Enhancement Of Biogas Production From Microalgae By Enzymatic Pretreatment

Abstract

Renewable energy sources have gained considerable attention due to increasing global energy demand and environmental concerns on fossil fuels. Anaerobic digestion is one of the renewable technologies that can produce gaseous energy from several types of substrates. Microalgal biomass is a good candidate for anaerobic digestion, however; rigid cell wall characteristics prevent accessibility of anaerobic bacteria to degrade microalgal biomass and limits methane production from microalgae. To enhance methane production from microalgae, application of pretreatment methods prior to anaerobic digestion provides rigid cell wall disruption and cell membrane solubilization. As a result, enhanced methane production potentials could be achieved by increasing the accessibility of microalgal biomass for hydrolytic bacteria. Main objective of this thesis was to enhance methane production from microalgae biomass by enzymatic pretreatment prior to anaerobic digestion. Enzymatic pretreatment was carried out with two different microalgae ( Scenedesmus sp. and ii Porphydium cruentum) at different pretreatment conditions including different enzymes (cellulase, protease, viscozyme, and enzyme mix including protease and visczoyme), enzyme doses (0.1-0.3-0.5 mL/ g biomass), temperatures (25-55°C), and time (1-24 hours). In order to determine the effectiveness of the enzymatic pretreatment, organic matter solubilization and cell wall disruption after enzymatic pretreatment were evaluated and methane production potential of pretreated biomass were determined by Biochemical Potential Tests (BMP) at both mesophilic (35°C) and thermophilic (55°C) range. The results showed that enzymatic pretreatment at 55 °C showed highest solubilization efficiencies and efficiency of enzyme used in pretreatment is directly related to characteristics of microalgae. In case of Scenedesmus sp., cellulase (8.8-12.3% solubility increase), protease (14.7-16 % solubility increase), viscozyme (8.2-16% solubility increase), and enzyme mix (22.5-37.8% solubility increase), were effective for cell wall degradation and biomass solubilization. In case of P. cruentum, protease (14.9-32.3 % solubility increase), viscozyme (27-30.4 % solubility increase), and enzyme mix (22.3-30.53 % solubility increase) were effective to improve biomass solubilization. After enzymatic pretreatment, highest methane improvements were achieved for Scenedesmus sp. as 109% after enzyme mix pretreatment (0.5 mL) at mesophilic temperature and 71.4% after protease pretreatment (0.5 mL dose) at thermophilic temperature. For P. cruentum, methane improvements were achieved as 71.7 % after protease pretreatment (0.5 mL dose) at mesophilic temperature and 100% (0.5 mL dose) after protease pretreatment at thermophilic temperature. According to the results, thermophilic digestion provided higher methane yields for both microalgae and highest methane improvements were achieved with enzyme mix and protease pretreatment for both microalgae Kinetic model studies showed that modified Gompertz model were able to fit experimental values than first order kinetic model. Energy assessment results indicated the ratio of energy input to the energy output were less than 1 for both untreated microalgae. However, the ratios were reduced by application of enzymatic pretreatment for both microalgae.