Efficient degradation of rice straw in the reactors packed by carbon fiber textiles

We have reported for the first time that agricultural and cellulosic waste, i.e., rice straw was directly applied to methanogenic bioreactors containing carbon fiber textiles (CFT) as supporting material. Addition of CFT to the methanogenic bioreactors enhanced the conversion of dichromate chemical oxygen demand of the substrate to methane (41%) to a greater extent than bioreactors without CFT (9%). In addition, removal of rice straw as a suspended solid was increased from 31% (in bioreactors without CFT) to 57% (in those with CFT). Methanogenic 16S rRNA gene analysis showed that the abundance of acetoclastic methanogen, genus Methanosarcina, was about 11 times higher in bioreactors with CFT (suspended fraction plus retained fraction to CFT) than in bioreactors without CFT (suspended fraction), resulting in lower concentration of acetate in bioreactors with CFT (0.4 mM) than in those without CFT (29.7 mM). On the other hand, the abundance of hydrogenotrophic methanogen, genus Methanobacterium, in bioreactors with CFT was similar to those without CFT. Bacterial communities in bioreactors with CFT were different from those in bioreactors without CFT. Our results indicated that specific microbial community and cooperative relationships between microorganisms in reactors containing CFT facilitated efficient decomposition of rice straw and its conversion to methane.     

Dry anaerobic ammonia–methane production from chicken manure

The effect of temperature on production of ammonia during dry anaerobic fermentation of chicken manure (CM), inoculated with thermophilic methanogenic sludge, was investigated in a batch condition for 8 days. Incubation temperature did not have a significant effect on the production of ammonia. Almost complete inhibition of production of methane occurred at 55 and 65°C while quite low yields of 8.45 and 6.34 ml g⁻¹ VS (volatile solids) were observed at 35 and 45°C due to a higher accumulation of ammonia. In order to improve the production of methane during dry anaerobic digestion of CM, stripping of ammonia was performed firstly on the CM previously fermented at 65°C for 8 days: the stripping for 1 day at 85°C and pH 10 removed 85.5% of ammonia. The first-batch fermentation of methane for 75 days was conducted next, using the ammonia-stripped CM inoculated with methanogenic sludge at different ratios, (CM: thermophilic sludge) of 1:2, 1:1, and 2:1 on volume per volume basis at both 35 and 55°C. Production of methane improved and was higher than that of the control (without stripping of ammonia) but the yield of 20.4 ml g⁻¹ VS was still low, so second stripping of ammonia was conducted, which resulted in 74.7% removal of ammonia. A great improvement in the production of methane of 103.5 ml g⁻¹ VS was achieved during the second batch for 55 days.     

Air-side ammonia stripping coupled to anaerobic digestion indirectly impacts anaerobic microbiome

Air-side stripping without a prior solid–liquid phase separation step is a feasible and promising process to control ammonia concentration in thermophilic digesters. During the process, part of the anaerobic biomass is exposed to high temperature, high pH and aerobic conditions. However, there are no studies assessing the effects of those harsh conditions on the microbial communities of thermophilic digesters. To fill this knowledge gap, the microbiomes of two thermophilic digesters (55°C), fed with a mixture of pig manure and nitrogen-rich co-substrates, were investigated under different organic loading rates (OLR: 1.1–5.2 g COD l⁻¹ day⁻¹), ammonia concentrations (0.2–1.5 g free ammonia nitrogen l⁻¹) and stripping frequencies (3–5 times per week). The bacterial communities were dominated by Firmicutes and Bacteroidetes phyla, while the predominant methanogens were Methanosarcina sp archaea. Increasing co-substrate fraction, OLR and free ammonia nitrogen (FAN) favoured the presence of genera Ruminiclostridium, Clostridium and Tepidimicrobium and of hydrogenotrophic methanogens, mainly Methanoculleus archaea. The data indicated that the use of air-side stripping did not adversely affect thermophilic microbial communities, but indirectly modulated them by controlling FAN concentrations in the digester. These results demonstrate the viability at microbial community level of air side-stream stripping process as an adequate technology for the ammonia control during anaerobic co-digestion of nitrogen-rich substrates.     

Diversity of the resident microbiota in a thermophilic municipal biogas plant

Biogas plants continuously convert biological wastes mainly into a mixture of methane, CO₂ and H₂O—a conversion that is carried out by a consortium of bacteria and archaea. Especially in the municipal plants dedicated towards waste treatment, the reactor feed may vary considerably, exposing the resident microbiota to a changing variety of substrates. To evaluate how and if such changes influence the microbiology, an established biogas plant (6,600 m³, up to 600 m³ biogas per h) was followed over the course of more than 2 years via polymerase chain reaction–denaturing gradient gel electrophoresis of 16S rRNA genes and subsequent sequencing. Both the bacterial and the archaeal community remained stable over the investigation. Of the bacterial consortium, about half of the sequences were in decreasing order of occurrence: Thermoacetogenium sp., Anaerobaculum mobile, Clostridium ultunense, Petrotoga sp., Lactobacillus hammesii, Butyrivibrio sp., Syntrophococcus sucromutans, Olsenella sp., Tepidanaerobacter sp., Sporanaerobacter acetigenes, Pseudoramibacter alactolyticus, Lactobacillus fuchuensis or Lactobacillus sakei, Lactobacillus parabrevis or Lactobacillus spicheri and Enterococcus faecalis. The other half matched closely to ones from similar habitats (thermophilic anaerobic methanogenic digestion). The archaea consisted of Methanobrevibacter sp., Methanoculleus bourgensis, Methanosphaera stadtmanae, Methanimicrococcus blatticola and uncultured Methanomicrobiales. The role of these species in methane production is discussed.     

Analysis of the methane production in thermophilic anaerobic reactors: use of autofluorescence microscopy

Methanogenic activity in thermophilic, anaerobic reactors was determined by comparing the amount of methane generated in single- and two-stage systems with the size of the methanogenic population, as determined by microscopy. The methanogenic activities were 2.71 × 10^–9 ml methane cell^–1 d^–1 and 1.10 × 10^–9 ml methane cell^–1 d^–1 for 10 and 4 days of the hydraulic retention time (HRT), in the single-stage system. In the two-stage system, 7.49 × 10^–9 ml methane cell^–1 d^–1 in the acidogenic reactor and 1.56 × 10^–9 ml methane cell^–1 d^–1 in the methanogenic reactor for 4 days of the HRT. A high correlation was evident between the methane production and methanogenic population [0.1354 ln(x) – 2.1375](R ² 0.8619).     

Microbial communities involved in biogas production from wheat straw as the sole substrate within a two-phase solid-state anaerobic digestion

Microbial communities involved in biogas production from wheat straw as the sole substrate were investigated. Anaerobic digestion was carried out within an up-flow anaerobic solid-state (UASS) reactor connected to an anaerobic filter (AF) by liquor recirculation. Two lab-scale reactor systems were operated simultaneously at 37 °C and 55 °C. The UASS reactors were fed at a fixed organic loading rate of 2.5 g L⁻¹ d⁻¹, based on volatile solids. Molecular genetic analyses of the bacterial and archaeal communities within the UASS reactors (digestate and effluent liquor) and the AFs (biofilm carrier and effluent liquor) were conducted under steady-state conditions. The thermophilic UASS reactor had a considerably higher biogas and methane yield in comparison to the mesophilic UASS, while the mesophilic AF was slightly more productive than the thermophilic AF. When the thermophilic and mesophilic community structures were compared, the thermophilic system was characterized by a higher Firmicutes to Bacteroidetes ratio, as revealed by 16S rRNA gene (rrs) sequence analysis. The composition of the archaeal communities was phase-separated under thermophilic conditions, but rather stage-specific under mesophilic conditions. Family- and order-specific real-time PCR of methanogenic Archaea supported the taxonomic distribution obtained by rrs sequence analysis. The higher anaerobic digestion efficiency of the thermophilic compared to the mesophilic UASS reactor was accompanied by a high abundance of Firmicutes and Methanosarcina sp. in the thermophilic UASS biofilm.     

Aerobic cometabolic degradation of trichloroethene by methane and ammonia oxidizing microorganisms naturally associated with <Emphasis Type="Italic">Carex comosa</Emphasis> roots

The degradation potential of trichloroethene by the aerobic methane- and ammonia-oxidizing microorganisms naturally associated with wetland plant (Carex comosa) roots was examined in this study. In bench-scale microcosm experiments with washed (soil free) Carex comosa roots, the activity of root-associated methane- and ammonia-oxidizing microorganisms, which were naturally present on the root surface and/or embedded within the roots, was investigated. Significant methane and ammonia oxidation were observed reproducibly in batch reactors with washed roots incubated in growth media, where methane oxidation developed faster (2 weeks) compared to ammonia oxidation (4 weeks) in live microcosms. After enrichment, the methane oxidizers demonstrated their ability to degrade 150 μg l⁻¹ TCE effectively at 1.9 mg l⁻¹ of aqueous CH₄. In contrast, ammonia oxidizers showed a rapid and complete inhibition of ammonia oxidation with 150 μg l⁻¹ TCE at 20 mg l⁻¹ of NH₄ ⁺-N, which may be attributed to greater sensitivity of ammonia oxidizers to TCE or its degradation product. No such inhibitory effect of TCE degradation was detected on methane oxidation at the above experimental conditions. The results presented here suggest that microorganisms associated with wetland plant roots can assist in the natural attenuation of TCE in contaminated aquatic environments.     

Illumination enhances methane production from thermophilic anaerobic digestion

Incandescent lamp illumination enhanced methane production from a thermophilic anaerobic digestion reactor (55°C) supplied with glucose. After 10 days of operation, the volume of methane produced from light reactors was approximately 2.5 times higher than that from dark reactors. A comparison of the carbon balance between light and dark conditions showed that methane produced from hydrogen and carbon dioxide in the light reactors was higher than that from the dark reactors. When hydrogen or acetate was fed into the reactors, methane production with added hydrogen was faster and higher under light conditions than under dark conditions. The use of blue light-emitting diodes also enhanced methane production over that under dark conditions. The 16S rRNA gene copy numbers for Methanothermobacter spp. in the light reactor and in the dark reactor were at the same level. The copy number for Methanosarcina spp. in the light reactors was approximately double than that in the dark reactors. These results suggest that blue light enhances the methanogenic activity of hydrogenotrophic methanogens.     

A bioelectrochemical reactor containing carbon fiber textiles enables efficient methane fermentation from garbage slurry

A packed-bed system includes supporting materials to retain microorganisms and a bioelectrochemical system influences the microbial metabolism. In our study, carbon fiber textiles (CFT) as a supporting material was attached onto a carbon working electrode in a bioelectrochemical reactor (BER) that degrades garbage slurry to methane, in order to investigate the effect of combining electrochemical regulation and packing CFT. The potential on the working electrode in the BER containing CFT was set to −1.0 V or −0.8 V (vs. Ag/AgCl). BERs containing CFT exhibited higher methane production, elimination of dichromate chemical oxygen demand, and the ratio of methanogens in the suspended fraction than reactors containing CFT without electrochemical regulation at an organic loading rate (OLR) of 27.8 gCODcr/L/day. In addition, BERs containing CFT exhibited higher reactor performances than BERs without CFT at this OLR. Our results revealed that the new design that combined electrochemical regulation and packing CFT was effective.     

Factors influencing the degradation of garbage in methanogenic bioreactors and impacts on biogas formation

Anaerobic digestion of garbage is attracting much attention because of its application in waste volume reduction and the recovery of biogas for use as an energy source. In this review, various factors influencing the degradation of garbage and the production of biogas are discussed. The surface hydrophobicity and porosity of supporting materials are important factors in retaining microorganisms such as aceticlastic methanogens and in attaining a higher degradation of garbage and a higher production of biogas. Ammonia concentration, changes in environmental parameters such as temperature and pH, and adaptation of microbial community to ammonia have been related to ammonia inhibition. The effects of drawing electrons from the methanogenic community and donating electrons into the methanogenic community on methane production have been shown in microbial fuel cells and bioelectrochemical reactors. The influences of trace elements, phase separation, and co-digestion are also summarized in this review.     

Molecular microbial ecology of stable versus failing rice straw anaerobic digesters

Waste rice straw (RS) is generated in massive quantities around the world and is often burned, creating greenhouse gas and air quality problems. Anaerobic digestion (AD) may be a better option for RS management, but RS is presumed to be comparatively refractory under anaerobic conditions without pre-treatment or co-substrates. However, this presumption assumes frequent reactor feeding regimes but less frequent feeding may be better for RS due to slow hydrolysis rates. Here, we assess how feeding frequency (FF) and organic loading rate (OLR) impacts microbial communities and biogas production in RS AD reactors. Using 16S rDNA amplicon sequencing and bioinformatics, microbial communities from five bench-scale bioreactors were characterized. At low OLR (1.0 g VS l⁻¹ day⁻¹), infrequently fed units (once every 21 days) had higher specific biogas yields than more frequent feeding (five in 7 days), although microbial community diversities were statistically similar (P > 0.05; ANOVA with Tukey comparison). In contrast, an increase in OLR to 2.0 g VS l⁻¹ day⁻¹ significantly changed Archaeal and fermenting Eubacterial sub-communities and the least frequency fed reactors failed. ‘Stable’ reactors were dominated by Methanobacterium, Methanosarcina and diverse Bacteroidetes, whereas ‘failed’ reactors saw shifts towards Clostridia and Christensenellaceae among fermenters and reduced methanogen abundances. Overall, OLR impacted RS AD microbial communities more than FF. However, combining infrequent feeding and lower OLRs may be better for RS AD because of higher specific yields.     

Detection of novel syntrophic acetate‐oxidizing bacteria from biogas processes by continuous acetate enrichment approaches

To enrich syntrophic acetate‐oxidizing bacteria (SAOB), duplicate chemostats were inoculated with sludge from syntrophic acetate oxidation (SAO)‐dominated systems and continuously supplied with acetate (0.4 or 7.5 g l^?1) at high‐ammonia levels. The chemostats were operated under mesophilic (37°C) or thermophilic (52°C) temperature for about six hydraulic retention times (HRT 28 days) and were sampled over time. Irrespective of temperature, a methane content of 64–69% and effluent acetate level of 0.4–1.0 g l^?1 were recorded in chemostats fed high acetate. Low methane production in the low‐acetate chemostats indicated that the substrate supply was below the threshold for methanization of acetate via SAO. Novel representatives within the family Clostridiales and genus Syntrophaceticus (class Clostridia) were identified to represent putative SAOB candidates in mesophilic and thermophilic conditions respectively. Known SAOB persisted at low relative abundance in all chemostats. The hydrogenotrophic methanogens Methanoculleus bourgensis (mesophilic) and Methanothermobacter thermautotrophicus (thermophilic) dominated archaeal communities in the high‐acetate chemostats. In line with the restricted methane production in the low‐acetate chemostats, methanogens persisted at considerably lower abundance in these chemostats. These findings strongly indicate involvement in SAO and tolerance to high ammonia levels of the species identified here, and have implications for understanding community function in stressed anaerobic processes.     

Changing Feeding Regimes To Demonstrate Flexible Biogas Production: Effects on Process Performance,Microbial Community Structure,and Methanogenesis Pathways

Flexible biogas production that adapts biogas output to energy demand can be regulated by changing feeding regimes. In this study, the effect of changes in feeding intervals on process performance, microbial community structure, and the methanogenesis pathway was investigated. Three different feeding regimes (once daily, every second day, and every 2 h) at the same organic loading rate were studied in continuously stirred tank reactors treating distiller''s dried grains with solubles. A larger amount of biogas was produced after feeding in the reactors fed less frequently (once per day and every second day), whereas the amount remained constant in the reactor fed more frequently (every 2 h), indicating the suitability of the former for the flexible production of biogas. Compared to the conventional more frequent feeding regimes, a methane yield that was up to 14% higher and an improved stability of the process against organic overloading were achieved by employing less frequent feeding regimes. The community structures of bacteria and methanogenic archaea were monitored by terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S rRNA and mcrA genes, respectively. The results showed that the composition of the bacterial community varied under the different feeding regimes, and the observed T-RFLP patterns were best explained by the differences in the total ammonia nitrogen concentrations, H₂ levels, and pH values. However, the methanogenic community remained stable under all feeding regimes, with the dominance of the Methanosarcina genus followed by that of the Methanobacterium genus. Stable isotope analysis showed that the average amount of methane produced during each feeding event by acetoclastic and hydrogenotrophic methanogenesis was not influenced by the three different feeding regimes.     

Change in ammonia-oxidizing microorganisms in enriched nitrifying activated sludge

In this study, sludge was taken from a municipal wastewater treatment plant that contained a nearly equal number of archaeal amoA genes (5.70 × 10⁶ ± 3.30 × 10⁵ copies mg sludge⁻¹) to bacterial amoA genes (8.60 × 10⁶ ± 7.64 × 10⁵ copies mg sludge⁻¹) and enriched in three continuous-flow reactors receiving an inorganic medium containing different ammonium concentrations: 2, 10, and 30 mM NH₄⁺–N (28, 140, and 420 mg N l⁻¹). The abundance and communities of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in enriched nitrifying activated sludge (NAS) were monitored at days 60 and 360 of the operation. Early on, between day 0 and day 60 of reactor operation, comparative abundance of AOA amoA genes to AOB amoA genes varied among the reactors depending on the ammonium levels found in the reactors. As compared to the seed sludge, the number of AOA amoA genes was unchanged in the reactor with lower ammonium level (0.06 ± 0.04 mgN l⁻¹), while in the reactors with higher ammonium levels (0.51 ± 0.33 and 0.25 ± 0.10 mgN l⁻¹), the numbers of AOA amoA genes were deteriorated. By day 360, AOA disappeared from the ammonia-oxidizing consortiums in all reactors. The majority of the AOA sequences from all NASs at each sampling period fell into a single AOA cluster, however, suggesting that the ammonium did not affect the AOA communities under this operational condition. This result is contradictory to the case of AOB, where the communities varied significantly among the NASs. AOB with a high affinity for ammonia were present in the reactors with lower ammonium levels, whereas AOB with a low affinity to ammonia existed in the reactors with higher ammonium levels.     

Co-cultivation of Thermoanaerobacter strains with a methanogenic partner enhances glycerol conversion

Glycerol-rich waste streams produced by the biodiesel, bioethanol and oleochemical industries can be treated and valorized by anaerobic microbial communities to produce methane. As current knowledge of the microorganisms involved in thermophilic glycerol conversion to methane is scarce, thermophilic glycerol-degrading methanogenic communities were enriched. A co-culture of Thermoanaerobacter and Methanothermobacter species was obtained, pointing to a non-obligately syntrophic glycerol degradation. This hypothesis was further studied by incubating Thermoanaerobacter brockii subsp. finnii and T. wiegelii with glycerol (10 mM) in pure culture and with different hydrogenotrophic methanogens. The presence of the methanogen accelerated glycerol fermentation by the two Thermoanaerobacter strains up to 3.3 mM day⁻¹, corresponding to 12 times higher volumetric glycerol depletion rates in the methanogenic co-cultures than in the pure bacterial cultures. The catabolic pathways of glycerol conversion were identified by genome analysis of the two Thermoanaerobacter strains. NADH and reduced ferredoxin formed in the pathway are linked to proton reduction, which becomes thermodynamically favourable when the hydrogen partial pressure is kept low by the hydrogenotrophic methanogenic partner.     

Methanogenic archaea are globally ubiquitous in aerated soils and become active under wet anoxic conditions

The prototypical representatives of the Euryarchaeota—the methanogens—are oxygen sensitive and are thought to occur only in highly reduced, anoxic environments. However, we found methanogens of the genera Methanosarcina and Methanocella to be present in many types of upland soils (including dryland soils) sampled globally. These methanogens could be readily activated by incubating the soils as slurry under anoxic conditions, as seen by rapid methane production within a few weeks, without any additional carbon source. Analysis of the archaeal 16S ribosomal RNA gene community profile in the incubated samples through terminal restriction fragment length polymorphism and quantification through quantitative PCR indicated dominance of Methanosarcina, whose gene copy numbers also correlated with methane production rates. Analysis of the δ¹³C of the methane further supported this, as the dominant methanogenic pathway was in most cases aceticlastic, which Methanocella cannot perform. Sequences of the key methanogenic enzyme methyl coenzyme M reductase retrieved from the soil samples before incubation confirmed that Methanosarcina and Methanocella are the dominant methanogens, though some sequences of Methanobrevibacter and Methanobacterium were also detected. The global occurrence of only two active methanogenic archaea supports the hypothesis that these are autochthonous members of the upland soil biome and are well adapted to their environment.     

Substrate Specificity of Methanogenic Communities from Lake Baikal Bottom Sediments Associated with Hydrocarbon Gas Discharge

Methane production by microbial communities from Lake Baikal bottom sediments with different chemical composition of pore water was studied. Methane production was more active in the media supplemented with H₂: CO₂ and H₂ + CH₃COONa, rather than on media with acetate as the sole source of carbon and energy. Addition of methanol stimulated methane production only in the case of microbial communities from upper silts. Ability of the communities to produce methane correlated reliably with the concentrations of the NO^3–, SO₄^2?, Cl^–, and CH₃COO^– ions in the pore water of the relevant sediments. Cultivation of communities from the mud volcano sediments resulted in development of methanogenic archaea of the family Methanocellaсеае in the media supplemented with H₂: CO₂ and H₂ + CH₃COONa, while methanogenic archaea in the communities cultivated without additional substrates belonged to the genera Methanoregula, Methanobacterium, and Methanosaeta.     

Production of surfactin and fengycin by Bacillus subtilis in a bubbleless membrane bioreactor

Surfactin and fengycin are lipopeptide biosurfactants produced by Bacillus subtilis. This work describes for the first time the use of bubbleless bioreactors for the production of these lipopeptides by B. subtilis ATCC 21332 with aeration by a hollow fiber membrane air–liquid contactor to prevent foam formation. Three different configurations were tested: external aeration module made from either polyethersulfone (reactor BB1) or polypropylene (reactor BB2) and a submerged module in polypropylene (reactor BB3). Bacterial growth, glucose consumption, lipopeptide production, and oxygen uptake rate were monitored during the culture in the bioreactors. For all the tested membranes, the bioreactors were of satisfactory bacterial growth and lipopeptide production. In the three configurations, surfactin production related to the culture volume was in the same range: 242, 230, and 188 mg l⁻¹ for BB1, BB2, and BB3, respectively. Interestingly, high differences were observed for fengycin production: 47 mg l⁻¹ for BB1, 207 mg l⁻¹ for BB2, and 393 mg l⁻¹ for BB3. A significant proportion of surfactin was adsorbed on the membranes and reduced the volumetric oxygen mass transfer coefficient. The degree of adsorption depended on both the material and the structure of the membrane and was higher with the submerged polypropylene membrane.     

Methanogenesis in thermophilic biogas reactors

Methanogenesis in thermophilic biogas reactors fed with different wastes is examined. The specific methanogenic activity with acetate or hydrogen as substrate reflected the organic loading of the specific reactor examined. Increasing the loading of thermophilic reactors stabilized the process as indicated by a lower concentration of volatile fatty acids in the effluent from the reactors. The specific methanogenic activity in a thermophilic pilot-plant biogas reactor fed with a mixture of cow and pig manure reflected the stability of the reactor. The numbers of methanogens counted by the most probable number (MPN) technique with acetate or hydrogen as substrate were further found to vary depending on the loading rate and the stability of the reactor. The numbers of methanogens counted with antibody probes in one of the reactor samples was 10 times lower for the hydrogen-utilizing methanogens compared to the counts using the MPN technique, indicating that other non-reacting methanogens were present. Methanogens that reacted with the probe againstMethanobacterium thermoautotrophicum were the most numerous in this reactor. For the acetate-utilizing methanogens, the numbers counted with the antibody probes were more than a factor of 10 higher than the numbers found by MPN. The majority of acetate utilizing methanogens in the reactor wereMethanosarcina spp. single cells, which is a difficult form of the organism to cultivatein vitro. No reactions were observed with antibody probes raised againstMethanothrix soehngenii orMethanothrix CALS-1 in any of the thermophilic biogas reactors examined. Studies using 2-¹⁴C-labeled acetate showed that at high concentrations (more than approx. 1 mM) acetate was metabolized via the aceticlastic pathway, transforming the methyl-group of acetate into methane. When the concentration of acetate was less than approx. 1 mM, most of the acetate was oxidized via a two-step mechanism (syntrophic acetate oxidation) involving one organism oxidizing acetate into hydrogen and carbon dioxide and a hydrogen-utilizing methanogen forming the products of the first microorganism into methane. In thermophilic biogas reactors, acetate oxidizing cultures occupied the niche ofMethanothrix species, aceticlastic methanogens which dominate at low acetate concentrations in mesophilic systems. Normally, thermophilic biogas reactors are operated at temperatures from 52 to 56° C. Experiments using biogas reactors fed with cow manure showed that the same biogas yield found at 55° C could be obtained at 61° C after a long adaptation period. However, propionate degradation was inhibited by increasing the temperature.     

Mesophilic and thermophilic anaerobic digestion of source-sorted organic wastes: effect of ammonia on glucose degradation and methane production

The wet organic fraction of household wastes was digested anaerobically at 37 °C and 55 °C. At both temperatures the volatile solids loading was increased from 1 g l⁻¹ day⁻¹ to 9.65 g l⁻¹ day⁻¹, by reducing the nominal hydraulic retention time from 93 days to 19 days. The volatile solids removal in the reactors at both temperatures for the same loading rates was in a similar range and was still 65% at 19 days hydraulic retention time. Although more biogas was produced in the thermophilic reactor, the energy conservation in methane was slightly lower, because of a lower methane content, compared to the biogas of the mesophilic reactor. The slightly lower amount of energy conserved in the methane of the thermophilic digester was presumably balanced by the hydrogen that escaped into the gas phase and thus was no longer available for methanogenesis. In the thermophilic process, 1.4 g/l ammonia was released, whereas in the mesophilic process only 1 g/l ammonia was generated, presumably from protein degradation. Inhibition studies of methane production and glucose fermentation revealed a K _i (50%) of 3 g/l and 3.7 g/l ammonia (equivalent to 0.22 g/l and 0.28 g/l free NH₃) at 37 °C and a K _i (50%) of 3.5 g/l and 3.4 g/l ammonia (equivalent to 0.69 g/l and 0.68 g/l free NH₃) at 55 °C. This indicated that the thermophilic flora tolerated at least twice as much of free NH₃ than the mesophilic flora and, furthermore, that the thermophilic flora was able to degrade more protein. The apparent ammonia concentrations in the mesophilic and in the thermophilic biowaste reactor were low enough not to inhibit glucose fermentation and methane production of either process significantly, but may have been high enough to inhibit protein degradation. The data indicated either that the mesophilic and thermophilic protein degraders revealed a different sensitivity towards free ammonia or that the mesophilic population contained less versatile protein degraders, leaving more protein undegraded. Received: 26 March 1997 / Received revision: 13 May 1997 / Accepted: 19 May 1997