Performance of an anoxic reactor proposed before BAF: Effect of ferrous sulfate on enhancing denitrification during simultaneous phosphorous removal

• A coagulant dosed pre-anoxic activated sludge reactor was proposed before the BAF.
• Phosphorus precipitation using Fe(II) was enhanced by anoxic activated sludge.
• Fe(II) addition enhanced denitrification process during phosphate precipitation.
• MLVSS concentration in anoxic basin increased with the Fe(II) dosing.
• Poor sedimentation of anoxic activated sludge was improved by Fe(II) addition.

A coagulant dosed pre-anoxic activated sludge reactor is proposed to configure prior to the BAF reactor to reduce the contents of phosphorus and suspend solids in wastewater, efficiently utilize the carbon source for denitrification, and avoid the problems caused by direct addition of coagulant into BAF reactor. A series of laboratory experiments were carried out to examine the feasibility and suitability of using Fe(II) as a coagulant. The performance on phosphorus removal and dinitrification, the determination of an optimal Fe(II), and any adverse effect brought by dosing Fe(II) are of interests. Base on the results of the continuously operated experiment, it was observed that dosing Fe(II) in the proposed pre-anoxic reactor demonstrated fair effectiveness in phosphorous removal and simultaneous denitrification. Enhanced denitrification efficiency was observed in the presence of Fe(II), probably resulting from the increased returned sludge amount and the promoted denitrification related to microbial Fe(II)-dependent nitrate reduction after dosing Fe(II). Overdosing Fe(II) would however result in suppressed TSS, TCOD and TP removal in the settling tank effluent due to the formation of fine microbial flocs. A Fe(II) dose with Fe:P molar ratio of 2–2.2:1 was selected as an optimal dose, which showed good performance in phosphorus removal, denitrification, and COD reduction, as well as, avoided the adverse effects caused by Fe(II) overdosing. Besides enhancing wastewater treatment in pre-anoxic reactor, dosing Fe(II) also improved sludge sedimentation and dewaterability, which were beneficial for the subsequent operation of dealing with excessive sludge.