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

Abstract

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.