Aggregates strength and regrowth in UASB-post-treated animal husbandry sewage using natural coagulants

Abstract Efficient animal husbandry sewage treatment is fundamental for environmental sustainability, with Upflow Anaerobic Sludge Blanket (UASB) reactors offering advanced secondary treatment. However, optimising post-treatment, particularly solid-liquid separation via coagulation and flocculation, requires a deeper understanding of aggregate strength and regrowth. This study investigated the performance of two natural-based coagulants (NBCs), Moringa oleifera **Lam. (MOL) and commercial tannin (Tanfloc SL ® ), in treating animal husbandry sewage (HS) and domestic sewage (DS) following UASB co-digestion. Batch experiments simulated mixing, flocculation, breakage, and regrowth under various shear conditions, with floc size monitored in real time. Computational Fluid Dynamics (CFD) modelling elucidated the influence of mechanical mixing and velocity gradients on agglomerate dynamics. Both NBCs produced robust aggregates, yet their regrowth potential under shear was limited, particularly in HS, highlighting the necessity for precise turbulence control in wastewater treatment plant (WWTP) design. CFD revealed significant spatial variation in shear forces near the mixer, suggesting that average velocity gradient (G) values may not adequately represent aggregate stress exposure. These findings reinforce the applicability of NBCs in post-UASB treatment and underscore the importance of localised flow characterisation for process optimisation.