Investigating the use of coagulants and Ca2+-rich steel residues as an anti-fouling strategy in the ultrafiltration of microalgae: Towards sustainable resource recovery from wastewater

•The effectiveness of tannin and calcium residues in microalgae UF antifouling strategies was investigated.•Steel industry residues enhance filtration over commercial coagulants.•Bag filter powder reduces fouling resistance by 30 %•Steel industry residues altered clake morphology, impacting cake layer structure and flow reversibility. This study aimed to assess the impact of various coagulants and alternative sources of Ca2+ as pre-treatment on a microalgae suspension undergoing ultrafiltration. Two commercial coagulants, namely aluminum polychloride (PAC) and Tanfloc SL, were compared against two calcium-rich residues from the steel industry: blast oxygen furnace slag and bag filter powder. The filterability of the samples was assessed through permeate flux measurement, analysis of added membrane resistances, and examination of fouling formation mechanisms. Additionally, the characteristics of the formed deposit, including membrane surface morphologies and functional groups, were characterized. Coagulation-flocculation significantly influenced floc characteristics, thereby impacting the filterability of the microalgae samples. Both aluminum polychloride and Tanfloc demonstrated a slight reduction in flux decay, except at 40 mg.L-1 doses, which notably increased normalized final flux. However, Tanfloc exhibited poorer performance compared to coagulated filtrations with calcium-rich residues in terms of fouling. Bag filter powder displayed a 30 % lower fouling resistance than the microalgae suspension without a coagulant. The predominant mechanism observed in fouling formation was attributed to cake layer formation, with the observed fouling being mostly reversible.