Comprehensive overview of numerical modeling of coupled landfill processes

Study of LFG bubble accumulation and discontinuous flow in the highly saturated region of landfill below the leachate level

Landfills in developing countries are typically characterized by high waste water content and elevated leachate levels. Despite the ongoing biodegradation of waste in the highly saturated regions of these landfills, which leads to gas accumulation and bubble formation, the associated gas pressure that poses a risk to landfill stability is often overlooked. This paper introduces a landfill gas (LFG) bubble generation model and a two-fluid model that considers bubble buoyancy and porous medium resistance. The entire process can be divided into two stages based on the force balance and velocity of bubbles: Bubble Development Stage and the Two-Fluid Flow Stage. The models were validated using a one-dimensional analytical solution of hydraulic distribution that considers bubble generation, as well as an experiment involving air injection into a saturated medium. The mechanisms of LFG accumulation and ascent, leachate level rise, and discontinuous leachate-gas flow were then investigated in conjunction with continuous flow in the unsaturated region. The results indicate that the generation of LFG bubbles below the leachate level can cause a rise in the level height of more than 20%. During the Bubble Development Stage, there is a critical height for bubble ascent, above which the buoyancy exceeds the combined forces of gravity and resistance, resulting in less than 10% of bubbles continuously flowing into the unsaturated zone for recovery. The developed model effectively captures the accumulation and flow of LFG bubbles below the leachate level and could be further utilized to study leachate-gas pumping in the future.

A comprehensive study of settlement during the filling and post-closure phases at a landfill in Québec, Canada: Field data and TMB modelling

In Northern climates, waste placed curbside the evening before waste collection can lead to partially frozen waste at placement, which delays biodegradation and biodegradation-induced settlement. A 12-year settlement dataset collected during the filling and post-closure phases at a landfill in Québec, Canada was analyzed. The dataset showed a delay in biodegradation-induced settlement due to the first three waste lifts being placed in the winter months and exhibited an increase in the settlement rate at later times when the waste temperatures increased to values that support biodegradation. The field data also demonstrated that the stiffness of MSW increased in response to confined stress as subsequent waste lifts were added. A thermal-mechanical-biological (TMB) model was developed, in COMSOL Multiphysics, to simulate the settlement dataset. TMB integrates a Generalized Kelvin-Voigt (GKV) model, simulating instantaneous and mechanical creep settlements, with a biodegradation-induced settlement model that relates heat/gas generation with time to biodegradation-induced settlement. The thermal model simulates heat transfer through conduction and includes a biodegradation heat generation source term. The GKV stiffness parameters are expressed as a function of the applied stress to account for waste compressibility effects on mechanical response, which is consistent with field data and the research literature. The paper focuses solely on the MSW settlement field data and model predictions, with thermal response analysis presented in a separate publication. The TMB model effectively predicted waste behaviour, including resistance to compressibility under higher stress and the delay in waste settlement for waste placed in winter. The temperature and settlement data provide a valuable dataset to validate different models that can be used to predict waste settlement in cold regions.

Semi-quantitative study on the secondary compression characteristics of municipal solid waste in aerobic and anaerobic bioreactors

Aeration plays a crucial role in accelerating the secondary compression of municipal solid waste (MSW) for the scientific implementation of aerobic bioreactor technology. There are few comparative reports on the secondary compaction characteristics of MSW in aerobic and anaerobic bioreactors. In this study, six long-term compression tests were conducted to analyze the impact of aeration on MSW compression characteristics, considering two degradation conditions (i.e. aerobic and anaerobic conditions) and three overburden stresses (i.e. 30, 50 and 100 kPa). Model-fitting analysis was employed to examine the data from the tests and exiting literatures. The results showed that aeration effectively increased the rate of secondary compression, and slightly enhanced the steady-state secondary compression strain. In addition, these enhancements tended to decrease with increasing stresses. The increment ratio of the secondary compression rate constant (Rk) was concentrated in the range of 25 % to 100 %, and increases with the increase of aeration rate. The increment ratio of the steady-state secondary compression strain (Rε) ranged from 10 % to 90 %, for the MSW with higher content of paper and wood exhibited higher Rε. The advance ratio of the secondary compression stabilization time (Rt) fell within the range of 20-50 %, and Rt is higher when the moisture content is in the range of 50-65 %. These findings provide valuable guidance on the accelerated stabilization in aerobic bioreactors, providing practical references for the application of aerobic technology to informal landfills.

Landfill bacteriology: Role in waste bioprocessing elevated landfill gaseselimination and heat management

This study delves into the critical role of microbial ecosystems in landfills, which are pivotal for handling municipal solid waste (MSW). Within these landfills, a complex interplay of several microorganisms (aerobic/anaerobic bacteria, archaea or methanotrophs), drives the conversion of complex substrates into simplified compounds and complete mineralization into the water, inorganic salts, and gases, including biofuel methane gas. These landfills have dominant biotic and abiotic environments where various bacterial, archaeal, and fungal groups evolve and interact to decompose substrate by enabling hydrolytic, fermentative, and methanogenic processes. Each landfill consists of diverse bio-geochemical environments with complex microbial populations, ranging from deeply underground anaerobic methanogenic systems to near-surface aerobic systems. These kinds of landfill generate leachates which in turn emerged as a significant risk to the surrounding because generated leachates are rich in toxic organic/inorganic components, heavy metals, minerals, ammonia and xenobiotics. In addition to this, microbial communities in a landfill ecosystem could not be accurately identified using lab microbial-culturing methods alone because most of the landfill's microorganisms cannot grow on a culture medium. Due to these reasons, research on landfills microbiome has flourished which has been characterized by a change from a culture-dependent approach to a more sophisticated use of molecular techniques like Sanger Sequencing and Next-Generation Sequencing (NGS). These sequencing techniques have completely revolutionized the identification and analysis of these diverse microbial communities. This review underscores the significance of microbial functions in waste decomposition, gas management, and heat control in landfills. It further explores how modern sequencing technologies have transformed our approach to studying these complex ecosystems, offering deeper insights into their taxonomic composition and functionality.

A review of existing methods for predicting leachate production from municipal solid waste landfills

Landfilling is one of the predominant methods of municipal solid waste (MSW) disposal worldwide, while the generation of leachate, a kind of toxic wastewater, is among the primary factors behind landfill instability and environmental contamination problems. Precise prediction of leachate production is crucial to landfill safety evaluation and design. This paper presents a comprehensive review of methods for predicting leachate production from MSW landfills. Firstly, compositional characteristics of MSW and leachate generation mechanism are analysed. Factors influencing leachate production are summarised based on the generation mechanism, including the components of MSW, climatic conditions, landfill structure, and environmental factors. Then, we classified the existing methods for predicting leachate production into four categories: water balance formula, water balance model, empirical formula, and artificial intelligence model methods. Advantages, disadvantages, and applicability of different leachate production prediction methods are compared and analysed. Furthermore, limitations in the existing leachate production prediction methods for MSW landfills and scope for future research are discussed.

Effects of initial temperature and moisture content on heat generation during degradation of municipal solid waste

Heat generation from degradation of organic matter in municipal solid waste (MSW) often leads to increased landfill temperature. However, it is difficult to measure environmental heat loss in laboratory and field tests; therefore, little research has been conducted to evaluate heat generation during waste degradation under different initial temperatures and moisture contents. In this study, tests were conducted to investigate the effects of initial temperature and moisture content on heat generation during waste degradation. A simple formula for calculating heat generation was proposed. Within 200 h, the waste temperature decreased by about 70%, and lower initial moisture contents were associated with greater temperature decreases. The smallest temperature decrease of 47% and the greatest heat generation occurred when the initial temperature was 40 °C. The initial moisture content increased from 30% to 60% and the heat generation increased from 5% to 36%. The heat generation per unit mass of organic matter during the aerobic and anaerobic stages were 19.44-23.77 and 0.27-0.50 MJ·kg-1, respectively, indicating that the proposed formula for calculation of heat generated from waste degradation was reasonable. The results presented herein provide theoretical support for the prediction of heat generation and the recycling of heat resources in MSW landfill sites.

A first-order kinetic model for simulating the aerobic degradation of municipal solid waste

Aerobic degradation models are important tools for investigating the aerobic degradation behavior of municipal solid waste (MSW). In this paper, a first-order kinetic model for aerobic degradation of MSW was developed. The model comprehensively considers the aerobic degradation of five substrates, i.e., holocellulose, non-cellulosic sugars, proteins, lipids and lignin. The proportion ranges of the five substrates are summarized with the recommended values given. The effects of temperature, moisture content, oxygen concentration and free air space (FAS) on the reaction rates are considered, and the effect of settlement is accounted for in the FAS correction function. The reliability of the model was verified by comparing simulations of the aerobic degradation of low food waste content (LFWC-) and high food waste content (HFWC-) MSWs to the literature. Afterwards, a sensitivity analysis was carried out to establish the relative importance of aeration rate (AR), volumetric moisture content (VMC), and temperature. VMC had the greatest influence on the aerobic degradation of LFWC-MSW, followed by temperature and then AR; for HFWC-MSW, temperature was the most important factor, then VMC and last was AR. The degradation ratio of LFWC-MSW can reach 98.0% after 100 days degradation under its optimal conditions (i.e., temperature: 55 °C, VMC: 40%, AR: 0.16 L min^-1 kg^-1 DM), while it is slightly higher as 99.5% for HFWC-MSW under its optimal conditions (i.e., temperature: 55 °C, VMC: 40%, AR: 0.20 L min^-1 kg^-1 DM).

Gas transport in landfill cover system: A critical appraisal

Landfill gas (LFG) emission is gaining more attention from the scientific fraternity and policymakers recently due to its threat to the atmosphere and human health of the populace living in surrounding premises. Though landfill cover (LFC) (viz., daily, intermittent and final cover) is widely used by landfill operators to mitigate or reduce these emissions, their overall performance is still under question. A critical analysis of available literature, primarily pertaining to (i) the composition of the landfill gases and their migration in the LFC system, (ii) experimental and mathematical investigations of the transport mechanism of gas and (iii) the impact of additives to cover soils on transport and fate of gas, has been conducted and presented in this manuscript. Investigation of the efficiency of modified soil was mainly focused on laboratory test. More field tests and application of amended cover soils should be conducted and promoted further. Studies on nitrous oxide and emerging pollutants, including poly-fluoroalkyl substances transport in landfill cover system are limited and need further research. The transport mechanisms of these unconventional contaminants should be considered regarding the selection of LFC materials including geomembrane and geosynthetic clay liners. The existing analytical and numerical models can provide a basic understanding of LFG transport mechanisms and are able to predict the migration behaviour of LFG; however, there are still knowledge gaps concerning the interaction between different species of the gas molecule when modeling multi-component gas transport. Gas transport through fractured cover should also be considered when evaluating LFG emission in the future. Simplified design method for landfill cover system regarding LFG emission based on analytical models should be proposed. Overall, mathematical models combined with experiments can facilitate more visualized and intensive insights, which would be instrumental in devising climate adaptive landfill covers.

A review on settlement models of municipal solid waste landfills

Currently, landfill is the most common, economical, and convenient method for municipal solid waste (MSW) disposal in countries around the world. MSW has a complex composition and special engineering characteristics, which lead to a very complex settlement mechanism in MSW landfills. This article reviews the description of this settlement mechanism in the existing literature and classifies it into stress-related mechanisms, biodegradation processes of organic substances, water-related mechanisms and physical and chemical processes of inorganic components. Based on the settlement mechanism, the influencing factors of landfill settlement were analysed, including the composition of MSW, physical parameters, environmental factors, and the operation mode of the landfill. Some practical engineering suggestions are obtained by analysing the influencing factors of MSW landfill settlement. Four commonmethods for studying the settlement of MSW landfills are presented, including laboratory experiments, in-situ settlement monitoring, theoretical analysis, and numerical simulation. We classified the existing settlement models into six categories: a soil mechanics, rheological, empirical, biodegradation, constitutive, and multiphase coupling models. Advantages and disadvantages of the different models and their applicability are compared and analysed. Moreover, limitations in the modelling process of MSW landfill settlement and future research directions are discussed.

Biochemical-thermal-hydro-mechanical coupling model for aerobic degradation of landfilled municipal solid waste

Ventilating solid waste landfills with an oxygen supply can effectively accelerate the degradation of waste, achieve rapid stabilization, and realize the sustainable utilization of landfills. Aiming to understand and verify the aerobic degradation process in landfills, this paper proposed a biochemical-thermal-hydro-mechanical coupling model. The model considers aerobic biochemical reactions, dissolved solute migration, heat transport, two-phase flow, and skeleton deformation. The model was verified by comparison with an in-situ experiment at Jinkou landfill. The results showed the model could accurately represent the observed degradation phenomena during the experiment. The modelling results indicated that the rate of temperature increase and peak temperature of the upper layer, which were lower than those of the middle layer, were affected by heat exchange at the landfill surface. The lowest temperatures occurred near the bottom because of high water content and low oxygen concentrations. The high temperature zone migrated out from the injection well during degradation, reflecting the degradation of degradable organic matter associated with oxygen diffusion rates and aerobic degradation reactions. The initial accumulated settlement value was fast, but slowed and finally stabilized. The surface subsidence also developed from the center around the injection well to the surrounding area, and 70% of the total subsidence occurred within 150 days. This newly developed model provides a theoretical framework for analyzing the multi-field coupling of aerobic degradation of landfilled municipal solid waste (MSW).

Genome-centric metagenomics provides new insights into the microbial community and metabolic potential of landfill leachate microbiota

Landfills are important sources of microorganisms associated with anaerobic digestion. However, the knowledge on microbiota along with their functional potential in this special habitat are still lacking. In this study, we recovered 1168 non-redundant metagenome-assembled genomes (MAGs) from nine landfill leachate samples collected from eight cities across China, spanning 42 phyla, 73 classes, 114 orders, 189 families, and 267 genera. Totally, 74.1% of 1168 MAGs could not be classified to any known species and 5.9% of these MAGs belonged to microbial dark matter phyla. Two putative novel classes were discovered from landfill leachate samples. The identification of thousands of novel carbohydrate-active enzymes showed similar richness level compared to the cow rumen microbiota. The methylotrophic methanogenic pathway was speculated to contribute significantly to methane production in the landfill leachate because of its co-occurrence with the acetoclastic and hydrogenotrophic methanogenic pathways. The genetic potential of dissimilatory nitrate reduction to ammonium (DNRA) was observed, implying DNRA may play a role in ammonium generation in landfill leachate. These findings implied that landfill leachate might be a valuable microbial resource repository and filled the previous understanding gaps for both methanogenesis and nitrogen cycling in landfill leachate microbiota. Our study provides a comprehensive genomic catalog and substantially provides unprecedented taxonomic and functional profiles of the landfill leachate microbiota.

Remediation of contaminated soil and groundwater using chemical reduction and solidification/stabilization method: a case study

This study presents a systematic on-site remediation case involving both heavy metal and organic contaminants in soil and groundwater in a historically industrial-used site in Shanghai, China. Lab-scale experiments and field tests were conducted to determine the optimum parameters for the removal of contaminants in soil and groundwater. It has been found that the remediation goal of hexavalent chromium in soil could be achieved with the mass content of added sodium hydrosulfite and ferrous sulfate reaching 3% + 6%. The total chromium in the groundwater was effectively removed, when the mass ratio of sodium metabisulfite was not less than 3 g/L, and the added quick lime made pH value not less than 9. The concentrations of arsenic and 1,2-dichloropropane in the groundwater decreased evidently after extraction and mixing of groundwater. The pH and calcium chloride dosage added should be larger than 9.5 and 5 g/L, respectively, to remove phosphate in groundwater. The removal efficiency of those contaminants was examined and evaluated after the on-site remediation. The results demonstrated that it was feasible to use the chemical reduction and solidification/stabilization methods for the on-site ex situ remediation of this site, which could be referenced for the realistic remediation of similar sites.