Biogas production from switchgrass under experimental conditions simulating U.S. digester operations

Energy Crops and Methane: Process Optimization of Ca(OH)2 Assisted Thermal Pretreatment and Modeling of Methane Production

Switchgrass earned its place globally as a significant energy crop by possessing essential properties such as being able to control erosion, low cost of production, biomass richness, and appeal for biofuel production. In this study, the impact of a Ca(OH)₂-assisted thermal pretreatment process on the switchgrass variety Shawnee for methane fuel production was investigated. The Ca(OH)₂-assisted thermal pretreatment process was optimized to enhance the methane production potential of switchgrass. Solid loading (3-7%), Ca(OH)₂ concentration (0-2%), reaction temperature (50-100 °C), and reaction time (6-16 h) were selected as independent variables for the optimization. Methane production was obtained as 248.7 mL CH₄ gVS^-1 under the optimized pretreatment conditions. Specifically, a reaction temperature of 100 °C, a reaction time of 6 h, 0% Ca(OH)₂, and 3% solid loading. Compared to raw switchgrass, methane production was enhanced by 14.5%. Additionally, the changes in surface properties and bond structure, along with the kinetic parameters from first order, cone, reaction curve, and modified Gompertz modeling revealed the importance of optimization.

OUP accepted manuscript

Rhamnolipids (RLs) are well-studied biosurfactants naturally produced by pathogenic strains of Pseudomonas aeruginosa. Current methods to produce RLs in native and heterologous hosts have focused on carbohydrates as production substrate; however, methane (CH₄) provides an intriguing alternative as a substrate for RL production because it is low cost and may mitigate greenhouse gas emissions. Here, we demonstrate RL production from CH₄ by Methylotuvimicrobium alcaliphilum DSM19304. RLs are inhibitory to M. alcaliphilum growth (<0.05 g/l). Adaptive laboratory evolution was performed by growing M. alcaliphilum in increasing concentrations of RLs, producing a strain that grew in the presence of 5 g/l of RLs. Metabolomics and proteomics of the adapted strain grown on CH₄ in the absence of RLs revealed metabolic changes, increase in fatty acid production and secretion, alterations in gluconeogenesis, and increased secretion of lactate and osmolyte products compared with the parent strain. Expression of plasmid-borne RL production genes in the parent M. alcaliphilum strain resulted in cessation of growth and cell death. In contrast, the adapted strain transformed with the RL production genes showed no growth inhibition and produced up to 1 μM of RLs, a 600-fold increase compared with the parent strain, solely from CH₄. This work has promise for developing technologies to produce fatty acid-derived bioproducts, including biosurfactants, from CH₄.

Comparing the Biomass Yield and Biogas Potential of Phragmites australis with Miscanthus x giganteus and Panicum virgatum Grown in Canada

The production of bioenergy from plant biomass has the potential to reduce fossil fuel use. The number of biogas facilities around the world has risen dramatically, increasing demand for feedstocks. In this study the invasive perennial grass Phragmites australis was evaluated as a biogas feedstock in comparison with Miscanthus x giganteus and Panicum virgatum. Results from three field sites for each species demonstrated that biomass yields for P. australis averaged approximately 1.82 ± 0.9 kg dry matter (DM) m−2, comparable to that of M. x giganteus. Yield of P. australis was greater than P. virgatum, which ranged from 0.49 ± 0.06 to 0.69 ± 0.07 kg DM m−2 in July and October, respectively. In mesophilic bench-top digester experiments, methane yields were greater for July-harvested material than for October, ranging from 172.4 ± 15.3 to 229.8 ± 15.2 L CH4 kg−1 volatile solids (VS) for all perennial grasses. Methane yields per hectare were highest for October-harvested M. x giganteus, followed by July-harvested M. x giganteus and P. australis, whereas methane yield from P. virgatum at both harvest times was lower than the other two species. These results suggest that P. australis is not an economically viable biogas feedstock without pre-treatment to improve methane yield.

Batch anaerobic co-digestion of waste activated sludge and microalgae (Chlorella sorokiniana) at mesophilic temperature

The microalgae Chlorella sorokiniana are used as co-substrate for waste activated sludge (WAS) anaerobic digestion. The specific objective of this research was to evaluate the feasibility of improving methane production from anaerobic digestion of WAS in co-digestion with this microalga, based on an optimized mixture percentage. Thus, the anaerobic co-digestion of both substrates aims to overcome the drawbacks of the anaerobic digestion of single WAS, simultaneously improving its management. Different co-digestion mixtures (0% WAS-100% microalgae; 25% WAS-75% microalgae; 50% WAS-50% microalgae; 75% WAS-25% microalgae; 100% WAS-0% microalgae) were studied. The highest methane yield (442 mL CH4/g VS) was obtained for the mixture with 75% WAS and 25% microalgae. This value was 22% and 39% higher than that obtained in the anaerobic digestion of the sole substrates WAS and microalgae, respectively, as well as 16% and 25% higher than those obtained for the co-digestion mixtures with 25% WAS and 75% microalgae and 50% WAS and 50% microalgae, respectively. The kinetic constant of the process increased 42%, 42% and 12%, respectively, for the mixtures with 25%, 50% and 75% of WAS compared to the substrate without WAS. Anaerobic digestion of WAS, together with C. sorokiniana, has been clearly improved by ensuring its viability, suitability and efficiency.

Low-heat, mild alkaline pretreatment of switchgrass for anaerobic digestion

This study examines the effectiveness of alkaline pretreatment under mild heat conditions (100°C or 212°F) on the anaerobic co-digestion of switchgrass. The effects of alkaline concentration, types of alkaline, heating time and rinsing were evaluated. In addition to batch studies, continuous-feed studies were performed in triplicate to identify potential digester operational problems caused by switchgrass co-digestion while accounting for uncertainty due to digester variability. Few studies have examined anaerobic digestion of switchgrass or the effects of mild heating to enhance alkaline pretreatment prior to biomass digestion. Results indicate that pretreatment can significantly enhance digestion of coarse-ground (≤ 0.78 cm particle size) switchgrass. Energy conversion efficiency as high as 63% was observed, and was comparable or superior to fine-grinding as a pretreatment method. The optimal NaOH concentration was found to be 5.5% (wt/wt alkaline/biomass) with a 91.7% moisture level. No evidence of operational problems such as solids build-up, poor mixing, or floating materials were observed. These results suggest the use of waste heat from a generator could reduce the concentration of alkaline required to adequately pretreat lignocellulosic feedstock prior to anaerobic digestion.

Performance and kinetic evaluation of the semi-continuous anaerobic digestion of sunflower oil cake pretreated with ultrasound

A study of the semi-continuous anaerobic digestion of sunflower oil cake previously sonicated (at a specific energy of 24,000 kJ/kg TS, constant sonication frequency of 20 kHz and ultrasonic power of 120 W) was carried out in laboratory-scale completely stirred tank reactors at mesophilic temperature (35°C). Two anaerobic inocula were used: a mixture of flocculant biomass (I) from a full-scale anaerobic reactor treating waste activated sludge and a granular inoculum (II) from an industrial UASB reactor treating brewery wastewater. Soluble COD (CODs) removal efficiencies ranged between 67.7% and 70.1% and between 61.3% and 67.7% at hydraulic retention times (HRTs) of between 24-10 days for inoculum I and 24-8 days for inoculum II. However, for HRTs lower than 8 days and 6.7 days, equivalent to organic loading rates (OLRs) higher than 2.62 and 3.15 g COD/(L·d), respectively, a sudden decrease in the CODs removal efficiency was observed in both cases. In any case, inoculum II allowed for a more stable and efficient operation for a wider range of both OLRs and HRTs, permitting an appropriate and reliable operation for OLRs as high as 3.15 g COD/(L·d) and HRTs as low as 6.7 days. The methane production rates achieved with inoculum II were always higher than those reached with inoculum I. The overall methane yield obtained with inoculum II was 13% higher than that achieved with inoculum I. In addition, this value was 1.9 times higher than the methane yield obtained with untreated (non-sonicated) SuOC. A second-order kinetic model was found to be adequate to fit the experimental results obtained for the two inocula used. The kinetic constant obtained with inoculum I was 3.5 times higher than that achieved with inoculum II.