Continuous treatment of fish processing wastewater using Chlorella sorokiniana in a photobioreactor: Comparison of chemical and mechanical pretreatments

Continuous microalgal photobioreactors (PBRs) are promising for industrial wastewater treatment; however, the influence of stepwise hydraulic retention time (HRT) reduction and pretreatment strategies remains unclear. This study evaluated a continuous PBR cultivated with Chlorella sorokiniana for treating real fish processing wastewater (FPWW) under decreasing HRTs and two pretreatment methods. In stage 1, chemically pretreated FPWW (coagulation and dissolved air flotation) was treated at HRTs of 20, 10, 5, and 2.5 days. In stage 2, mechanically pretreated FPWW (centrifugation and filtration) with significantly higher nutrient and organic load was treated at HRTs of 20 and 10 days. The best performance occurred in stage 1 at an HRT of 2.5 days, achieving total nitrogen (TN) and total phosphorus (TP) removal efficiencies of 95% and 98%, respectively. Mechanically pretreated FPWW at an HRT of 20 days achieved similar nutrient removal efficiencies (95% TN and 98% TP). Biomass composition shifted from 57.2% protein in stage 1 to 32.0% in stage 2, reflecting changes in nutrient availability and HRT. Bacterial diversity increased in stage 1 but declined in stage 2, with stress-tolerant genera such as Cyanobium PCC-6307.