Effects of different thermal and thermochemical pretreatments on the anaerobic digestion of algal biomass cultivated in urban wastewater and collected with and without chemical coagulants

Abstract

Algal biomass cultivated in wastewater treatment systems is a promising feedstock for biogas production through anaerobic digestion. However, its rigid cell wall inhibits the hydrolysis of intracellular substances and limits its anaerobic digestibility. To improve anaerobic biodegradability, this study investigated the effects of different thermal and thermochemical pretreatments on the performance of anaerobic digestion of the obtained biomass. High-rate algal ponds fed with UASB effluent treating domestic sewage were used to grow algal biomass. Two methods were used for biomass harvesting: a physical method, through filtration, and a physico-chemical method, using coagulants, Tanfloc SL and Aluminum Sulfate (Al2(SO4)3). Three types of biomasses were obtained, one without coagulant (AB-P), one with Tanfloc (AB-T), and one with Al2(SO4)3 (AB-AS), which were subjected to different thermal and thermochemical pretreatments. Fourier-transform infrared spectroscopy analysis confirmed the effectiveness of the pretreatment methods, showing modifications in the structures of proteins, carbohydrates, and lipids in the microalgae cells. The alkaline thermochemical pretreatment (90 °C-3 h-pH 11) of AB-AS resulted in higher organic matter solubilization, with a 32 % increase in soluble COD. This pretreatment also resulted in the highest biogas yield, producing 460.64 mL CH4.gVS−1, with a 229 % increase, and the highest maximum production rate, with Rmax = 78.98 mL CH4.gVS−1.day−1. Acidic thermochemical pretreatments that resulted in biomass with pH ≤ 4.2 inhibited biogas production. In terms of energy, the best pretreatment was 78 °C-7 h on AB-P, with an EROI of 2.20, which demonstrates a surplus of energy. These findings underscore the potential for optimizing the anaerobic digestibility of algal biomass, thus contributing to more efficient and sustainable biogas production.