(620bj) Impact of Nitrogen Removal Pretreatments on Algal Biomethane Production

Authors: 
Das, K., University of Georgia

Microalgal anaerobic digestion (AD) has been receiving increasing attention in the recent past owing to the versatility of algal biomass and the benefits of biomethane fuel.  Algal methane production is known to be species dependent. The current work investigated the biochemical methane potential of 3 algal species namely Chlorella pyrenoidosa (fresh water green alga), Tetraselmis chuii (marine green alga), and Phaeodactylum tricornutum (marine diatom), which differ significantly in the cell wall structure and biochemical composition. The results indicated that biomethane yield was the highest for P. tricornutum (259.6 mL gVS -1), followed by C. pyrenoidosa (234 mL gVS -1) and the least for T. chuii (160.4 mL gVS -1). Infact, the experimentally obtained biomethane yield was only about 33 % of the stoichiometric (theoretical maximum) yield for T. chuii and around 50-60 % of the respective stoichiometric yields for the other two species.  These results clearly indicate a limitation of the AD process. Factors such as cell wall recalcitrance and the presence of inhibitory compounds such as ammonia significantly contribute to the limitation.  Since algae have high nitrogen (and protein) content, they are capable of producing ammonia at inhibitory concentrations during AD process. Thus, pretreatments aimed at achieving nitrogen removal from the algal biomass may help in enhancing biomethane production from the residual biomass.  Investigations are underway to validate this hypothesis for the 3 species listed above using chemical and hydrothermal protein/nitrogen removal pretreatments.  Chemical protein/nitrogen removal was carried out by the alkali-acid method which involved solubilisation of proteins at alkaline pH followed by precipitation at acidic pH. Hydrothermal nitrogen removal was carried out by subcritical water extraction of algal proteins and amino acids at two different temperatures (125 °C and 160 °C) and 300 psi in a high pressure reactor.