Elemental sulfur redox bioconversion for selective recovery of phosphorus from Fe/Al-bound phosphate-rich anaerobically digested sludge: Sulfur oxidation or sulfur reduction?

The biological oxidation of elemental sulfur (S) to sulfate and the reduction of S to sulfide provide a potential route for extracting and reclaiming phosphorus (P) from anaerobically digested sludge (ADS). However, the treatment performance, stability, and cost-effectiveness of the two opposing bioprocesses based on S° for selective P recovery from ADS remain unclear. This study aimed to compare the roles of S-oxidizing bacteria (SOB) and S-reducing bacteria (SRB) in liberating insoluble P from ADS through single-batch and consecutive multibatch experiments. Changes in P speciation in the sludge during the biological extraction processes were analyzed by using complementary sequential extraction and P X-ray absorption near-edge spectroscopy. Results showed that SOB treatment extracted more phosphate from the sludge compared with SRB treatment, but it also released a considerable amount of metal cations (e.g., heavy metals, Mg, Al, Ca) and negatively affected sludge dewaterability due to intense sludge acidification and cell lysis. At pH 1.2, the SOB treatment released 92.9% of P from the sludge, with the dissolution of HAP, Fe-PO, Mg(PO), and P-fehrrihy contributing 26.8%, 22.1%, 12.8%, and 10.5%, respectively. The SRB treatment released 63.6% of P from the sludge at pH 7.0, with negligible dissolution of metal cations, thereby avoiding costly purification of the extract and alkali neutralization for pH adjustment. This treatment involved the replacement of phosphates bounded with Fe-PO (FePO and P-fehrrihy) and Al-PO (P-Alumina and AlPO) with biogenic sulfides, with contributions of 72.7%, and 20.9%, respectively. Consecutive bioprocesses for P extraction were achieved by recirculating the treated sludge. Both SOB and SRB treatments did not affect the extent of sludge dewatering but considerably weakened the dewatering rate. The SOB-treated sludge exhibited prolonged filtration time (from 3010 s to 9150 s) and expressing time (from 795 s to 4690 s) during compression dewatering. After removing metal cations using cation exchange resin (CER) and neutralizing using NaOH, a vivianite product Fe(PO)·8HO (purity: 84%) was harvested from the SOB-treated extract through precipitation with FeSO·7HO. By contrast, a vivianite product Fe(PO)·8HO (purity: 81%) was directly obtained from the SRB-treated extract through precipitation with FeSO·7HO. Ultimately, 79.8 and 57.9wt% of P were recovered from ADS through SOB extraction-CER purification-alkali neutralization-vivianite crystallization, and SRB extraction-vivianite crystallization, respectively. Collectively, biological S reduction is more applicable than biological S oxidation for selectively reclaiming P from Fe/Al-associated phosphate-rich ADS due to better cost-effectiveness and process simplicity. These findings are of significance for developing sludge management strategies to improve P reclamation with minimal process inputs.