Impact of hydrothermal pretreatments on microalgal biomethane production

Microalgae are regarded as attractive feedstock for the fermentative production of biomethane fuel through anaerobic digestion (AD) process. However, microalgal AD is limited by 2 significant factors: (1) high cell wall recalcitrance resulting in lower access to intracellular organic components by AD bacteria, and (2) high nitrogen (protein) content of microalgae leading to the formation of ammonia at inhibitory concentrations. Subjecting microalgae to pretreatment using hot compressed water is expected to decrease recalcitrance and improve anaerobic digestibility of microalgae.  A further phase separation using centrifugation results in a solid fraction with lower nitrogen content than the aqueous phase. The use of this solid fraction as a feedstock for AD process could help reduce ammonia inhibition. The current study investigated the impact of low temperature hydrothermal pretreatment (LTHTP) process, with or without phase separation on microalgal AD. The results indicated that for Chlorella pyrenoidosa (CP) LTHTP without phase separation at 125 °C enhanced methane production by 26.2 % while that at 225 °C decreased methane production. For the wall-less diatom, Phaeodactylum tricornutum (PT), pretreatment at 160 °C improved methane production by 100% and that at 125 °C and 225 °C had a negative effect. The use of the solid phase after LTHTP at 200 °C as feedstock for AD enhanced methane production by 43.8% for CP but resulted in a significant decrease for PT.