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.

Use of additives to improve collective biogas plant performances: A comprehensive review.

Nowadays, anaerobic digestion (AD) is being increasingly encouraged to increase the production of biogas and thus of biomethane. Due to the high diversity among feedstocks used, the variability of operating parameters and the size of collective biogas plants, different incidents and limitations may occur (e.g., inhibitions, foaming, complex rheology). To improve performance and overcome these limitations, several additives can be used. This literature review aims to summarize the effects of the addition of various additives in co-digestion continuous or semi-continuous reactors to fit as much as possible with collective biogas plant challenges. The addition of (i) microbial strains or consortia, (ii) enzymes and (iii) inorganic additives (trace elements, carbon-based materials) in digester is analyzed and discussed. Several challenges associated with the use of additives for AD process at collective biogas plant scale requiring further research work are highlighted: elucidation of mechanisms, dosage and combination of additives, environmental assessment, economic feasibility, etc.

Impact of salinities, metals and organic compounds found in saline oil & gas produced water on microalgae and cyanobacteria.

This work evaluates the impact of salinity and the toxicity of some metals and organic compounds commonly found in produced waters on the growth of model photosynthetic organisms. Five strains of marine microalgae and one cyanobacteria (i.e. Dunaliella salina, Nannochloropsis oceanica, Tetraselmis suecica, Picochlorum costavermella, Coccomyxa simplex and Synechococcus rubescens) were tested in microplates as well as the freshwater Chlorella vulgaris selected as reference. Results revealed that D.salina was able to growth at high salinity (up to 135 g·L-1). Copper was the most toxic metal for all strains (half maximal effective concentration between 0.1 and 10 mg·L-1) except for D.salina and C.simplex. These two strains were the most resistant to all metals tested. All organic compounds presented half maximal effective concentration above 10 mg·L-1, none of them being very toxic for the studied microorganisms. P.costavermella and C.simplex were the most resistant strains to organic compounds. Looking at tolerance to salinity, metals and organic compounds, D.salina appeared to be the best choice for biomass production in produced waters. In addition, growths in 80% artificial produced water supplemented with f medium confirm the feasibility to use this medium to produce biomass.

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.

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).

Use of solid digestate for lignocellulolytic enzymes production through submerged fungal fermentation.

Studies were performed on the use of the solid fraction of digestate (D) for the production of lignocellulolytic enzymes (endo- and exo-glucanase, xylanase, ß-glucosidase and laccase) by fungi, in comparison with wheat straw (benchmark) (W). To date, this is the first report on the use of such an inexpensive substrate in a liquid environment. Submerged instead of solid state fermentation was applied to overcome pH inhibition and increase surface accessibility. A total of 21 fungal strains were tested: the most performing ones were Irpex lacteus DSM1183 for both ß-glucosidase (52 IU/g with D, + 400% compared to W) and endo-glucanase (236 IU/g with D, + 470% compared to W), Schizophyllum commune CBS30132 for xylanase (715 IU/g with W, + 145% compared to D) and Pleurotus ostreatus ATCC96997 for laccase (124 IU/g with D, +230% compared to D). Cultures from S. commune and P. ostreatus were analyzed at the beginning and at the end of the growth test to determine soluble COD, total (TS) and volatile (VS) solids. COD was always lower at the end of the test suggesting a faster uptake than hydrolysis. P. ostreatus evidenced a higher VS reduction (-11% rather than -32%), suggesting a more effective growth of this strain on D. Results may open up new avenues for the utilization of solid digestate, an inexpensive agricultural by-product, for the production of value-added products as well as to increase biodegradation of lignocellulosic materials.

Harmonization of the quantitative determination of volatile fatty acids profile in aqueous matrix samples by direct injection using gas chromatography and high-performance liquid chromatography techniques: Multi-laboratory validation study.

The performance parameters of volatile fatty acids (VFAs) measurements were assessed for the first time by a multi-laboratory validation study among 13 laboratories. Two chromatographic techniques (GC and HPLC) and two quantification methods such as external and internal standard (ESTD/ISTD) were combined in three different methodologies GC/ESTD, HPLC/ESTD and GC/ISTD. Linearity evaluation of the calibration functions in a wide concentration range (10-1000mg/L) was carried out using different statistical parameters for the goodness of fit. Both chromatographic techniques were considered similarly accurate. The use of GC/ISTD, despite showing similar analytical performance to the other methodologies, can be considered useful for the harmonization of VFAs analytical methodology taking into account the normalization of slope values used for the calculation of VFAs concentrations. Acceptance criteria for VFAs performance parameters of the multi-laboratory validation study should be established as follows: (1) instrument precision (RSDINST≤1.5%); (2) linearity (R(2)≥0.998; RSDSENSITIVITY≤4%; REMAX≤8%; REAVER≤ 3%); (3) precision (RSD≤1.5%); (4) trueness (recovery of 97-103%); (5) LOD (≤3mg/L); and (6) LOQ (10mg/L).

Comparative performance of enzymatic and combined alkaline-enzymatic pretreatments on methane production from ensiled sorghum forage.

This study investigated the effect of enzymatic and combined alkaline-enzymatic pretreatments on chemical composition and methane production from ensiled sorghum forage. Four commercial enzymatic preparations were tested and the two yielding the highest sugars release were added to evaluate any hydrolytic effect on both untreated and alkaline pretreated samples. In the combined alkaline-enzymatic pretreatment trials, the highest sugar release was found with Primafast and BGL preparations (added at a final concentration 0.12 and 0.20 mL/g TS, respectively), with a total monomeric content of 12 and 6.5 g/L. Fibre composition analysis confirmed that the combined alkaline-enzymatic pretreatment led to cellulose (up to 32 %) and hemicelluloses (up to 56 %) solubilisation, compared to the enzymatic pretreatment alone. BMP tests were performed on both untreated and pretreated samples, and time courses of methane production were fitted. Both enzymatic and combined alkaline-enzymatic pretreatment led to a methane production increase (304 and 362 mL CH4/g VS), compared to that of untreated sorghum (265 mL CH4/g VS), as  +15 and  +37 %, respectively. Moreover, higher specific methane production rates, compared to that of untreated sorghum (20.31 mL CH4/g VS/d), were obtained by applying the enzymatic and combined alkaline-enzymatic pretreatment (33.94 and 31.65 mL CH4/g VS/d), respectively.

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.