Evaluation and statistical optimization of methane oxidation using rice husk amended dumpsite soil as biocover

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.

Validation and optimization of key biochar properties through iron modification for improving the methane oxidation capacity of landfill cover soil

Knowledge of various BC properties in stimulating the methane (CH₄) oxidation capacity of landfill cover soil (LCS) is still limited, restricting the optimization of BC performance. To validate key BC properties and seek a feasible way for enhancing BC performance, this study prepared BCs with distinctly varying characteristics through iron (Fe) modification. Then, batch incubation experiments under different CH₄ and oxygen concentrations were conducted. Pore volume, cation exchange capacity (CEC), and surface area of BC collectively accounted for 78.5% of the variances in the microbial community structures, with pore volume being the most important factor. These correlated well with the differences in the CH₄ oxidation capacities among the different BC-amended LCS. At a low ratio of 15% (v/v) in LCS, BCs' pH not affected their performance but homogeneity could be a limiting factor. Fe modification proved a promising approach to more efficiently improve the three key BC properties (especially pore volume) and thus optimize BC performance than increasing pyrolysis temperature did. Fe-modified BCs encouraged a bacterial consortium (methanotroph, methylotrophs, and nitrogen-fixing bacteria) in the soil with significantly improved CH₄ oxidation capacities by up to 26%-74% compared to that of pristine BC.

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.

Exploring the efficiency and pollutant emission of a dual fuel CI engine using biodiesel and producer gas: An optimization approach using response surface methodology

The present study focuses on optimizing the engine operating parameters of a dual-fuel (DF) engine. Producer gas (PG) and Honge oil methyl ester (HOME) are used as primary fuel and pilot fuel respectively for the operation. An experimental design matrix of 20 different combinations was considered using Design of Experiments (DoE), based on the central composite design (CCD) of response surface methodology (RSM). The effects of these combinations were experimentally investigated to calculate the performance and emission characteristics of the engine. The objective of the work is to maximize the Brake thermal efficiency (BTE) and minimize the exhaust gas temperature (EGT), nitrogen oxide (NOx), hydrocarbon (HC), and carbon monoxide (CO) emissions. The RSM model is developed using the experimental data and further, the operating parameters were optimized using the desirability approach. The optimized combination of operating parameters was obtained at 61.10% engine load, compression ratio (CR) of 18, and injection timing (IT) of 23.30° before top dead center (BTDC). The optimum responses corresponding to these operating conditions were found as 14.23%, 354.29 °C, 52.18 ppm, 39.53 ppm, and 0.51% for BTE, EGT, NOx, HC, and CO respectively with an overall desirability of 0.962. The optimized responses were validated experimentally at optimum input conditions and found to be within acceptable error levels. Further, an economic analysis of the optimized DF system is also carried out.

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.

In-situ biodegradation of volatile organic compounds in landfill by sewage sludge modified waste-char

VOCs are the major harmful pollutants released from MSW landfills, which are toxicity to human health. In order to in-situ biodegradation of VOCs released from landfill, two novel laboratory-scale biocovers, including waste-char obtained from MSW pyrolysis (WC), and sewage sludge modified the WC (SWC), are used to degradate VOCs. The removal performances of VOCs as well as the bacterial community in the WC and SWC are investigated in a simulated landfill systems with the contrast experiment of a landfill cover soil (LCS) for 60 days. Meanwhile, the adsorption-biodegradation of VOCs model compounds over the LCS, WC, and SWC are also tested in fixed-bed adsorption reactor and in-situ FTIR. The VOCs removal efficiencies by the SWC are maintained above 85% for a long-term, much higher than that of the LCS and WC. The higher removal efficiencies and long-term stability for VOCs degradation in SWC are attributed to a strongly positive synergistic between adsorption and biodegradation that the gaseous VOCs released from MSW is effectively adsorbed by the SWC due to its higher VOCs adsorption capacity, and then the adsorbed-VOCs is converted into CO₂ and H₂O by the microorganisms that consuming the adsorbed-VOCs as energy and carbon sources. Subsequently, the decrease of the adsorbed-VOCs in SWC would also promote the transformation of the gaseous VOCs into the adsorbed VOCs and accelerate the growth of microorganisms by taking the adsorbed-VOCs as the energy and carbon source, resulted in a higher adsorption rate and degradation rate for VOCs.

In-situ biodegradation of harmful pollutants in landfill by sludge modified biochar used as biocover

MSW landfill releases a lot of harmful pollutants such as H₂S, NH₃, and VOCs. In this study, two laboratory-scale biocovers such as biochar (BC) derived from agricultural & forestry wastes (AFW) pyrolysis, and sludge modified the biochar (SBC) were designed and used to remove the harmful pollutants. In order to understand in-situ biodegradation mechanism of the harmful pollutants by the SBC, the removal performances of the harmful pollutants together with the bacterial community in the BC and SBC were investigated in simulated landfill systems for 60 days comparing with the contrast experiment of a landfill cover soil (LCS). Meanwhile, the adsorption capacities of representative harmful pollutants (hydrogen sulfide, toluene, acetone and chlorobenzene) in the LCS, BC, and SBC were also tested in a fixed bed reactor. The removal efficiencies of the harmful pollutants by the SBC ranged from 95.43% to 100.00%, which was much higher than that of the LCS. The adsorption capacities of the harmful pollutants in the SBC were 4 times higher than that of the LCS since the SBC exhibited higher BET surface and N-containing functional groups. Meanwhile, the biodegradation rates of the harmful pollutants in the SBC were also much higher than that of the LCS since the populations of the bacterial community in the SBC were more abundant due to its facilitating the growth and activity of microorganisms in the porous structure of the SBC. In addition, a synergistic combination of adsorption and biodegradation in the SBC that enhanced the reproduction rate of microorganisms by consuming the absorbed-pollutants as carbon sources, which also contributed to enhance the biodegradation rates of the harmful pollutants.

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.

Comparison of the methane-oxidizing capacity of landfill cover soil amended with biochar produced using different pyrolysis temperatures

The in-situ mitigation of methane (CH₄) in landfill gas using landfill cover soil (LCS) is a cost-effective approach, but its efficiency needs to be enhanced. In this study, we incorporated an enriched methane-oxidizing bacteria (MOB) consortium into LCS and established four biochar-amended LCS groups with biochar produced at 300 °C (BC300), 400 °C (BC400), 500 °C (BC500), and 600 °C (BC600). The purpose was to evaluate the CH₄ oxidation capacity of biochar-amended LCS after inoculation with MOB and to investigate how the physicochemical properties of biochar that are influenced by the pyrolysis temperature affect the performance and microbial activity of biochar-amended LCS. It was found that a 15% volume ratio (representing a mass ratio of 2.49%-2.78%) for biochar amendment in LCS enhanced CH₄ removal efficiency, with the highest removal observed to be 46% for BC400-amended LCS compared to 30% for the original LCS. In addition, CH₄ adsorption by the biochar was not observed, and a 15% mass ratio for biochar in the LCS had no or a negative impact. Besides improving the water-holding capacity and gas permeability of LCS, other possible advantages of biochar amendment in terms of CH₄ oxidization include greater retention of nutrients, electron acceptors, and exchangeable cations, as well as introducing iron ions. It was also found that CH₄ oxidation capacity and the methanotroph activity of biochar-amended LCS did not continue to increase with higher pyrolysis temperatures, even though higher micropore volumes and surface areas were obtained at higher pyrolysis temperatures. From this study, BC400 was identified as the optimal choice for the best performance in terms of enhancing both the CH₄ oxidation capacity of the amended LCS and the growth of type II methanotroph Methylocystaceae, which can possibly be attributed to having the highest cation exchange capacity of the four biochars.

Optimization of torrefaction conditions of coffee industry residues using desirability function approach

The aim of the present study is to analyze the influence of independent process variables such as temperature, residence time, and heating rate on the torrefaction process of coffee chaff (CC) and spent coffee grounds (SCGs). Response surface methodology and a three-factor and three-level Box-Behnken design were used in order to evaluate the effects of the process variables on the weight loss (W_L) and the Higher Heating Value (HHV) of the torrefied materials. Results showed that the effects of the three factors on both responses were sequenced as follows: temperature>residence time>heating rate. Data obtained from the experiments were analyzed by analysis of variance (ANOVA) and fitted to second-order polynomial models by using multiple regression analysis. Predictive models were determined, able to obtain satisfactory fittings of the experimental data, with coefficient of determination (R²) values higher than 0.95. An optimization study using Derringer's desired function methodology was also carried out and the optimal torrefaction conditions were found: temperature 271.7°C, residence time 20min, heating rate 5°C/min for CC and 256.0°C, 20min, 25°C/min for SCGs. The experimental values closely agree with the corresponding predicted values.

Statistical assessment of dumpsite soil suitability to enhance methane bio-oxidation under interactive influence of substrates and temperature

Biocovers are considered as the most effective and efficient way to treat methane (CH₄) emission from dumpsites and landfills. Active methanotrophs in the biocovers play a crucial role in reduction of emissions through microbiological methane oxidation. Several factors affecting methane bio-oxidation (MOX) have been well documented, however, their interactive effect on the oxidation process needs to be explored. Therefore, the present study was undertaken to investigate the suitability of a dumpsite soil to be employed as biocover, under the influence of substrate concentrations (CH₄ and O₂) and temperature at variable incubation periods. Statistical design matrix of Response Surface Methodology (RSM) revealed that MOX rate up to 69.58μgCH₄g^-1_dwh^-1 could be achieved under optimum conditions. MOX was found to be more dependent on CH₄ concentration at higher level (30-40%, v/v), in comparison to O₂ concentration. However, unlike other studies MOX was found in direct proportionality relationship with temperature within a range of 25-35°C. The results obtained with the dumpsite soil biocover open up a new possibility to provide improved, sustained and environmental friendly systems to control even high CH₄ emissions from the waste sector.