Enhancing algal biomass and biofuels recovery from open culture systems

Abstract

Growth of algae for the purposes of generating biofuels and/or other bioproducts holds promise due to numerous advantages unique to algae. But there are serious barriers to commercial scale algae cultivation and downstream processing. Two barriers addressed in this thesis are: (i) carbon limitation in open pond algal cultivation, and (ii) difficulties for downstream processing due to the robust nature of algal cells. Hence the overall aims of this thesis were to: • Understand and model the contribution of carbon cycling by heterotrophic bacteria to algal growth, and • Facilitate effective downstream processing by applying a novel free nitrous acid (FNA) pre-treatment to disrupt algal cells. To understand the contribution of heterotrophic bacteria to algal growth, carbon cycling driven by bacteria was evaluated in open algal culture systems. Since pH controls the proportion of carbon in the various inorganic pools, two different pHs were trialled. The contribution of bacteria to carbon cycling was determined by quantifying algae growth with and without supplementation of bacteria. It was found that adding heterotrophs to an open algal culture dramatically enhances algae productivity. Increases in algae productivity due to supplementation of bacteria of 4.8 and 3.4 times were observed in two batch tests operating at two different pH values over 7 days. A simplified kinetic model was proposed which described carbon limited algal growth. Simulation of the model highlighted how carbon limitation can be overcome by bacteria re-mineralising photosynthetic end products. The model was extended into a full Algae-Bacteria Model (ABM) to describe carbon, oxygen and nitrogen flows in open algal systems. The integrated model, presented in an easy to use Petersen’s matrix format, provides the first description of algal cultivation processes considering the effect of heterotrophic bacteria driven carbon cycling. The second aim of the thesis was addressed through the development and use of a novel pre-treatment technology – FNA pre-treatment – to enhance biofuels production, including (i) enhancing lipids, and specifically triacylglycerides (TAG) recovery for biodiesel production, and (ii) enhancing methane yield during anaerobic digestion of algae. For enhancing lipids, and specifically TAG recovery, laboratory batch tests, with a range of FNA conditions and pre-treatment time, were conducted to disrupt algal cells prior to lipid extraction by organic solvents. Total lipid (TL) quantified by the Bligh and Dyer method was found to increase with longer pre-treatment time (48 h) and higher FNA concentration (up to 2.19 mg HNO2-N L-1). Lipid extraction kinetic analysis was also conducted by Soxhlet extraction apparatus. The mass transfer coefficient (k) for lipid extraction from algae treated with 2.19 mg HNO2-N L-1 FNA was found to increase dramatically to 0.96 h-1 over the untreated algae (0.39 h-1). The quantity of TAG among total lipid recovered was boosted with increasing FNA pre-treatment up to an FNA concentration of 2.25 mg HNO2-N L-1. But higher FNA concentrations (13.49 and 26.98 mg HNO2-N L-1) were detrimental to polyunsaturated fatty acid (PUFA) recovery, which resulted in a decrease in TAG recovery. For enhancing methane yield during algal digestion, the feasibility of using FNA pre-treatment was investigated through laboratory biochemical methane potential tests. It was demonstrated that methane production was dramatically enhanced by FNA pre-treatment (2.31 mg HNO2-N L-1), with a 53 % increase in the methane yield (from 161 to 250 L CH4 per kg VS). A two substrate model was used to describe digestion to account for the apparent presence of rapid and slowly degradable material. Model-based analysis revealed that with FNA pre-treatment, the availability of both rapid and slowly biodegradable substrates were increased.