Effects of free long-chain fatty acids on thermophilic anaerobic digestion

Summary Low concentrations of the long-chain fatty acids oleate and stearate inhibited all steps of the anaerobic thermophilic biogas process during digestion of cattle manure. The lag phase increased when the concentrations of oleate and stearate were 0.2 g/l and 0.5 g/l, respectively, and no growth was found at concentrations of 0.5 g/l for oleate and 1.0 g/l for stearate. The toxic effect of these acids was permanent as growth did not occur when inhibited cultures were diluted to a non-inhibitory concentration. No adaptation to the fatty acids toxicity was observed by pre-exposing the cultures to non-inhibitory concentrations and the inhibitory response was the same as for cultures not pre-exposed to the fatty acids. Oleate was less inhibitory when added as a neutral oil in the form of the glycerol ester. This indicates that it is the free fatty acid that influences the bacterial activity. Correspondence to: B. K. Ahring     

Screening Escherichia coli, Enterococcus faecalis, and Clostridium perfringens as Indicator Organisms in Evaluating Pathogen-Reducing Capacity in Biogas Plants

This study was conducted to identify an indicator organism(s) in evaluating the pathogen-reducing capacity of biogas plants. Fresh cow manure containing 10⁴ to 10⁵ colony forming unit (CFU) per milliliter of Escherichia coli and Enterococcus faecalis along with an inoculated Clostridium perfringens strain were exposed to 37°C for 15 days, 55°C for 48 h, and 70°C for 24 h. C. perfringens was the most heat-resistant organism followed by E. faecalis, while E. coli was the most heat-sensitive organism. E. coli was reduced below detection limit at all temperatures with log₁₀ reductions of 4.94 (10 s), 4.37 (40 min), and 2.6 (5 days) at 70°C, 55°C, and 37°C, respectively. Maximum log₁₀ reductions for E. faecalis were 1.77 at 70°C (1 day), 1.7 at 55°C (2 days) and 3.13 at 37°C (15 days). For C. perfringens, maximum log₁₀ reduction at 37°C was 1.35 log₁₀ units (15 days) compared to less than 1 unit at 55 and 70°C. Modeling results showed that E. faecalis and C. perfringens had higher amount of heat-resistant fraction than E. coli. Thus, E. faecalis and C. perfringens can be used as indicator organisms to evaluate pathogen-reducing capacity in biogas plants at high temperatures of 55°C and 70°C while at 37°C E. coli could also be included as indicator organism.     

Acetate oxidation is the dominant methanogenic pathway from acetate in the absence of Methanosaetaceae

The oxidation of acetate to hydrogen, and the subsequent conversion of hydrogen and carbon dioxide to methane, has been regarded largely as a niche mechanism occurring at high temperatures or under inhibitory conditions. In this study, 13 anaerobic reactors and sediment from a temperate anaerobic lake were surveyed for their dominant methanogenic population by using fluorescent in situ hybridization and for the degree of acetate oxidation relative to aceticlastic conversion by using radiolabeled [2-14C]acetate in batch incubations. When Methanosaetaceae were not present, acetate oxidation was the dominant methanogenic pathway. Aceticlastic conversion was observed only in the presence of Methanosaetaceae.     

Biogas production from potato-juice, a by-product from potato-starch processing, in upflow anaerobic sludge blanket (UASB) and expanded granular sludge bed (EGSB) reactors

In this study, the utilization of potato-juice, the organic by-product from potato-starch processing, for biogas production was investigated in batch assay and in high rate anaerobic reactors. The maximum methane potential of the potato-juice determined by batch assay was 470 mL-CH₄/gVS-added. Anaerobic digestion of potato-juice in an EGSB reactor could obtain a methane yield of 380 mL-CH₄/gVS-added at the organic loading rate of 3.2 gCOD/(L-reactor.d). In a UASB reactor, higher organic loading rate of 5.1 gCOD/(L-reactor.d) could be tolerated, however, it resulted in a lower methane yield of 240 mL-CH₄/gVS-added. The treatment of reactor effluent was also investigated. By acidification with sulfuric acid to pH lower than 5, almost 100% of the ammonia content in the effluent could be retained during the successive up-concentration process step. The reactor effluent could be up-concentrated by evaporation to minimize its volume, and later be utilized as fertilizer.     

Long-term effect of inoculum pretreatment on fermentative hydrogen production by repeated batch cultivations: homoacetogenesis and methanogenesis as competitors to hydrogen production

Long-term effects of inoculum pretreatments (heat, acid, loading-shock) on hydrogen production from glucose under different temperatures (37 °C, 55 °C) and initial pH (7 and 5.5) were studied by repeated batch cultivations. Results obtained showed that it was necessary to investigate the long-term effect of inoculum pretreatment on hydrogen production since pretreatments may just temporarily inhibit the hydrogen consuming processes. After long-term cultivation, pretreated inocula did not enhance hydrogen production compared to untreated inocula under mesophilic conditions (initial pH 7 and pH 5.5) and thermophilic conditions (initial pH 7). However, pretreatment could inhibit lactate production and lead to higher hydrogen yield under thermophilic conditions at initial pH 5.5. The results further demonstrated that inoculum pretreatment could not permanently inhibit either methanogenesis or homoacetogenesis, and methanogenesis and homoacetogenesis could only be inhibited by proper control of fermentation pH and temperature. Methanogenic activity could be inhibited at pH lower than 6, both under mesophilic and thermophilic conditions, while homoacetogenic activity could only be inhibited under thermophilic condition at initial pH 5.5. Microbial community analysis showed that pretreatment did not affect the dominant bacteria. The dominant bacteria were Clostridium butyricum related organisms under mesophilic condition (initial pH 7 and 5.5), Thermoanaerobacterium sp. related organisms under thermophilic condition (initial pH 7), and Thermoanaerobacterium thermosaccharolyticum related organisms under thermophilic condition (initial pH 5.5). Results from this study clearly indicated that the long-term effects of inoculum pretreatments on hydrogen production, methanogenesis, homoacetogenesis and dominant bacteria were dependent on fermentation temperature and pH.     

Effect of temperature and active biogas process on passive separation of digested manure

The objective of the study was to identify the optimum time interval for effluent removal after temporarily stopping stirring in otherwise continuously stirred tank reactors. Influence of temperature (10 and 55 degrees C) and active biogas process on passive separation of digested manure, where no outside mechanical or chemical action was used, within the reactor was studied in three vertical settling columns (100 cm deep). Variations in solids and microbial distribution at top, middle and bottom layers of column were assessed over a 15 day settling period. Results showed that best solids separation was achieved when digested manure was allowed to settle at 55 degrees C with active biogas process (pre-incubated at 55 degrees C) compared to separation at 55 degrees C without active biogas process (autoclaved at 120 degrees C, for 20 min) or at 10 degrees C with active biogas process. Maximum solids separation was noticed 24h after settling in column incubated at 55 degrees C, with active biogas process. Microbiological analyses revealed that proportion of Archaea and Bacteria, absent in the autoclaved material, varied with incubation temperature, time and sampling depth. Short rod shaped bacteria dominated at 55 degrees C, while long rod shaped bacteria dominated at 10 degrees C. Methanosarcinaceae were seen more abundant in the surface layer at 55 degrees C while it was seen more common in the top and bottom layers at 10 degrees C. Thus, passive separation of digester contents within the reactor can be used effectively as an operating strategy to optimize biogas production by increasing the solids and biomass retention times. A minimum of 1-2h "non-stirring" period appears to be optimal time before effluent removal in plants where extraction is batch-wise 2-4 times a day.     

Anaerobic biodegradation of spent sulphite liquor in a UASB reactor

Anaerobic biodegradation of fermented spent sulphite liquor, SSL, which is produced during the manufacture of sulphite pulp, was investigated. SSL contains a high concentration of lignin products in addition to hemicellulose and has a very high COD load (173 g COD l(-1)). Batch experiments with diluted SSL and pretreated SSL indicated a potential of 12-22 l methane per litre SSL, which corresponds to 0.13-0.22 l methane (g VS)(-1) and COD removal of up to 37%. COD removal in a mesophilic upflow anaerobic sludge blanket, UASB. reactor ranged from 10% to 31% at an organic loading rate, OLR, of 10-51 g (1 d)(-1) and hydraulic retention time from 3.7 to 1.5 days. The biogas productivity was 3 1 (l(reactor d)(-1), with a yield of 0.05 l gas (g VS)(-1). These results suggest that anaerobic digestion in UASB reactors may provide a new alternative for the treatment of SSL to other treatment strategies such as incineration. Although the total COD reduction achieved is limited, bioenergy is produced and readily biodegradable matter is removed causing less load on post-treatment installations.