• Energy Research

Excess consumption of energy by humans is compounded by environmental pollution, the greenhouse effect and climate change impacts. Current developments in the use of algae for bioenergy production offer several advantages. Algal biomass is hence considered a new bio-material which holds the promise to fulfil the rising demand for energy. Microalgae are used in effluents treatment, bioenergy production, high value added products synthesis and CO2 capture. This review summarizes the potential applications of algae in bioelectrochemically mediated oxidation reactions in fully biotic microbial fuel cells for power generation and removal of unwanted nutrients. In addition, this review highlights the recent developments directed towards developing different types of microalgae MFCs. The different process factors affecting the performance of microalgae MFC system and some technological bottlenecks are also addressed.

The interest in microalgae for wastewater treatment and liquid bio-fuels production (i.e. biodiesel and bioethanol) is steadily increasing due to the energy demand of the ultra-modern technological world. The associated biomass and by-product residues generated from these processes can be utilized as a feedstock in anaerobic fermentation for the production of gaseous bio-fuels. In this context, dark fermentation coupled with anaerobic digestion can be a potential technology for the production of hydrogen and methane from these residual algal biomasses. The mixture of these gaseous bio-fuels, known as hythane, has superior characteristics and is increasingly regarded as an alternative to fossil fuels. This review provides the current developments achieved in the conversion of algal biomass to bio-hythane (H2+CH4).

Abstract This study investigated the effects of pretreatment methods (such as autoclave, ultrasonication and electrolysis) of mixed microalgae consortia (predominantly composed of Scenedesmus followed by Chlorella species) from natural ecological niche. In addition, the cultivated biomass (wet) was subsequently utilized for fermentative H2 production in mesophilic regime. The results showed that peak hydrogen production rate (HPR) and hydrogen yield (HY) were achieved from electrolysis pretreated algal consortia as 236 ± 14 mL/L-d and 37.7 ± 0.4 mL/g (volatile solids) VSadded, whereas the untreated algal consortia resulted in the turnout as 64 ± 5 mL/L-d and 9.5 ± 0.0 mL/g VSadded, respectively. The significant increment observed in HPR and HY values were nearly 4 times higher. The solubilization of organic matter during the pretreatment showed positive correlation with the H2 production. The energy generation rate and yield of the corresponding pretreatment methods were as follows, 1.44, 1.79 and 2.65 kJ/L-d for autoclave, ultra-sonication and electrolysis, the corresponding yields also fell in the range of 0.32, 0.41 and 0.43 kJ/g VSadded, respectively.

This review provides the alternative routes towards the valorization of dark H2 fermentation effluents that are mainly rich in volatile fatty acids such as acetate and butyrate. Various enhancement and alternative routes such as photo fermentation, anaerobic digestion, utilization of microbial electrochemical systems, and algal system towards the generation of bioenergy and electricity and also for efficient organic matter utilization are highlighted. What is more, various integration schemes and two-stage fermentation for the possible scale up are reviewed. Moreover, recent progress for enhanced performance towards waste stabilization and overall utilization of useful and higher COD present in the organic source into value-added products are extensively discussed.

Microbial electrolysis cells (MECs) are perceived as a potential and promising innovative biotechnological tool that can convert carbon-rich waste biomass or wastewater into hydrogen (H2) or other value-added chemicals. Undesired methane (CH4) producing H2 sinks, including methanogens, is a serious challenge faced by MECs to achieve high-rate H2 production. Methanogens can consume H2 to produce CH4 in MECs, which has led to a drop of H2 production efficiency, H2 production rate (HPR) and also a low percentage of H2 in the produced biogas. Organized inference related to the interactions of microbes and potential processes has assisted in understanding approaches and concepts for inhibiting the growth of methanogens and profitable scale up design. Thus, here in we review the current developments and also the improvements constituted for the reduction of microbial H2 losses to methanogens. Firstly, the greatest challenge in achieving practical applications of MECs; undesirable microorganisms (methanogens) growth and various studied techniques for eliminating and reducing methanogens activities in MECs were discussed. Additionally, this extensive review also considers prospects for stimulating future research that could help to achieve more information and would provide the focus and path towards MECs as well as their possibilities for simultaneously generating H2 and waste remediation.

Hydrogen production from lignocellulosic biomass (LCB) using dark fermentation is an interesting research niche being developed over the last decade. This review analyses the relevant studies which focused on biohydrogen production from LCB using dark fermentation techniques in terms of substrate characterization, bottlenecks associated with the pretreatment and its subsequent utilization, possible remedies for the scale-up of the most adapted processes and finally the prospects and suggestions which may be envisaged. Studies dealing primarily with the utilization of raw and pretreated LCB have been assessed in terms of biohydrogen production performance for production rate and yield. Energy production analysis and prospecting of suitable cellulosic biomass and efficient cellulolytic microbes have been elucidated towards better cellulose hydrolysis and efficient conversion of LCB to H2 in addition to process economics.

Generation of food waste is a serious issue that needs to be addressed worldwide. Developing suitable treatment methods while generating energy (methane) is a common practice for sustainable treatment of waste. In this study, methane generation by food waste was investigated in mesophilic and thermophilic regimes at various hydraulic retention times (HRTs) and organic loading rates (OLR). In temperature regimes, influent concentrations and HRTs ranged from 30 to 110 g COD/L and 18 to 30 days, respectively, which corresponding to an OLR of 1.0 to 6.1 kg COD/㎥-d. Better methane production and organic removal was observed under thermophilic conditions because of the enhanced hydrolysis of complex polymers and microbial activity at higher temperature. The peak methane productivities attained in thermophilic and mesophilic regimes were 1.30 and 0.99 ㎥/㎥-d, respectively. The maximum methane yields were achieved at 50 g COD/L and HRT of 24 d in both cases, and the values were 264 and 221 ㎥/ton COD, respectively. The results of this study will facilitate the development of sustainable methane production technologies using food waste as a feedstock.