Incorporating saline microalgae biomass in anaerobic digester treating sewage sludge: Impact on performance and microbial populations.

The aim of this study was to acclimate anaerobic prokaryotes to saline microalgae biomass. Semi-continuous experiments were conducted using two 1.5 L mesophilic reactors for 10 weeks, (hydraulic retention time of 21 days). The first reactor was solely fed with sewage sludge (control), while the second received a mixture of sewage sludge and microalgal biomass (80/20 %w/w) cultivated at 70 g·L-1 salinity. The in-reactor salinity reached after the acclimation phase was 14 g·L-1. Biomethane production was comparable between the control and acclimated reactors (205 ± 29 NmLMethane·gVolatileSolids-1). Salinity tolerance assessment of methanogenic archaea revealed that salinity causing 50% inhibition of methane production increased from 10 to 27 g·L-1 after acclimation. Microbial diversity analyses revealed notable changes in methanogenic archaea populations during co-digestion of saline microalgae biomass, particularly methylotrophic (+27%) and acetotrophic (-26%) methanogens. This study has highlighted the possibility of treating efficiently saline microalgae in co-digestion with sewage sludge in future industrial biogas plants.

Innovative and sustainable cultivation strategy for the production of Spirulina platensis using anaerobic digestates diluted with residual geothermal water.

The following study investigates the possibility of growing the Spirulina platensis (S. platensis) cyanobacteria on two agro-industrial anaerobic digestion (AD) digestates diluted with geothermal water. The two digestates (FAWD: Food and Agricultural Wastes Digestate and CDD: Cheese Diary Digestate) were selected based on their different chemical characteristics, attributed to the type of feedstock and the operating conditions used during the AD process. In the first part of the study, a screening experiment was performed in 200 mL glass tubes to evaluate the appropriate dilution factor to generate the maximum S. platensis growth using both AD digestates individually and geothermal water as sustainable alternative dilution agent. Based on the different growth parameters measured, dilution rates of 5x and 40x were chosen for CDD and FAWD respectively, as a trade-off between growth performances and quantity of water to use. Volumetric productivities of 33 ± 1 mg/L/d and 56 ± 8 mg/L/d combined with maximal concentrations of 0.52 ± 0.02 g/L and 0.69 ± 0.02 g/L were achieved when cultivating S. platensis on CDD and FAWD, respectively. In the second part, the selected experimental results were scaled-up to 6 L flat panels bioreactors and S. platensis biomass productivities of 71 and 101 mg/L/d were obtained for CDD and FAWD, respectively using sodium bicarbonate as inorganic carbon source. When regulating the pH to 8.5 with carbon dioxide (CO2) injection, cultures were able to produce up to 1.13 g/L and 0.79 g/L of S. platensis corresponding to biomass productivities of 81 and 136 mg/L/d for CDD and FAWD, respectively. In addition, S. platensis properly assimilated the ammonium present in the digestate-based culture media, with removal efficiency up to 98% in the case of the CDD substrate. The characterization of the final S. platensis biomass revealed the presence of high concentration of carbohydrates (48.6-70.3 % of dry weight) in the culture supplemented with both AD digestates. The experimental findings show the potential of reusing liquid digestate, CO2 as well as geothermal water for the sustainable production of carbohydrate-rich S. platensis biomass.

Impact of Operational Factors, Inoculum Origin, and Feedstock Preservation on the Biochemical Methane Potential.

Anaerobic digestion for the valorization of organic wastes into biogas is gaining worldwide interest. Nonetheless, the sizing of the biogas plant units require knowledge of the quantity of feedstock, and their associated methane potentials, estimated widely by Biochemical Methane Potential (BMP) tests. Discrepancies exist among laboratories due to variability of protocols adopted and operational factors used. The aim of this study is to verify the influence of some operational factors (e.g., analysis frequency, trace elements and vitamins solution addition and flushing gas), feedstock conservation and the source of inoculum on BMP. Among the operational parameters tested on cellulose degradation, only the type of gas used for flushing headspace of BMP assays had shown a significant influence on methane yields from cellulose. Methane yields of 344 ± 6 NL CH4 kg-1 VS and 321 ± 10 NL CH4 kg-1 VS obtained from assays flushed with pure N2 and N2/CO2 (60/40 v/v). The origin of inoculum (fed in co-digestion) only significantly affected the methane yields for straw, 253 ± 3 and 333 ± 3 NL CH4 kg-1 VS. Finally, freezing/thawing cycle effect depended of the substrate (tested on biowaste, manure, straw and WWTP sludge) with a possible effect of water content substrate.

Industrial symbiosis of anaerobic digestion and pyrolysis: Performances and agricultural interest of coupling biochar and liquid digestate.

The sustainability of the anaerobic digestion industry is closely related to proper digestate disposal. In this study, an innovative cascading biorefinery concept coupling anaerobic digestion and subsequent pyrolysis of the digestate was investigated with the aim of enhancing the energy recovery and improving the fertilizers from organic wastes. Continuous anaerobic co-digestion of quinoa residues with wastewater sludge (45/55% VS) exhibited good stability and a methane production of 219 NL CH4/kg VS. Subsequent pyrolysis of the solid digestate was carried out (at 500 °C, 1 h, and 10 °C/min), resulting in a products distribution of 40 wt% biochar, 36 wt% bio-oil, and 24 wt% syngas. The organic phase (OP) of bio-oil and syngas exhibited higher and lower heating values of 34 MJ/kg and 11.8 MJ/Nm3, respectively. The potential synergy of coupling biochar with liquid digestate (LD) for agronomic purposes was investigated. Interestingly, coupling LD (at 170 kg N/ha) with biochar (at 25 tons/ha) improved the growth of tomato plants up to 25% compared to LD application alone. In parallel, co-application of biochar with LD significantly increased the ammonia volatilization (by 64%) compared to LD application alone, although their simultaneous use did not impact the C and N mineralization rates.

Production and Dry Mechanochemical Activation of Biochars Derived from Moroccan Red Macroalgae Residue and Olive Pomace Biomass for Treating Wastewater: Thermodynamic, Isotherm, and Kinetic Studies.

This study aimed to produce activated biochars (BCs) from Moroccan algae residue (AG) and olive pomace (OP) using mechanochemical activation with NaOH and ball milling (BM) for treating artificial textile wastewater containing methylene blue (MeB). The produced OP-activated BC by BM showed the highest absolute value of ζ-potential (-59.7 mV) and high removal efficiency of MeB compared to other activated BCs. The nonlinear pseudo-first-order kinetic model was the most suitable model to describe the kinetics of adsorption of MeB onto biochars produced from AG and the NaOH-activated BC from OP, whereas the nonlinear pseudo-second-order kinetic model suits the OP raw biochar and BM-activated BC. The nonlinear Langmuir isotherm model was the most suitable model for describing MeB adsorption onto BCs, compared to the nonlinear Freundlich isotherm model. The maximum adsorption capacities of AG-activated BCs with NaOH and BM were 13.1 and 9.1 mg/g, respectively, while those of OP-activated BCs were 2.6 and 31.8 mg/g, respectively. The thermodynamic study indicates the spontaneous and endothermic nature of the adsorption process of most activated BCs. In addition, ΔS° values indicate the increase of randomness at the solid-liquid interface during MeB sorption onto BC.

Coupling anaerobic digestion and pyrolysis processes for maximizing energy recovery and soil preservation according to the circular economy concept.

After press separation of the liquid and solid digestate from an agricultural biogas plant, pyrolysis of solid anaerobic digestate was carried out (i.e., at 500 °C, 1h, and 10 °C/min) to produce biochar (37.6 wt%), bio-oil (33.7 wt%) and syngas (29.3 wt%). The organic phase of bio-oil and syngas exhibited high and low heating values of 28.4 MJ/kg and 12.9 MJ/Nm3, respectively. Then, the synergy of coupling biochar with liquid digestate for agronomic purposes was investigated by leaching experiment and growth plant tests on wheat. Leaching experiments using combination of liquid digestate (170 kg N/ha) and biochar demonstrated that biochar addition increases the cumulative leaching of all nutrients, except nitrate, that have a significant decrease of 82% and 91%, respectively at 50 and 100 t/ha, compared to soil treated only with liquid digestate. The co-application of biochar with liquid digestate on growth wheat plant tests demonstrated that biochar application at 50 t/ha did not exhibit a negative impact on the relative seed germination and improved aerial dry biomass production (up to 27.5%) compared to soil with only liquid digestate addition.

Effect of coupling alkaline pretreatment and sewage sludge co-digestion on methane production and fertilizer potential of digestate.

This study aims at investigating how organic waste co-digestion coupled with alkaline pretreatment can impact the methane production and agronomic value of produced digestates. For this purpose, sludge alone and mixed with olive pomace or macroalgal residues were subjected to anaerobic digestion with and without alkaline pretreatment. In addition, co-digestion of pretreated sludge with raw substrates was also carried out and compared to the whole mixture pretreatment. KOH pretreatment enhanced methane production by 39%, 15% and 49% from sludge, sludge mixed with olive pomace and sludge mixed with macroalgal residues, respectively. The digestates were characterised according to their physico-chemical and agronomic properties. They were then applied as biofertilizers for tomato growth during the first vegetative stage (28 days of culture). Concentrations in chlorophyll a and carotenoids in tomato plants, following sludge digestate addition, rose by 46% and 41% respectively. Sludge digestate enhanced tomato plant dry weight by 87%, while its nitrogen content increased by 90%. The impact of nitrogen and phosphorus contents in the digestate was strongest on tomato plant dry weight, thus explaining the efficiency of sludge digestate relative to other types of digestate. However, when methane production is considered, the combination of pre-treatment with co-digestion of macroalgal residues and sludge appears most beneficial for maximizing energy recovery and for biofertilizer generation.

Evaluation of agronomic properties of digestate from macroalgal residues anaerobic digestion: Impact of pretreatment and co-digestion with waste activated sludge.

The aim of this paper is to investigate the impact of pretreating macroalgal residue (MAR) from agar-agar extraction and its co-digestion with sewage sludge on methane production and the agronomic quality of the digestates produced. First, different pretreatments were assessed on BMP tests. Among milling technologies used, knife milling with a 4 mm-screen improved methane production by 25%. The MAR was then knife milled before alkaline, acid and thermal pretreatment. KOH pretreatment (5% TS basis, 25 °C for 2 days) led to the highest methane improvement. It was applied to semi-continuous anaerobic digestion and methane production achieved 237 Nml/gVS which was 20% higher than the control (198 Nml/gVS). In comparison to MAR mono-digestion, co-digestion with thickened activated sludge produced less methane (184 Nml/gVS) but reduced H2S emission by 91%. None of the digestates was toxic for the germination or growth of wheat and tomato plants. Particularly, co-digestion had the highest impact on tomato plant dry weight (+94% compared to soil alone) mainly due to the phosphorous brought by sludge. However, the impact of alkaline pretreatment on plant growth was not significant.

Comparison of Dry Versus Wet Milling to Improve Bioethanol or Methane Recovery from Solid Anaerobic Digestate.

Biogas plants for waste treatment valorization are presently experiencing rapid development, especially in the agricultural sector, where large amounts of digestate are being generated. In this study, we investigated the effect of vibro-ball milling (VBM) for 5 and 30 min at a frequency of 20 s-1) on the physicochemical composition and enzymatic hydrolysis (30 U g-1 total solids (TS) of cellulase and endo-1,4-xylanase from Trichoderma longibrachiatum) of dry and wet solid digestates from an agricultural biogas plant. We found that VBM of dry solid digestate improved the physical parameters as both the particle size and the crystallinity index (from 27% to 75%) were reduced. By contrast, VBM of wet solid digestate had a minimal effect on the physicochemical parameters. The best results in terms of cellulose and hemicelluloses hydrolysis were noted for 30 min of VBM of dry solid digestate, with hydrolysis yields of 64% and 85% for hemicelluloses and cellulose, respectively. For the condition of 30 min of VBM, bioethanol and methane production on the dry solid separated digestate was investigated. Bioethanol fermentation by simultaneous saccharification and fermentation resulted in an ethanol yield of 98 geth kg-1 TS (corresponding to 90% of the theoretical value) versus 19 geth kg-1 TS for raw solid digestate. Finally, in terms of methane potential, VBM for 30 min lead to an increase of the methane potential of 31% compared to untreated solid digestate.

Fast prediction of organic wastes methane potential by near infrared reflectance spectroscopy: A successful tool for farm-scale biogas plant monitoring.

Currently, there is a growing worldwide interest for the treatment of wastes, and especially farm wastes, by anaerobic digestion. Biochemical methane potential is a key parameter for the design, optimisation and monitoring of the anaerobic digestion process, but it is also time consuming (4-7 weeks). Near infrared reflectance spectroscopy seems a promising method to predict the biochemical methane potential of a wide range of organic substrates. This study compares a 'global' predictive model mainly built with biogas plant feedstocks, and a more 'agricultural' specific one built with farm wastes only (e.g. manures and crop residues). The global model was calibrated with 245 samples and the specific one with 171 samples. In parallel, validation sets composed of 36 farm wastes and eight other wastes (sludge, fruit residues and vegetables) were used to evaluate and compare both models. Satisfying results were obtained on the validation sets considering, respectively for the global and the specific models, a root mean square error of prediction of 44 and 34 NL CH4 kg-1 volatile solid, a coefficient of determination of 0.76 and 0.83, and a ratio of performance to deviation of 2.0 and 2.4. In general rules, the specific model was better than the global one in the prediction of farm wastes methane potential. However, thanks to its larger sample variability, the global one was more robust, especially towards the 'other' wastes, which can be introduced punctually in agricultural biogas plant.

Mechano-Enzymatic Deconstruction with a New Enzymatic Cocktail to Enhance Enzymatic Hydrolysis and Bioethanol Fermentation of Two Macroalgae Species.

The aim of this study was to explore the efficiency of a mechano-enzymatic deconstruction of two macroalgae species for sugars and bioethanol production, by using a new enzymatic cocktail (Haliatase) and two types of milling modes (vibro-ball: VBM and centrifugal milling: CM). By increasing the enzymatic concentration from 3.4 to 30 g/L, the total sugars released after 72 h of hydrolysis increased (from 6.7 to 13.1 g/100 g TS and from 7.95 to 10.8 g/100 g TS for the green algae U. lactuca and the red algae G. sesquipedale, respectively). Conversely, total sugars released from G. sesquipedale increased (up to 126% and 129% after VBM and CM, respectively). The best bioethanol yield (6 geth/100 g TS) was reached after 72 h of fermentation of U. lactuca and no increase was obtained after centrifugal milling. The latter led to an enhancement of the ethanol yield of G. sesquipedale (from 2 to 4 g/100 g TS).

Assessment of hydrothermal pretreatment of various lignocellulosic biomass with CO2 catalyst for enhanced methane and hydrogen production.

Hydrothermal pretreatment of five lignocellulosic substrates (i.e. wheat straw, rice straw, biomass sorghum, corn stover and Douglas fir bark) were conducted in the presence of CO2 as a catalyst. To maximize disintegration and conversion into bioenergy (methane and hydrogen), pretreatment temperatures and subsequent pressures varied with a range of 26-175 °C, and 25-102 bars, respectively. Among lignin, cellulose and hemicelluloses, hydrothermal pretreatment caused the highest reduction (23-42%) in hemicelluloses while delignification was limited to only 0-12%. These reductions in structural integrity resulted in 20-30% faster hydrolysis rates during anaerobic digestion for the pretreated substrates of straws, sorghum, and corn stover while Douglas fir bark yielded 172% faster hydrolysis/digestion due to its highly refractory nature in the control. Furans and phenolic compounds formed in the pretreated hydrolyzates were below the inhibitory levels for methane and hydrogen production which had a range of 98-340 ml CH4/g volatile solids (VS) and 5-26 ml H2/g VS, respectively. Results indicated that hydrothermal pretreatment is able to accelerate the rate of biodegradation without generating high levels of inhibitory compounds while showing no discernible effect on ultimate biodegradation.

Application of optimized alkaline pretreatment for enhancing the anaerobic digestion of different sunflower stalks varieties.

The use of lignocellulosic residues such as sunflower stalks (SS) for the production of bioenergy such as methane is a promising alternative to fossil fuels. However, their recalcitrant structure justifies the use of pretreatment to enhance the accessibility of holocelluloses and their further conversion into methane. First, different conditions of alkaline pretreatment (i.e. duration and NaOH concentration (g/100 g TS) at a fixed temperature of 55 degrees C) were tested to enhance the methane potential of the stalks of the Serin sunflower (193 mL of methane per gram of volatile solids (VS)). The greatest improvement to the methane potential (262 mL CH4 g(-1) VS) was observed at 55 degrees C, 24 h, 4 g NaOH/100 g TS. Fourier Transform Infrared spectra highlighted an accumulation of lignin in the digestate and the degradation of holocelluloses during the anaerobic process, both for pretreated and untreated SS. In a second stage, this optimum condition for alkaline pretreatment (55 degrees C, 24 h, 4 g NaOH/100 g TS) was applied to the stalks of three other varieties of sunflower. Alkaline pretreatment was effective in the delignification of the stalks of the different sunflower varieties, with lignin reduction varying from 23.3% to 36.3% VS. This reduction of lignin was concomitant with the enhancement of methane potential as compared to that of raw SS, with an increase ranging from 29% to 44% for the different SS.

Two-stage alkaline-enzymatic pretreatments to enhance biohydrogen production from sunflower stalks.

Because of their rich composition in carbohydrates, lignocellulosic residues represent an interesting source of biomass to produce biohydrogen by dark fermentation. Nevertheless, pretreatments should be applied to enhance the solubilization of holocelluloses and increase their further conversion into biohydrogen. The aim of this study was to investigate the effect of thermo-alkaline pretreatment alone and combined with enzymatic hydrolysis to enhance biohydrogen production from sunflower stalks. A low increase of hydrogen potentials from 2.3 ± 0.9 to 4.4 ± 2.6 and 20.6 ± 5.6 mL of H2 g(-1) of volatile solids (VS) was observed with raw sunflower stalks and after thermo-alkaline pretreatment at 55 °C, 24 h, and 4% NaOH and 170 °C, 1 h, and 4% NaOH, respectively. Enzymatic pretreatment alone showed an enhancement of the biohydrogen yields to 30.4 mL of H2 g(-1) of initial VS, whereas it led to 49 and 59.5 mL of H2 g(-1) of initial VS when combined with alkaline pretreatment at 55 and 170 °C, respectively. Interestingly, a diauxic effect was observed with sequential consumption of sugars by the mixed cultures during dark fermentation. Glucose was first consumed, and once glucose was completely exhausted, xylose was used by the microorganisms, mainly related to Clostridium species.

Predictive models of biohydrogen and biomethane production based on the compositional and structural features of lignocellulosic materials.

In an integrated biorefinery concept, biological hydrogen and methane production from lignocellulosic substrates appears to be one of the most promising alternatives to produce energy from renewable sources. However, lignocellulosic substrates present compositional and structural features that can limit their conversion into biohydrogen and methane. In this study, biohydrogen and methane potentials of 20 lignocellulosic residues were evaluated. Compositional (lignin, cellulose, hemicelluloses, total uronic acids, proteins, and soluble sugars) as well as structural features (crystallinity) were determined for each substrate. Two predictive partial least square (PLS) models were built to determine which compositional and structural parameters affected biohydrogen or methane production from lignocellulosic substrates, among proteins, total uronic acids, soluble sugars, crystalline cellulose, amorphous holocelluloses, and lignin. Only soluble sugars had a significant positive effect on biohydrogen production. Besides, methane potentials correlated negatively to the lignin contents and, to a lower extent, crystalline cellulose showed also a negative impact, whereas soluble sugars, proteins, and amorphous hemicelluloses showed a positive impact. These findings will help to develop further pretreatment strategies for enhancing both biohydrogen and methane production.

Effect of lignin-derived and furan compounds found in lignocellulosic hydrolysates on biomethane production.

Hydrolysates resulting from the lignocellulosic biomass pretreatment in bioethanol production may be used to produce biogas. Such hydrolysates are rich in xylose but also contain lignin polymers or oligomers as well as phenolic and furan compounds, such as syringaldehyde, vanillin, HMF, furfural. The aim of this study was to investigate the impact of these byproducts on biomethane production from xylose. The anaerobic digestion of the byproducts alone was also investigated. No inhibition of the anaerobic digestion of xylose was observed and methane was obtained from furans: 430 mL CH(4)/g of furfural and 450 mL CH(4)/g of HMF; from phenolic compounds: 453 mL CH(4)/g of syringaldehyde and 105 mL CH(4)/g of vanillin; and, to a lesser extent, from lignin polymers: from 14 to 46 mL CH(4)/g MV. The use of different natural polymers (lignosulfonates, organosolv and kraft lignins) and synthetic dehydrogenative polymers showed that higher S/G ratios and lower molecular weights in lignin polymers led to greater methane production.