Use of macroalgae in a three-stage bioreactor for the biogas production

Abstract

An excess of macroalgae growing in water bodies is an environmental problem, as it is difficult to remove the excessive amounts of them in natural ways. It was observed previously that mixing macroalgae with the other organic substrates can produce high yields of biogas (mostly methane). Cladophora glomerata, macroalgae growing in the Lithuanian freshwaters, was used for this purpose. They were mixed with cattle manure and an activated sewage sludge as source of microorganisms for the biogas production in a three-stage bioreactor. Bradford, Folch or Dinitro Salicylic Acid (DNS) methods were used for the determination of concentrations of total proteins, lipids, and glucose (reduced sugars), respectively. Concentrations of those were determined at different bioreactor organic loads: 2.87, 4.06 and 8.13 kg VS/m3 d. The highest total lipid concentration (6.69%) was reached when the organic load was 8.13 kg VS/m3 d. The average biogas yield at this load was 198.7±2.2 L/Kg VSadded, and the methane yield was 154.1±5.4 L CH4/Kg VSadded. Concentration of reduced sugards decreased from 9.68 to 6.67% during this proccess indicating that some of it has been consumed by microorganisms. The lowest total protein concentration (1.12%) was found when the organic load of the bioreactor was 4.06 kg VS/m3 d. The decreased concentration of total proteins showed the activity of microorganisms during which the yield of biogas and methane increased to 386.4±3.8 L/Kg VSadded and 281.5±8.2 L CH4/Kg VSadded, respectively. Our studies have shown that the yield of biogas and methane in a three-stage bioreactor depends on the organic load of the bioreactor. Such dependence was determined by the concentration of glucose, fat and protein in the substrate. This project has received funding from the Research Council of Lithuania (LMTLT, grant number S-LU-22-2) and Ministry of Education and Science of Ukraine.