Evaluation of respiration in compost landfill biocovers intended for methane oxidation

Precipitation and evaporation affecting landfill gas migration into passive methane oxidation biosystems: Models development and verification

Passive methane oxidation biosystems (PMOBs) are developed as an innovative and cost-effective solution to reduce methane (CH4) emissions from municipal solid waste landfills. A PMOB consists of a methane oxidation layer (MOL) and an underlying gas distribution layer (GDL). The length of unrestricted gas migration (LUGM) has been recently proposed as the design criterion for PMOBs where the LUGM is calculated as the horizontal length along the MOL-GDL interface with the volumetric gas content (θa) exceeding the threshold volumetric gas content (θa,occ). This paper examined water and gas migration within three PMOBs with different MOL-GDL interfaces subject to precipitation and evaporation using verified numerical models. The results show that the use of a single-phase flow model underestimates the LUGM values of the PMOB for heavy precipitation events, and a two-phase flow model should be used to calculate both the LUGM and the total gas mass flow rate into the MOL when designing PMOBs. Both zig-zag and trapezoidal MOL-GDL interfaces can redistribute the gas mass flow rate at the MOL-GDL interface, while the trapezoidal MOL-GDL interface slightly outperforms the zig-zag MOL-GDL interface for enhancing the total gas mass flow rate into the MOL when comparing with the planar MOL-GDL interface. The zig-zag and trapezoidal MOL-GDL interfaces allow gas migration in the upper part of each PMOB segment even when the lower part of each PMOB segment was filled with water, and thus have a potential to minimize hotspot formation.

Evaluating the efficacy of biogeochemical cover system in mitigating landfill gas emissions: A large-scale laboratory simulation

Municipal solid waste (MSW) landfills are a significant source of methane (CH4) emissions in the United States, contributing to global warming. Current landfill gas (LFG) management methods, like the landfill cover system and LFG collection system, do not entirely prevent LFG release. Biocovers have the potential to reduce CH4 emissions through microbial oxidation. However, LFG also contains carbon dioxide (CO2) and trace hydrogen sulfide (H2S) depending on waste composition, temperature, moisture content, and age of waste. An innovative biogeochemical cover (BGCC) was developed to tackle these concerns. This cover comprises a biochar-based biocover layer overlaid with a basic oxygen furnace (BOF) steel slag layer. The biochar-based biocover layer oxidizes CH4 emissions, while the BOF slag layer reduces CO2 and H2S through carbonation and sulfidation reaction mechanisms. The BGCC system's field performance remains unexamined. Therefore, a large-scale tank setup simulating near-field conditions was developed to evaluate the BGCC system's ability to mitigate CH4, CO2, and H2S from LFG simultaneously. Synthetic LFG was passed through the BGCC in five distinct phases, each designed to simulate the varying gas compositions and flux rates typical of MSW landfill. Gas profiles along the depth were monitored during each phase, and gas removal efficiency was measured. After testing, biocover and BOF slag samples were extracted to analyze physico-chemical properties. Batch tests were also conducted on samples extracted from the biocover and BOF slag layers to determine potential CH4 oxidation rates and residual CO2 sequestration capacity. The results showed that the BGCC system's CH4 removal efficiency decreased with higher CH4 flux rates, achieving its highest removal (74.7-79.7%) at moderate influx rates (23.9-25.5 g CH4/m2-day) and reducing to its lowest removal (27.4%) at the highest influx rate (57.5 g CH4/m2-day). Complete H2S removal occurred during Phase 3 in the biocover layer of BGCC system. CH4 oxidation rates were highest near the upper (277.9 µg CH4/g-day) and lowest in the deeper region of the biocover layer. In the tank experiment, CO2 breakthrough occurred after 156 days due to drying of the BOF slag layer, with an average residual carbonation capacity of 46 gCO2/kg slag after moisture adjustment. Overall, the BGCC system effectively mitigated LFG emissions, including CH4, CO2, and H2S, at moderate flux rates, showing promise as a comprehensive solution for LFG management.

The use of cellulolytic Aspergillus sp. inoculum to improve the quality of Pineapple compost

Pineapple litter has a complex polymer of cellulose, hemicellulose, and lignin, which makes them difficult to decompose. However, pineapple litter has great potential to be a good organic material source for the soil when completely decomposed. The addition of inoculants can facilitate the composting process. This study investigated whether the addition of cellulolytic fungi inoculants to pineapple litters improves the efficiency of the composting processes. The treatments were KP1 = pineapple leaf litter: cow manure (2:1), KP2 = pineapple stem litter: cow manure (2:1), KP3 = pineapple leaf litter: pineapple stem litter: cow manure P1 (leaf litter and 1% inoculum), P2 (stem litter and 1% inoculum), and P3 (leaf + stem litters and 1% inoculum). The result showed that the number of Aspergillus sp. spores on corn media was 5.64 x 10⁷ spores/mL, with viability of 98.58%. Aspergillus sp. inoculum improved the quality of pineapple litter compost, based on the enhanced contents of C, N, P, K, and the C/N ratio, during the seven weeks of composting. Moreover, the best treatment observed in this study was P1. The C/N ratios of compost at P1, P2, and P3 were within the recommended range of organic fertilizer which was 15-25%, with a Carbon/Nitrogen proportion of 11.3%, 11.8%, and 12.4% (P1, P2, and P3), respectively.

Mitigation of methane emissions from three Danish landfills using different biocover systems

The establishment of biocover systems is an emerging methodology in reducing methane (CH₄) emissions from landfills. This study investigated the performance of three biocover systems with different designs (biowindow and passively and actively loaded biofilters) in mitigating CH₄ emissions from three landfills in Denmark. A series of field tests were carried out to evaluate the functionality of each system, and total CH₄ emissions from relevant landfill sections or the entire landfill were measured before and after biocover implementation. Surface CH₄ concentration screening and local CH₄ fluxes showed generally low emissions from the biowindow/biofilters (mostly < 5 g CH₄ m^-2 d^-1), although some hotspots were identified on two actively loaded biofilters. One passively loaded biofilter exhibited high CH₄ emissions, mainly due to gas overloading into the system. Gas concentration profiles measured at different locations suggested uneven gas distribution in the biofilters, and significant CH₄ oxidation occurred in both the gas distribution layer (when oxygen was fed into the system) and the CH₄ oxidation layer. High CH₄ oxidation efficiencies of above 95% were found in all systems except for one biofilter (55%). Whole-site emission measurements showed CH₄ reduction efficiencies between 29 and 72% after implementing biocover systems at the three landfills, suggesting that they were efficient in reducing CH₄ emissions. The most challenging task for the passively loaded biocover systems was to control gas flow and secure homogenous gas distribution, while for actively loaded biocovers, it might be more important to eliminate emission hotspots for better functionality.

Methane Emission Reduction and Biological Characteristics of Landfill Cover Soil Amended With Hydrophobic Biochar

Biochar-amended landfill cover soil (BLCS) can promote CH₄ and O₂ diffusion, but it increases rainwater entry in the rainy season, which is not conducive to CH₄ emission reduction. Hydrophobic biochar–amended landfill cover soil (HLCS) was prepared to investigate the changes in CH₄ emission reduction and biological characteristics, and BLCS was prepared as control. Results showed that rainwater retention time in HLCS was reduced by half. HLCS had a higher CH₄ reduction potential, achieving 100% CH₄ removal at 25% CH₄ content of landfill gas, and its main contributors to CH₄ reduction were found to be at depths of 10–30 cm (upper layer) and 50–60 cm (lower layer). The relative abundances of methane-oxidizing bacteria (MOB) in the upper and lower layers of HLCS were 55.93% and 46.93%, respectively, higher than those of BLCS (50.80% and 31.40%, respectively). Hydrophobic biochar amended to the landfill cover soil can realize waterproofing, ventilation, MOB growth promotion, and efficient CH₄ reduction.

Revisiting the passive biocover system at Klintholm landfill, six years after construction

A biocover system was established at Klintholm landfill in Denmark in 2009 to mitigate methane emissions, and the system exhibited high mitigation efficiency during the first year after implementation. The biocover system was revisited in 2016/2017, and a series of field and laboratory tests were carried out to evaluate functionality about six years after establishment. Three field campaigns were executed in three different barometric pressure conditions, namely increasing, stable and decreasing. Local surface flux measurements and gas concentration profiles in the methane oxidation layer showed that barometric pressure changes had a significant effect on gas emission and methane oxidation. Elevated concentrations of oxygen were observed in the gas distribution layer, and field data showed that significant methane oxidation took place in this location. This finding was verified in laboratory-based methane oxidation incubation tests. Temperatures higher than ambient temperature were observed throughout the methane oxidation layer, with average temperatures ranging between 13 and 27 °C, even in the coldest month of the year. Field measurements showed that total methane emissions from the whole landfill cell were at the same level or lower than measurements performed in 2009/2010 after implementation of the biocover system, and laboratory tests showed methane oxidation potential approximately equal to former tests. In spite of an inhomogeneous distribution of landfill gas load to the methane oxidation layer, the performance of the biocover system had not declined over the 6-7 years since its establishment, even though no maintenance had been carried out in the intervening years.

Surface Hydrophobic Modification of Biochar by Silane Coupling Agent KH-570

Biochar-amended soil cover (BSC) in landfills can improve the oxidation of methane. However, adding biochar can cause a larger amount of rainwater to enter the soil cover and landfill because it increases the permeability of the soil cover, which increases leachate production. Improving the hydrophobicity and waterproof ability of BSC is expected to reduce rainwater that goes into landfills. Silane coupling agent KH-570 is used to modify biochar to improve its hydrophobicity and waterproof ability after being added to the soil cover. The waterproofness of hydrophobic biochar-amended soil cover (HBSC) was studied by conducting a precipitation simulation test. Results showed that the optimum hydrophobicity of the surface-modified biochar was obtained when the mass fraction of KH-570 was 7%, the biochar dosage was 7 g, and the modification temperature was 60 °C. In these conditions, the contact angle was 143.99° and the moisture absorption rate was 0.10%. The analysis results of thermogravimetric, X-ray diffractometer and scanning electron microscopy before and after the biochar modification showed that KH-570 formed a hydrophobic organic coating layer on the biochar surface, indicating that the surface hydrophobic modification of biochar was successfully carried out by silane coupling agent. The waterproof ability of HBSC was significantly better than that of BSC in the simulated precipitation test.

Current status of global warming potential reduction by cleaner composting

The global living standards are currently undergoing a stage of growth; however, such improvement also brings some challenges. Global warming is the greatest threat to all living things and attracts more and more attention on a global scale due to the rapid development of economy. Carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) are the common components of greenhouse gases, which contribute to the global warming. Mitigation technologies for these gas emissions are urgently needed in every industry for the aim of cleaner production. Traditional agriculture also contributes significantly to enhance the greenhouse gases emission. Composting is a novel and economic greenhouse gases mitigation strategy compared to other technologies in terms of the organic waste disposal. Some of the European countries showed an increase of more than 50% in the composting rate. The microbial respiration, nitrification and denitrification processes, and the generation of anaerobic condition makes the emission of greenhouse gases inevitable during composting. However, although there have been a lot of papers that focused on the reduction of greenhouse gases emission in composting, none of these has summarized the methods of reducing the emission of greenhouse gases during the composting. This review discusses the benefit of composting in greenhouse gases mitigation in the organic waste management and the current methods to improve mitigation efficiency during cleaner composting. Key physical, chemical, and biological parameters related to greenhouse gases mitigation strategies were precisely studied to give a deep understanding about the emission of greenhouse gases during cleaner composting. Furthermore, the mechanism of greenhouse gases emission mitigation strategies for cleaner composting based on various external measures would be helpful for the exploration of novel and effective mitigation strategies.

Simultaneous removal of ammonia and volatile organic compounds from composting of dead pigs and manure using pilot-scale biofilter

Odor emission is one of the most common problems associated with dead animals composting. Biofiltration treatment for eliminating exhaust odors formed during dead pigs and manure composting has been studied. The composting and biofiltration process consisted of two series of tests. Composting experimental trials lasted 6 weeks, and composting was performed using six pilot-scale reactor vessels. A total of 37 kinds of volatile organic compounds (VOCs) present in the air were identified, and temporal variations were determined during the 42 days of composting. Dimethyl sulfide (DMS), dimethyl disulfide (DMDS), dimethyl trisulfide (DMTS), and trimethylamine (TMA) were identified as the main odors VOCs component according to odor active values (OAVs). Nine biofilter vessels containing mature compost were used in studying the effect of different (30, 60, and 100 s) empty bed retention times (EBRT) on the simultaneous removal efficiencies (REs) of NH₃, DMS, DMDS, DMTS, and TMA. Results indicated that the inlet concentration of NH₃ applied was 12-447 mg m^-3, and the average removal efficiencies were 85.4%, 88.7%, and 89.0% for EBRTs of 30, 60, and 100 s, respectively. The average REs of DMS, DMDS, DMTS, and TMA were 79.2%-95.4%, 81.9%-94.0%, 76.7%-99.1%, and 92.9%-100%, respectively, and their maximum elimination capacity (ECs) were 220, 1301, 296, and 603 mg m^-3 h^-1, respectively. The optimal EBRT for the stimulation removal of NH₃, DMS, DMDS, DMTS, and TMA was 60 s.Implications: Dimethyl sulfide (DMS), dimethyl disulfide (DMDS), dimethyl trisulfide (DMTS), and trimethylamine (TMA) were identified as the main odors VOCs component during dead pigs and manure composting. Biofilter with mature as media can be used to stimulation remove NH₃, DMS, DMDS, DMTS, and TMA, the optimal empty bed retention times EBRT was 60 s.

Municipal Solid Waste Characterization and Landfill Gas Generation in Kakia Landfill, Makkah

In many countries, open dumping is considered the simplest, cheapest, and most cost-effective way of managing solid wastes. Thus, in underdeveloped economies, Municipal Solid Wastes (MSW) are openly dumped. Improper waste disposal causes air, water, and soil pollution, impairing soil permeability and blockage of the drainage system. Solid Waste Management (SWM) can be enhanced by operating a well-engineered site with the capacity to reduce, reuse, and recover MSW. Makkah city is one of the holiest cities in the world. It harbors a dozen of holy places. Millions of people across the globe visit the place every year to perform Hajj, Umrah, and tourism. In the present study, MSW characterization and energy recovery from MSW of Makkah was determined. The average composition of solid waste in Makkah city is organic matter (48%), plastics (25%), paper and cardboard (20%), metals (4%), glass (2%), textiles (1%), and wood (1%). In order to evaluate energy recovery potential from solid waste in Kakia open dumpsite landfill, the Gas Generation Model (LandGEM) was used. According to LandGEM results, landfill gas (methane and carbon dioxide) generation potential and capacity were determined. Kakia open dump has a methane potential of 83.52 m3 per ton of waste.

Enhancement of the methane removal efficiency via aeration for biochar-amended landfill soil cover

Methane (CH₄) mitigation of biocovers or biofilters for landfills is influenced by the bed material and oxygen availability. The improvement of active aeration for the CH₄ oxidation efficiency of biochar-amended landfill soil cover was investigated over a period of 101 days. There were column 1 as the control group, column 2 with biochar amending the soil cover, and column 3 with daily active aeration besides the same biochar amendment. All groups were inoculated with enriched methane oxidation bacteria (MOB). The average CH₄ removal efficiency was up to 78.6%, 85.2% and 90.6% for column 1, 2, and 3, respectively. The depth profiles of CH₄ oxidation efficiencies over the whole period also showed that the stimulation of CH₄ oxidation by biochar amendment was apparent in the top 35 cm but became very faint after two months. This probably was due to the rapid depletion of nitrogen nutrition caused by enhanced methanotrophic activities. While through aeration, CH₄ oxidation efficiency was further improved for column 3 than column 2. This enhancement also lasted for the whole period with a reduced decline of CH₄ oxidation. Finally, the major MOB Methylocystis, commonly found in the three columns, were most abundant in the top 35 cm for column 3. A more balanced ratio of MOB and more homogeneous microbial community structures across different soil depths were also the results of active aeration.

Field Study on the Efficiency of a Methane Degradation Layer Composed of Fine Fraction Soil from Landfill Mining

The main components of landfill gas are methane and carbon dioxide. Emissions of methane, a strong greenhouse gas, can be minimized by in situ oxidation in the bioactive cover layer. Typically, organic-rich porous materials such as compost are used for this process. In this study, the material for a biocover was obtained from the same landfill by landfill mining. The objective was to study the spatial distribution of gases and the efficiency of methane degradation in the biocover. The methane and carbon dioxide emissions were measured at 29 measuring points six times on the surface and once at a depth of 0.5 m. The highest values of both gases from the surface were recorded in July 2015: 1.0% for CO2 and 2.1% for CH4. Deeper in the cover layer, higher values of methane concentration were recorded. The results showed that (a) methane from the waste deposit was entering the biocover, (b) the migration of methane to the atmosphere was low, (c) fluctuations in the composition of gases are seasonal, and (d) the trend in the concentration of CH4 over time was an overall decrease. The described cover design reduces the CH4 emissions in landfills using elements of circular economy—instead of wasting natural soils and synthetic liners for the construction of the final cover layer, functional waste-derived materials can be used.

Model-based interpretation of methane oxidation and respiration processes in landfill biocovers: 3-D simulation of laboratory and pilot experiments

Landfill biocovers are an efficient strategy for the mitigation of greenhouse gas emissions from landfills. A complex interplay between key physical and reactive processes occurs in biocovers and affects the transport of gas components. Therefore, numerical models can greatly help the understanding of these systems, their design and optimal operation. In this study, we developed a 3-D multicomponent modeling approach to quantitatively interpret experimental datasets measured in the laboratory and in pilot-scale landfill biocovers. The proposed model is able to reproduce the observed spatial and temporal dynamics of CH₄, O₂ and CO₂ migration in biocovers under different operating conditions and demonstrates the importance of dimensionality in understanding the propagation of gas flow and migration of gas components in such porous media. The model allowed us to capture the coupled transport behavior of gas components, to evaluate the exchange of gas fluxes at the interface between the biocover surface and free air flow, and to investigate the effects of different gas injection patterns on the distribution of gas components within biocovers. The model also helps elucidating the dynamics and competition between methane oxidation and respiration processes observed in the different experimental setups. The simulation outcomes reveal that increasing availability of methane (i.e., higher injection flow rates or higher fractions of CH₄ in the injected gas composition) results in progressive dominance of methane oxidation in the biocovers and moderates the impact of respiration.

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.

Methane Emission Reduction Enhanced by Hydrophobic Biochar-Modified Soil Cover

The microbial oxidation of CH4 in biochar-modified soil cover is considered a potent option for the mitigation of emissions from old landfills or sites containing wastes full of low CH4 generation rates. The mechanism of methane oxidizing bacteria (MOB) can be enhanced by amending the landfill cover soil with biochar, which is recalcitrant to biological degradation and can adsorb CH4 while facilitating the growth and activity of MOB within its porous structure. However, the increase in the permeability coefficient and water content of the cover due to the addition of biochar also affects the methane removal efficiency. A hydrophobic biochar modified by KH-570 was employed to reduce the water content and to promote the diffusion and oxidation of CH4 in the cover. Several series of small-scale column tests were conducted to quantify the CH4 oxidation properties of the landfill cover soil amended with biochar and hydrophobic biochar under different levels of exposed CH4 concentrations (5% and 15%), heights (10–66 cm), and temperatures (15–40 °C). After 30 days of domestication, the removal rate of the hydrophobic biochar-modified soil cover reached 98.8%. The water holding capacity of the cover and the CH4 oxidation efficiency under different moisture contents were investigated in different columns. The hydrophobic biochar-modified soil cover has a weak water holding capacity, low saturated water content, and optimal CH4 oxidation efficiency at this time.

Use of mature compost as filter media and the effect of packing depth on hydrogen sulfide removal from composting exhaust gases by biofiltration

A study was conducted to investigate the utilization of mature compost as a biofilter medium for the removal of hydrogen sulfide (H₂S) from the exhaust gases of the composting process. Source-selected kitchen waste from municipal solid waste was composted in a reactor, and the exhaust gas was passed through a biofilter packed with a 1:4 (wet weight) mixture of mature compost and sand. Two treatments were applied under sterilized and unsterilized conditions to quantify the contribution of microbial activity. The effect of packing depth on H₂S removal efficiency was also studied. A global H₂S removal efficiency of 51% was obtained in the biofilter for loading rates in the range of 0-429 mg H₂S m^-3 h^-1. The adsorption capacity was the main factor affecting H₂S removal efficiency, contributing 64.2% to the total removal efficiency, with microbial activity contributing 35.8%. The relationship between the cumulative amount of H₂S removed and the packing height was well-described by a linear equation. The equation indicated that 99% H₂S removal efficiency could be achieved using a packing height of 96 cm for unsterilized packing material or 158 cm for sterilized packing material.

Effects of digestion time in anaerobic digestion on subsequent digestate composting

Digestion time (DT) in anaerobic digestion (AD) on performance of subsequent digestate composting regarding compost maturity and greenhouse gas (GHG) emission was investigated. Digestates for composting were obtained after anaerobically digested mixture of dairy manure, corn stalks, and tomato residues (48:32:20, volatile solids based) with DT of 15, 30, and 45 days, respectively. Digestates were composted with corn stalks (85:15, wet weight based). Results showed approximately 30% and 70% of biochemical methane potential (342.0 L/kg VS_feedstock) were obtained when DT of 15 and 30 days. Digestate co-composting with cornstalks could be initiated effectively and reduced GHG emissions by 18.9-29.0% compared to compost with raw materials. DT of 30 and 45 days digestate composting cause benefit on germination index. DT of 45 days had the highest net power production in combine AD and composting system. DT of 30 days digestate composting was optimum choice for compost maturity and GHG emissions.

Methodology to determine the extent of anaerobic digestion, composting and CH4 oxidation in a landfill environment

An examination of the processes contributing to the production of landfill greenhouse gas (GHG) emissions is required, as the actual level to which waste degrades anaerobically and aerobically beneath covers has not been differentiated. This paper presents a methodology to distinguish between the rate of anaerobic digestion (r_AD), composting (r_COM) and CH₄ oxidation (r_OX) in a landfill environment, by means of a system of mass balances developed for molecular species (CH₄, CO₂) and stable carbon isotopes (δ¹³C-CO₂ and δ¹³C-CH₄). The technique was applied at two sampling locations on a sloped area of landfill. Four sampling rounds were performed over an 18 month period after a 1.0 m layer of fresh waste and 30-50 cm of silty clay loam had been placed over the area. Static chambers were used to measure the flux of the molecular and isotope species at the surface and soil gas probes were used to collect gas samples at depths of approximately 0.5, 1.0 and 1.5 m. Mass balances were based on the surface flux and the concentration of the molecular and isotopic species at the deepest sampling depth. The sensitivity of calculated rates was considered by randomly varying stoichiometric and isotopic parameters by ±5% to generate at least 500 calculations of r_OX, r_AD and r_COM for each location in each sampling round. The resulting average value of r_AD and r_COM indicated anaerobic digestion and composting were equally dominant at both locations. Average values of r_COM: ranged from 9.8 to 44.5 g CO₂ m^-2 d^-1 over the four sampling rounds, declining monotonically at one site and rising then falling at the other. Average values of r_AD: ranged from 10.6 to 45.3 g CO₂ m^-2 d^-1. Although the highest average r_AD value occurred in the initial sampling round, all subsequent r_AD values fell between 10 and 20 g CO₂ m^-2 d^-1. r_OX had the smallest activity contribution at both sites, with averages ranging from 1.6 to 8.6 g CO₂ m^-2 d^-1. This study has demonstrated that for an interim cover, composting and anaerobic digestion of shallow landfill waste can occur simultaneously.

Effects of biochar and wood pellets amendments added to landfill cover soil on microbial methane oxidation: A laboratory column study

Alternate landfill covers designed to enhance microbial methane (CH₄) oxidation and reduce the negative impacts of landfill gas emissions on global climate have recently been proposed and investigated. In this study, the use of biochar as a soil amendment is examined in order to assess the feasibility and effectiveness for enhanced CH₄ removal in landfill covers when incorporated under high compaction conditions and relatively low soil moisture. Four different cover configurations were tested in large soil columns for ∼510 days and potential CH₄ oxidation rates were determined following long-term incubation in small batch assays. Cover designs tested include: a thin biochar layer at 15-18 cm; 2% mixed soil-biochar layer at 20-40 cm; 2% mixed soil-uncharred wood pellets at 20-40 cm; and soil obtained from intermediate cover at an active landfill site. The placement of a thin biochar layer in the cover significantly impacted moisture distribution and infiltration, which in turn affected CH₄ oxidation potential with depth. An increase in CH₄ removal rates was observed among all columns over the 500 day incubation period, with steady-state CH₄ removal efficiencies ranging from ∼60 to 90% in the final stages of incubation (inlet load ∼80 g CH₄ m^-2 d^-1). The thin biochar layer had the lowest average removal efficiency as a result of reduced moisture availability below the biochar layer. The addition of 2% biochar to soil yielded similar CH₄ oxidation rates in terminal assays as the 2% uncharred wood pellet amendment. CH₄ oxidation rates in terminal assays were positively correlated with soil moisture, which was affected by the materials' water holding capacity. The high water holding capacity of biochar led to higher oxidation rates within the thin biochar layer, supporting the initial hypothesis that biochar may confer more favorable physical conditions for methanotrophy. Ultimate performance was apparently affected by soil type and CH₄ exposure history, with the highest oxidation rates observed in the unamended field soil with higher initial methanotrophic activity.

A mass balance model to estimate the rate of composting, methane oxidation and anaerobic digestion in soil covers and shallow waste layers

Although CH₄ oxidation in landfill soil covers is widely studied, the extent of composting and CH₄ oxidation in underlying waste layers has been speculated but not measured. The objective of this study was to develop and validate a mass balance model to estimate the simultaneous rates of anaerobic digestion (r_AD), CH₄ oxidation (r_OX) and composting (r_COM) in environments where O₂ penetration is variable and zones of aerobic and anaerobic activity are intermingled. The modelled domain could include, as an example, a soil cover and the underlying shallow waste to a nominated depth. The proposed model was demonstrated on a blend of biogas from three separate known sources of gas representing the three reaction processes: (i) a bottle of laboratory grade 50:50% CH₄:CO₂ gas representing anaerobic digestion biogas; (ii) an aerated 250mL bottle containing food waste that represented composting activity; and (iii) an aerated 250mL bottle containing non-degradable graphite granules inoculated with methanotrophs and incubated with CH₄ and O₂ to represent methanotrophic activity. CO₂, CH₄, O₂ and the stable isotope ¹³C-CO₂ were chosen as the components for the mass balance model. The three reaction rates, r (=r_AD, r_OX, r_COM) were calculated as fitting parameters to the overdetermined set of 4mass balance equations with the net flux of these components from the bottles q (= [Formula: see text] , [Formula: see text] , [Formula: see text] and [Formula: see text] ) as inputs to the model. The coefficient of determination (r²) for observed versus modelled values of r were 1.00, 0.97, 0.98 when the stoichiometry of each reaction was based on gas yields measured in the individual bottles and q was calculated by summing yields from the three bottles. r² deteriorated to 0.95, 0.96, 0.87 when using an average stoichiometry from 11 incubations of each of the composting and methane oxidation processes. The significant deterioration in the estimation of r_COM showed that this output is highly sensitive to the evaluated stoichiometry coefficients for the reactions. r² deteriorated further to 0.86, 0.77, 0.74 when using the average stoichiometry and experimental measurement of the composition and volume of the blended biogas to determine q. This was primarily attributed to average errors of 8%, 7%, 11% and 14% in the measurement of [Formula: see text] , [Formula: see text] , [Formula: see text] and [Formula: see text] relative to the measurement of the same quantities from the individual bottles.

Mitigation of methane emissions in a pilot-scale biocover system at the AV Miljø Landfill, Denmark: 2. Methane oxidation

Greenhouse gas mitigation at landfills by methane (CH₄) oxidation in engineered biocover systems is believed to be a cost effective technology but so far a full quantitative evaluation of the efficiency of the technology in full scale has only been carried out in a few cases. A third generation semi-passive biocover system was constructed at the AV Miljø Landfill, Denmark. The biocover was fed by landfill gas pumped out of three leachate collection wells. An innovative gas distribution system was used to overcome the often observed uneven gas distribution to the active CH₄ oxidation layer resulting in overloaded areas causing CH₄ emission hot spot areas in the biocover surface. The whole biocover CH₄ oxidation efficiency was determined by measuring the CH₄ inlet load and CH₄ surface fluxes. In addition, CH₄ oxidation was determined for single points in the biocover using two different methods; the carbon mass balance method (based on CH₄ and carbon dioxide (CO₂) concentrations in the deeper part of the cover and CH₄ and CO₂ surface flux measurements) and a new-developed tracer gas mass balance method (based on CH₄ and tracer inlet fluxes and CH₄ and tracer surface flux measurements). Overall, the CH₄ oxidation efficiency of the whole biocover varied between 81 and 100% and showed that the pilot plant biocover system installed at AV Miljø landfill was very efficient in oxidizing the landfill CH₄. The average CH₄ oxidation rate measured at nine campaigns was approximately 13gm^-2d^-1. Extrapolating laboratory measured CH₄ oxidation rates to the field showed that the biocover system had a much larger CH₄ oxidation potential in comparison to the tested CH₄ load. The carbon mass balance approach compared reasonably well with the tracer gas mass balance approach when applied for quantification of CH₄ oxidation in single points at the biofilter giving CH₄ oxidation efficiencies in the range of 84 to a 100%. CH₄ oxidation rates where however much higher using the tracer gas balance method giving CH₄ oxidation rates between 7 and 124gm²d^-1 compared to the carbon mass balance, which gave CH₄ oxidation rates -0.06 and 40gm²d^-1. The study also revealed that the compost respiration contributed significantly to the measured CO₂ surface emission, and that the contribution of the compost respiration decreased significantly with time probably due to further maturation of the compost material.

Effects of preconditioning the rhizosphere of different plant species on biotic methane oxidation kinetics

The rhizosphere is known as the most active biogeochemical layer of the soil. Therefore, it could be a beneficial environment for biotic methane oxidation. The aim of this study was to document - by means of batch incubation tests - the kinetics of CH4 oxidation in rhizosphere soils that were previously exposed to methane. Soils from three pre-exposure to CH4 zones were sampled: the never-before pre-exposed (NEX), the moderately pre-exposed (MEX) and the very pre-exposed (VEX). For each pre-exposure zone, the rhizosphere of several plant species was collected, pre-incubated, placed in glass vials and submitted to CH4 concentrations varying from 0.5% to 10%. The time to the beginning of CH4 consumption and the CH4 oxidation rate were recorded. The results showed that the fastest CH4 consumption occurred for the very pre-exposed rhizosphere. Specifically, a statistically significant difference in CH4 oxidation half-life was found between the rhizosphere of the VEX vegetated with a mixture of different plants and the NEX vegetated with ryegrass. This difference was attributed to the combined effect of the preconditioning level and plant species as well as to the organic matter content. Regardless of the preconditioning level, the oxidation rate values obtained in this study were comparable to those reported in the reviewed literature for mature compost.

Adsorption and transport of methane in biochars derived from waste wood

Mitigation of landfill gas (LFG) is among the critical aspects considered in the design of a landfill cover in order to prevent atmospheric pollution and control global warming. In general, landfill cover soils can partially remove methane (CH4) through microbial oxidation carried out by methanotrophic bacteria present within them. The oxidizing capacity of these landfill cover soils may be improved by adding organic materials, such as biochar, which increase adsorption and promote subsequent or simultaneous oxidation of CH4. In this study, seven wood-derived biochars and granular activated carbon (GAC) were characterized for their CH4 adsorption capacity by conducting batch and small-scale column studies. The effects of influential factors, such as exposed CH4 concentration, moisture content and temperature on CH4 adsorption onto biochars, were determined. The CH4 transport was modeled using a 1-D advection-dispersion equation that accounted for sorption. The effects of LFG inflow rates and moisture content on the combined adsorption and transport properties of biochars were determined. The maximum CH4 adsorption capacity of GAC (3.21mol/kg) was significantly higher than that of the biochars (0.05-0.9mol/kg). The CH4 gas dispersion coefficients for all of the biochars ranged from 1×10(-3) to 3×10(-3)m(2)s(-1). The presence of moisture significantly suppressed the extent of methane adsorption onto the biochars and caused the methane to break through within shorter periods of time. Overall, certain biochar types have a high potential to enhance CH4 adsorption and transport properties when used as a cover material in landfills. However, field-scale studies need to be conducted in order to evaluate the performance of biochar-based cover system under a more dynamic field condition that captures the effect of seasonal and temporal changes.

Adsorption and transport of methane in landfill cover soil amended with waste-wood biochars

The natural presence of methane oxidizing bacteria (MOB) in landfill soils can stimulate the bio-chemical oxidation of CH4 to CO2 and H2O under suitable environmental conditions. This mechanism can be enhanced by amending the landfill cover soil with organic materials such as biochars that are recalcitrant to biological degradation and are capable of adsorbing CH4 while facilitating the growth and activity of MOB within their porous structure. Several series of batch and small-scale column tests were conducted to quantify the CH4 sorption and transport properties of landfill cover soil amended with four types of waste hardwood biochars under different levels of amendment percentages (2, 5 and 10% by weight), exposed CH4 concentrations (0-1 kPa), moisture content (dry, 25% and 75% water holding capacity), and temperature (25, 35 and 45 °C). The linear forms of the pseudo second-order kinetic model and the Langmuir isotherm model were used to determine the kinetics and the maximum CH4 adsorption capacity of cover materials. The maximum CH4 sorption capacity of dry biochar-amended soils ranged from 1.03 × 10(-2) to 7.97 × 10(-2) mol kg(-1) and exhibited a ten-fold increase compared to that of soil with 1.9 × 10(-3) mol kg(-1). The isosteric heat of adsorption for soil was negative and ranged from -30 to -118 kJ/mol, while that of the biochar-amended soils was positive and ranged from 24 to 440 kJ/mol. The CH4 dispersion coefficients for biochar-amended soils obtained through predictive transport modeling indicated that amending the soil with biochar enhanced the methane transport rates by two orders of magnitude, thereby increasing their potential for enhanced exchange of gases within the cover system. Overall, the use of hardwood biochars as a cover soil amendment to reduce methane emissions from landfills appears to be a promising alternative to conventional soil covers.

Rapid assessment of methanotrophic capacity of compost-based materials considering the effects of air-filled porosity, water content and dissolved organic carbon

Since the global warming potential of CH4 is 25 times that of CO2 on a 100-year time horizon, the development of methanotrophic applications for the conversion of CH4 to CO2 is emerging as an area of interest for researchers and practicing engineers. Compost exhibits most of the characteristics required for methanotroph growth media and has been used in several projects. This paper presents results from a study that was undertaken to assess the influence of physical and chemical characteristics of compost-based materials on the biological oxidation of CH4 when used in methane biofilters. The results showed that easily-measurable parameters, such as air filled porosity, water content and dissolved organic carbon, are correlated with maximum CH4 removal rates. The results obtained were used to develop an empirical relationship that could be regarded as a rapid assessment tool for the estimation of the performance of compost-based materials in engineered methanotrophic applications.

Effects of mixing and covering with mature compost on gaseous emissions during composting

This study investigated effects of mature compost on gaseous emissions during composting using pig manure amended with corn stalks. Apart from a control treatment, three treatments were conducted with the addition of 5% (wet weight of raw materials) of mature compost: (a) mixing raw materials with mature compost at the beginning of composting; (b) covering raw materials with mature compost throughout the experimental period; and (c) covering raw materials with mature compost at the start of composting, but incorporating it into composting pile on day 6 of composting. Mature compost used for the last treatment was inoculated with 2% (wet weight) of raw materials of strain M5 (a methanotrophic bacterium) solution. During 30-d of composting, three treatments with the addition of mature compost could reduce CH4 emission by 53-64% and N2O emission by 43-71%. However, covering with mature compost throughout the experimental period increased cumulative NH3 emission by 61%, although it could reduce 34% NH3 emission in the first 3d. Inoculating strain M5 in mature compost covered on the top of composting pile within first 6d enhanced CH4 oxidation, but simultaneously increased N2O emission. In addition, mixing with mature compost could improve compost maturity. Given the operational convenience in practice, covering with mature compost and then incorporating it into composting pile is a suitable approach to mitigate gaseous emissions during composting.

Effects of nitrogen conversion and environmental factors on landfill CH4 oxidation and N2O emissions in aged refuse

We determined the effects of nitrification capacity and environmental factors on landfill methane oxidation potential (MOP) using an aged refuse in laboratory batch assays and compared it with two different types of soils. The nitrogen conversion in the three experimental materials after 120 h incubation yielded first-order reaction kinetics at an initial concentration of 200 mg kg(-1) NH4(+)-N. The net nitrification rate for the aged refuse was 1.50 (p < 0.05) and 2.08 (p < 0.05) times that of the clay soil and the sandy soil, respectively. The net NO3(-)-N generation rate by the aged refuse was 1.93 (p < 0.05) and 2.57 (p < 0.05) times that of the clay soil and the sandy soil, respectively. When facilitated by ammonia-oxidizing bacteria during CH4 co-oxidation, the average value of the MOP in the aged refuse at a temperature range of 4-45 °C was 2.34 (p < 0.01) and 4.71 (p < 0.05) times greater than that of the clay soil and the sandy soil, respectively. When the moisture content ranged from 8 to 32% by mass, the average values for the MOP in the aged refuse were 2.08 (p < 0.01) and 3.15 (p < 0.01) times greater than that of the clay soil and the sandy soil, respectively. The N2O fluxes in the aged refuse at 32% moisture content were 5.33 (p < 0.05) and 12.00 (p < 0.05) times more than in the clay and the sandy soil, respectively. The increase in N2O emissions from a municipal solid waste landfill can be neglected after applying an aged refuse bio-cover because of the much higher MOP in the aged refuse. The calculated maximum MOP value in the aged refuse was 12.45 μmol g(-1) d.w. h(-1), which was much higher than the documented data.

Design of top covers supporting aerobic in situ stabilization of old landfills--an experimental simulation in lysimeters

Landfill aeration by means of low pressure air injection is a promising tool to reduce long term emissions from organic waste fractions through accelerated biological stabilization. Top covers that enhance methane oxidation could provide a simple and economic way to mitigate residual greenhouse gas emissions from in situ aerated landfills, and may replace off-gas extraction and treatment, particularly at smaller and older sites. In this respect the installation of a landfill cover system adjusted to the forced-aerated landfill body is of great significance. Investigations into large scale lysimeters (2 × 2 × 3m) under field conditions have been carried out using different top covers including compost materials and natural soils as a surrogate to gas extraction during active low pressure aeration. In the present study, the emission behaviour as well as the water balance performance of the lysimeters has been investigated, both prior to and during the first months of in situ aeration. Results reveal that mature sewage sludge compost (SSC) placed in one lysimeter exhibits in principle optimal ambient conditions for methanotrophic bacteria to enhance methane oxidation. Under laboratory conditions the mature compost mitigated CH(4) loadings up to 300 lCH(4)/m(2)d. In addition, the compost material provided high air permeability even at 100% water holding capacity (WHC). In contrast, the more cohesive, mineral soil cover was expected to cause a notably uniform distribution of the injected air within the waste layer. Laboratory results also revealed sufficient air permeability of the soil materials (TS-F and SS-Z) placed in lysimeter C. However, at higher compaction density SS-Z became impermeable at 100% WHC. Methane emissions from the reference lysimeter with the smaller substrate cover (12-52 g CH(4)/m(2)d) were significantly higher than fluxes from the other lysimeters (0-19 g CH(4)/m(2)d) during in situ aeration. Regarding water balance, lysimeters covered with compost and compost-sand mixture, showed the lowest leachate rate (18-26% of the precipitation) due to the high water holding capacity and more favourable plant growth conditions compared to the lysimeters with mineral, more cohesive, soil covers (27-45% of the precipitation). On the basis of these results, the authors suggest a layered top cover system using both compost material as well as mineral soil in order to support active low-pressure aeration. Conventional soil materials with lower permeability may be used on top of the landfill body for a more uniform aeration of the waste due to an increased resistance to vertical gas flow. A compost cover may be built on top of the soil cover underlain by a gas distribution layer to improve methane oxidation rates and minimise water infiltration. By planting vegetation with a high transpiration rate, the leachate amount emanating from the landfill could be further minimised. The suggested design may be particularly suitable in combination with intermittent in situ aeration, in the later stage of an aeration measure, or at very small sites and shallow deposits. The top cover system could further regulate water infiltration into the landfill and mitigate residual CH(4) emissions, even beyond the time of active aeration.

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.