Temperature-driven carboxylic acid production from waste activated sludge and food waste: Co-fermentation performance and microbial dynamics

This work aims to improve the continuous co-fermentation of waste activated sludge (WAS) and food waste (FW) by investigating the long-term impact of temperature on fermentation performance and the underpinning microbial community. Acidogenic co-fermentation of WAS and FW (70:30 % VS-basis) to produce volatile fatty acids (VFA) was studied in continuous fermenters at different temperatures (25, 35, 45, 55 °C) at an organic loading rate of 11 gVS/(L·d) and a hydraulic retention time of 3.5 days. Two batches of WAS (A and B) were collected from the same wastewater treatment plant at different periods to understand the impact of the WAS microbioota on the fermenters' microbial communities. Solubilisation yield was higher at 45 °C (575 ± 68 mgCOD/gVS) followed by 55 °C (508 ± 45 mgCOD/gVS). Fermentation yield was higher at 55 °C (425 ± 28 mgCOD/gVS) followed by 35 °C (327 ± 17 mgCOD/gVS). Temperature also had a noticeable impact on the VFA profile. At 55 °C, acetic (40 %) and butyric (40 %) acid dominated, while acetic (37 %), butyric acid (31 %), and propionic acid (17 %) dominated at 35 °C. At 45 °C, an accumulation of caproic acid was detected which did not occur at other temperatures. Each temperature had a distinct microbial community, where the WAS microbiota played an important role. The biomass mass-balance showed the highest growth of microorganisms (51 %) at 35 °C and WAS_B, where a consumption of acetic acid was observed. Therefore, at 35 °C, there is a higher risk of acetic acid consumption probably due to the proliferation of methanogens imported from WAS.

Mesophilic and thermophilic fermentation of activated sludge for volatile fatty acids production: focusing on anaerobic degradation of carbohydrate and protein

The volatile fatty acids (VFAs) productions, as well as particulate organics decomposition, soluble chemical oxygen demand (SCOD) yield, and the VFAs production pathways from mesophilic and thermophilic anaerobic fermentation in waste activated sludge were investigated. Batch experiments showed that the decomposition rate of volatile suspended solids (VSS), particulate carbohydrate (P-C) and particulate protein (P-P) followed the first-order kinetic model at different temperatures. However, the intermediates, accumulated in the process of protein or carbohydrate digestion had a more significant inhibitory effect on the production of VFAs during the mesophilic anaerobic acidification process. The production of VFAs by thermophilic anaerobic fermentation is 2086.05 mg COD/L, which is about twice the production under mesophilic conditions. Among them, the concentration and proportion of high molecular weight organic acids such as isobutyric acid (320.29 mgCOD/L) and isovaleric acid (745.75 mgCOD/L) are relatively high. Then 13C stable isotope labelling experiment demonstrated that, the decomposition of carbohydrates yields 77% acetic acid and 86% butyric acid, while protein breakdown produces 85% propionic acid and 99% valeric acid. This confirms that carbohydrates are more favourable for the formation of even-carbon organic acids, while proteins tend to yield odd-carbon organic acids. Additionally, this helps refine the pathway for valeric acid formation during anaerobic acidogenesis.

Long-term biofiltration of gaseous N,N-dimethylformamide: Operational performance and microbial diversity analysis at different conditions

N,N-Dimethylformamide (DMF) is an organic solvent produced in large quantities worldwide. It is considered as a hazardous air pollutant and its emission should be controlled. However, only a limited number of studies have been performed on the removal of gaseous DMF by biological technologies. In this paper, we evaluate the removal of DMF under mesophilic and thermophilic conditions in a lab-scale biofilter for 472 days. The results show that, at ambient temperature, the biofilter achieved an average removal efficiency (RE) of 99.7 ± 0.3 % at Inlet Loads (ILs) up to 297 ± 52 g DFM m^-3 h^-1 (Empty Bed Residence Time (EBRTs) of 10.7 s). However, a decrease in EBRT (6.4 s) led to an unstable outlet concentration and, thus, to a drop in the biofilter performance (average RE: 90 ± 9 %). Moreover, an increase in temperature up to 65 °C led to a gradual decrease in RE (till 91 ± 7 %). Microbial analysis indicates that once the microorganisms encountered DMF, Rhizobiaceae dominated followed by Alcaligenaceae. Afterwards, a strong decrease in Rhizobiaceae was observed at every increase in temperature, and at 65 °C, the taxa were more heterogeneous. Overall, our experimental results indicate that biofiltration is a promising technique to remove DMF from waste gas streams.

Biodegradation of polylactic acid by a mesophilic bacteria Bacillus safensis

A mesophilic bacterial strain, Bacillus safensis PLA1006, was isolated from landfill soil and was tested for growth on polylactic acid (PLA) emulsion medium. The strain formed clear zones on the medium and produced protease and lipase. The macroscopic morphology of the PLA films was not changed significantly after treatment with Bacillus safensis PLA1006 but the films were whitened. Weight loss of PLA films was about 8% after 30 days of incubation with Bacillus safensis PLA1006 in mineral salt medium. Scanning electron microscopy revealed etching on the surface of PLA film treated by Bacillus safensis PLA1006. This also caused an increase in hydrophilicity of the PLA films surface. Attenuated total reflectance - Fourier transform infrared spectroscopy analysis of PLA films after treated by Bacillus safensis PLA1006 showed no new absorption peaks but a decrease in the intensity of all absorption peaks. The hydrolysis products of PLA by the strain contained monomers and oligomers of lactic acid. Zymogram detection showed that proteases may play a role in the degradation of PLA.

Biochemical methane potential and active microbial communities during anaerobic digestion of biodegradable plastics at different inoculum-substrate ratios

The influence of the inoculum-substrate ratio (ISR) on the mesophilic and thermophilic biochemical methane potential test of two biodegradable plastics was evaluated. Poly(lactic acid) (PLA) and polyhydroxybutyrate (PHB) were selected for this study, the first for being recalcitrant to mesophilic anaerobic digestion (AD) and the second, by contrast, for being readily biodegradable. Several ISRs, calculated on the basis of volatile solids (VS), were tested: 1, 2, 2.85, 4, and 10 g(VS of inoculum).g(VS of substrate)^-1. A high ISR was associated with an enhanced methane production rate (i.e., biodegradation kinetics). However, the ultimate methane production did not change, except when inhibition was observed. Indeed, applying the lowest ISR to readily biodegradable plastics such as PHB resulted in inhibition of methane production. Based on these experiments, in order to have reproducible degradation kinetics and optimal methane production, an ISR between 2.85 and 4 is recommended for biodegradable plastics. The active microbial communities were analyzed, and the active bacteria differed depending on the plastic digested (PLA versus PHB) and the temperature of the process (mesophilic versus thermophilic). Previously identified PHB degraders (Ilyobacter delafieldii and Enterobacter) were detected in PHB-fed reactors. Thermogutta and Tepidanaerobacter were detected during the thermophilic AD of PLA, and they are probably involved in PLA hydrolysis and lactate conversion, respectively.

Enhancement of anaerobic digestion by co-digesting food waste and water hyacinth in improving treatment of organic waste and bio-methane recovery

In Kenya, 57% of the municipal solid waste generated is Food waste (FW) which has high organic content. However, the treatment and bioconversion of FW to biogas have always been challenging due to its rapid biodegradation, resulting from rapid hydrolysis and accumulation of volatile fatty acids and lowering pH in the bioreactor. In this study, the anaerobic digestibility of FW as a mono substrate was compared to co-digestion of FW with water hyacinth (WH) for improved biogas production and organic matter removal efficiency in a laboratory batch reactor. Different mix proportions of FW and WH were co-digested under mesophilic conditions (37 °C) at a dilution of 6% (w/v) Total Solids (TS) content. The TS of the substrates (Food waste and Water Hyacinth) were pre-processed to have a concentration of TS at 6% (60 g/L) to operate a wet AD which requires the substrate to be less than 15% TS. The proportions of WH: FW (v/v) were 100:0, 85:15, 70:30, 55:45, 30:70, 15:85, and 0:100. In the batch rectors the anaerobic co-digestion was conducted with Substrate to Inoculum (S/I) ratio of 1:1. FW is generally considered to have high volatile solids which hydrolyze rapidly lowering pH arising from excess production of Hydrogen which in presence of CO₂ and acetogenic bacteria leads to more production of acetate, formate and other long chain fatty acids which inhibits methanogenesis as a result of rapid acidification. The rapid acidification of the bioreactors that are used to treat FW results in the inhibition of the methanogenesis process. The co-digestion of the substrates could have improved the process parameters by reducing acidity caused by the high C/N ratio, reducing the inhibitory range, and increasing the buffer capacity which enhanced the bio-methane potential and the microbial activity. The batch experiments were set in triplicate for both cases of FW, WH, mixtures, and Inoculum. The results showed that the average gas yields after 81 days for the various mix proportions were 256.27and 357.69 ml/g-VS for mono-digestion of WH and FW respectively. For the mixtures of WH: FW the average reported biogas production were 305.01, 280.27, 548.91,616.01 and 270.87 ml/g-VS for mixtures of 15:85, 30:70, 55:45,70:30 and 85:15 respectively. The modified Gompertz model showed that the digesters with WH and FW alone had lag times of 2.599 and 1.052 days respectively. The mix substrates of WH: FW 85:15, 70:30, 55:45, 30:70 and 15:85 shown lag times of 2.456, 3.777, 2.574, 1.956 and 1.75 days respectively. A mix (WH: FW) of 70:30 had the highest maximum specific biogas production Rmax and the maximum biogas production potential of 18.19 mlCH₄/gVS per day and 607.7mlCH₄/gVS respectively. The R² and RSME values ranged from 0.9867 to 0.9963 and 2.663 to 9.359 respectively in all the digesters. The study shows that the co-digestion of WH and FW in the mix ratio of 70:30 improved the volume of biogas produced and organic matter removal efficiency reached 79%.

In-situ biogas upgrading with H2 addition in an anaerobic membrane bioreactor (AnMBR) digesting waste activated sludge

Biological in-situ biogas upgrading is a promising approach for sustainable energy-powered technologies. This method increases the CH₄ content in biogas via hydrogenotrophic methanogenesis with an external H₂ supply. In this study, an anaerobic membrane bioreactor (AnMBR) was employed for in-situ biogas upgrading. The AnMBR was operated in semi-batch mode using waste activated sludge as the substrate. Pulsed H₂ addition into the reactor and biogas recirculation effectively increased the CH₄ content in the biogas. The addition of 4 equivalents of H₂ relative to CO₂ did not lead to appreciable biogas upgrading, although the acetate concentration increased significantly. When 11 equivalents of H₂ were introduced, the biogas was successfully upgraded, and the CH₄ content increased to 92%. The CH₄ yield and CH₄ production rate were 0.31 L/g-VS_input and 0.086 L/L/d, respectively. In this phase of the process, H₂ addition increased the acetate concentration and the pH because of CO₂ depletion. Compared with a continuously-stirred tank reactor, the AnMBR system attained higher CH₄ content, even without the addition of H₂. The longer solid retention time (100 d) in the AnMBR led to greater degradation of volatile solids. Severe membrane fouling was not observed, and the transmembrane pressure remained stable under 10 kPa for 117 d of continuous filtration without cleaning of the membrane. The AnMBR could be a promising reactor configuration to achieve in-situ biogas upgrading during sludge digestion.

Impact of mechanical and thermo-chemical pretreatments to enhance anaerobic digestion of poly(lactic acid)

To date, the introduction of biodegradable plastics such as PLA in anaerobic digestion systems has been limited by a very low rate of biodegradation. To overcome these limitations, pretreatment technologies can be applied. In this study, the impact of pretreatments (mechanical, thermal, thermo-acid, and thermo-alkaline) was investigated. Mechanical pretreatment of PLA improved its biodegradation rate but did not affect the ultimate methane potential (430-461 NL CH₄ kg^-1 VS). In parallel, thermal and thermo-acid pretreatments exhibited a similar trend for PLA solubilization. Both of these pretreatments only achieved substantial solubilization (>60%) at higher temperatures (120 and 150 °C). At lower temperatures (70 and 90 °C), negligible solubilization (between 1 and 6%) occurred after 48 h. By contrast, coupling of thermal and alkaline pretreatment significantly increased solubilization at the lower temperatures (70 and 90 °C). In terms of biodegradation, thermo-alkaline pretreatment (with 5% w/v Ca(OH)₂) of PLA resulted in a similar methane potential (from 325 to 390 NL CH₄ kg^-1 VS) for 1 h at 150 °C, 6 h at 120 °C, 24 h at 90 °C, and 48 h at 70 °C. Reduction of the Ca(OH)₂ concentration (from 5% to 0.5% w/v) highlighted that a concentration of 2.5% w/v was sufficient to achieve a substantial level of biodegradation. Pretreatment at 70 and 90 °C using 2.5% w/v Ca(OH)₂ for 48 h resulted in biodegradation yields of 73% and 68%, respectively. Finally, a good correlation (R² = 0.90) was found between the PLA solubilization and its biodegradation.

A meta-analysis reveals that operational parameters influence levels of antibiotic resistance genes during anaerobic digestion of animal manures

Anaerobic digestion (AD) has shown the potential to reduce the numbers and types of antibiotic-resistance genes (ARG) present in animal manures. However, the variability of the results has limited the ability to draw solid conclusions. To address this issue, we performed a series of meta-analyses to evaluate how AD of pig, cattle, and dairy manures affects ARG levels and how different parameters, such as temperature, pH, digestion times, and the addition of other substances (e.g., solids, antibiotics) influence ARG changes. Twenty studies with enough details on changes in ARG levels during the AD process were identified and used for the meta-analyses. The results suggested that AD could significantly reduce ARG levels regardless of the conditions of the process. Also, thermophilic AD was more effective than mesophilic AD at reducing ARGs, although this difference was only significant for pig manures. The results also suggested that long digestion times (>50 days) yielded better ARG reduction rates, and that the addition of solids from an external source (co-digestion) negatively affected the efficiency of ARG reduction. In general, the results suggested that ARG changes during AD could be linked to the abundance and activity of hydrolytic communities.

Dry anaerobic digestion of the fine particle fraction of mechanically-sorted organic fraction of municipal solid waste in laboratory and pilot reactor

High-solid anaerobic digestion of the very small particle fraction of mechanically-sorted organic fraction of municipal solid waste (OFMSW) was examined in mesophilic digestion tests in a conventional laboratory (0.013 m³) and a pilot (0.300 m³) reactor. The non-biodegradable and recalcitrant molecules together with the low protein and starch contents of the small-particles of OFMSW limited the methane generation potential of substrate. In the conventional AD system, methane yields remained low at 0.139 m³kg_VS^-1 due to formation of a non-reacting layer on digestate surface, which restricted utilization of the available in OFMSW digestible organics. The absence of surface solid crust in the pilot unit favoured consumption of a greater proportion of volatile solids of the OFMSW. Dry AD was remarkably stable over the entire period and negligibly effected by the toxic H₂S yields. Methane generation (0.167 m³kg_VS^-1) was increased 1.2-fold compared to the conventional system due to a better mixing of substrate and microorganisms achieved inside the pilot reactor, which led to an increase of the digested volatile organics. Digestate presented low stability and high heavy metal content, both of which restrain its implementation as soil conditioner or fertilizer in agriculture. A secondary co-digestion treatment may be required for the neutralization of digestate.

One waste and two products: choosing the best operational temperature and hydraulic retention time to recover hydrogen or 1,3-propanediol from glycerol fermentation

This study aimed to compare the production of hydrogen and 1,3-propanediol from crude glycerol (10 g/L) in mesophilic (30 °C) and thermophilic (55 °C) anaerobic fluidized bed reactors, namely AFBR_30 °C and AFBR_55 °C, respectively, at hydraulic retention times (HRT) reduced from 8 to 1 h. In AFBR_30 °C, the absence or low hydrogen yields can be attributed to the production of 1,3-propanediol (maximum of 651 mmol/mol glycerol), and the formation of caproic acid (maximum of 1097 mg/L) at HRTs between 8 and 2 h. In AFBR_55 °C, the hydrogen yield of 1.20 mol H₂/mol glycerol consumed was observed at the HRT of 1 h. The maximum yield of 1,3-propanediol in AFBR_55 °C was equal to 804 mmol/mol glycerol at the HRT of 6 h and was concomitant with the production of hydrogen (0.87 mol H₂/mol glycerol consumed) and butyric acid (1447 mg/L).

l-arabinose isomerase for d-galactose isomerase activity under mesophilic conditions

d-Tagatose, a potential low calorific substitute for sucrose, can be produced by bioconversion of d-galactose catalysed by l-arabinose isomerase. l-Arabinose isomerase from Shewanella sp. ANA-3 is unique for its ability to catalyse bioconversion reactions under mesophilic conditions. However, d-galactose not being a natural substrate for l-arabinose isomerase is catalysed at a slower rate. We attempted to increase the biocatalytic efficiency of Shewanella sp. l-arabinose isomerase by rational design to enhance galactose isomerisation activity. In silico molecular docking, analysis has revealed that F279 is sterically hindering the binding of d-galactose at the C6 position. Substitution of bulky Phe residue with smaller hydrophilic residues such as Asn and Thr increased the galactose isomerase activity by 86 % and 12 % respectively. At mesophilic conditions, F279N mutant catalysed the bioconversion of d-galactose more efficiently than l-arabinose, indicating a shift in substrate preference.

Identification of microorganisms responsible for foam formation in mesophilic anaerobic digesters treating surplus activated sludge

Foaming is a common operational problem in anaerobic digestion (AD) systems, where hydrophobic filamentous microorganisms are usually considered to be the major cause. However, little is known about the identity of foam-stabilising microorganisms in AD systems, and control measures are lacking. This study identified putative foam forming microorganisms in 13 full-scale mesophilic digesters located at 11 wastewater treatment plants in Denmark, using 16S rRNA gene amplicon sequencing with species-level resolution and fluorescence in situ hybridization (FISH) for visualization. A foaming potential aeration test was applied to classify the digester sludges according to their foaming propensity. A high foaming potential for sludges was linked to the abundance of species from the genus Candidatus Microthrix, immigrating with the feed stream (surplus activated sludge), but also to several novel phylotypes potentially growing in the digester. These species were classified to the genera Ca. Brevefilum (Ca. B. fermentans) and Tetrasphaera (midas_s_5), the families ST-12K33 (midas_s_22), and Rikenellaceae (midas_s_141), and the archaeal genus Methanospirillum (midas_s_2576). Application of FISH showed that these potential foam-forming organisms all had a filamentous morphology. Additionally, it was shown that concentrations of ammonium and total nitrogen correlated strongly to the presence of foam-formers. This study provided new insight into the identity of putative foam-forming microorganisms in mesophilic AD systems, allowing for the subsequent surveillance of their abundances and studies of their ecology. Such information will importantly inform the development of control measures for these problematic microorganisms.

Optimization of hydrothermal pretreatment conditions for mesophilic and thermophilic anaerobic digestion of high-solid sludge

Hydrothermal pretreatment (HTP) conditions were optimized for continuous mesophilic (MAD) and thermophilic (TAD) anaerobic digestion of high-solid sludge (10-11% total solids). COD solubilization increased with prolonged HTP durations, and became not significant after 210 min. According to the methane production rate and energy consumption, the optimal HTP temperature was determined at 160 °C. Regarding continuous operation without HTP, TAD achieved higher methane yield and volatile solids (VS) reduction, at 0.12 L/g VS_added and 23.9%, respectively. After HTP, methane yield and VS reduction in MAD and TAD were increased by 400% and 191% (MAD), 67% and 72% (TAD), respectively. TAD was limited due to the inhibition from about 2800 mg/L of NH₄⁺-N concentration. The methanogenic activity of MAD was enhanced, whereas TAD displayed a reduced value owing to ammonia inhibition. Ultimately, MAD with HTP and TAD without HTP achieved the higher energy balance, 5.25 and 3.27 kJ/g VS, respectively.

Assessment of high-solid mesophilic and thermophilic anaerobic digestion of mechanically-separated municipal solid waste

Mechanically-sorted organic fraction of municipal solid waste (OFMSW) was tested to determine its biogas and biomethane generation efficiency. Methane production capability of OFMSW was examined in biochemical methane potential (BMP) tests. The factors affecting the high-solid anaerobic digestion (AD) of feedstock were investigated in a series of long-term semi-continuous digestion tests performed at dry mesophilic and thermophilic conditions in a continuously rotating drum reactor with working volume of 0.013 m³. OFMSW presented low biogas and methane generation capacity due to its contained non-biodegradable components and the low proteins and starch proportions. Dry mesophilic AD allowed only a relatively limited fraction of OFMSW volatile solids to be consumed for biogas and methane production. Reducing particle size favoured utilization of higher proportions of the available digestible organic substances, and concurrently promoted biogas and biomethane generation rate. Stability of methane generation was also significantly improved by particle downsizing. Small particles compensated the limited mass transfer and restricted distribution of methane production intermediate metabolites caused by water absence in the dry AD system. Dry thermophilic AD converted sufficient quantity of OFMSWs biodegradable content. The average methane released from dry thermophilic AD (0.176 m³kg_VS^-1) was higher than that of dry mesophilic AD of fine particles (0.148 m³kg_VS^-1) and much higher than that of dry mesophilic AD of same grain size (0.114 m³kg_VS^-1). High temperature proved more suitable for anaerobically digesting mechanically-sorted OFMSW.

Dynamics of Clostridium genus and hard-cheese spoiling Clostridium species in anaerobic digesters treating agricultural biomass

Biogas plants are a widespread renewable energy technology. However, the use of digestate for agronomic purposes has often been a matter of concern. It is controversial whether biogas plants might harbor some pathogenic clostridial species, which represent a biological risk. Moreover, the inhabitance of Clostridium hard-cheese spoiling species in anaerobic digesters can be problematic for hard-cheese manufacturing industries, due to the issue of cheese blowing defects. This study investigated the effect of mesophilic anaerobic digestion processes on the Clostridium consortia distribution over time. Specifically, three lab-scale CSTRs treating agricultural biomass were characterized by considering both the whole microbial community and the cultivable clostridial spores. It is assessed an overall reduction of the Clostridium genus during the anaerobic digestion process. Moreover, it was evidenced a slight, but steady decrease of the cultivable clostridial spores, mainly represented by two pathogenic species, C. perfringens and C. bifermentans, and one hard-cheese spoiling species, C. butyricum. Thus, it is revealed an overall reduction of the clostridial population abundance after the mesophilic anaerobic digestion treatment of agricultural biomass.

Comparison of mesophilic and thermophilic methane production potential of acids rich and high-strength landfill leachate at different initial organic loadings and food to inoculum ratios

Landfill leachate (LL), which can contaminate both ground and surface water is a major global environmental issue. The aim of the present study was to investigate the biomethane potential (BMP) of a high-strength LL with low pH (5.0), high solids concentration (16%), and high organic matter (170 g/L of chemical oxygen demand (COD); 55 g/L of volatile fatty acids (VFA)) with ammonia nitrogen (NH₃-N) (17 g/L). We investigated the BMP of LL at four different initial organic loadings (IOL) of 170 g/L, 85 g/L, 42.5 g/L and 21 g/L of COD and Food to inoculum (F/I) ratios of 0.5; 1; 2 and 3 at mesophilic (35 ± 2 °C) and thermophilic temperatures (55 ± 2 °C). We found that the highest cumulative CH₄ could be obtained at an IOL of 42.5 g/L of COD regardless of the F/I ratio and temperature. The highest methane content results in biogas at an IOL of 42.5 g/L were 72% and 74% at mesophilic and thermophilic temperatures respectively. About 80-100% of cumulative methane was produced within 15 days in thermophilic reactors, and 40-72% in mesophilic reactors. The kinetic study revealed a fourfold reduction of lag phase in thermophilic compared to mesophilic reactors. The methane yield and organic matter removal rate increased as the concentration of IOL in LL decreased from 170 g/L to 21 g/L regardless of temperature. There exists an inverse correlation between IOL and organic matter removal efficiency. About 80% COD reduction was obtained at mesophilic temperature, and 90% at thermophilic temperature, at an IOL of 42.5 g/L and 21 g/L of COD. The modified Gompertz model showed a good fit to the experimental data, with R² > 0.98 in all cases. Overall, the findings of this study conclude that treatment of acids rich and high-strength LL both at mesophilic and thermophilic temperature is feasible at an optimum IOL of 42.5 g/L of COD. However, treatment of LL at thermophilic temperature outperformed compared to mesophilic over the digestion time.

Impact of low loading on digestion of the mechanically-separated organic fraction of municipal solid waste

Changing waste management practice, introduction of new technologies, and population demographics and behaviour will impact on both quantity and composition of future waste streams. Laboratory-scale anaerobic digestion of the mechanically-separated organic fraction of municipal solid waste (ms-OFMSW) was carried out at relatively low organic loading rates (OLR), and results analysed using an energy modelling tool. Thermophilic operation with water addition and liquor recycle was compared to co-digestion with dilution water replaced by sewage sludge digestate (SSD); thermophilic and mesophilic mono-digestion were also tested at low OLR. All thermophilic conditions showed stable operation, with specific methane production (SMP) from 0.203 to 0.296 m³ CH₄ kg^-1 volatile solids (VS). SSD addition increased biogas production by ~20% and there was evidence of further hydrolysis and degradation of the SSD. Long-term operation at 1 kg VS m^-3 day^-1 had no adverse effect except in mesophilic conditions where SMP was lower at 0.256 m³ CH₄ kg^-1 VS and stability was reduced, especially during OLR increases. This was probably due to low total ammonia nitrogen, which stabilised at ~0.2 g N kg^-1 and limited the buffering capacity. Energy analysis showed thermophilic operation at OLR 2 g VS L^-1 day^-1 gave 42% of the theoretical methane potential and 38% of the higher heating value, reducing to 37% and 34% respectively in mesophilic conditions. Scenario modelling indicated that under low ms-OFMSW load even an energy-depleted co-substrate such as SSD could contribute to the energy balance, and would be a better diluent than water due to its nutrient and buffering capacity.

A comparative study of mesophilic and thermophilic anaerobic digestion of municipal sludge with high-solids content: Reactor performance and microbial community

This study involved a comparison between mesophilic (MAD) and thermophilic anaerobic digestion (TAD) of municipal sludge with high (10%) solids content; the reactor performance and the response of total and active microbial communities to changes in sludge properties were monitored. Both TAD and MAD were stably maintained. TAD performed better than MAD in biogas production and volatile total solids reduction upon feeding sludge 1. TAD was slightly inhibited by ammonia, whereas the performance of MAD was improved when sludge 2 was used as the feedstock. Alpha- and beta-diversity analyses revealed significant differences in the microbial community based on DNA and RNA datasets, indicating that not all microbes function in AD. The active microbial community diversity and composition in MAD and TAD were also driven by sludge properties. Moreover, MAD showed significantly higher richness and diversity of the active microbial community compared with TAD, regardless of changes in sludge properties.

Effects of temperature and mixing modes on the performance of municipal solid waste anaerobic slurry digester

PURPOSE

Anaerobic digestion is a promising technology for simultaneous treatment of biodegradable organic matter of municipal solid waste (MSW) and production of renewable energy. Mixing modes and temperature have influences on biogas production in anaerobic digesters treating MSW. Therefore, in this study, digester was operated at different modes of mixing and temperatures to obtain design criteria.

METHODS

The experiments were carried out in a semi-continuous digester. In the first part of the investigation, temperature was set at 25, 28, 31 and 34 °C. During this step, digester content was mixed in an intermittent mode by mechanical mixers. In the second part of the study, mixing condition of the digester was set at various modes: continuous, intermittent (15 min on and 30 min off) and minimal (twice in a batch).

RESULTS

Digestion with a temperature in this range resulted in biogas yield of 0.23-0.33 m³ biogas/kg VS, with a methane content of 60.2-71.8% in biogas. The methane content and yield decreased with reduction of digestion temperature. However, this reduction was almost negligible from 34 to 31 °C. In addition, in comparison to intermittent mixing, continuous and minimal mixing modes reduced the biogas production by 40% and 50%, respectively. Therefore, in this digester greatest biogas yield of 0.33 ± 0.02 m³ biogas/kg VS were obtained at 34 °C and intermittent mixing mode.

CONCLUSIONS

Based on the data obtained from this study, temperature in the range of 31-34 °C and intermittent mixing is suggested as a base for design purposes.

Current time-temperature relationships for thermal inactivation of Ascaris eggs at mesophilic temperatures are too conservative and may hamper development of simple, but effective sanitation

Ascaris eggs are commonly used as indicators for pathogen inactivation during the treatment of fecal sludge and wastewater due to their highly resistant lipid membrane and ability to survive in the environment for long periods of time. Current guidelines suggest that thermal treatment alone cannot inactivate Ascaris eggs at temperatures below 45 °C, although some evidence in the literature suggests this to be incorrect. Here, we performed a controlled experiment to test the effect of mesophilic temperatures on Ascaris inactivation. We exposed Ascaris suum eggs to a temperature gradient between 34°C and 45 °C under anaerobic and aerobic conditions to observe the required exposure times for a 3-log reduction. Indeed, we found that temperatures lower than 45 °C did inactivate these eggs, and the required exposure times were up to two orders of magnitude shorter than suggested by current guidelines. Results from the anaerobic exposures were used to develop a time-temperature relationship that is appropriate for Ascaris inactivation at mesophilic temperatures. Data from the literature demonstrated that our relationship is conservative, with faster inactivation occurring under environmental conditions when Ascaris eggs were suspended in fecal sludge or manure. A specific aerobic relationship was not developed, but we demonstrated that aerobic conditions cause faster inactivation than anaerobic conditions. Therefore, the anaerobic relationship provides a conservative guideline for both conditions. We demonstrate that relatively low temperatures can considerably impact Ascaris viability and suggest that mesophilic temperatures can be used in waste treatment processes to inactivate pathogens. The development of safe, low-input, mesophilic treatment processes is particularly valuable for ensuring universal access to safe sanitation and excreta management.

Change to biogas production in solid-state anaerobic digestion using rice straw as substrates at different temperatures

This work investigated the effects of different temperatures on methane production, kinetics, and microbial communities during solid-state anaerobic digestion (SS-AD) using rice straw. The results indicated that thermophilic anaerobic digestion led to the faster methane production (13.74 L/kg) and a shorter biogas production cycle (34 days) than mesophilic anaerobic digestion (5.48 L/kg, 58 days). SS-AD under thermophilic conditions resulted in more intense lignocellulose degradation and better fitting results. The species of microorganisms did not differ when the temperature was altered; however, the abundances of various phyla, particularly Firmicutes, differed. Overall, the findings suggested that thermophilic SS-AD had higher methanogenic efficiency and dramatically altered the structure of the microbial community during solid-state anaerobic digestion. Moreover, a potential effective strategy for agricultural waste management by SS-AD was proposed.

Enhanced mesophilic anaerobic digestion of waste sludge with the iron nanoparticles addition and kinetic analysis

As the functional material, iron nanoparticles effectively promote anaerobic digestion (AD) process, including the hydrolysis-acidification process and the biogas production. In this study, nano zero-valent iron (nZVI) and Fe₃O₄ nanoparticles (Fe₃O₄ NPs) were added to AD reactors respectively. The AD process was evaluated by the reactors performances, including pH, biogas yields and compositions, as well as the removal ratio of total solids (TS), volatile solids (VS) and soluble chemical oxygen demand (sCOD). Three models (first-order kinetic model, transfer function model and Cone model) were used to explore the kinetics of AD biogas production. The results showed that adding appropriate dose of nZVI or Fe₃O₄ NPs enhanced anaerobic digestibility of sludge. The highest cumulative biogas yield of 140.34 L with 0.5 g L^-1 nZVI and 137.13 L with 1 g L^-1 Fe₃O₄ NPs were obtained by the 80 days of mesophilic operation, respectively. Cumulative biogas productions of these two reactors were significantly enhanced up to 15.70% and 13.44%. TS removal rates reached >70% in all AD reactors with iron nanoparticles, and the highest sCOD removal rates of nZVI and Fe₃O₄ NPs digesters on the 80th day were 88.22% and 77.63%, respectively. The results of the three-day fermentation experiment and the kinetic parameters showed that the nZVI or Fe₃O₄ NPs enhanced the hydrolysis-acidification process of the AD, which eventually promoted biogas production. The Cone model was satisfied with the experimental results, which could be used to evaluate the kinetics of AD with iron nanoparticles more reasonably.

Mesophilic and thermophilic anaerobic digestion of soybean curd residue for methane production: Characterizing bacterial and methanogen communities and their correlations with organic loading rate and operating temperature

To find the optimal operation parameters and provide an explanation of methanogenic pathway for methane production in mesophilic (35 °C) and thermophilic (55 °C) anaerobic digestion (MAD, TAD) of soybean curd residue (SCR), MAD and MAD were contrastively investigated for 95 days. The maximum available OLR was identified as 3.3 gVS/L for both MAD and TAD. Compared to MAD, TAD exhibited a 20% higher average methane yield (0.591 L/gVS) and a 7.5% higher volatile solids removal efficiency (74.1 ± 10.4%). Bacterial phyla Bacteroidetes, Firmicutes and Proteobacteria dominated in MAD digesters while genus Defluviitoga was selectively enriched in TAD digesters due to higher temperature and organic loading pressure. Principal coordinates analysis of methanogen community showed that both temperature and OLR were crucial environmental variables shifting the taxonomic patterns of the methanogens. The enriched methanogen genus Methanothermobacter (93%) with a hydrogenotrophic methanogenic pathway had a close correlation with the TAD performance.

Effects of H2:CO2 ratio and H2 supply fluctuation on methane content and microbial community composition during in-situ biological biogas upgrading

BACKGROUND

Commercial biogas upgrading facilities are expensive and consume energy. Biological biogas upgrading may serve as a low-cost approach because it can be easily integrated with existing facilities at biogas plants. The microbial communities found in anaerobic digesters typically contain hydrogenotrophic methanogens, which can use hydrogen (H₂) as a reducing agent for conversion of carbon dioxide (CO₂) into methane (CH₄). Thus, biological biogas upgrading through the exogenous addition of H₂ into biogas digesters for the conversion of CO₂ into CH₄ can increase CH₄ yield and lower CO₂ emission.

RESULTS

The addition of 4 mol of H₂ per mol of CO₂ was optimal for batch biogas reactors and increased the CH₄ content of the biogas from 67 to 94%. The CO₂ content of the biogas was reduced from 33 to 3% and the average residual H₂ content was 3%. At molar H₂:CO₂ ratios > 4:1, all CO₂ was converted into CH₄, but the pH increased above 8 due to depletion of CO₂, which negatively influenced the process stability. Additionally, high residual H₂ content in these reactors was unfavourable, causing volatile fatty acid accumulation and reduced CH₄ yields. The reactor microbial communities shifted in composition over time, which corresponded to changes in the reactor variables. Numerous taxa responded to the H₂ inputs, and in particular the hydrogenotrophic methanogen Methanobacterium increased in abundance with addition of H₂. In addition, the apparent rapid response of hydrogenotrophic methanogens to intermittent H₂ feeding indicates the suitability of biological methanation for variable H₂ inputs, aligning well with fluctuations in renewable electricity production that may be used to produce H₂.

CONCLUSIONS

Our research demonstrates that the H₂:CO₂ ratio has a significant effect on reactor performance during in situ biological methanation. Consequently, the H₂:CO₂ molar ratio should be kept at 4:1 to avoid process instability. A shift toward hydrogenotrophic methanogenesis was indicated by an increase in the abundance of the obligate hydrogenotrophic methanogen Methanobacterium.

Mesophilic, thermophilic and hyperthermophilic acidogenic fermentation of food waste in batch: Effect of inoculum source

Distinctions in hydrolysis and acidogenesis were examined for a series of anaerobic batch reactors inoculated with three different anaerobic mixed cultures (mesophilic, thermophilic and hyperthermophilic anaerobic sludge) and operated at the temperature of inoculum's origin and additionally at 70 °C. Hyperthermophilic temperatures led to increased hydrolysis rates during the start-up stage but a rapid drop in pH limited the overall hydrolysis efficiency, indicating the importance of pH control to sustain the high reaction rates at higher temperatures. No significant difference (P > 0.05) was observed among hydrolysis efficiencies obtained for different reactors which ranged between 27 ± 3% and 40 ± 14%. The highest fermentation yield of 0.44 g COD of fermentation products/g VSS-COD_added was obtained under thermophilic conditions, followed by mesophilic (0.33 g COD ferm. prod./g VSS-COD_added) and hyperthermophilic conditions (0.05-0.08 g COD ferm. prod./g VSS-COD_added). Fermentative performance was better at mesophilic and thermophilic conditions as indicated by improved production of volatile fatty acids (VFA). VFAs accounted for 60-71% of the solubilised matter at thermophilic and mesophilic conditions. Acetic acid formed the primary VFA (70%) at mesophilic temperatures, while butyric acid was the major VFA at thermophilic (60%) conditions. Hyperthermophilic conditions led to increased production of lactic acid, which comprised up to 32% of the solubilised matter. Overall, the results indicate that different operating temperatures may not significantly affect the substrate degradation efficiency but clearly influence the biotransformation pathways.

Temperature susceptibility of a mesophilic anaerobic membrane bioreactor treating saline phenol-containing wastewater

This study examined the temperature susceptibility of a continuous-flow lab-scale anaerobic membrane bioreactor (AnMBR) to temperature shifts from 35 °C to 55 °C and its bioconversion robustness treating synthetic phenolic wastewater at 16 gNa^+.L^-1. During the experiment, the mesophilic reactor was subjected to stepwise temperature increases by 5 °C. The phenol conversion rates of the AnMBR decreased from 3.16 at 35 °C to 2.10 mgPh^.gVSS^-1.d^-1 at 45 °C, and further decreased to 1.63 mgPh^.gVSS^-1.d^-1 at 50 °C. At 55 °C, phenol conversion rate stabilized at 1.53 mgPh^.gVSS^-1.d^-1 whereas COD removal efficiency was 38% compared to 95.5% at 45 °C and 99.8% at 35 °C. Interestingly, it was found that the phenol degradation process was less susceptible for the upward temperature shifts than the methanogenic process. The temperature increase implied twenty-one operational taxonomic units from the reactor's microbial community with significant differential abundance between mesophilic and thermophilic operation, and eleven of them are known to be involved in aromatic compounds degradation. Reaching the upper-temperature limits for mesophilic operation was associated with the decrease in microbial abundance of the phyla Firmicutes and Proteobacteria, which are linked to syntrophic phenol degradation. It was also found that the particle size decreased from 89.4 μm at 35 °C to 21.0 μm at 55 °C. The accumulation of small particles and higher content of soluble microbial protein-like substances led to increased transmembrane pressure which negatively affected the filtration performance. Our findings indicated that at high salinity a mesophilic AnMBR can tolerate a temperature up to 45 °C without being limited in the phenol conversion capacity.

An explanation of the methanogenic pathway for methane production in anaerobic digestion of nitrogen-rich materials under mesophilic and thermophilic conditions

The impact of temperature on the anaerobic digestion of chicken manure was investigated by studying the process performance and pathway for continuously-fed digesters under mesophilic and thermophilic conditions. The mesophilic digester obtained a 15% higher methane yield compared with the thermophilic digester. Mesophilic and thermophilic digester had free ammonia of 31 and 145 mg/L, respectively. The stable carbon isotope analysis indicated that 41% and 50% of acetate was converted to methane through the syntrophic acetate oxidation and hydrogenotrophic methanogenesis (SAO-HM) pathway under mesophilic and thermophilic conditions, respectively. The genus Pseudomonas represented 10% and 16% under mesophilic and thermophilic conditions, respectively. A high abundance of the methanogens genus Methanoculleus (94% of total methanogens) in mesophilic and the genus Methanothermobacter (96%) in thermophilic digesters indicated they were the main hydrogenotrophic partners in SAO. The present study therefore illustrated that methanogenic pathway shifting, induced by free ammonia, closely correlated to the process performance.

Investigation of the bacteriophage community in induced lysates of undefined mesophilic mixed-strain DL-cultures using classical and metagenomic approaches

To investigate the notion that starter cultures can be a reservoir of bacteriophages (phages) in the dairy environment, strains of three DL-starters (undefined mesophilic mixed-strain starters containing Lactococcus lactis subsp. lactis biovar. diacetylactis and Leuconostoc species) were selected and induced by mitomycin C, and the whole starters were induced spontaneously as well as by mitomycin C. Frequency of induction of 17%, 26% and 12% was estimated among the isolates of the three starters, with majority of the induced phages mostly showing morphological similarity to known P335 phages, and with a fraction of them showing atypical features. Sequences of P335 quasi-species phages were found to be the most frequent entities in almost all metaviromes derived from the induced lysates. However, sequences of Sk1virus phages (previously 936 phages) were emerged as the predominant entities following spontaneous induction of one of the starters, suggesting a phage-carrier state. Sequences of other phages such as 949, 1706, C2virus (previously c2 phages) and Leuconostoc species could also be observed but with a lower relative frequency. Taken together, the majority of the P335 quasi-species phages could represent the induced viral community of the starters and the remaining phage groups mainly represent the background ambient viral community.

Strategies for the sustainable management of orange peel waste through anaerobic digestion

The processing of oranges is a major industry worldwide and leads to the production of large amounts of orange peel waste (OPW). Energy production through anaerobic digestion of OPW is a promising option; however, the high content of essential oil, mainly composed of d-limonene, a well-known antioxidant, can cause the inhibition of the biological activity. In this paper, different pretreatment methods were tested (e.g. ensiling, aeration, thermal and alkaline treatments) to optimize the anaerobic digestion of OPW focusing on d-limonene removal. The raw and pretreated substrates were characterized and their biochemical methane production was measured. The results demonstrated the ability of some of the treatments to reduce d-limonene content up to 80%. A relatively high biomethane potential production of OPW (up to about 500 NmL CH₄ g^-1VS) was measured. The importance of the acclimation of inoculum and the risk connected to the accumulation of inhibiting substances in the reactor is discussed.

Effect of temperature and organic loading rate on siphon-driven self-agitated anaerobic digestion performance for food waste treatment

The effects of organic loading rate (OLR) and operating temperature on the performance of siphon-driven self-agitated anaerobic reactor (SDSAR) in an on-site food waste (FW) treatment system were investigated. Two reactors were operated in parallel for comparison between mesophilic condition (35 ± 1 °C) and thermophilic condition (55 ± 1 °C). With HRT above 15 d and OLR below 4.8 kg-COD/m³/d, relatively high COD removal in the range of 84.5-92.3% was obtained in both reactors. The limits of the loading capacity of the mesophilic SDSAR were observed when OLR was further increased to 7.3 kg-COD/m³/d by shortening HRT. Blocking and gas production reduction occurred and COD removal decreased sharply to 75.9% in the mesophilic reactor. In contrast, the thermophilic reactor can be operated at this OLR with satisfactory COD removal and biogas production. Furthermore, at OLR of 14.4 kg-COD/m³/d, the COD removal was maintained as high as 87.5% in the thermophilic reactor. The conversion of influent COD to methane was maintained above 80% at all the OLR applied in both reactors. The results of this study indicated that thermophilic SDSAR is preferred for the on-site FW treatment.

Performance of direct anaerobic digestion of dewatered sludge in long-term operation

Direct anaerobic digestion of dewatered sludge with total solids (TS) content of 15-20% was tested in a horizontal digester for one and half years. The system kept stable with pH 7-8. The concentration of volatile fatty acids was lower than 800 mg/L, free ammonia nitrogen was lower than 200 mg/L, and total alkalinity kept higher than 6000 mg/L. The performance was influenced by organic load rate (OLR) and organic content in feed sludge. When volatile solids (VS) in TS of feed sludge reached 60-65% at OLR 3.50-3.70 g/(L·d), the process exhibited the best performance with organic removal rate of 32.19 ± 7.73% and methane production of 156.86 ± 13.05 ml/g VS added. Microbial analyses indicated that Methanosarcina became predominant and Methanosaeta almost disappeared. Moreover, hydrogenotrophic and methylotrophic methanogens accounted for 18.13-29.40% and 11.58-29.56% of the total, respectively. These provide a new guideline for small-scale or centralized sludge treatment.

Mesophilic and thermophilic anaerobic co-digestion of winery wastewater sludge and wine lees: An integrated approach for sustainable wine production

In this work, winery wastes generated by a cellar producing approximately 300,000 hL of wine per year was monitored for a period of one year. On average, 196 L of wastewater, 0.1 kg of waste activated sludge (dry matter) and 1.6 kg of wine lees were produced per hectoliter of wine produced. Different winery wastes, deriving from different production steps, namely waste activated sludge from wastewater treatment and wine lees, were co-treated using an anaerobic digestion process. Testing was conducted on a pilot scale for both mesophilic and thermophilic conditions. The process was stable for a long period at 37 °C, with an average biogas production of 0.386 m³/kg COD_fed. On the other hand, for thermophilic conditions, volatile fatty acids accumulated in the reactor and the process failed after one hydraulic retention time (23 days). In order to fix the biological process, trace elements (iron, cobalt and nickel) were added to the feed of the thermophilic reactor. Metals augmentation improved process stability and yields at 55 °C. The pH ranged between 7.8 and 8.0, and specific gas production was 0.450 m³/kg COD_fed, which corresponded to dry matter and COD removals of 34% and 88%, respectively. Although the observed performances in terms of biogas production were good, the thermophilic process exhibited some limitations related to both the necessity of metals addition and the worse dewaterability properties. In fact, while the mesophilic digestates reached a good dewatering quality via the addition of 6.5 g of polymer per kg of dry matter, the required dosage for the thermophilic sludge was greater than 10 g/kg of dry matter.

Backbone and side-chain 1H, 15N and 13C resonance assignments of two Sac10b family members Mvo10b and Mth10bTQQA from archaea

The Sac10b family proteins, also named as Alba, are small, basic, nucleic acid-binding proteins widely distributed in archaea. They possess divergent physiological functions such as binding to both DNA and RNA with a high affinity and involving in genomic DNA compaction, RNA transactions and transcriptional regulations. The structures of many Sac10b family proteins from hyperthermophilic archaea have been reported, while those from thermophilic and mesophilic archaea are largely unknown. As was pointed out, the homologous members from thermophilic and mesophilic archaea may have functions different from the hyperthermophilic members. Therefore, comparison of these homologous members can provide biophysical and structural insight into the functional diversity and thermal adaptation mechanism. The present work mainly focused on the NMR study of two Sac10b family members, Mvo10b and Mth10b, from the mesophilic and thermophilic archaea, respectively. To overcome the difficulties caused by the oligomerization and conformation heterogeneity of Mth10b, a M13T/L17Q/I20Q/P56A mutant Mth10b (Mth10bTQQA) was constructed and used together with Mvo10b for multi-dimensional NMR experiments. The resonance assignments of Mvo10b and Mth10bTQQA are reported for further structural determination which is a basis for understanding the functional diversity and their thermal adaption mechanisms.

Seeking to enhance the bioenergy of municipal sludge: Effect of alkali pre-treatment and soluble organic matter supplementation

The aim of this research is to enhance the mesophilic anaerobic digestion of municipal sludge from Cadiz-San Fernando (Spain) wastewater treatment plant at 20days hydraulic retention time (HRT). Two different strategies were tested to improve the process: co-digestion with the addition of soluble organic matter (1% v/v); and alkali sludge pre-treatment (NaOH) prior to co-digestion with glycerine (1% v/v). Methane production (MP) was substantially enhanced (from 0.36±0.09 L CH₄ l/d to 0.85±0.16 L CH₄ l/d), as was specific methane production (SMP) (from 0.20±0.05 L CH₄/g VS to 0.49±0.09 L CH₄/g VS) when glycerine was added. The addition of glycerine does not seem to affect sludge stability, the quality of the effluent in terms of pH and organic matter content, i.e. volatile fatty acids (VFA), soluble organic matter and total volatile solid, or process stability (VFA/Alkalinity ratio<0.4). Alkali pre-treatment prior to co-digestion resulted in a high increase in soluble organic loading rates (more than 20%) and acidification yield (more than 50%). At 20days HRT, however, it led to overload of the system and total destabilization of the mesophilic anaerobic co-digestion of sewage sludge and glycerine.

Biochemical methane potential of oil-extracted microalgae and glycerol in co-digestion with chicken litter

The objective of this work was to evaluate the technical feasibility of using both oil-extracted microalgae (M) and glycerol (G) in co-digestion with chicken litter (CL), thereby improving biochemical methane potential (BMP). Different feedstock ratios of M (0-30%), G (0-3%) and CL (67-100%) were investigated to determine the best co-digestion condition under mesophilic conditions. According to the modified Gompertz model, the best BMP (131.1mLCH₄g_VSfed^-1) was obtained with the triple co-digestion (M:G:CL) in a proportion of 30:3:67. This yielded a methane production rate (μ_m) of 3.3mLCH₄g_VSfed^-1d^-1 and a lag time (λ) of 17.4d. This treatment reduced chemical oxygen demand (COD) by 91.02% and increased the methane yield 15.8% with respect to the CL control.

Correlation analysis of enzyme activities and deconstruction of ammonia-pretreated switchgrass by bacterial-fungal communities

The mixed microbial communities that occur naturally on lignocellulosic feedstocks can provide feedstock-specific enzyme mixtures to saccharify lignocelluloses. Bacterial-fungal communities were enriched from switchgrass bales to deconstruct ammonia-pretreated switchgrass (DSG). Correlation analysis was carried out to elucidate the relationship between microbial decomposition of DSG by these communities, enzymatic activities produced and enzymatic saccharification of DSG using these enzyme mixtures. Results of the analysis showed that β-glucosidase and xylosidase activities limited the extent of microbial deconstruction and enzymatic saccharification of DSG. The results also underlined the importance of ligninase activity for the enzymatic saccharification of pretreated lignocellulosic feedstock. The bacterial-fungal communities developed in this research can be used to produce enzyme mixtures to deconstruct DSG, and the results from the correlation analysis can be used to optimize these enzyme mixtures for efficient saccharification of DSG to produce second-generation biofuels.

Mesophilic and thermophilic alkaline fermentation of waste activated sludge for hydrogen production: Focusing on homoacetogenesis

The present study compared the mesophilic and thermophilic alkaline fermentation of waste activated sludge (WAS) for hydrogen production with focus on homoacetogenesis, which mediated the consumption of H2 and CO2 for acetate production. Batch experiments showed that hydrogen yield of WAS increased from 19.2 mL H2/gVSS at 37 °C and pH 10-80.1 mL H2/gVSS at 55 °C and pH 10. However, the production of volatile fatty acids (mainly acetate) was higher at 37 °C and pH 10 by comparison with 55 °C and pH 10. Hydrogen consumption due to homoacetogenesis was observed at 37 °C and pH 10 but not 55 °C and pH 10. Higher expression levels of genes relating with homoacetogenesis and lower expression levels of genes relating with hydrogen production were found at 37 °C and pH 10 compared to 55 °C and pH 10. The continuous experiment demonstrated the steady-state hydrogen yield of WAS was comparable to that obtained from batch experiments at 55 °C and pH 10, and homoacetogenesis was still inhibited. However, the steady-state hydrogen yield of WAS (6.5 mL H2/gVSS) was much lower than that (19.2 mL H2/gVSS) obtained from batch experiments at 37 °C and pH 10 due to the gradual enrichment of homoacetogens as demonstrated by qPCR analysis. The high-throughput sequencing analysis of 16S rRNA genes showed that the abundance of genus Clostridium, containing several homoacetogens, was 5 times higher at 37 °C and pH 10 than 55 °C and pH 10.

Comparison of microbial communities during the anaerobic digestion of Gracilaria under mesophilic and thermophilic conditions

Mesophilic and thermophilic anaerobic digesters (MD and TD, respectively) utilizing Gracilaria and marine sediment as the substrate and inoculum, respectively, were compared by analyzing their performances and microbial community changes. During three successive transfers, the average cumulative methane yields in the MD and TD were 222.6 ± 17.3 mL CH4/g volatile solids (VS) and 246.1 ± 11 mL CH4/g VS, respectively. The higher hydrolysis rate and acidogenesis in the TD resulted in a several fold greater accumulation of volatile fatty acids (acetate, propionate, and butyrate) followed by a larger pH drop with a prolonged recovery than in the MD. However, the operational stability between both digesters remained comparable. Pyrosequencing analyses revealed that the MD had more complex microbial diversity indices and microbial community changes than the TD. Interestingly, Methanomassiliicoccales, the seventh methanogen order was the predominant archaeal order in the MD along with bacterial orders of Clostridiales, Bacteriodales, and Synergistales. Meanwhile, Coprothermobacter and Methanobacteriales dominated the bacterial and archaeal community in the TD, respectively. Although the methane yield is comparable, both MD and TD show a different profile of pH, VFA and the microbial communities.

Comparative analysis of the digestibility of sewage fine sieved fraction and hygiene paper produced from virgin fibers and recycled fibers

Sewage fine sieved fraction (FSF) is a heterogeneous substrate consisting of mainly toilet paper fibers sequestered from municipal raw sewage by a fine screen. In earlier studies, a maximum biodegradation of 62% and 57% of the sewage FSF was found under thermophilic (55°C) and mesophilic (35°C) conditions, respectively. In order to research this limited biodegradability of sewage FSF, this study investigates the biodegradation of different types of cellulosic fibers-based hygiene papers including virgin fibers based toilet paper (VTP), recycled fiber based toilet paper (RTP), virgin pulp for paper production (VPPP) as a raw material, as well as microcrystalline cellulose (MCC) as a kind of fiberless reference material. The anaerobic biodegradation or digestibility tests were conducted under thermophilic and mesophilic conditions. Results of the experiments showed different biomethane potential (BMP) values for each tested cellulose fiber-based substrate, which might be associated with the physical characteristics of the fibers, type of pulping, presence of lignin encrusted fibers, and/or the presence of additive chemicals and refractory compounds. Higher hydrolysis rates (Kh), higher specific methane production rates (SMPR) and shorter required incubation times to achieve 90% of the BMP (t90%CH4), were achieved under thermophilic conditions for all examined substrates compared to the mesophilic ones. Furthermore, the biodegradability of all employed cellulose fiber-based substrates was in the same range, 38-45%, under both conditions and less than the observed FSF biodegradability, i.e. 57-62%. MCC achieved the highest BMP and biodegradability, 86-91%, among all cellulosic substrates.

Biological Pretreatment of Chicken Feather and Biogas Production from Total Broth

Chicken feathers are available in large quantities around the world causing environmental challenges. The feathers are composed of keratin that is a recalcitrant protein and is hard to degrade. In this work, chicken feathers were aerobically pretreated for 2-8 days at total solid concentrations of 5, 10, and 20 % by Bacillus sp. C₄, a bacterium that produces both α- and β-keratinases. Then, the liquid fraction (feather hydrolysate) as well as the total broth (liquid and solid fraction of pretreated feathers) was used as substrates for biogas production using anaerobic sludge or bacteria granules as inoculum. The biological pretreatment of feather waste was productive; about 75 % of feather was converted to soluble crude protein after 8 days of degradation at initial feather concentration of 5 %. Bacteria granules performed better during anaerobic digestion of untreated feathers, resulting in approximately two times more methane yield (i.e., 199 mlCH₄/gVS compared to 105 mlCH₄/gVS when sludge was used). Pretreatment improved methane yield by 292 and 105 % when sludge and granules were used on the hydrolysate. Bacteria granules worked effectively on the total broth, yielded 445 mlCH₄/gVS methane, which is 124 % more than that obtained with the same type of inoculum from untreated feather.

Lignocellulose-derived thin stillage composition and efficient biological treatment with a high-rate hybrid anaerobic bioreactor system

BACKGROUND

This study aims to chemically characterize thin stillage derived from lignocellulosic biomass distillation residues in terms of organic strength, nutrient, and mineral content. The feasibility of performing anaerobic digestion on these stillages at mesophilic (40 °C) and thermophilic (55 °C) temperatures to produce methane was demonstrated. The microbial communities involved were further characterized.

RESULTS

Energy and sugar cane stillage have a high chemical oxygen demand (COD of 43 and 30 g/L, respectively) and low pH (pH 4.3). Furthermore, the acetate concentration in sugar cane stillage was high (45 mM) but was not detected in energy cane stillage. There was also a high amount of lactate in both types of stillage (35-37 mM). The amount of sugars was 200 times higher in energy cane stillage compared to sugar cane stillage. Although there was a high concentration of sulfate (18 and 23 mM in sugar and energy cane stillage, respectively), both thin stillages were efficiently digested anaerobically with high COD removal under mesophilic and thermophilic temperature conditions and with an organic loading rate of 15-21 g COD/L/d. The methane production rate was 0.2 L/g COD, with a methane percentage of 60 and 64, and 92 and 94 % soluble COD removed, respectively, by the mesophilic and thermophilic reactors. Although both treatment processes were equally efficient, there were different microbial communities involved possibly arising from the differences in the composition of energy cane and sugar cane stillage. There was more acetic acid in sugar cane stillage which may have promoted the occurrence of aceticlastic methanogens to perform a direct conversion of acetate to methane in reactors treating sugar cane stillage.

CONCLUSIONS

Results showed that thin stillage contains easily degradable compounds suitable for anaerobic digestion and that hybrid reactors can efficiently convert thin stillage to methane under mesophilic and thermophilic conditions. Furthermore, we found that optimal conditions for biological treatment of thin stillage were similar for both mesophilic and thermophilic reactors. Bar-coded pyrosequencing of the 16S rRNA gene identified different microbial communities in mesophilic and thermophilic reactors and these differences in the microbial communities could be linked to the composition of the thin stillage.

Mesophilic and thermophilic anaerobic digestion of the liquid fraction of pressed biowaste for high energy yields recovery

Deep separate collection of the organic fraction of municipal solid waste generates streams with relatively low content of inert material and high biodegradability. This material can be conveniently treated to recovery both energy and material by means of simplified technologies like screw-press and extruder: in this study, the liquid fraction generated from pressed biowaste from kerbside and door-to-door collection was anaerobically digested in both mesophilic and thermophilic conditions while for the solid fraction composting is suggested. Continuous operation results obtained both in mesophilic and thermophilic conditions indicated that the anaerobic digestion of pressed biowaste was viable at all operating conditions tested, with the greatest specific gas production of 0.92m(3)/kgVSfed at an organic loading rate of 4.7kgVS/m(3)d in thermophilic conditions. Based on calculations the authors found that the expected energy recovery is highly positive. The contents of heavy metals and pathogens of fed substrate and effluent digestates were analyzed, and results showed low levels (below End-of-Waste 2014 criteria limits) for both the parameters thus indicating the good quality of digestate and its possible use for agronomic purposes. Therefore, both energy and material were effectively recovered.

Influence of digestion temperature and organic loading rate on the continuous anaerobic treatment of process liquor from hydrothermal carbonization of sewage sludge

This experimental study investigates the use of process liquor from hydrothermal carbonization (HTC) of sewage sludge as sole substrate for anaerobic digestion (AD). The process performance at 37°C (mesophilic) and 55°C (thermophilic) was determined based on two identical continuously-fed anaerobic filters (26 L each) operated for 20 weeks. During operation, the organic loading rate was stepwise increased from 1 to 5 gCOD L(-1)d(-)(1). This decreased the hydraulic retention time from 34 to 5 days. Significant differences in methane production were not observed as both reactors yielded up to 0.18 LCH4 gCOD(-)(1). Increased temperature had no effect on the steady-state COD removal efficiency with both reactors stabilized at 68-75%. Macro- and micronutrients were provided by HTC liquor in sufficient concentrations throughout the experiment. Methanogenesis was identified as the speed-limiting step in anaerobic digestion of HTC liquor. Hydrolysis was of an order of magnitude faster than methanogenesis.

Identification and characterization of a mesophilic phytase highly resilient to high-temperatures from a fungus-garden associated metagenome

Phytases are enzymes degrading phytic acid and thereby releasing inorganic phosphate. While the phytases reported to date are majorly from culturable microorganisms, the fast-growing quantity of publicly available metagenomic data generated in the last decade has enabled bioinformatic mining of phytases in numerous data mines derived from a variety of ecosystems throughout the world. In this study, we are interested in the histidine acid phosphatase (HAP) family phytases present in insect-cultivated fungus gardens. Using bioinformatic approaches, 11 putative HAP phytase genes were initially screened from 18 publicly available metagenomes of fungus gardens and were further overexpressed in Escherichia coli. One phytase from a south pine beetle fungus garden showed the highest activity and was then chosen for further study. Biochemical characterization showed that the phytase is mesophilic but possesses strong ability to withstand high temperatures. To our knowledge, it has the longest half-life time at 100 °C (27 min) and at 80 °C (2.1 h) as compared to all the thermostable phytases publicly reported to date. After 100 °C incubation for 15 min, more than 93 % of the activity was retained. The activity was 3102 μmol P/min/mg at 37 °C and 4135 μmol P/min/mg at 52.5 °C, which is higher than all the known thermostable phytases. For the high activity level demonstrated at mesophilic temperatures as well as the high resilience to high temperatures, the phytase might be promising for potential application as an additive enzyme in animal feed.

Draft genome sequence of Mesotoga strain PhosAC3, a mesophilic member of the bacterial order Thermotogales, isolated from a digestor treating phosphogypsum in Tunisia

Mesotoga strain PhosAc3 was the first mesophilic cultivated member of the order Thermotogales. This genus currently contain two described species, M. prima and M. infera. Strain PhosAc3, isolated from a Tunisian digestor treating phosphogypsum, is phylogenetically closely related to M. prima strain MesG1.Ag.4.2(T). Strain PhosAc3 has a genome of 3.1 Mb with a G+C content of 45.2%. It contains 3,051 protein-coding genes of which 74.6% have their best reciprocal BLAST hit in the genome of the type species, strain MesG1.Ag.4.2(T). For this reason we propose to assign strain PhosAc3 as a novel ecotype of the Mesotoga prima species. However, in contrast with the M. prima type strain, (i) it does not ferment sugars but uses them only in the presence of elemental sulfur as terminal electron acceptor, (ii) it produces only acetate and CO2 from sugars, whereas strain MesG1.Ag.4.2(T) produces acetate, butyrate, isobutyrate, isovalerate, 2-methyl-butyrate and (iii) sulfides are also end products of the elemental sulfur reduction in theses growth conditions.

Performance of mesophilic biohydrogen-producing cultures at thermophilic conditions

In this study, batch tests were conducted to investigate the performance of mesophilic anaerobic digester sludge (ADS) at thermophilic conditions and estimate kinetic parameters for co-substrate fermentation. Starch and cellulose were used as mono-substrate and in combination as co-substrates (1:1 mass ratio) to conduct a comparative assessment between mesophilic (37 °C) and thermophilic (60 °C) biohydrogen production. Unacclimatized mesophilic ADS responded well to the temperature change. The highest hydrogen yield of 1.13 mol H2/mol hexose was observed in starch-only batches at thermophilic conditions. The thermophilic cellulose-only yield (0.42 mol H2/mol hexose) was three times the mesophilic yield (0.13 mol H2/mol hexose). Interestingly, co-fermentation of starch-cellulose at mesophilic conditions enhanced the hydrogen yield by 26% with respect to estimated mono-substrate yields, while under thermophilic conditions no enhancement in the overall yield was observed. Interestingly, the estimated overall Monod kinetic parameters showed higher rates at mesophilic than thermophilic conditions.

Effect of limited air exposure and comparative performance between thermophilic and mesophilic solid-state anaerobic digestion of switchgrass

Switchgrass is an attractive feedstock for biogas production via anaerobic digestion (AD). Many studies have used switchgrass for liquid anaerobic digestion (L-AD), but few have used switchgrass for solid-state anaerobic digestion (SS-AD). Limited air exposure to the reactor headspace has been adopted in commercial scale anaerobic digesters for different applications. However, little research has examined the effect of limited air exposure on biogas production during SS-AD. In this study, the effects of air exposure and total solids (TS) content on SS-AD performance were evaluated under mesophilic (36±1°C) and thermophilic (55±0.3°C) conditions. Limited air exposure did not significantly influence the methane yield during SS-AD. Thermophilic SS-AD had greater methane yields (102-145LCH4kg(-1)VSadded) than mesophilic SS-AD (88-113LCH4kg(-1)VSadded). Both mesophilic SS-AD (73-136GJ) and thermophilic SS-AD (2-95GJ) produced positive net energy based on a theoretical 'garage-type' SS-AD digester operating in a temperate climate.

Kinetic characterization of thermophilic and mesophilic anaerobic digestion for coffee grounds and waste activated sludge

This study was conducted to characterize the kinetics of an anaerobic process (hydrolysis, acetogenesis, acidogenesis and methanogenesis) under thermophilic (55 °C) and mesophilic (35 °C) conditions with coffee grounds and waste activated sludge (WAS) as the substrates. Special focus was given to the kinetics of propionic acid degradation to elucidate the accumulation of VFAs. Under the thermophilic condition, the methane production rate of all substrates (WAS, ground coffee and raw coffee) was about 1.5 times higher than that under the mesophilic condition. However, the effects on methane production of each substrate under the thermophilic condition differed: WAS increased by 35.8-48.2%, raw coffee decreased by 76.3-64.5% and ground coffee decreased by 74.0-57.9%. Based on the maximum reaction rate (Rmax) of each anaerobic stage obtained from the modified Gompertz model, acetogenesis was found to be the rate-limiting step for coffee grounds and WAS. This can be explained by the kinetics of propionate degradation under thermophilic condition in which a long lag-phase (more than 18 days) was observed, although the propionate concentration was only 500 mg/L. Under the mesophilic condition, acidogenesis and hydrolysis were found to be the rate-limiting step for coffee grounds and WAS, respectively. Even though reducing the particle size accelerated the methane production rate of coffee grounds, but did not change the rate-limiting step: acetogenesis in thermophilic and acidogenesis in mesophilic.

Rheology evolution of sludge through high-solid anaerobic digestion

The main purpose of this study was to investigate the rheology evolution of sludge through high-solid anaerobic digestion (AD) and its dependency on sludge retention time (SRT) and temperature of AD reactor. The operation performance of high-solid AD reactors were also studied. The results showed that sludge became much more flowable after high-solid AD. It was found that the sludge from reactors with long SRT exhibited low levels of shear stress, viscosity, yield stress, consistency index, and high value of flow behaviour index. While the flowability of sludge from thermophilic AD reactors were better than that of sludge from mesophilic AD reactors though the solid content of the formers were higher than that of the latters, which could be attributed to the fact that the formers had more amount of free and interstitial moisture. It might be feasible to use sludge rheology as an AD process controlling parameter.