Kinetics of biological methane oxidation in the presence of non-methane organic compounds in landfill bio-covers

Exploring the potential of biofiltration for mitigating harmful gaseous emissions from small or old landfills: a review

Landfills are widely employed as the primary means of solid waste disposal. However, this practice generates landfill gas (LFG) which contains methane (CH4), a potent greenhouse gas, as well as various volatile organic compounds and volatile inorganic compounds. These emissions from landfills contribute to approximately 25% of the total atmospheric CH4, indicating the imperative need to valorize or treat LFG prior to its release into the atmosphere. This review first aims to outline landfills, waste disposal and valorization, conventional gas treatment techniques commonly employed for LFG treatment, such as flares and thermal oxidation. Furthermore, it explores biotechnological approaches as more technically and economically feasible alternatives for mitigating LFG emissions, especially in the case of small and aged landfills where CH4 concentrations are often below 3% v/v. Finally, this review highlights biofilters as the most suitable biotechnological solution for LFG treatment and discusses several advantages and challenges associated with their implementation in the landfill environment.

Nutrient starvation intensifies chlorine disinfection-stressed biofilm formation

Bacterial surface attachment and subsequent biofilm expansion represent an essential adaptation to environmental signals and stresses, which are of great concern for many natural and engineered ecosystems. Yet the underlying mechanisms and driving forces of biofilm formation in a chlorinated and nutrient-restricted system remain sketchy. In this study, we coupled an experimental investigation and modeling simulation to understand how chlorination and nutrient limitation conspire to form biofilm using Pseudomonas aeruginosa as a model bacterium. Experimental results showed that moderate chlorination at 1.0 mg/L led to biofilm development amplified to 2.6 times of those without chlorine, while additional nutrient limitation (of 1/50-diluted or 0.4 g/L LB broth culture) achieved 4.6 times increment as compared to those of undiluted scenarios (of 20 g/L LB broth culture) with absence of chlorination after 24 h exposure. Meanwhile, intermediate chlorination stimulated instant flagellar motility and subsequently extracellular polymeric substances (EPS) secretion, particularly under limited nutrient condition (of 1/50-diluted or 0.4 g/L LB broth culture) that retarded chlorine consumption and provoked bacterial nutrient-limitation response. From our simulations, chlorine and resource levels along with associated spatio-temporal variations collectively drove bacterial cell movement and EPS excretion. Our results demonstrated that restraining nutrient intensified chlorination-excited cell movement and EPS production that reinforced biological and cell-surface interactions, thereby encouraging bacterial surface attachment and subsequent biofilm development. The findings provide the insights into the linkage of disinfectant and nutrient-regulated bacterial functional responses with consequent micro-habitats and biofilm dynamics.

Impact of hydrogen sulfide on biochar in stimulating the methane oxidation capacity and microbial communities of landfill cover soil

Hydrogen sulfide (H₂S) can influence methanotrophic activities and be adsorbed by biochar (BC); however, the impact of H₂S on BC in stimulating the methane (CH₄) oxidation capacity of landfill cover soil (LCS) has not been clarified. Thus, batch incubation experiments were conducted to observe the effect of H₂S on the CH₄ oxidation capacity of and microbial communities in BC-amended LCS. Three landfill gas conditions were considered: 5 % CH₄ and 15 % oxygen (O₂) (5 M), 10 % CH₄ and 10 % O₂, and 20 % CH₄ and 5 % O₂ (20 M) by volume, with H₂S concentrations of 0, 100, 250, and 1000 ppm, respectively. Another series was conducted using LCS subjected to pre-H₂S saturation under the 20 M gas condition. In the 5 M gas condition suitable for the dominant methanotroph Methylocaldum (type I), the BC retained its ability to stimulate the CH₄ oxidation capacity of LCS (enhancement of 41-108 %) in the presence of H₂S. Additionally, when H₂S ≤ 250 ppm, the BC exhibited a relatively consistent impact of H₂S on both CH₄ oxidation capacity and microbial communities in LCS, independent of the CH₄ or O₂ concentrations. This result could be attributed to the different pathways of H₂S metabolism for the LCS and BC-amended LCS. Furthermore, when saturated adsorption of H₂S occurred for the LCS, the CH₄ oxidation capacity for BC-amended LCS was higher than that for non-amended LCS, which demonstrated the ability of BC in alleviating the inhibition of H₂S on CH₄ oxidation due to its excellent H₂S adsorption under even anoxic environments.

Methane in wastewater treatment plants: status, characteristics, and bioconversion feasibility by methane oxidizing bacteria for high value-added chemicals production and wastewater treatment

Methane is a type of renewable fuel that can generate many types of high value-added chemicals, however, besides heat and power production, there is little methane utilization in most of the wastewater treatment plants (WWTPs) all round the world currently. In this review, the status of methane production performance from WWTPs was firstly investigated. Subsequently, based on the identification and classification of methane oxidizing bacteria (MOB), the key enzymes and metabolic pathway of MOB were presented in depth. Then the production, extraction and purification process of high value-added chemicals, including methanol, ectoine, biofuel, bioplastic, methane protein and extracellular polysaccharides, were introduced in detail, which was conducive to understand the bioconversion process of methane. Finally, the use of methane in wastewater treatment process, including nitrogen removal, emerging contaminants removal as well as resource recovery was extensively explored. These findings could provide guidance in the development of sustainable economy and environment, and facilitate biological methane conversion by using MOB in further attempts.

Biological Waste Air and Waste Gas Treatment: Overview, Challenges, Operational Efficiency, and Current Trends

International contracts to restrict emissions of climate-relevant gases, and thus global warming, also require a critical reconsideration of technologies for treating municipal, commercial, industrial, and agricultural waste gas emissions. A change from energy- and resource-intensive technologies, such as thermal post-combustion and adsorption, as well to low-emission technologies with high energy and resource efficiency, becomes mandatory. Biological processes already meet these requirements, but show restrictions in case of treatment of complex volatile organic compound (VOC) mixtures and space demand. Innovative approaches combining advanced oxidation and biofiltration processes seem to be a solution. In this review, biological processes, both as stand-alone technology and in combination with advanced oxidation processes, were critically evaluated in regard to technical, economical, and climate policy aspects, as well as present limitations and corresponding solutions to overcome these restrictions.

Simultaneous biodegradation of methane and styrene in biofilters packed with inorganic supports: Experimental and macrokinetic study

Four upflow 0.018 m³ biofilters (3 beds), B-ME, B-200, B-500 and B-700, all packed with inorganic materials, were operated at a constant air flow rate of 0.18 m³ h^-1 to eliminate methane (CH₄), a harmful greenhouse gas (GHG), and styrene (C₈H₈), a carcinogenic volatile organic compound (VOC). The biofilters were irrigated with 0.001 m³ of recycled nutrient solution (NS) every day (flow rate of 60 × 10^-3 m³ h^-1). Styrene inlet load (IL) was kept constant in each biofilter. Different CH₄-ILs varying in the range of 7-60 gCH₄ m^-3 h^-1 were examined in B-ME (IL of 0 gC₈H₈ m^-3 h^-1), B-200 (IL of 9 gC₈H₈ m^-3 h^-1), B-500 (IL of 22 gC₈H₈ m^-3 h^-1) and B-700 (IL of 32 gC₈H₈ m^-3 h^-1). Finally, the effect of C₈H₈ on the macrokinetic parameters of CH₄ biofiltration was studied based on the Michaelis-Menten model. Average C₈H₈ removal efficiencies (RE) varying between 64 and 100% were obtained at CH₄-ILs increasing from 7 to 60 gCH₄ m^-3 h^-1 and for C₈H₈-ILs range of 0-32 gC₈H₈ m^-3 h^-1. More than 90% of C₈H₈ was removed in the bottom and middle beds of the biofilters. By increasing C₈H₈-IL from 0 to 32 gC₈H₈ m^-3 h^-1, maximal EC in Michaelis-Menten model and macrokinetic saturation constant declined from 311 to 39 g m^-3 h^-1 and from 19 to 2.3 g m^-3, respectively, which confirmed that an uncompetitive inhibition occurred during CH₄ biofiltration in the presence of C₈H₈.

Real-time monitoring of methane oxidation in a simulated landfill cover soil and MiSeq pyrosequencing analysis of the related bacterial community structure

Real-time CH₄ oxidation in a landfill cover soil was studied using automated gas sampling that determined biogas (CH₄ and CO₂) and O₂ concentrations at various depths in a simulated landfill cover soil (SLCS) column reactor. The real-time monitoring system obtained more than 10,000 biogas (CH₄ and CO₂) and O₂ data points covering 32 steady states of CH₄ oxidation with 32 different CH₄ fluxes (0.2-125mol·m^-2·d^-1). The kinetics of CH₄ oxidation at different depths (0-20cm, 20-40cm, and 40-60cm) of SLCS were well fit by a CH₄-O₂ dual-substrate model based on 32 values (averaged, n=5-15) of equilibrated CH₄ concentrations. The quality of the fit (R² ranged from 0.90 to 0.96) was higher than those reported in previous studies, which suggests that real time monitoring is beneficial for CH₄ oxidation simulations. MiSeq pyrosequencing indicated that CH₄ flux events changed the bacterial community structure (e.g., increased the abundance of Bacteroidetes and Methanotrophs) and resulted in a relative increase in the amount of type I methanotrophs (Methylobacter and Methylococcales) and a decrease in the amount of type II methanotrophs (Methylocystis).

Methane oxidation in a landfill cover soil reactor: Changing of kinetic parameters and microorganism community structure

Changing of CH₄ oxidation potential and biological characteristics with CH₄ concentration was studied in a landfill cover soil reactor (LCSR). The maximum rate of CH₄ oxidation reached 32.40 mol d^-1 m^-2 by providing sufficient O₂ in the LCSR. The kinetic parameters of methane oxidation in landfill cover soil were obtained by fitting substrate diffusion and consumption model based on the concentration profile of CH₄ and O₂. The values of [Formula: see text] (0.93-2.29%) and [Formula: see text] (140-524 nmol kg_soil-DW^-1·s^-1) increased with CH₄ concentration (9.25-20.30%), while the values of [Formula: see text] (312.9-2.6%) and [Formula: see text] (1.3 × 10^-5 to 9.0 × 10^-3 nmol mL^-1 h^-1) were just the opposite. MiSeq pyrosequencing data revealed that Methylobacter (the relative abundance was decreased with height of LCSR) and Methylococcales_unclassified (the relative abundance was increased expect in H 80) became the key players after incubation with increasing CH₄ concentration. These findings provide information for assessing CH₄ oxidation potential and changing of biological characteristics in landfill cover soil.

Sensitivity of methanotrophic community structure, abundance, and gene expression to CH4 and O2 in simulated landfill biocover soil

Pressure on mitigating CH4 emission in landfill requires better understanding of methanotrophs in landfill biocovers. Most previous studies focused on CH4 as the sole substrate. This study aims to understand the sensitivity of methanotrophs to both substrates CH4 and O2 concentrations in landfill biocovers. The estimated CH4 oxidation rates (4.66-98.7 × 10(-16) mol cell(-1) h(-1)) were evidently higher than the previous reports, suggesting that activity of methanotrophs was enhanced with both the increasing of O2 and CH4 concentrations. Denaturing gradient gel electrophoresis based on the amplification of pmoA genes suggested that methanotrophs were more sensitive to CH4 than O2. Quantification of methanotrophs using pmoA- and mmoX-targeted real-time polymerase chain reaction showed that Mbac and Mcoc as well as Mcys groups were significantly dominant. Mbac group with pmoA gene transcription was dominant. Results indicate that CH4 mitigation would have higher potential by increasing O2 at appropriate CH4 concentrations.

Effect of trichloroethylene and tetrachloroethylene on methane oxidation and community structure of methanotrophic consortium

The methane oxidation rate and community structure of a methanotrophic consortium were analyzed to determine the effects of trichloroethylene (TCE) and tetrachloroethylene (PCE) on methane oxidation. The maximum methane oxidation rate (Vmax ) of the consortium was 326.8 μmol·g-dry biomass(-1)·h(-1), and it had a half-saturation constant (Km ) of 143.8 μM. The addition of TCE or PCE resulted in decreased methane oxidation rates, which were decreased from 101.73 to 5.47-24.64 μmol·g-dry biomass(-1)·h(-1) with an increase in the TCE-to-methane ratio, and to 61.95-67.43 μmol·g-dry biomass(-1)·h(-1) with an increase in the PCE-to-methane ratio. TCE and PCE were non-competitive inhibitors for methane oxidation, and their inhibition constants (Ki ) were 33.4 and 132.0 μM, respectively. When the methanotrophic community was analyzed based on pmoA using quantitative real-time PCR (qRT-PCR), the pmoA gene copy numbers were shown to decrease from 7.3 ± 0.7 × 10(8) to 2.1-5.0 × 10(7) pmoA gene copy number · g-dry biomass(-1) with an increase in the TCE-to-methane ratio and to 2.5-7.0 × 10(7) pmoA gene copy number · g-dry biomass(-1) with an increase in the PCE-to-methane ratio. Community analysis by microarray demonstrated that Methylocystis (type II methanotrophs) were the most abundant in the methanotrophic community composition in the presence of TCE. These results suggest that toxic effects caused by TCE and PCE change not only methane oxidation rates but also the community structure of the methanotrophic consortium.

Dynamics of CH4 oxidation in landfill biocover soil: effect of O2/CH4 ratio on CH4 metabolism

The CH(4) oxidation dynamics was investigated by observing the CH(4) oxidation rates at concentrations (from 1.0 × 10(4) ppmv to 2.0 × 10(5) ppmv) mixed with O(2) (from 5.0 × 10(4) ppmv to 7.5 × 10(5) ppmv). The CH(4)-O(2) dual-substrate model based on Michaelis-Menten equation (K(m, CH4) = 1.4 × 10(5) ppmv; V(max) = 7.6 × 10(2) μmol kg(-1) d(-1); K(m, O2) = 5.5 × 10(4) ppmv) was got and agreed well with the experimental data when the initial O(2)/CH(4) ratio reached 3:1, indicating full aerobic CH(4) oxidization. Anoxic CH(4) oxidation gradually became predominant with decreasing O(2)/CH(4) ratios. The effect of CH(4) is more significant than O(2), as evidenced by higher slope (0.58 kg(-1) d(-1)) of V(CH4) - [S(CH4)] line graph compared with that of V(CH4) - [S(CH4)] line graph (0.062 kg(-1) d(-1)). The paper presents the dynamics of CH(4) oxidation and proposes that ratio of O(2)/CH(4) need to be considered for their dynamically changing in environmental habitats. The findings provide an important parameter for optimizing the operations of breathing biocover systems.