Waste biorefineries — integrating anaerobic digestion and microalgae cultivation for bioenergy production

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Highlights

  • Microalgae can be grown with liquid (COD, N, P) and gaseous (CO2) effluents of anaerobic digestion.

  • The organic acids present in the effluent are utilized efficiently in a mixotrophic mode of cultivation.

  • Simultaneous biogas upgrading with over 90% methane content could be obtained.

  • Lipid content in microalgae grown on anaerobic digestion effluents can be improved for biofuel applications.

  • Integration of sewage sludge AD and microalgal cultivation is an interesting waste biorefinery option.

Commercialization of microalgal cultivation has been well realized in recent decades with the use of effective strains that can yield the target products, but it is still challenged by the high costs arising from mass production, harvesting, and further processing. Recently, more interest has been directed towards the utilization of waste resources, such as sludge digestate, to enhance the economic feasibility and sustainability of microalgae production. Anaerobic digestion for waste disposal and phototrophic microalgal cultivation are well-characterized technologies in both fields. However, integration of anaerobic digestion and microalgal cultivation to achieve substantial economic and environmental benefits is extremely limited, and thus deserves more attention and research effort. In particular, combining these two makes possible an ideal ‘waste biorefinery’ model, as the C/N/P content in the anaerobic digestate can be used to produce microalgal biomass that serves as feedstock for biofuels, while biogas upgrading can simultaneously be performed by phototrophic CO2 fixation during microalgal growth. This review is thus aimed at elucidating recent advances as well as challenges and future directions with regard to waste biorefineries associated with the integration of anaerobic waste treatment and microalgal cultivation for bioenergy production.

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.

References (61)

  • L. Meier et al.

    Photosynthetic CO2 uptake by microalgae: an attractive tool for biogas upgrading

    Biomass Bioenergy

    (2015)
  • J.M. Prandini et al.

    Enhancement of nutrient removal from swine wastewater digestate coupled to biogas purification by microalgae Scenedesmus spp

    Bioresour Technol

    (2016)
  • M. Bahr et al.

    Microalgal–biotechnology as a platform for an integral biogas upgrading and nutrient removal from anaerobic effluents

    Environ Sci Technol

    (2014)
  • J. Fan et al.

    A chloroplast pathway for the de novo biosynthesis of triacylglycerol in Chlamydomonas reinhardtii

    FEBS Lett

    (2011)
  • C. Baroukh et al.

    Dynamic metabolic modeling of heterotrophic and mixotrophic microalgal growth on fermentative wastes

    PLoS Comput Biol

    (2017)
  • S. Venkata Mohan et al.

    Fatty acid rich effluent from acidogenic biohydrogen reactor as substrate for lipid accumulation in heterotrophic microalgae with simultaneous treatment

    Bioresour Technol

    (2012)
  • A. Elmekawy et al.

    Bioelectro-catalytic valorization of dark fermentation effluents by acetate oxidizing bacteria in bioelectrochemical system (BES)

    J Power Sources

    (2014)
  • M. Moon et al.

    Mixotrophic growth with acetate or volatile fatty acids maximizes growth and lipid production in Chlamydomonas reinhardtii

    Algal Res

    (2013)
  • V. Turon et al.

    Growth of Chlorella sorokiniana on a mixture of volatile fatty acids: the effects of light and temperature

    Bioresour Technol

    (2015)
  • O. Perez-Garcia et al.

    Heterotrophic cultures of microalgae: metabolism and potential products

    Water Res

    (2011)
  • C. Yan et al.

    Performance of photoperiod and light intensity on biogas upgrade and biogas effluent nutrient reduction by the microalgae Chlorella sp

    Bioresour Technol

    (2013)
  • Y. Zhao et al.

    Performance of three microalgal strains in biogas slurry purification and biogas upgrade in response to various mixed light-emitting diode light wavelengths

    Bioresour Technol

    (2015)
  • C. Yan et al.

    Performance of mixed LED light wavelengths on biogas upgrade and biogas fluid removal by microalga Chlorella sp

    Appl Energy

    (2014)
  • J.-M. Lv et al.

    Enhanced lipid production of Chlorella vulgaris by adjustment of cultivation conditions

    Bioresour Technol

    (2010)
  • K.L. Yeh et al.

    Nitrogen starvation strategies and photobioreactor design for enhancing lipid content and lipid production of a newly isolated microalga Chlorella vulgaris ESP-31: implications for biofuels

    Biotechnol J

    (2011)
  • L. Jiang et al.

    Biomass and lipid production of marine microalgae using municipal wastewater and high concentration of CO2

    Appl Energy

    (2011)
  • N.R. Boyle et al.

    Three acyltransferases and nitrogen-responsive regulator are implicated in nitrogen starvation-induced triacylglycerol accumulation in Chlamydomonas

    J Biol Chem

    (2012)
  • H.U. Cho et al.

    Effects of pH control and concentration on microbial oil production from Chlorella vulgaris cultivated in the effluent of a low-cost organic waste fermentation system producing volatile fatty acids

    Bioresour Technol

    (2015)
  • W. Tongprawhan et al.

    Biocapture of CO2 from biogas by oleaginous microalgae for improving methane content and simultaneously producing lipid

    Bioresour Technol

    (2014)
  • G.O. James et al.

    Temperature modulation of fatty acid profiles for biofuel production in nitrogen deprived Chlamydomonas reinhardtii

    Bioresour Technol

    (2013)
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