The anaerobic digestion of halophytic microalgae

Abstract

The anaerobic digestion of microalgae is a potential environmentally feasible option for creating a renewable source of energy for industrial and domestic needs. Microalgae anaerobic digestion is a key unit process that integrates efficiency and beneficially into the production of microalgae derived biofuels. Anaerobic digestion culminating in methane fermentation improves the economic viability of microalgae liquid biofuel production and presents an opportunity for power generation from wastewater derived microalgae. However the anaerobic digestion of halophytic microalgae biomass is not straight forward due to several technical restraints including low concentration of digestible biodegradable substrate, recalcitrant substrate constituents, cell wall degradability and effects from salinity and associated metal ions. To address the quantification of low biodegradable substrate associated with microalgae cultures, development of a high throughput methodology to determine the quantification of suspended microalgae biomass content and other water quality parameters via turbidity measurements was determined. The development of the new management tool allows faster operational control from a simple turbidity analysis, reducing time delays to fewer than 5 minutes and avoids expensive laboratory testing. Further development of this management tool will support the operational control for biofuel pond management and wastewater treatment plants. This management tool provides a rapid quantification of biomass and allows harvesting volumes to be calculated to allow consistent volatile solid and chemical oxygen demand loading to anaerobic digesters. The anaerobic digestion of halophytic microalgae biomass however, has a significant challenge to be mitigated before this technology can be beneficial for the burgeoning microalgae industry. The halophytic microalgae biomass as a potential substrate feedstock for anaerobic digestion will have salinities > 35 ppt. To address this issue the first section of my PhD research focussed on the changes undertaken in the bacterial community associated with the anaerobic digestion of piggery effluent under increasing saline conditions with the aim of establishing a saline tolerant anaerobic digestion inoculum capable of digesting feedstock’s under high salinity conditions. Favourable results from this inoculum development study allowed the second part of the PhD research to be investigated where the anaerobic digestion of halophytic biomass was investigated utilising the inoculum established from the initial component of the reported study. Results of the later study demonstrated that a hyper saline inoculum was achieved and subsequent DGGE fingerprinting of the bacterial community detected several high salinity methanogens at a salinity of 7% and validated the establishment of a halo-tolerant anaerobic digestion community. Establishment of a halo tolerant anaerobic digestion community was further validated by significant methane production at the high 7% salinities. This inoculum was then used for all other reported studies. Another major difficulty associated with the anaerobic digestion of microalgae is the need to disrupt the cell wall allowing the cell contents to be processed by the bacterial community. In this study I compared the methane production from lipid extracted, pre-treated disrupted and non-pretreated Tetraselmis sp. microalgae respectively. Results demonstrate that a methane production of 122 mL per g VS for the lipid extracted Tetraselmis sp. biomass. This result demonstrates that after the extraction of lipid for use in biofuel production residual lipid extracted microalgae biomass is a viable feedstock for methane production. A methane production of 252 mL per g VS and 248 mL per g VS was reported for the non-disrupted algae and pretreated disrupted Tetraselmis sp. respectively. This study also identified the ability of the anaerobic digestion microbial community to undertake cell lyses via microbial degradation of the Tetraselmis sp. microalgae. Cell lyses by the anaerobic digestion microbial community can offer a direct conversion pathway for energy production were whole biomass can be harvested and concentrated and directly fed to the anaerobic digester without energy intensive pre-treatment or processing being required. Investigation was also undertaken to quantify the suitability of anaerobically digested halophytic Tetraselmis sp. microalgae digestate as a nutrient feed stock to form a closed loop nutrient system. To determine microalgae digestate suitability I established that the following factors needed to be observed: growth, lipid content, and the bacterial community diversity. Microalgae digestate was diluted according to the concentration of NH₄⁺ content (20, 40, 60, 80 mg/L) and compared against a standard medium for Tetraselmis sp.. The growth rate on the microalgae digestate media was not as rapid as the F/2 standard medium and the high microalgae digestate media concentrations correlated with lower total lipid contents, additionally acyl carrier proteins (ACP) gene expression rates displayed lower lipid gene expression within high microalgae digestate treatments. Lastly, higher concentrations of microalgae digestate were correlated with a higher bacterial diversity in the bacterial community throughout the investigation. No significant difference in lipid production and satisfactory growth was recorded for the lower microalgae digestate treatments. These results confirmed the suitability of microalgae digestate as a suitable nutrient source for use in the production of Tetraselmis sp. biomass for lipid and biofuel production.