Waste biorefineries — integrating anaerobic digestion and microalgae cultivation for bioenergy production
Graphical abstract
Introduction
Anaerobic digestion (AD) is one of the most widely applied biological processes for the conversion of organic biomass to bioenergy (e.g. H2 and CH4). Dark fermentation (DF) with anaerobic bacteria (e.g. Clostridium spp.) is the major process for the conversion of biomass to hydrogen but during the DF process, most of the carbon matters remain in the liquid phase in the form of volatile fatty acids, alcohols and acetone. During methanogenesis process, COD reduction is more efficient, whereas most H2 generated from acidogenesis phase is consumed but nitrogen and phosphorus contents still remain to certain extent. In addition, the biogas generated from anaerobic digestion still contains a significant amount of CO2, which decreases the efficiency of power generation with the biogas and causes global warming when emitting to the atmosphere. Thus, these interrelated anaerobic processes generate: first, an effluent with high chemical oxygen demand (COD) contributed by the high concentrations of volatile fatty acids (VFAs), total nitrogen (TN) and total phosphorus (TP) [1]; and second, low biogas (CH4 or H2) yield and purity of due to the incomplete conversion of organic carbon. The biogas from AD contains approximately 20–60% CO2 and 0.005–2% H2S, and thus does not meet fuel gas specifications unless a proper purification process is employed [2]. Therefore, developing a low-cost strategy to treat fermentation effluents and upgrade biogas quality to meet fuel specifications is essential. In this review, microalgal cultivation is presented as a valorization method for the utilization of fermentation effluents, including both liquid and gases components. Integration of AD with microalgal cultivation has the dual benefits of reducing the carbon footprint of AD and managing the high production costs associated with conventional microalgal cultivation.
Section snippets
Mechanism and major metabolites of anaerobic digestion and dark fermentation
Fermentation of complex organic materials by anaerobic bacteria results in the decomposition of the carbon in the biomass to either CO2 or CH4. AD is a multi-step process, with four different phases: hydrolysis, acidogenesis, acetogenesis and methanogenesis, and the initial organic compounds are decomposed to methane and VFAs, with concomitant release of gaseous products [1]. DF is the acidogenesis phase of AD, where VFAs, hydrogen and CO2 are generated as the main products [3••]. These two
Growth of microalgae on gaseous and liquid fermentation effluents
Microalgae own the advantages of high growth rate, superior environmental adaptability, high nutrient-removal ability, no competition with food or arable land, year-round cultivation, higher lipid productivities and photosynthetic efficiencies compared with other terrestrial plants or microorganisms [5, 6], which are regarded as a potential solution for the valorization of AD waste. Biogas is the gaseous counterpart of the AD products, and the liquid effluent or slurry is rich in organic
Factors affecting metabolites removal from fermentation effluents by microalgae and microalgal lipid accumulation
Light irradiance plays an important role in the assimilation of organic carbon in fermentation effluents by microalgae, and optimal light intensity enhances VFA removal and biomass production. The mixotrophic mode of cultivation is suitable for the growth of Chlamydomonas reinhardtii on VFAs, enhancing its lipid accumulation [28], and an optimal light supply could alleviate butyrate inhibition in C. sorokiniana and C. vulgaris [9, 29]. However, the high light intensities required for
Developing a waste biorefinery by integrating anaerobic digestion and microalgal cultivation
Recent progress concerning the development of integrated systems that utilize the anaerobic fermentation byproducts from anaerobic digestion for microalgal growth is described in Table 3. This shows that the combining dark fermentation, acidogenic fermentation and waste sludge digestion with heterotrophic or mixotrophic microalgal cultivation has been successful, and the combined systems appear to enable efficient removal of COD and nutrients from the fermentation effluents with simultaneous
Conclusions and future perspectives
The integration of sludge digestion and microalgal cultivation can address the issues of both energy sustainability and waste disposal. From an economic perspective, the utilization of waste for microalgal cultivation and multi-level processing for the extraction of total energy from the mix can reduce the cultivation costs and so reduce the associated biofuel prices. Several bottlenecks in this process need to be addressed prior to implementation on a large scale: first, identification of a
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
This work was supported by the State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (No. 2016TS07) and by the Project of Thousand Youth Talents. The funding from Taiwan's Ministry of Science and Technology (MOST) under grant numbers of MOST 106-3113-E-006-011, 106-3113-E-006-004-CC2, 104-2221-E-006-227-MY3, and 103-2221-E-006-190-MY3 is also acknowledged.
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