Effect of Pre-treatment Methods on Phosphorus Recovery from Anaerobic Co-digestion of Food Waste with Digested Sludge

Abstract

A lab-scale anaerobic co-digestion process was developed to achieve simultaneous biogas production and phosphorus (P) recovery through supernatant crystallization. The process operated at a 30-day hydraulic retention time (HRT) using strategically selected co-substrates: food waste (FW) and digested sludge (DS) with the addition of corn stover (CS). These substrates were chosen for their beneficial characteristics, i.e. FW contributed high moisture content (73%) and biodegradability with volatile solids (VS) at 95% of total solids (TS), while CS provided structural balance through low moisture (6.6%) and high TS (934 mg/g) with similar VS proportion (93%). The combination achieved an optimal C/N ratio of 24, with DS serving the dual purpose of microbial inoculum and P source. Besides, corn stover with two different pre-treatment methods (chemically and thermally) were also investigated. Results showed that the integration of FW with CS significantly enhanced process performance by increasing biogas yield from 0.249 L/g VS (only FW) to 0.381 L/g VS (FW with CS). However, corn stover addition showed little impact on P released proportion in P mass balance, as evidenced by similar percentages of 23 and 22 observed in the experiment with only FW and that with FW plus CS, respectively. Nevertheless, the anaerobic co-digestion process in this study could maintain favorable Mg:P molar ratios (1.18–1.43), requiring only pH adjustment to achieve around 70% P recovery through struvite precipitation. These precipitates contained phosphorus in the promising range of 8–10% in comparison to some commercial single superphosphate fertilizer (8.8% P).