Hexane biodegradation in two-liquid phase bioreactors: High-performance operation based on the use of hydrophobic biomass

Are Si-C bonds cleaved by microorganisms? A critical review on biodegradation of methylsiloxanes

Methylsiloxanes, compounds that contain H₃C-Si-O subunits in their molecular structure, are emerging ubiquitous pollutants now detected in many environmental compartments. These compounds and generally Si-C bonds do not occur in living nature, but are industrially produced worldwide in millions of tons per annum and are widely used, resulting in their release to the environment. It is an open question whether or to what extent microorganisms are able to decompose these compounds. The presence of methylsiloxanes in many biogases adds to the economic relevance of this question. We here review and critically discuss, for the first time, the evidence obtained for and against degradation of methylsiloxanes by microorganisms, and in particular for microbial cleavage of Si-CH₃ bonds. As a result, no convincing demonstration of Si-C cleavage by native environmental microorganisms has been found.

Simultaneous degradation of n-hexane and production of biosurfactants by Pseudomonas sp. strain NEE2 isolated from oil-contaminated soils

The presence of surfactants in biofilters could enhance hydrophobic VOC removal. In this study, blood agar plate, methylene blue agar plate and a culture with n-hexane as the only carbon source were used to screen strains that could biodegrade n-hexane and produce biosurfactants simultaneously. The effects of n-hexane concentration on n-hexane removal and biosurfactant production were also investigated. Results showed that such a strain identified to be Pseudomonas sp. Strain NEE2 was successfully isolated from oil-polluted soils. The biosurfactants production by this strain were dependent on the initial concentration of n-hexane (132-2640 mg/L). At the concentration of 2640 mg/L of n-hexane, the biosurfactants promoted n-hexane removal. At 132 mg/L of n-hexane, n-hexane removal efficiency on day 2 exceeded 60%. The synergistic mechanisms of n-hexane removal and biosurfactant production by Pseudomonas sp. Strain NEE2 were discussed including the enhanced mass transfer from gas to liquid phase, within the biofilm phase and biodegradation at the presence of biosurfactants as well as the consequently enhanced production of the biosurfactants. These results in this study proved that it is possible for microorganisms utilizing the synergistic effect of hydrophobic VOC degradation and biosurfactant production for cost-effective hydrophobic VOC removal in biofilters.

The impact of environmental parameters on the conversion of toluene to CO2 and extracellular polymeric substances in a differential soil biofilter

The fraction of pollutant converted to CO₂ versus biomass in biofiltration influences the process efficacy and the lifetime of the bed due to pressure drop increases. This work determined the relative quantitative importance and potential interactions between three critical environmental parameters: toluene concentration (Tol), matric potential (ψ) and temperature (T) on % CO₂, elimination capacity (EC) and the production rate of non-CO₂ products. These parameters are the most variable in typical biofilter operation. The data was fit to a non-linear model of the form y=a(Tol)^bT^cψ^d. A rigorous carbon balance (100.5 ± 7.0%) tracked the fate of degraded toluene as CO₂ and non-CO₂ carbon endpoints. The % CO₂ mineralization varied from (34-91%) with environmental parameters: temperature (20-40 °C), matric potential, (-10 to -100 cm_H2O) and residual toluene, (20-180 ppm). The highest conversion to CO₂ was at the wettest conditions (-10 cm_H2O) and lowest residual toluene concentration (18 ppm). Matric potential had twice the impact of toluene concentration on % CO₂, while temperature had less impact. The elimination capacity varied from 11 to 50 g_C⋅m^-3h^-1 and was highest at 40 °C, the wettest conditions with limited impact by toluene concentrations. Temperature increased the EC and non-CO₂ production rates strongly while matric potential and toluene concentration had less influence (4x - 10x less). This study illustrated the quantitative significance and simultaneous interaction between critical environmental parameters on carbon endpoints and biofilter performance. This kind of multivariable parameter study provides valuable insights which can address performance and clogging issues in biofilters.

Enhancing Chlorobenzene Biodegradation by Delftia tsuruhatensis Using a Water-Silicone Oil Biphasic System

In this study, a water-silicone oil biphasic system was developed to enhance the biodegradation of monochlorobenzene (CB) by Delftia tsuruhatensis LW26. Compared to the single phase, the biphasic system with a suitable silicone oil fraction (v/v) of 20% allowed a 2.5-fold increase in the maximum tolerated CB concentration. The CB inhibition on D. tsuruhatensis LW26 was reduced in the presence of silicone oil, and the electron transport system activity was maintained at high levels even under high CB stress. Adhesion of cells to the water-oil interface at the water side was observed using confocal laser scanning microscopy. Nearly 75% of cells accumulated on the interface, implying that another interfacial substrate uptake pathway prevailed besides that initiated by cells in the aqueous phase. The 8-fold increase in cell surface hydrophobicity upon the addition of 20% (v/v) silicone oil showed that silicone oil modified the surface characteristics of D. tsuruhatensis LW26. The protein/polysaccharide ratio of extracellular polymeric substances (EPS) from D. tsuruhatensis LW26 presented a 3-fold enhancement. These results suggested that silicone oil induced the increase in the protein content of EPS and rendered cells hydrophobic. The resulting hydrophobic cells could adhere on the water-oil interface, improving the mass transfer by direct CB uptake from silicone oil.

Treatment of hydrophobic volatile organic compounds using two-liquid phase biofilters

The traditional one-liquid phase biofilter (OLPB), with water as the selected liquid phase, demonstrated low performance to volatile hydrophobic organic compounds. In this study, a novel two-liquid phase biofilter (TLPB) using silicone oil and water was established to treat gaseous dichloromethane (DCM). A comprehensive investigation of removal performance, kinetic analysis, biomass accumulations, pressure drops, CO₂ productions, and microbial communities of the two biofilters was compared. Results showed that TLPB presented an average removal efficiency of 85% during 200 days of operation, which was higher than that of OLPB (63%). Owing to the buffering effects caused by silicone oil, TLPB demonstrated a superior fluctuation resistance capability than OLPB. TLPB was determined at a higher actual mass distribution coefficient of 6.00 than that of the OLPB (3.99), thereby suggesting a significantly more effective mass transfer process inside TLPB compared with that in OLPB. Furthermore, a rapid biomass accumulation process was observed in TLPB. The specific growth rates of biomass in OLPB and TLPB were calculated as 0.035 and 0.026 g of dry biomass/g of dry filter per day, respectively. The carbon balances were analyzed in the two biofilters. The yield coefficients (Y) were determined at 1.449 and 1.143 g of dry biomass/g of removed VOC for OLPB and TLPB, respectively. However, the corresponding CO₂ production fraction was 0.263 g and 0.316 g per 1 g of DCM for OLPB and TLPB, respectively. The variations in fraction of carbon in DCM transformation to biomass and to CO₂ suggested distinct microbial transformation pathways of utilizing DCM in the two biofilters, which were mainly caused by the different microbial communities and metabolic activities.

Two-liquid phase partitioning biotrickling filters for methane abatement: exploring the potential of hydrophobic methanotrophs

The potential of two-liquid phase biotrickling filters (BTFs) to overcome mass transfer limitations derived from the poor aqueous solubility of CH4 has been scarcely investigated to date. In this context, the abatement of diluted methane emissions in two-liquid phase BTFs was evaluated using two different inocula: a type II methanotrophs culture in BTF 1 and a hydrophobic microbial consortium capable of growing inside silicone oil in BTF 2. Both BTFs supported stable elimination capacities above 45 g m(-3) h(-1) regardless of the inoculum, whereas no improvement derived from the presence of hydrophobic microorganisms compared to the type II metanotrophs culture was observed. Interestingly, the addition of silicone oil mediated a reduced metabolites concentration in the recycling aqueous phase, thus decreasing the needs for mineral medium renewal. Moreover, a 78% similarity was recorded between the microbial communities enriched in both BTFs at the end of the experimental period in spite of the differences in the initial inoculum structure. The results obtained confirmed the superior performance of two-liquid phase BTFs for CH4 abatement compared with conventional biotrickling filters.

Effect of saponins on n-hexane removal in biotrickling filters

Saponins was applied to enhance the removal of n-hexane in a biotrickling filter (BTF) in this study. Comparison experiments were carried out to examine the effect of saponins on n-hexane removal in two BTFs at various saponins concentrations, n-hexane loading rates (LRs) and gas empty bed contact times (EBCTs). Results show that the optimum concentration of saponins in nutrient feed was 50.0mgL(-1). When organic LR of n-hexane increased from 47.8 to 120.0gm(-3)h(-1), the removal efficiency (RE) for BTF1 (with saponins) and BTF2 (without saponins) decreased from 91.3% to 83.3% and from 62.8% to 56.8%, respectively. As gas EBCT decreased from 30.0 to 7.5s, the RE declined from 88.4% to 64.5% for BTF1 and from 61.4% to 38.3% for BTF2. Saponins could also decrease the biomass accumulation rate within the medium bed. These results could be referred in the design and operation of BTFs for hydrophobic VOC removal.

Assessment of methane biodegradation kinetics in two-phase partitioning bioreactors by pulse respirometry

Biological methane biodegradation is a promising treatment alternative when the methane produced in waste management facilities cannot be used for energy generation. Two-phase partitioning bioreactors (TPPBs), provided with a non-aqueous phase (NAP) with high affinity for the target pollutant, are particularly suitable for the treatment of poorly water-soluble compounds such as methane. Nevertheless, little is known about the influence of the presence of the NAP on the resulting biodegradation kinetics in TPPBs. In this study, an experimental framework based on the in situ pulse respirometry technique was developed to assess the impact of NAP addition on the methane biodegradation kinetics using Methylosinus sporium as a model methane-degrading microorganism. A comprehensive mass transfer characterization was performed in order to avoid mass transfer limiting scenarios and ensure a correct kinetic parameter characterization. The presence of the NAP mediated significant changes in the apparent kinetic parameters of M. sporium during methane biodegradation, with variations of 60, 120, and 150% in the maximum oxygen uptake rate, half-saturation constant and maximum specific growth rate, respectively, compared with the intrinsic kinetic parameters retrieved from a control without NAP. These significant changes in the kinetic parameters mediated by the NAP must be considered for the design, operation and modeling of TPPBs devoted to air pollution control.

Treatment of O₂-free toluene emissions by anoxic biotrickling filtration

Toluene biotrickling filtration under anoxic denitrifying conditions was evaluated in two identical bioreactors (R1 and R2) operated at liquid recycling rates of 1.3, 2.7 and 5.3 m h−1 and liquid renewal rates of 0 and 0.17 d−1. R1 and R2 achieved a similar maximum elimination capacity (EC ∼30 g m−3 h−1) at the same toluene inlet load (∼50 g m−3 h−1), which was approximately 7 times higher compared with available literature on continuous toluene removal under anoxic conditions. Nevertheless, higher metabolite accumulation was observed in the bioreactor operated without periodical liquid phase renewal (R2), leading to intermittent drops in its toluene removal performance. This is the first work operating an anoxic biotrickling filter at empty bed residence time of 3 min, which is comparable with those employed in conventional aerobic systems. A characterization of the metabolites accumulated in the liquid phase revealed a dynamic metabolite production and degradation.

Simultaneous biodegradation of volatile and toxic contaminant mixtures by solid-liquid two-phase partitioning bioreactors

Microbial inhibition and stripping of volatile compounds are two common problems encountered in the biotreatment of contaminated wastewaters. Both can be addressed by the addition of a hydrophobic auxiliary phase that can absorb and subsequently re-release the substrates, lowering their initial aqueous concentrations. Such systems have been described as Two Phase Partitioning Bioreactors (TPPBs). In the current work the performances of a solid-liquid TPPB, a liquid-liquid TPPB and a single phase reactor for the simultaneous degradation of butyl acetate (the volatile component) and phenol (the toxic component) have been compared. The auxiliary phase used in the solid-liquid TPPB was a 50:50 polymer mixture of styrene-butadiene rubber and Hytrel 8206, with high affinities for butyl acetate and phenol, respectively. The liquid-liquid TPPB employed silicone oil which has fixed physical properties, and had no capacity to absorb the toxic contaminant (phenol). Butyl acetate degradation was enhanced in both TPPBs relative to the single phase, arising from its sequestration into the auxiliary phase, thereby reducing volatilization losses. The solid-liquid TPPB additionally showed a substantial increase in the phenol degradation rate, relative to the silicone oil system, demonstrating the superiority and versatility of polymer based systems.