Biohydrogen and methane production via a two-step process using an acid pretreated native microalgae consortium
Introduction
The energy crisis has driven a search for renewable fuels that can be produced using substrates such as photosynthetic biomass or wastes. The term microalga often generalizes all photosynthetic unicellular or simple multi-cellular prokaryotic and eukaryotic microorganisms, such as cyanobacteria, green and red algae, and diatoms. Microalgae are a potential biomass for biofuel production because of their fast growth rate, their lipid and carbohydrate content, and their cultivation in wastewater, which is coupled to their effective role in nutrient removal. When compared to terrestrial crops, microalgae cultures consume less water, reaching higher productivities per culture area, and do not compromise the production of food (Brennan and Owende, 2010). In this sense, a sustainable biofuel production based on microalgae is only possible under a biorefinery approach, producing gaseous biofuels and other value-added products from microalgae biomass (Sarkar et al., 2015).
Microalgae cultivation in wastewater will promote the development of a consortium, contrasting with the mono-algal cultures evaluated in most studies of fuel production. The importance of evaluating a native microalgae consortium lies in the wide diversity of cell wall composition among microalgae species (Domozych et al., 2012), implying different grades of resistance between species. The high carbohydrate content in microalgae makes them a suitable substrate for fermentative fuel production, producing fuels such as biohydrogen, bioethanol, and methane. However, carbohydrates are difficult to extract from microalgae because they are part of the microfibrillar polysaccharides embedded in matrix of polysaccharides and proteoglycans, making necessary a pretreatment step to liberate them (Domozych et al., 2012, Günerken et al., 2015). Different pretreatment technologies have been suggested to break down complex biopolymers in microalgae cells, among them biological, mechanical or chemical.
Biological pretreatments involve the cell degradation by purified enzymes or by microorganisms with enzymatic activity capable of hydrolyzing the microalgae cell wall (Carrillo-Reyes et al., 2016). In this sense, significant differences were observed in the fermentative step when microalgae biomass (Chlorella vulgaris) was used directly or received an enzymatic pretreatment. Specifically, the former resulted in a yield of 11.3 mL H2 g(volatile solids) VS−1 (Lakaniemi et al., 2011), whereas enzymatic pretreatment produced 135 mL H2 g VS−1 (Wieczorek et al., 2014).
Methane production from microalgal biomass has been improved by applying pretreatments to solubilize the microalgae and digest their organic content. For instance, applying a thermal pretreatment there was an increase of 50% in the methane production from Chlorella vulgaris (Mendez et al., 2014).
Chemical pretreatment has an economic advantage over enzymatic pretreatment or thermal pretreatments; however, its application to native microalgae biomass for hydrogen and methane production is still limited (Passos et al., 2014). To the best of our knowledge, the scarce works applying chemical pretreatments are combined with harsh physical disruption strategies, such as ultrasonic or high pressure (Cheng et al., 2014, Liu et al., 2012, Yun et al., 2013). Among chemical pretreatments for microalgae, acidic hydrolysis has been successful in carbohydrate recovery for bioethanol production. For instance, thermal-acidic hydrolysis, under optimized conditions, achieved 95.6% sugar extraction from Scenedesmus obliquus (Miranda et al., 2012), and 97% from Chlorella vulgaris (Ho et al., 2013). Regarding biohydrogen production, acidic hydrolysis recovered almost 100% of the carbohydrate concentrations as reducing sugars; however, this procedure has only been optimized for pure microalgae strains (Liu et al., 2012), which is different from the microalgae consortium that could be recovered from wastewater treatment. Moreover, the acidic hydrolysate concentration is a key parameter to evaluate for increasing the specific hydrogen-producing potential from microalgae biomass, since it has been observed the generation of inhibitors such as furans and 5-hydroxymethyl furfural (HMF) (Yun et al., 2013).
Two-step processes have been proposed to improve the energetic gain from microalgal biomass (Yang et al., 2011, Lü et al., 2013, Wieczorek et al., 2014). In such processes, the carbohydrates are first fermented producing hydrogen and volatile fatty acids (VFA). Then, in a second step, the VFA are easily digested under methanogenic conditions to generate methane. This two-step strategy has been applied in lipid-extracted microalgal biomass residues increasing the methane yield by 22% (Yang et al., 2011), and up to 67% compared to methanogenesis using a single step (Wieczorek et al., 2014). Lü et al. (2013) found a 9.4% increase in the energy yield in a two-step process when compared with the one-step process, using bacterial bioaugmentation. Despite the advances on microalgal pretreatments, most of the previously cited works used mono-algal cultures as feedstock, leaving unresolved the potential barriers of hydrolyzing mixed cultures, such as the developed in wastewater.
Therefore, the aim of the present work was to evaluate the energy recovery through the hydrogen and methane production using a two-step process. A thermal acidic pretreated native microalgae consortium was evaluated under different hydrolysate conditions.
Section snippets
Microalgae biomass
A native microalgae consortium enriched from a local lake in Queretaro, Mexico (20°42′07.0″N 100°27′36.7″W) was used as the biomass source. The microalgae culture was enriched in Bold’s medium in tubular plastic bags (8 L) as reactors illuminated by 12 h light-dark cycles supplied via a 54 W daylight neon lamp with an intensity of 100 μmol m−2 s−1 (LT 300, Extech Instruments, Nashua, NH, USA) and aeration flow rate of 1 L min−1 (Cea-Barcia et al., 2014). The culture composition was determined by
Thermal acidic microalgal pretreatment
The percentage of COD solubilization (CODSOL) during pretreatment was 37.7%, 39.8% and 50.5% at different hydrolysate conditions of 40, 20 and 10 g TS L−1, respectively. A release of volatile fatty acids (Table 1) was observed and explained by cellulose and hemicellulose hydrolysis (Hendriks and Zeeman, 2009). During pretreatment, the sugars solubilization reached 85% and 100% of the total sugar content using 2% HCl and 1% HCl (Table 1), respectively. A higher recovery of reducing sugars (close to
Conclusions
Thermal-acidic microalgal hydrolysis increases the hydrogen and methane production potential of a native consortium compared to those that were not pretreated. The acid concentration was a crucial parameter. In that sense, it was observed that lower acid concentration presented higher hydrogen and methane production potential due to a high recovery of reducing sugars. Results from diluted hydrolysate showed a high energy conversion and methane yield of 15.9 kJ g SV−1 and 432 CH4 g SV−1, respectively,
Acknowledgements
This research was supported through the Dirección General de Asuntos del Personal Académico, Universidad Nacional Autónoma de México – Mexico (PAPIIT IN 101716) and CONACYT – Mexico (249590) projects. The authors are grateful to Jaime Perez and Gloria Moreno for the technical support and fruitful discussions.
References (36)
- et al.
Biochemical methane potential of microalgae: influence of substrate to inoculum ratio, biomass concentration and pretreatment
Bioresour. Technol.
(2012) - et al.
Combining urban wastewater treatment with biohydrogen production – an integrated microalgae-based approach
Bioresour. Technol.
(2015) - et al.
Scenedesmus obliquus as feedstock for biohydrogen production by Enterobacter aerogenes and Clostridium butyricum
Fuel
(2014) - et al.
Biofuels from microalgae—a review of technologies for production, processing, and extractions of biofuels and co-products
Renewable Sustainable Energy Rev.
(2010) - et al.
Biohydrogen production from Tequila vinasses in an anaerobic sequencing batch reactor: effect of initial substrate concentration, temperature and hydraulic retention time
Bioresour. Technol.
(2010) - et al.
Biological pretreatments of microalgal biomass for gaseous biofuel production and the potential use of rumen microorganisms: a review
Algal Res.
(2016) - et al.
Optimization of wastewater microalgae saccharification using dilute acid hydrolysis for acetone, butanol, and ethanol fermentation
Appl. Energy
(2015) - et al.
A cost-effective strategy for the bio-prospecting of mixed microalgae with high carbohydrate content: diversity fluctuations in different growth media
Bioresour. Technol.
(2014) - et al.
Cogeneration of hydrogen and methane from the pretreated biomass of algae bloom in Taihu Lake
Int. J. Hydrogen Energy
(2014) - et al.
Screening microalgae strains for their productivity in methane following anaerobic digestion
Appl. Energy
(2013)
Cell disruption for microalgae biorefineries
Biotechnol. Adv.
Influence of acid pre-treatment on microalgal biomass for bioethanol production
Process Biochem.
Pretreatments to enhance the digestibility of lignocellulosic biomass
Bioresour. Technol.
Bioethanol production using carbohydrate-rich microalgae biomass as feedstock
Bioresour. Technol.
Anaerobic conversion of microalgal biomass to sustainable energy carriers – a review
Bioresour. Technol.
Fermentative hydrogen production by Clostridium butyricum CGS5 using carbohydrate-rich microalgal biomass as feedstock
Int. J. Hydrogen Energy
Methane production of thermally pretreated Chlorella vulgaris and Scenedesmus sp. biomass at increasing biomass loads
Appl. Energy
Pre-treatment optimization of Scenedesmus obliquus microalga for bioethanol production
Bioresour. Technol.
Cited by (39)
Insights into biohydrogen production from algal biomass: Challenges, recent advancements and future directions
2024, International Journal of Hydrogen EnergySubstituting microalgae with fecal sludge for biohythane production enhancement and cost saving through two-stage anaerobic digestion
2023, Journal of Cleaner ProductionFactors affecting biohydrogen production: Overview and perspectives
2023, International Journal of Hydrogen EnergyA review on pretreatment methods, photobioreactor design and metabolic engineering approaches of algal biomass for enhanced biohydrogen production
2023, International Journal of Hydrogen EnergyMicroalgal farming for biofuel production: Extraction, conversion, and characterization
2023, Microalgal Biomass for Bioenergy ApplicationsRole of microalgae in achieving sustainable development goals and circular economy
2023, Science of the Total Environment