Dynamic bioreactor operation: effects of packing material and mite predation on toluene removal from off-gas

Recent studies have focused on using vapor-phase bioreactors for the treatment of volatile organic compounds from contaminated air streams. Although high removal capacities have been achieved in many studies, long-term operation is often unstable at high pollutant loadings due to biomass accumulation and drying of the packing medium. In this study, three bench-scale bioreactors were operated to determine the effect of packing material and fungal predation on toluene removal efficiency and pressure drop. Toluene elimination capacities (mass toluene removed per unit packing per unit time) above 100 g m(-3) h(-1) were obtained in the fungal bioreactors packed with light-weight, artificial medium, and submersion of the packing in mineral medium once per week was found to provide sufficient moisture and nutrients to the biofilm. The use of mites as fungal predators improved performance by increasing the overall mineralization of toluene to CO(2), and by dislodging biomass along the bioreactor.     

Degradation and Fate of Carbon Tetrachloride in Unadapted Methanogenic Granular Sludge

The potential of granular sludge from upflow anaerobic sludge blanket (UASB) reactors for bioremediation of chlorinated pollutants was evaluated by using carbon tetrachloride (CT) as a model compound. Granular sludges cultivated in UASB reactors on methanol, a volatile fatty acid mixture, or sucrose readily degraded CT supplied at a concentration of 1,500 nmol/batch (approximately 10 μM) without any prior exposure to organohalogens. The maximum degradation rate was 1.9 μmol of CT g of volatile suspended solids⁻¹ day⁻¹. The main end products of CT degradation were CO₂ and Cl⁻, and the yields of these end products were 44 and 68%, respectively, of the initial amounts of [¹⁴C]CT and CT-Cl. Lower chlorinated methanes accumulated in minor amounts temporarily. Autoclaved (dead) sludges were capable of degrading CT at rates two- to threefold lower than those for living sludges, indicating that abiotic processes (mediated by cofactors or other sludge components) played an important role in the degradation observed. Reduced components in the autoclaved sludge were vital for CT degradation. A major part (51%) of the CT was converted abiotically to CS₂. The amount of CO₂ produced (23%) was lower and the amount of Cl⁻ produced (86%) was slightly higher with autoclaved sludge than with living sludge. Both living and autoclaved sludges could degrade chloroform. However, only living sludge degraded dichloromethane and methylchloride. These results indicate that reductive dehalogenation, which was mediated better by living sludge than by autoclaved sludge, is only a minor pathway for CT degradation. The main pathway involves substitutive and oxidative dechlorination reactions that lead to the formation of CO₂. Granular sludge, therefore, has outstanding potential for gratuitous dechlorination of CT to safe end products.     

Isolation and characterization of a new CO-utilizing strain, Thermoanaerobacter thermohydrosulfuricus subsp. carboxydovorans, isolated from a geothermal spring in Turkey

A novel anaerobic, thermophilic, Gram-positive, spore-forming, and sugar-fermenting bacterium (strain TLO) was isolated from a geothermal spring in Ayaş, Turkey. The cells were straight to curved rods, 0.4–0.6 μm in diameter and 3.5–10 μm in length. Spores were terminal and round. The temperature range for growth was 40–80°C, with an optimum at 70°C. The pH optimum was between 6.3 and 6.8. Strain TLO has the capability to ferment a wide variety of mono-, di-, and polysaccharides and proteinaceous substrates, producing mainly lactate, next to acetate, ethanol, alanine, H₂, and CO₂. Remarkably, the bacterium was able to grow in an atmosphere of up to 25% of CO as sole electron donor. CO oxidation was coupled to H₂ and CO₂ formation. The G + C content of the genomic DNA was 35.1 mol%. Based on 16S rRNA gene sequence analysis and the DNA–DNA hybridization data, this bacterium is most closely related to Thermoanaerobacter thermohydrosulfuricus and Thermoanaerobacter siderophilus (99% similarity for both). However, strain TLO differs from Thermoanaerobacter thermohydrosulfuricus in important aspects, such as CO-utilization and lipid composition. These differences led us to propose that strain TLO represents a subspecies of Thermoanaerobacter thermohydrosulfuricus, and we therefore name it Thermoanaerobacter thermohydrosulfuricus subsp. carboxydovorans.     

Exocellular electron transfer in anaerobic microbial communities

Exocellular electron transfer plays an important role in anaerobic microbial communities that degrade organic matter. Interspecies hydrogen transfer between microorganisms is the driving force for complete biodegradation in methanogenic environments. Many organic compounds are degraded by obligatory syntrophic consortia of proton-reducing acetogenic bacteria and hydrogen-consuming methanogenic archaea. Anaerobic microorganisms that use insoluble electron acceptors for growth, such as iron- and manganese-oxide as well as inert graphite electrodes in microbial fuel cells, also transfer electrons exocellularly. Soluble compounds, like humic substances, quinones, phenazines and riboflavin, can function as exocellular electron mediators enhancing this type of anaerobic respiration. However, direct electron transfer by cell-cell contact is important as well. This review addresses the mechanisms of exocellular electron transfer in anaerobic microbial communities. There are fundamental differences but also similarities between electron transfer to another microorganism or to an insoluble electron acceptor. The physical separation of the electron donor and electron acceptor metabolism allows energy conservation in compounds as methane and hydrogen or as electricity. Furthermore, this separation is essential in the donation or acceptance of electrons in some environmental technological processes, e.g. soil remediation, wastewater purification and corrosion.     

Gratuitous dechlorination of chloroethanes by methanogenic granular sludge

The dechlorinating activity of a methanogenic granular sludge from a methanol-fed upflow anaerobic sludge blanket reactor was investigated with chlorinated ethanes. This unadapted methanogenic consortium degraded all chloroethanes tested. The product formation rates decreased with the number of chlorine substituents. The more highly chlorinated ethanes were also converted, although at a lower rate, in the presence of autoclaved (dead) sludge, indicating the involvement of reduced heat-stable cofactors like vitamin B₁₂ and F₄₃₀. Direct chemical dechlorination of hexa-, penta- and tetrachloroethanes was also observed in medium without sludge, although at a much lower rate. The results show the importance of cometabolic and abiotic (chemical) conversions for the transformation of chlorinated ethanes by the methanogenic consortium. The types of reaction and the products formed were correlated with the Gibbs free-energy change (ΔG ^0′). Reductive hydrogenolysis and dichloroelimination were important dechlorinating mechanisms. Generally, these reactions have a higher ΔG ^0′ value than dehydrochlorination reactions, which occurred less frequently during the transformation of chloroethanes by the methanogenic granular sludge. Received: 8 June 1998 / Received revision: 7 September 1998 / Accepted: 13 September 1998     

Concurrent hexachlorobenzene and chloroethene transformation by endogenous dechlorinating microorganisms in the Ebro River sediment

The ability of Dehalococcoides spp. to reduce chlorinated compounds offers a great potential for bioremediation and/or bioaugmentation of contaminated environments. So far, however, our knowledge of the activity of Dehalococcoides spp. in situ is limited to only a few subsurface environments. The aim of this study was to broaden this knowledge to other environments, and we investigated the role of Dehalococcoides spp. in the transformation of chlorinated benzenes and chlorinated ethenes in the Ebro River (Spain) sediments. Lab-scale batch microcosms were used to follow the growth and abundance of Dehalococcoides spp. during the transformation of selected chlorinated compounds. We applied biomolecular tools targeting the 16S rRNA, the 16S rRNA gene and several functional genes involved in dechlorination in combination with chemical measurements. The growth of Dehalococcoides spp. and the differential expression of several reductive dehalogenase genes during the dechlorination process could be demonstrated. Furthermore, 16S rRNA gene-based clone libraries of dechlorinating river sediment showed a complex community structure and indicated the involvement of several additional bacterial genera in the transformation process, underlining the remarkable potential of this rivers' sediment to transform different halo-organic pollutants.     

Degradation of BTEX by anaerobic bacteria: physiology and application

Pollution of the environment with aromatic hydrocarbons, such as benzene, toluene, ethylbenzene and xylene (so-called BTEX) is often observed. The cleanup of these toxic compounds has gained much attention in the last decades. In situ bioremediation of aromatic hydrocarbons contaminated soils and groundwater by naturally occurring microorganisms or microorganisms that are introduced is possible. Anaerobic bioremediation is an attractive technology as these compounds are often present in the anoxic zones of the environment. The bottleneck in the application of anaerobic techniques is the lack of knowledge about the anaerobic biodegradation of benzene and the bacteria involved in anaerobic benzene degradation. Here, we review the existing knowledge on the degradation of benzene and other aromatic hydrocarbons by anaerobic bacteria, in particular the physiology and application, including results on the (per)chlorate stimulated degradation of these compounds, which is an interesting new alternative option for bioremediation.     

Constitutive dechlorination of chlorinated ethenes by a methanol degrading methanogenic consortium

The ability of granular methanogenic sludge to dechlorinate chloroethenes was investigated with unadapted sludge from an upflow anaerobic sludge blanket (UASB) reactor fed with methanol. The sludge degraded chlorinated ethenes, but the degradation rates were low. The addition of primary substrate was necessary to sustain dechlorination. The dechlorinating activity seemed to be constitutively present in the anaerobic bacteria. Usually, one chlorine atom was removed via reductive hydrogenolysis. Only trichloroethene (TCE) was converted to substantial amounts of vinylchloride (VC). 1,1-Dichloroethene (1,1DCE) was observed to be an important intermediate in the dechlorination by unadapted granular sludge, although previously this compound had not been commonly observed. Furthermore, the dechlorination of 1,1DCE was faster than the dechlorination of the other chloroethenes.