Influence of nutrients on oxidation of low level methane by mixed methanotrophic consortia

Iron and nitrogen regulate carbon transformation in a methanotroph-microalgae system

Nutrient supply is important for maintaining a methanotroph and microalgae (MOB-MG) system for biogas valorization. However, there is a lack of understanding regarding how key elements regulate the growth of a MOB-MG coculture. In this study, a MOB-MG coculture with high protein content (0.47 g/g biomass) was established from waste activated sludge using synthetic biogas. An increase in iron availability substantially stimulated the specific growth rate (from 0.18 to 0.62 day-1) and biogas conversion rate (from 26.81 to 106.57 mg-C L-1 day-1) of the coculture. Moreover, the protein content remained high (0.51 g/g biomass), and the total lipid content increased (from 0.09 to 0.14 g/g biomass). Nitrogen limitation apparently constrained the specific growth rate (from 0.64 to 0.28 day-1) and largely reduced the protein content (from 0.51 to 0.31 g/g biomass) of the coculture. Intriguingly, the lipid content remained unchanged after nitrogen was depleted. The eukaryotic community was consistently dominated by MG belonging to Chlorella, while the populations of MOB shifted from Methylococcus/Methylosinus to Methylocystis due to iron and nitrogen amendment. In addition, diverse non-methanotrophic heterotrophs were present in the community. Their presence neither compromised the performance of the coculture system nor affected the protein content of the biomass. However, these heterotrophs may contribute to high carbon conversion efficiency by utilizing the dissolved organic carbon released by MOB and MG. Overall, the findings highlight the vital roles of iron and nitrogen in achieving efficient conversion of biogas, fast growth of cells, and optimal biomass composition in a MOB-MG coculture system.

Characterization of the Bacterial Community Associated with Methane and Odor in a Pilot-Scale Landfill Biocover under Moderately Thermophilic Conditions

A pilot-scale biocover was constructed at a sanitary landfill and the mitigation of methane and odor compounds was compared between the summer and non-summer seasons. The average inlet methane concentrations were 22.0%, 16.3%, and 31.3%, and the outlet concentrations were 0.1%, 0.1%, and 0.2% during winter, spring, and summer, respectively. The odor removal efficiency was 98.0% during summer, compared to 96.6% and 99.6% during winter and spring, respectively. No deterioration in methane and odor removal performance was observed even when the internal temperature of the biocover increased to more than 40°C at midday during summer. During summer, the packing material simultaneously degraded methane and dimethyl sulfide (DMS) under both moderately thermophilic (40-50°C) and mesophilic conditions (30°C). Hyphomicrobium and Brevibacillus, which can degrade methane and DMS at 40°C and 50°C, were isolated. The diversity of the bacterial community in the biocover during summer did not decrease significantly compared to other seasons. The thermophilic environment of the biocover during summer promoted the growth of thermotolerant and thermophilic bacterial populations. In particular, the major methane-oxidizing species were Methylocaldum spp. during summer and Methylobacter spp. during the nonsummer seasons. The performance of the biocover remained stable under moderately thermophilic conditions due to the replacement of the main species and the maintenance of bacterial diversity. The information obtained in this study could be used to design biological processes for methane and odor removal during summer and/or in subtropical countries.

Suitability assessment of dumpsite soil biocover to reduce methane emission from landfills under interactive influence of nutrients

Biocovers are known for their role as key facilitator to reduce landfill methane (CH₄) emission on improving microbial methane bio-oxidation. Methanotrophs existing in the aerobic zone of dumped wastes are the only known biological sinks for CH₄ being emitted from the lower anaerobic section of landfill sites and even from the atmosphere. However, their efficacy remains under the influence of landfill environment and biocover characteristics. Therefore, the present study was executed to explore the suitability and efficacy of dumpsite soil as biocover to achieve enhanced methane bio-oxidation under the interactive influence of nutrients, carbon source, and environmental factors using statistical-mathematical models. The Placket-Burman design (PBD) was employed to identify the significant factors out of 07 tested factors having considerable impact on CH₄ bio-oxidation. The normal plot and Student's t test of PBD indicated that ammonical nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N), methane (CH₄), and copper (Cu) concentration were found significant. A three-level Box-Behnken design (BBD) was further applied to optimize the significant factors identified from PBD. The BBD results revealed that interactive interaction of CH₄ with NH₄⁺-N and NO₃^--N affected the CH₄ bio-oxidation significantly. The sequential statistical approach predicted that maximum CH₄ bio-oxidation of 27.32 μg CH₄ h^-1 could be achieved with CH₄ (35%), NO₃^--N (250 μg g^-1), NH₄+-N (25 μg g^-1), and Cu (50 mg g^-1) concentration. Conclusively, waste dumpsite soil could be a good alternative over conventional soil cover to improve CH₄ bio-oxidation and lessen the emission of greenhouse gas from waste sector.

Simultaneous mitigation of methane and odors in a biowindow using a pipe network

In this study, a biowindow with a piped gas collection network is proposed as an area-efficient landfill gas treatment system. A 9-m² biowindow was constructed for treating landfill gas collected from an area of 450 m² in a sanitary landfill, and its performance was evaluated for 224 days. The methane removal efficiency was 59-100% at 146.3-675.1 g-CH₄ m^-2 d^-1. Odorous compounds were also removed by the biowindow, with a complex odor intensity removal rate of 93-100%. In particular, the removal efficiency for hydrogen sulfide and methanethiol, major contributors to the complex odor intensity, was 97% and 91%, respectively. Metagenomic analysis showed that the dominant bacterial genera shifted from Acinetobacter and Pseudomonas to Methylobacter and Methylocaldum due to the high concentration of methane. A high bacterial diversity was maintained, which may have contributed to the robust performance of the biowindow against environmental fluctuations. At 1/50th of the size of conventional biocovers, the proposed biowindow can greatly reduce the required installation area and represents a competitive method for the simultaneous treatment of methane and odor in landfills.

Optimisation of bioscrubber systems to simultaneously remove methane and purify wastewater from intensive pig farms

The use of bioscrubber is attracting increasing attention for exhaust gas treatment in intensive pig farming. However, the challenge is to improve the methane (CH₄) removal efficiency as well as the possibility of pig house wastewater treatment. Three laboratory-scale bioscrubbers, each equipped with different recirculation water types, livestock wastewater (10-times-diluted pig house wastewater supernatant), a methanotroph growth medium (10-times-diluted), and tap water, were established to evaluate the performance of CH₄ removal and wastewater treatment. The results showed that enhanced CH₄ removal efficiency (25%) can be rapidly achieved with improved methanotrophic activity due to extra nutrient support from the wastewater. The majority of the CH₄ was removed in the middle to end part of the bioscrubbers, which indicated that CH₄ removal could be potentially optimised by extending the length of the reactor. Moreover, 52-86% of the ammonium (NH₄⁺-N), total organic carbon (TOC), and phosphate (PO₄^3--P) removal were simultaneously achieved with CH₄ removal in the present study. Based on these results, this study introduces a low-cost and simple-to-operate method to improve CH₄ removal and simultaneously treat pig farm wastewater in bioscrubbers.

Ammonium influences kinetics and structure of methanotrophic consortia

The literature is conflicted on the influence of ammonium on the kinetics and microbial ecology of methanotrophy. In this study, methanotrophic cultures were enriched, under ammonium concentrations ranging from 0 to 200 mM, from an inoculum comprising leachate and top-cover soil from a landfill. Specific CH₄ biodegradation rates were highest (7.8 × 10^-4 ± 6.0 × 10^-5 gCH₄ g_X^-1 h^-1) in cultures enriched at 4 mM NH₄⁺, which were mainly dominated by type II methanotrophs belonging to Methylocystis spp. Lower specific CH₄ oxidation rates (average values of 1.8-3.6 × 10^-4 gCH₄ g_X^-1 h^-1) were achieved by cultures enriched at higher NH₄⁺ concentrations (20 and 80 mM), and had higher affinity for CH₄ compared to 4 mM enrichments. These lower affinities were attributed to lower diversity dominated by type I methanotrophs, of the Methylosarcina, Methylobacter and Methylomicrobium genera, encountered with increasing concentrations of NH₄⁺. The study indicates that CH₄ oxidation biotechnologies applied at low NH₄⁺ concentrations can support efficient abatement of CH₄ and high diversity of methanotrophic consortia, whilst enriching type II methanotrophs.

Biofiltration of methane from cow barns: Effects of climatic conditions and packing bed media acclimatization

The performance of biofiltration to mitigate CH₄ emissions from cow barns was investigated in the laboratory using two flow-through columns constructed with an acclimatized packed bed media composed of inexpensive materials and readily available in an agricultural context. The biofilters were fed with artificial exhaust gas at a constant rate of 0.036 m³ h^-1 and low inlet CH₄ concentration (0.22 g m^-3 = 300 ppm). The empty-bed residence time (EBRT) was equal to 0.21 h. Additionally, in order to simulate temperature changes under natural conditions and determine the influence of such cycles on CH₄ removal efficiency, the upper part of the biofilters were submitted to temperature oscillations over time. The maximum oxidation rate (1.68 μg _CH4 g_dw^-1h^-1) was obtained with the commercial compost mixed with straw. Accordingly, it was considered as packing bed media for the biofilters. The CH₄ removal efficiency was affected by the temperature prevailing within the biofilters, by the way in which the cooling-warming cycles were applied and by the acclimatization process. The shorter the cooling-warming cycles, the more oxidation rates varied. With longer cycles, CH₄ removal rates stabilized and CH₄ removal efficiencies attained nearly 100% in both biofilters, and remained at this level for more than 100 days, irrespective of the temperature at the top of the biofilter, which was - at times - adverse for microbiological activity. The first order rate constant for CH₄ oxidation kinetics of the entire system was estimated at 15 h^-1. If such rate could be transposed to real field conditions in Canada, home to nearly 945,000 dairy cows, biofiltration may be applied to efficiently abate between 2 × 10⁶ and 3 × 10⁶ t yr^-1 of CO₂ equivalent (depending on how estimates are performed) from bovine enteric fermentation alone.

Response of mixed methanotrophic consortia to different methane to oxygen ratios

Methane (CH₄) and oxygen (air) concentrations affect the CH₄ oxidation capacity (MOC) and mixed methanotrophic community structures in compost (fresh) and landfill (age old) top cover soils. A change in the mixed methanotrophic community structure in response has implications for landfill CH₄ bio-filter remediation and possible bio-product outcomes (i.e., fatty acid methyl esters (FAME) content and profiles and polyhydroxybutyrate (PHB) contents). Therefore the study aimed to evaluate the effect of variable CH₄ to oxygen ratios (10-50% CH₄ in air) on mixed methanotrophic community structures enriched from landfill top cover (LB) and compost soils (CB) and to quantify flow on impacts on MOC, total FAME contents and profiles, and PHB accumulation. A stable consortium developed achieving average MOCs of 3.0±0.12, 4.1±0.26, 6.9±0.7, 7.6±1.3 and 9.2±1.2mgCH₄g^-1DW_biomassh^-1 in LB and 2.9±0.04, 5.05±0.32, 6.7±0.31, 7.9±0.61 and 8.6±0.48mgCH₄g^-1DW_biomassh^-1 in CB for a 20day cultivation period at 10, 20, 30, 40 and 50% CH₄, respectively. CB at 10% CH₄ had a maximal FAME content of 40.5±0.8mgFAMEg^-1DW_biomass, while maximal PHB contents (25mgg^-1DW_biomass) was observed at 40% CH₄ in LB. Despite variable CH₄/O₂ ratios, the mixed methanotrophic community structures in both LB and CB were relatively stable, dominated by Methylosarcina sp., and Chryseobacterium, suggesting that a resilient consortium had formed which can now be tested in bio-filter operations for CH₄ mitigations in landfills.

Culture scale-up and immobilisation of a mixed methanotrophic consortium for methane remediation in pilot-scale bio-filters

Robust methanotrophic consortia for methane (CH₄) remediation and by-product development are presently not readily available for industrial use. In this study, a mixed methanotrophic consortium (MMC), sequentially enriched from a marine sediment, was assessed for CH₄ removal efficiency and potential biomass-generated by-product development. Suitable packing material for bio-filters to support MMC biofilm establishment and growth was also evaluated. The enriched MMC removed ∼7-13% CH₄ under a very high gas flow rate (2.5 L min^-1; 20-25% CH₄) in continuous-stirred tank reactors (∼10 L working volume) and the biomass contained long-chain fatty acids (i.e. C₁₆ and C₁₈). Cultivation of the MMC on plastic bio-balls abated ∼95-97% CH₄ in pilot-scale non-sterile outdoor-operated bio-filters (0.1 L min^-1; 1% CH₄). Contamination by cyanobacteria had beneficial effects on treating low-level CH₄, by providing additional oxygen for methane oxidation by MMC, suggesting that the co-cultivation of MMC with cyanobacterial mats does not interfere with and may actually be beneficial for remediation of CH₄ and CO₂ at industrial scale.