• Energy Research

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.

Membrane inlet mass spectrometry (MIMS) is useful for on-line monitoring of fermentation processes. However, readings are affected by the complex and dynamic matrix in which biological processes occur, making MIMS calibration a challenge. In this work, two calibration strategies were evaluated for measurement of typical products of acidogenic fermentation, i.e., ethanol, H(2), and CO(2) in the liquid phase, and H(2) and CO(2) in the gas phase: (1) "standard calibration", which was performed independent of fermentation experiments with sterile standards in water with a N(2) headspace, and (2) "in-process calibration" whereby fermentation was monitored concurrent with off-line analysis. Fermentation was operated in batch and continuous modes. In-process calibration was shown to be most effective for measurements of H(2) and CO(2) in both gas and liquid phases; standard calibration gave erroneous results. In the gas phase, this was due to a lower sensitivity during experiments compared to the independent standard calibration, believed to be caused by formation of a liquid film on the surface of the probe. In the liquid phase, moving from the standard calibration environment to the fermentation caused the linear relationship between the H(2) concentration and MIMS signal to change in intercept, and the relationship for CO(2) to change in slope, possibly due to dissolved ions, and related non-ideality. For ethanol, standard calibration results were fairly consistent with in-process calibration results. The main limitation with in-process calibration is the potential for a lack of variability in target concentration. This could be addressed by spiking the targeted compound at the end of the experiment. Regardless, MIMS is an ideal instrument for analysing fermentation experiments, due to its ability to measure targeted compounds semi-continuously, and due to a lack of drift over long periods.

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.

Phosphorus removal in waste stabilization ponds (WSP) is highly variable, but the reasons for this are not well understood. Luxury uptake of phosphorus by microalgae has been studied in natural systems such as lakes but not under the conditions found in WSP. This work reports on the effects of phosphate concentration, light intensity, and temperature on luxury uptake of phosphorus by WSP microalgae in continuous culture bioreactors. Increasing temperature had a statistically significant "positive effect" on intracellular acid-insoluble polyphosphate concentration. It is likely that elevated temperature increased the rate of polyphosphate accumulation, but because the biomass was not starved of phosphate, the stored acid-insoluble polyphosphate was not utilized. Increasing light intensity had no effect on acid-insoluble polyphosphate but had a "negative effect" on the acid-soluble polyphosphate. A possible explanation for this is that the faster growth rate at high light intensity results in this form of polyphosphate being utilized by the cells for synthesis of cellular constituents at a rate that exceeds replenishment. The variability in the phosphorus content of the microalgal biomass shows that with this new understanding ofthe luxury uptake mechanism there is the potential to optimize WSP for biological phosphorus removal.

The use of mixed microbial cultures for the production of polyhydroxyalkanoates (PHAs) is emerging as a viable technology. In this study, 16S rRNA gene amplicon pyrosequencing was used to analyse fluctuations in populations over a 63-day period within a PHA-storing mixed microbial community enriched on fermented whey permeate. This community was dominated by the genera Flavisolibacter and Zoogloea as well as an unidentified organism belonging to the phylum Bacteroidetes. The population was observed to cycle through an increase in Zoogloea followed by a return to a community composition similar to the initial one (highly enriched in Flavisolibacter). It was found that the PHA accumulation capacity of the community was robust to population flux during enrichment and even PHA accumulation, with final polymer composition dependent on the overall proportion of acetic to propionic acids in the feed. This community adaptation suggests that mixed culture PHA production is a robust process.