Treatment of Polysilicon Production Wastewater by Ferrate(VI) Microcapsule Oxidation and Biological Aerated Biofilter

Abstract

Polysilicon production wastewater (PPW) is characterized by complex composition, high chemical oxygen demand (COD) concentration and poor biodegradability. An integrated process comprising of ferrate(VI) oxidation and biological aerated biofilter (BAF) was developed at lab scale for treating PPW with an initial COD of 3630 mg/L, biochemical oxygen demand (BOD5) of 350 mg/L, suspended solids (SS) of 440 mg/L, and turbidity of 430 nephelometric turbidity units (NTU). Firstly, the potassium ferrate(VI) (K2FeO4) microcapsules were synthesized by using the phase separation method in cyclohexane, and ethylcellulose was used as the microcapsule wall materials (WM). The stability could be enhanced greatly when ferrate(VI) was encapsulated in the microcapsules with a mass ratio of K2FeO4:WM of 1:4 in the air compared with pure K2FeO4. The microcapsules exhibited sustained release behaviour and higher oxidation efficiency than pure K2FeO4. The microcapsules were used to pretreat PPW. Under the oxidation conditions of pH 6.0, microcapsule dosage 5.0 g/L and reaction time 70 min, the removal efficiency of COD, suspended solids (SS) and turbidity reached 55.1%, 61.3% and 74.2%, respectively. Subsequently, the oxidation effluent was subjected to BAF treatment. Under a hydraulic residence time of 48 h and a gas:water ratio of 6:1, the final effluent values of COD, SS and turbidity were 308 mg/L, 35 mg/L and 28 NTU, respectively, corresponding to total removal of 91.5%, 92.0% and 93.5%, respectively. Thus, this work demonstrates the feasible and potential application of encapsulated ferrate(VI) samples in the degradation of various toxic effluents.