Sequential wet extraction of phosphorus from sewage sludge using alum sludge: Reassessing the aluminium-phosphorus speciation using experimental and simulation approach

Sewage sludge (SS) is a reclaimable phosphorus (P) source for fertilizer production, but it has a high toxic trace element (TE) concentration. Using alkaline leaching of SS, P can be extracted without simultaneous extraction of TEs. However, low P recovery efficiency during alkaline leaching is a major challenge. This study explored alkaline leaching of P from alum sludge (AS)-amended dry SS, with or without acidic pretreatment. The amendment with AS was intended to promote apatite to non-apatite inorganic phosphorus conversion for a higher alkaline P recovery. The highest orthophosphate (ortho-P) alkaline recovery was 74 % for the acid-pretreated SS + AS mix. Amending SS with AS averted the loss of ortho-P during acidic pretreatment by 59 % compared to the reference case (unamended SS). Standards Measurements and Testing (SMT) analysis revealed apatite to non-apatite inorganic phosphorus conversion during acidic pretreatment, especially in the amended SS. This conversion was due to the dissolution of Ca-P and adsorption of dissolved ortho-P on Al(OH)₃ during acidic pretreatment. Visual MINTEQ modelling, solid-state NMR (³¹P, ²⁷Al and ³¹P-¹H CP-HETCOR) and Al 2p XPS data indicated a high abundance of Al(OH)_3(s), which enhanced the ortho-P adsorption. There was no formation of AlPO₄, possibly because the acid-sludge mixture reached the metastable equilibrium and not the true equilibrium during acidic pretreatment. ³¹P liquid NMR, SEM, FTIR and XRD provided additional insight into the effects of acidic pretreatment on sludges. Overall, Ca-P to Al-P conversion enhanced alkaline ortho-P recovery by about 12 % compared to the reference case (unamended + non-pretreated). This study demonstrated that P leaching efficiency from SS can be improved (with appropriate process modifications) irrespective of the intrinsic property of SS, thus making alkaline P recovery applicable in many regions globally. Additionally, the chemical co-treatment of SS + AS proposed is a prospective dual waste management strategy.