Fate and distribution of phosphorus in coking wastewater treatment: From sludge to its derived biochar

Abstract

Due to the phosphorus (P) deficiency in coking wastewater, sufficient P needs to be provided in the treatment process to maintain biotic activity. However, most of the dosed P sources are transferred to the sludge phase out of the chemical equilibrium. After an in-depth investigation of P morphology changes in coking wastewater treatment, it is found that above 71.6 % P applied to the full-scale O/H/H/O (oxic-hydrolytic & denitrification-hydrolytic & denitrification-oxic) process for coking wastewater treatment is ended up in the sludge phase of the aerobic reactors in the forms of non-apatite inorganic phosphorus (NAIP). Theoretical simulations suggest that the P forms precipitates such as FePO₄·2H₂O, AlPO₄·2H₂O, MnHPO₄ at pH < 7, and Ca₅(PO₄)₃OH at pH > 7. Microbial utilization of P in coking wastewater treatment is swayed by precipitation, pH and sludge retention time (SRT). By pyrolysis treatment of the waste sludge at 700 °C, phosphoric substances in coking sludge are enriched and converted into Ca₅(PO₄)₃OH, Ca₅(PO₄)₃Cl, Ca₃(PO₄)₂, etc. with apatite phosphorus (AP) accounting for 65.7 % of total phosphorus. Moreover, the heavy metals in biochar were below the national standard limits for discharge. This study shows that hazardous waste (coking sludge) can be transformed into bioavailable products (P-rich biochar) through comprehensive management of the fate of P. Combined with the O/H/H/O process, the mechanisms of phosphorus consumption in coking wastewater treatment are revealed for the first time, which will facilitate a reduced consumption of phosphorus and provide a demonstration for other phosphorus-deficient industrial wastewater treatment.

Keywords: Coking sludge; Phosphorus forms; Phosphorus recovery; Pyrolysis biochar; Resource utilization; Wastewater treatment.