Densification of Synechococcus subsalsus biomass by chitosan coagulation for biogas production

This study evaluated the coagulation/flocculation process using chitosan as a natural coagulant to concentrate suspensions of the cyanobacterium Synechococcus subsalsus and enable biogas production from concentrated biomass. The chitosan performance was tested and compared with the inorganic ferric chloride (FeCl₃) coagulant. Using the liquid fraction of the coagulation/flocculation process in subsequent biomass cultivations proved viable, with similar growths in culture media with up to 80% of the liquid fraction. At pH 6 and 400 mg/L FeCl₃, the biomass concentrated almost seven times, increasing the total suspended solids (TSS) of the suspension from 0.4-0.6 g/L to 2.6-4.0 g/L. With 80 mg/L chitosan and pH 7, the TSS concentration attained values in the range of 7.0-9.7 g/L, an increase of more than 30 times, clearly showing that chitosan has a much higher capacity for biomass concentration at a lower concentration. A ratio of 0.3 g chitosan/g dry mass of the biomass was established to reach the maximum densification. The production of methane from chitosan-densified biomass proved to be feasible. Chitosan-densified biomass showed a two-phase cumulative methane production when digested, with slower methane production and 23% lower methane yield after 30 days of digestion (207 NmL CH₄/g COD_i) compared to the biomass from cultivation (non-densified, 270 NmL CH₄/g COD_i). However, optimizing the digestion conditions of the densified biomass should increase the methane yield and reduce process time.