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Coagulants are widely used for solids (uneaten food, faeces, etc.) management in recirculating aquaculture systems (RAS), but no recent research has been performed on the effectiveness of different coagulants in treatment of aquaculture sludge. This study examined the effectivity of selected inorganic (polyaluminium chloride, PAC) and organic products (polyamine- and starch-based) as coagulant agents for solids management in RAS. Reductions in residual concentrations of total phosphorus (tot-P), phosphate‑phosphorus (PO 4 -P), suspended solids (SS) total nitrogen (tot-N), nitrate‑nitrogen (NO 3 -N), ammonium‑nitrogen (NH 4 -N), aluminium (Al) and chemical oxygen demand (COD) in reject water were determined. The effect of process parameters (coagulant type, dose, mixing and sedimentation time) on sludge treatment was also evaluated. The PAC products tested were most effective at concentrating pollutants (Tot-P, PO 4 -P, SS, COD) in RAS sludge into the solid phase. The organic products tested, especially a high-molecular-weight polyamine product (pAmine1), achieved good performance and can be considered a valid alternative to inorganic salts. At optimum dose, PAC (dose 32 mg/L) and pAmine1 (dose 15 mg/L) removed, respectively, 99.4% and 82.8% of turbidity, 98.2% and 65.4% of PO 4 -P and 97.7% and 73.6% of SS. The mixing time applied in flocculation and the time allowed for sedimentation had significant effects on coagulant performance, with the organic coagulants being most affected. Flocculation times of 5–15 min and sedimentation times of 15–60 min showed good results and can be used as a starting point in process optimisation with both inorganic and organic coagulants. The use of coagulants for treatment of RAS sludge enhances flock formation and improves particle settling characteristics, substantially decreasing nutrient, organics and solids concentration in reject water. Unlabelled Image • Coagulants improve solids treatment in RAS reducing SS and P discharge. • Inorganic coagulants (PAC) achieved best SS and P removal from RAS sludge. • High MW organic products can replace inorganic coagulants is RAS solids treatment. [ABSTRACT FROM AUTHOR]

Dredged sediment contains various contaminants that are released during the process of dewatering and subsequent utilization. In this study, two organic polymers—chitosan (CS) and cationic polyacrylamide (CPAM) both in samples of varying molecular weights (MWs)—were used as flocculants to improve dewatering and rheological behaviors of dredged sediment, and floc properties were characterized to unravel the mechanisms of flocculation treatment. Moreover, pollutant transfer and release in the flocculation-dewatering process was investigated. Compared to CPAM, CS had better performance in dredged sediment dewatering, and more compact flocs were produced after treatment. The flocculated sediment belonged to the type of yield dilatant fluid and showed good shear resistance. Three-dimensional excitation–emission matrix spectroscopy and PARAFAC showed that protein-like substances were removed after treatment. The MW of CS had insignificant effects on flocculation performance, whereas CPAM removal efficiency for protein-like substances was increased at higher MWs, which may be related to the adsorption bridging effect of CPAM polymer chains. There were significant correlations between the dewatering performance of sediments, MW distribution of organics and rheological properties. CS and CPAM treatments caused the transformation of Fe/Al-P into Ca P, which could reduce phosphorus release and its ecological risk. The flocculants contributed to the formation of carbonate-bound forms of As, Cr, Pb, and Ni. Ecological risk assessment results of the geo-accumulation index showed that medium- and low-MW CS reduced risk of sediment contamination, whereas CPAM and high-MW CS increased the ecological risk. CS had a greater effect on the release of VOCs than CPAM, with an increased release of total VOCs at higher flocculant MWs. The study was helpful to understand the dewatering mechanism of dredged sediment and provided a new strategy for pollution release management in sediment dewatering. Unlabelled Image • CS performed better in improving dredged sediment dewaterability than CPAM. • The floc produced by CS treatment was more compact than that flocculated with CPAM. • Sediment dewatering efficiency was related to protein-like substances removal. • Flocculation conditioning affected the form and transferring of pollutants in sediment. • Pollutant release and transferring behavior in flocculation conditioning was related to MW of polymers. [ABSTRACT FROM AUTHOR]