Anna Schnürer

Mesophilic syntrophic acetate oxidation during methane formation by a triculture at high ammonium concentration

In a mesophilic (37°C) triculture at a high ammonium concentration and pH8, methanogenesis from acetate occurred via syntrophic acetate oxidation. Studies with 14C-labelled substrates showed that the amount of labelled methane formed from 1-14C-labelled acetate was equal to that formed from 2-14C-labelled acetate. Labelled methane was also formed from H14CO3-. These results clearly showed that both the methyl and carboxyl groups of acetate were oxidized to CO2 and that CO2 was reduced to CH4 through hydrogenotrophic methanogenesis. During growth of the triculture, a significant isotopic exchange between the carboxyl group of acetate and bicarbonate occurred. As a result, there was a decrease in the specific activity of 1-14C-acetate, and the production of 14CO2 was slightly higher from 1-14C- than from 2-14C-acetate. For each mole acetate degraded, 0.94 mol methane was formed; 9.2 mmol acetate was metabolized during the 294 days of incubation.

Improved bio-energy yields via sequential ethanol fermentation and biogas digestion of steam exploded oat straw.

Using standard laboratory equipment, thermochemically pretreated oat straw was enzymatically saccharified and fermented to ethanol, and after removal of ethanol the remaining material was subjected to biogas digestion. A detailed mass balance calculation shows that, for steam explosion pretreatment, this combined ethanol fermentation and biogas digestion converts 85-87% of the higher heating value (HHV) of holocellulose (cellulose and hemicellulose) in the oat straw into biofuel energy. The energy (HHV) yield of the produced ethanol and methane was 9.5-9.8 MJ/(kg dry oat straw), which is 28-34% higher than direct biogas digestion that yielded 7.3-7.4 MJ/(kg dry oat straw). The rate of biogas formation from the fermentation residues was also higher than from the corresponding pretreated but unfermented oat straw, indicating that the biogas digestion could be terminated after only 24 days. This suggests that the ethanol process acts as an additional pretreatment for the biogas process.

Impact of trace element addition on degradation efficiency of volatile fatty acids, oleic acid and phenyl acetate and on microbial populations in a biogas digester.

The effect of trace element addition on anaerobic digestion of food industry- and household waste was studied using two semi-continuous lab-scale reactors, one (R30+) was supplied with Fe, Co and Ni, while the other (R30) acted as a control. Tracer analysis illustrated that methane production from acetate proceeded through syntrophic acetate oxidation (SAO) in both digesters. The effect of the trace elements was also evaluated in batch assays to determine the capacity of the microorganisms of the two digesters to degrade acetate, phenyl acetate, oleic acid or propionate, butyrate and valerate provided as a cocktail. The trace elements addition improved the performance of the process giving higher methane yields during start-up and early operation and lower levels of mainly acetate and propionate in the R30+ reactor. The batch assay showed that material from R30+ gave effects on methane production from all substrates tested. Phenyl acetate was observed to inhibit methane formation in the R30 but not in the R30+ assay. A real-time PCR analysis targeting methanogens on the order level as well as three SAO bacteria showed an increase in Methanosarcinales in the R30+ reactor over time, even though SAO continuously was the dominating pathway for methane production. Possibly, this increase explains the low VFA-levels and higher degradation rates observed in the R30+ batch incubations. These results show that the added trace elements affected the ability of the microflora to degrade VFAs as well as oleic acid and phenyl acetate in a community, where acetate utilization is dominated by SAO.

Conversion of phenols during anaerobic digestion of organic solid waste – A review of important microorganisms and impact of temperature

During anaerobic digestion of organic waste, both energy-rich biogas and a nutrient-rich digestate are produced. The digestate can be used as a fertiliser in agricultural soils if the levels of hazardous compounds and pathogens are low. This article reviews the main findings about phenols in anaerobic digestion processes degrading organic solid wastes, and examines the effect of process temperature on the anaerobic degradation of phenols, the microbial community and the quality of the digestate. The degradation efficiency of a number of different phenols has been shown to be correlated to the process temperature. Higher degradation efficiency is observed at mesophilic process temperature than at thermophilic temperature. Possible explanations for this variation in the degradation of phenols include differences in diversity, particularly of the phenol-degrading bacteria, and/or the presence of temperature-sensitive enzymes. Chemical analysis of digestate from bioreactors operating at thermophilic temperature detected a higher content of phenols compared to mesophilic bioreactors, verifying the degradation results. Digestate with the highest phenol content has the greatest negative impact on soil microbial activity.

Syntrophic acetate oxidation in industrial CSTR biogas digesters

The extent of syntrophic acetate oxidation (SAO) and the levels of known SAO bacteria and acetate- and hydrogen-consuming methanogens were determined in sludge from 13 commercial biogas production plants. Results from these measurements were statistically related to the prevailing operating conditions, through partial least squares (PLS) analysis. This revealed that high abundance of microorganisms involved in SAO was positively correlated with relatively low abundance of aceticlastic methanogens and high concentrations of free ammonia (>160 mg/L) and volatile fatty acids (VFA). Temperature was identified as another influencing factor for the population structure of the syntrophic acetate oxidising bacteria (SAOB). Overall, there was a high abundance of SAOB in the different digesters despite differences in their operating parameters, indicating that SAOB are an enduring and important component of biogas-producing consortia.

Characterization of microbial community structure during continuous anaerobic digestion of straw and cow manure

Responses of bacterial and archaeal communities to the addition of straw during anaerobic digestion of manure at different temperatures (37°C, 44°C and 52°C) were investigated using five laboratory‐scale semi‐continuous stirred tank reactors. The results revealed that including straw as co‐substrate decreased the species richness for bacteria, whereas increasing the operating temperature decreased the species richness for both archaea and bacteria, and also the evenness of the bacteria. Taxonomic classifications of the archaeal community showed that Methanobrevibacter dominated in the manure samples, while Methanosarcina dominated in all digesters regardless of substrate. Increase of the operating temperature to 52°C led to increased relative abundance of Methanoculleus and Methanobacterium. Among the bacteria, the phyla Firmicutes and Bacteroidetes dominated within all samples. Compared with manure itself, digestion of manure resulted in a higher abundance of an uncultured class WWE1 and lower abundance of Bacilli. Adding straw to the digesters increased the level of Bacteroidia, while increasing the operating temperature decreased the level of this class and instead increased the relative abundance of an uncultured genus affiliated to order MBA08 (Clostridia). A considerable fraction of bacterial sequences could not be allocated to genus level, indicating that novel phylotypes are resident in these communities.

Improved biogas production from whole stillage by co-digestion with cattle manure.

Whole stillage, as sole substrate or co-digested with cattle manure, was evaluated as substrate for biogas production in five mesophilic laboratory-scale biogas reactors, operating semi-continuously for 640 days. The process performance was monitored by chemical parameters and by quantitative analysis of the methanogenic and acetogenic population. With whole stillage as sole substrate the process showed clear signs of instability after 120 days of operation. However, co-digestion with manure clearly improved biogas productivity and process stability and indicated increased methane yield compared with theoretical values. The methane yield at an organic loading rate (OLR) at 2.8 g VS/(L×day) and a hydraulic retention time (HRT) of 45 days with a substrate mixture 85% whole stillage and 15% manure (based on volatile solids [VS]) was 0.31 N L CH(4)/gVS. Surprisingly, the abundance of the methanogenic and acetogenic populations remained relatively stable throughout the whole operation and was not influenced by process performance.

Methanogenic Population and CH4 Production in Swedish Dairy Cows Fed Different Levels of Forage

ABSTRACT Methanogenic community structure, methane production (CH4), and volatile fatty acid (VFA) profiles were investigated in Swedish dairy cows fed a diet with a forage/concentrate ratio of 500/500 or 900/100 g/kg of dry matter (DM) of total DM intake (DMI). The rumen methanogenic population was evaluated using terminal restriction fragment length polymorphism (T-RFLP) analysis, 16S rRNA gene libraries, and quantitative real-time PCR (qRT-PCR). Mean CH4 yields did not differ (P > 0.05) between diets, being 16.9 and 20.2 g/kg DMI for the 500/500 and 900/100 diets, respectively. The T-RFLP analysis revealed that populations differed between individual cows and that each individual population responded differently to the diets. The 16S rRNA gene libraries revealed that Methanobrevibacter spp. dominated for both diets. CH4 production was positively correlated with a dominance of sequences representing T-RFs related to Methanobrevibacter thaueri, Methanobrevibacter millerae, and Methanobrevibacter smithii relative to Methanobrevibacter ruminantium and Methanobrevibacter olleyae. Total numbers of methanogens and total numbers of Methanobacteriales were significantly higher with the 500/500 diet (P < 0.0004 and P < 0.002, respectively). However, no relationship was found between CH4 production and total number of methanogens. No differences were seen in total VFA, propionic acid, or acetic acid contents, but the molar proportion of butyric acid in the rumen was higher for the 500/500 diet than for the 900/100 diet (P < 0.05). Interestingly, the results also revealed that a division of the identified methanogenic species into two groups, suggested in the work of King et al. (E. E. King, R. P. Smith, B. St-Pierre, and A. D. G. Wright, Appl. Environ. Microbiol. 77:5682–5687, 2011), increased the understanding of the variation in CH4 production between different cows.

In situ ammonia production as a sanitation agent during anaerobic digestion at mesophilic temperature

Aim:  To measure the sanitizing effect of mesophilic (37°C) anaerobic digestion in high ammonia concentrations produced in situ.

Comparison of operating strategies for increased biogas production from thin stillage.

The effect of increasing organic loading rate (OLR) and simultaneously decreasing hydraulic retention time (HRT) during anaerobic digestion of sulphur- and nitrogen-rich thin stillage was investigated during operation of continuously stirred tank laboratory reactors at two different temperatures. The operating strategies and substrate were set in order to mimic an existing full-scale commercial biogas plant in Sweden. The reactors were operated for 554-570 days with a substrate mixture of thin stillage and milled grain, resulting in high ammonium concentrations (>4.5gL(-1)). Initially, one reactor was operated at 38°C, as in the full-scale plant, while in the experimental reactor the temperature was raised to 44°C. Both reactors were then subjected to increasing OLR (from 3.2 to 6.0gVSL(-1)d(-1)) and simultaneously decreasing HRT (from 45 to 24 days) to evaluate the effects of these operational strategies on process stability, hydrogen sulphide levels and microbial composition. The results showed that operation at 44°C was the most successful strategy, resulting in up to 22% higher methane yield compared with the mesophilic reactor, despite higher free ammonia concentration. Furthermore, kinetic studies revealed higher biogas production rate at 44°C compared with 38°C, while the level of hydrogen sulphide was not affected. Quantitative PCR analysis of the microbiological population showed that methanogenic archaea and syntrophic acetate-oxidising bacteria had responded to the new process temperature while sulphate-reducing bacteria were only marginally affected by the temperature-change.