Phosphorus recovery from wastewater using waste gypsum via sulfur-metabolizing bacteria: Influence of gypsum type on performance and mechanisms

With global phosphorus depletion, a novel strategy for phosphorus recovery from phosphorus-laden wastewater combined with phosphogypsum recycling to form hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 ) via sulfur-metabolizing bacteria was recently developed. However, enhancing recovery efficiency and reducing costs in complex wastewater systems remain key challenges, primarily due to variations in binding energy and impurity species within waste-gypsum from different sources. Therefore, the effects of three waste gypsum types (phosphogypsum [PG], titanium gypsum [TG], and flue gas desulfurization gypsum [FGD]) on recovery efficiency and mechanisms were evaluated/elucidated, with analytical-grade CaSO 4 ·2H 2 O (ARG) as the reference material. Results revealed that waste gypsum selection is pivotal for efficient phosphorus recovery in sulfur-metabolizing microbial systems. Although a negative correlation was observed between phosphorus recovery efficiency and gypsum stability (FGD > TG > PG > ARG), deviations from this trend occurred due to the specific impurity composition of the waste gypsum. TG, containing abundant Fe impurities, inhibited sulfur-metabolizing bacterial activity and impeded Ca-P precipitation. Despite intermediate stability, PG emerged as the optimal substrate, achieving 62.86% phosphorus recovery efficiency. This superior performance was attributed to PG’s phosphorus-containing impurity species, which actively promoted Ca-P formation (72% proportion in sediments). Final economic evaluation demonstrated that utilizing PG as the treatment additive for wastewater phosphorus recovery achieved optimal waste gypsum valorization, delivering a net economic benefit of $1.73 per ton of wastewater - significantly higher than other gypsum types ($0.89-1.30 per ton).