• Energy Research
• 7. Clean energy close

Coal seam gas (CSG) is being touted as a transition fuel as the world moves towards low-carbon economies. However, the development of CSG reserves will generate enormous volumes of saline water. In this work, we investigate the potential of using this saline water to support mass algae production. Water and brine from a CSG water treatment facility (1.6 and 11.6 g total dissolved solids per litre (TDS L(-1)) respectively) were inoculated with algal biomass from freshwater and seawater environments and supplemented with nutrients in open, fed-batch reactors. Significant algal growth was recorded, with maximum specific growth rates in CSG water and CSG brine of 0.20 +/- 0.05 d(-1) and 0.26 +/- 0.04 d(-1) respectively. These maximum specific growth rates were equal to or greater than specific growth rates in deionized water and seawater diluted to the same salinity. However, algal growth lag time in CSG brine was between 7 and 9 times longer than in other waters. Microscopy and terminal-restriction fragment length polymorphism (T-RFLP) were used to monitor community structure in the reactors. The same few algal species dominated all of the reactors, except for the CSG brine reactor at day 15. This result indicates that conditions in CSG brine select for different species of algae compared to seawater of the same salinity and other waters tested. The findings suggest that mass algae production in CSG water is feasible but algae community composition may be a function of CSG water chemistry. This has implications for the downstream use of algae.

The methane yield from the digestion of algae is typically much lower than the theoretical methane yield, and lower than yields reported for other organic substrates. This study presents a novel free nitrous acid (FNA) pre-treatment technique to improve methane production from algal biomass. The methane production yield through anaerobic digestion was found to be dramatically enhanced by FNA pre-treatment (2.31 mg HNO2–N L−1), with a 51% increase in the methane yield (from 161 to 250 L CH4 per kg VS added). A two substrate model was used to describe the apparent presence of rapid and slowly degradable material. Model-based analysis revealed that with FNA pre-treatment (2.31 mg HNO2–N L−1), the availability of both rapid and slowly biodegradable substrates were increased. Higher levels of nitrite (159 and 1006 mg N L−1) had an inhibitory/toxic effect. For this reason, coupled with the fact that denitrification of nitrite consumes organic substrate, it is concluded that pre-treatment liquor should be removed before digestion.

Abstract The sustainable, efficient production of biofuel can lead to reductions in greenhouse gas emissions, lowered climate change impact and increased security owing to the fulfilment of global energy demands. Microalgae have been shown as an attractive feedstock for renewable fuel production, such as biodiesel and biogas. To date, more effort has been put towards the production of biodiesel using the lipid contents in algal cells, while less attention has been placed on biogas production through anaerobic digestion. However, anaerobic digestion has the potential to generate energy from waste residues and to mobilize nutrients enabling subsequent recovery and/or recycling. Therefore, anaerobic digestion is an area with strong potential for novel research focusing on the development of a sustainable integrated system of biodiesel and biogas production. The result is essentially a solar power plant, producing fuel with minimal inputs and a closed nutrient loop, a necessity for sustainable and cost-efficient production of biofuel. In this review we discuss relevant studies on biodiesel and biomethane production, including the potential improvements and advantages when using an integrated approach for biodiesel and biogas production with special focus on nutrient recycling.

Microalgal lipid recovery for biodiesel production is currently considered suboptimal, but pre-treatment of algal biomass, the use of solvent mixtures and the positioning of transesterification can lead to increased yields. Here, the effect of various reportedly successful pre-treatments and solvent mixtures were directly compared to each other and combined with direct and indirect transesterification methods using the oleaginous microalga Tetraselmis sp. M8. Microwave and thermal pre-treatments were applied and the total lipid and fatty acid methyl ester (FAME) recoveries were investigated. The application of pre-treatments increased FAME recovery through indirect transesterification when a Soxhlet system was used but they had no significant effect for direct transesterification. Gravimetric analyses of total lipids revealed that lipid recovery was highest when utilizing the chloroform-based Bligh and Dyer extraction method; however FAME yield was the highest when applying a Soxhlet system utilizing a solvent mixture of hexane-ethanol (3:1). Total lipid recovery did not necessarily correlate with the recovery of FAMEs. The highest FAME recovery was achieved from thermal or microwave pre-treated biomass followed by indirect transesterification through Soxhlet extraction. FAME recovery could be more than doubled (increase of up to 171%) under these conditions. We conclude that a simple thermal pre-treatment (80°C for 10 min) in combination with solvent mixture extraction through indirect transesterification may present a cost-effective and scalable option for large-scale lipid extraction from microalgae.

Acidogenic fermentation is an anaerobic process of double purpose, while treating organic residues it produces chemical compounds, such as hydrogen, ethanol and organic acids. Therefore, acidogenic fermentation arises as an attractive biotechnology process towards the biorefinery concept. Moreover, this process does not need sterile operating conditions and works under a wide range of pH. Changes of operating conditions produce metabolic shifts, inducing variability on acidogenic product yield. To induce those changes, experiments, based on reactor headspace N(2)-flushing (gas phase), were designed. A major result was the hydrogen yield increase from 1 to 3.25±0.4 ( [Formula: see text] ) at pH 4.5 and N(2)-flushing of 58.4 (L·d(-1)). This yield is close to the theoretical acidogenic value (4 [Formula: see text] ). The mechanisms that explain this increase on hydrogen yield shifts are related to the thermodynamics of three metabolic reactions: lactate hydrogenase, NADH hydrogenase and homoacetogenesis, which are affected by the low hydrogen partial pressures.

Abstract Triacylglyceride (TAG) recovery from algal biomass is primarily limited by the rigid algal cell envelope (cell wall and cell membrane). In this work, the effect of free nitrous acid (FNA) pre-treatment on TAG recovery from algal biomass with six different FNA concentrations is reported. Results show that at a range of low FNA concentrations (0.24–2.25 mg HNO2–N L−1) TAG recovery was strongly enhanced with increasing FNA concentration. An FNA concentration of around 2 mg HNO2–N L−1 resulted in a 3.3-fold increase in fatty acid recovery over untreated algae, but higher FNA concentrations (13.49 and 26.98 mg HNO2–N L−1) were detrimental to TAG recovery. Analysis of the fatty acid profile revealed that the higher FNA concentrations caused a reduction in polyunsaturated fatty acids. Also, the ratio of extracted fatty acids to total lipids was significantly reduced when high FNA concentration were applied, and only non-fatty acid lipids potentially benefited from more intense FNA pre-treatments.

AbstractA method for detailed investigation of aerobic carbon degradation processes by microorganisms is presented. The method relies on an integrated use of the respirometric, titrimetric, and off‐gas CO2 measurements. The oxygen uptake rate (OUR), hydrogen ion production rate (HPR), and the carbon dioxide transfer rate (CTR) resulting from the biological as well as physicochemical processes, coupled with a metabolic model characterizing both the growth and carbon storage processes, enables the comprehensive study of the carbon degradation processes. The method allows the formation of carbon storage products and the biomass growth rates to be estimated without requiring any off‐line biomass or liquid‐phase measurements, although the practical identifiability of the system could be improved with additional measurements. Furthermore, the combined yield for biomass growth and carbon storage is identifiable, along with the affinity constant with respect to the carbon substrate. However, the individual yields for growth and carbon storage are not identifiable without further knowledge about the metabolic pathways employed by the microorganisms in the carbon conversion. This is true even when more process variables are measured. The method is applied to the aerobic carbon substrate degradation by a full‐scale sludge using acetate as an example carbon source. The sludge was able to quickly take up the substrate and store it as poly‐β‐hydroxybutyrate (PHB). The PHB formation rate was a few times faster than the biomass growth rate, which was confirmed by off‐line liquid‐ and solid‐phase analysis. The estimated combined yield for biomass growth and carbon storage compared closely to that determined from the theoretical yields reported in literature based on thermodynamics. This suggests that the theoretical yields may be used as default parameters for modeling purposes. © 2004 Wiley Periodicals, Inc.

This paper investigates anaerobic co-digestion of pig manure and algae (Scenedesmus sp.) with and without extraction of intracellular algal co-products, with views towards the development of a biorefinery concept for lipid, protein and/or biogas production. Protein and/or lipids were extracted from Scenedesmus sp. using free nitrous acid pre-treatments and solvent-based Soxhlet extraction, respectively. Processing increased algae methane yield between 29% and 37% compared to raw algae (VS basis), but reduced the amount of algae available for digestion. Co-digestion experiments showed a synergy between pig manure and raw algae that increased raw algae methane yield from 0.163 to 0.245 m(3) CH4 kg(-1)VS. No such synergy was observed when algal residues were co-digested with pig manure. Finally, experimental results were used to develop a high-level concept for an integrated biorefinery processing pig manure and onsite cultivated algae, evaluating methane production and co-product recovery per mass of pig manure entering the refinery.