect of floated sludge recycling on phosphorus removal in dissolved air flotation

The total phosphorus (TP) content allowed in effluent water in many countries has been reduced to below 0·1 mg/l, which is about one tenth of the TP concentration in effluent water discharged from plants using conventional biological processes. Research and advanced treatment processes are being actively applied to remove phosphorus from municipal wastewater, with most research focusing on stable and efficient chemical treatments. To enhance the removal of phosphorus, a series of experiments was conducted based on a pilot plant of the dissolved air flotation (DAF) process using effluent from a municipal wastewater treatment plant, which applies a biological process for nutrient removal. Specifically, this study evaluated the effect of floated sludge recycling in the DAF process in addition to the treatment conditions for TP removal. The applied treatment led to rapid TP removal with [Al]/[P] molecular ratios of less than 2·0. Slight TP removal occurred with [Al]/[P] ratios greater than 2·0, indicating the inrush of the equilibrium control stage. When the floated sludge was recycled, increased removal of TP was achieved. The recycling of floated sludge thus improved the quality of the treated water and reduced the coagulant dosage, generating an efficient process for TP removal.