Anaerobic digestion of microalgae and cyanobacteria was first proposed as a destination for algal biomass accumulated on stabilization ponds since it could not be disposed of directly in the environment. Now, the versatility of algal biomass makes them a suitable candidate to produce biofuels and other biomolecules in biorefineries. Anaerobic digestion of biomass is advantageous because it does not require the extraction of specific cellular constituents or drying of the biomass. Nevertheless, challenges remain regarding biomass concentration and their resistant cell walls, which are factors that could hamper methane yield. Many pretreatment methods, including chemical and thermochemical, have been proposed to break down the complex polymers present on the cell wall into smaller molecules. Unfortunately, the relationship between biomass solubilization and methane yield is not well defined. This article intends to review the anaerobic digestion of algal biomass and the role of chemical and thermochemical pretreatments in enhancing methane production. Several pretreatment conditions selected from the scientific literature were compared to verify which conditions actually improve methane yield. The severity of the selected pretreatments was also assessed using the combined severity factor. Results suggest that thermochemical pretreatment in less severe conditions is the most efficient, leading to a greater increase in methane yield. Only enzymatic pretreatments and some thermal pretreatments result in a positive energy balance. The large-scale implementation of pretreatment methods requires technological innovations to reduce energy consumption and its integration with other processes in wastewater treatment plants.

In this study, the integration of enzymatic pretreatment with anaerobic co-digestion (ACoD) was evaluated as a strategy to enhance methane production from waste activated sludge (WAS) and food waste (FW). Hydrolytic enzymes were produced via solid-state fermentation (SSF) using Aspergillus niger and babassu cake as substrate, resulting in a crude enzyme extract with high xylanase and protease activities, indicating strong hydrolytic potential for sludge pretreatment, along with lower activities of amylase and endoglucanase. Enzymatic pretreatment of WAS at 0.2 U/mg total suspended solids promoted up to 95% organic matter solubilization. ACoD was evaluated under three conditions: (I) FW with untreated WAS, (II) FW with enzymatically pretreated WAS, and (III) FW with WAS and direct enzyme addition. Enzymatic pretreatment increased methane yield by approximately 30%, from 210 to 270 NmL CH₄/gVS, while direct enzyme addition led to more moderate improvements. Kinetic modeling using modified Gompertz and logistic models confirmed enhanced methane production but indicated longer lag phases in enzyme-treated systems, attributed to intermediate product accumulation. Under these conditions, elevated volatile acidity/total alkalinity ratios and increased ammonia concentrations suggested process stress. Overall, SSF-derived enzymes combined with co-digestion represent a promising strategy to boost methane production, while requiring careful control to avoid acidification.

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 CODi) compared to the biomass from cultivation (non-densified, 270 NmL CH4/g CODi). However, optimizing the digestion conditions of the densified biomass should increase the methane yield and reduce process time.