Optimisation of chemical harvesting of microalgae from fish processing wastewater and economic assessment based on process modelling

Chemical flocculation with bio-based coagulants offers an effective method for harvesting microalgae after wastewater treatment. Nevertheless, limited information exists on how the coagulants affect treated wastewater and microalgae biomass quality, and only a few studies have assessed their economic feasibility in industrial operations. This study optimised microalgae ( Chlorella sorokiniana ) harvesting from fish processing wastewater (FPWW) in laboratory-scale experiments and evaluated the economic performance of coagulation-flocculation-sedimentation process followed by minimal centrifugation (case study 1) versus conventional centrifugation alone (case study 2) using process simulation. Chitosan achieved 97 % harvesting efficiency at 15 mg/L concentration, while tannin coagulant (TC) achieved 98 % at 10 mg/L and was less sensitive to overdosing. TC was further examined for harvesting C. sorokiniana during FPWW treatment. The best performance (94 %) was achieved on day 15 at pH 5 using only 15 mg/L TC. Under these conditions, treated water met wastewater discharge standards for total nitrogen (8.5 mg/L) and total phosphorus (0.4 mg/L), and the biomass obtained high protein content (52.9 %), with minimal impact from TC on its quality. Techno-economic analysis showed that, while centrifugation (case study 2) yielded slightly higher biomass quality, the TC-based harvesting method with minimal centrifugation (case study 1) achieved a reduction of roughly 10 % in operating and annual capital costs. A sensitivity analysis indicated that labour cost, the cost of capital and the electricity price were the key cost drivers for both case studies in the base case scenario. Additionally, scale-up calculations with 10- and 100-fold increases in flow rate showed that case study 1 resulted in more moderate growth in operating and annual capital costs, showing a more economically sustainable option for large-scale microalgae recovery. • Tannin coagulant at 15 mg/L efficiently harvested C. sorokiniana from FPWW at pH 5. • Treated FPWW had TN (8.5 mg/L), TP (0.4 mg/L) and biomass contained 52.9 % protein. • TC-based harvesting had ∼10 % lower base case operating and annual capital costs. • Harvesting cost is driven by labour cost, cost of capital and electricity price. • TC-based harvesting showed lower cost escalation with scale-ups than centrifugation.