Pilot scale evaluation of a model to distinguish the rates of simultaneous anaerobic digestion, composting and methane oxidation in static waste beds

Study on the effect of landfill gas on aerobic municipal solid waste degradation: Lab-scale model and tests

Aeration is of great importance in landfill remediation. However, most existing studies on aerobic waste degradation ignore the presence of landfill gases. In this study, gas characteristics during aerobic waste degradation in the presence of landfill gas in lab-scale lysimeters were investigated. Oxygen (O₂) was intermittently injected into municipal solid waste. Changes in the gas concentration and reaction rate of methane (CH₄), carbon dioxide (CO₂), and O₂ during the reaction process were monitored and calculated. The results showed that all reactions, including aerobic degradation, CH₄ oxidation, and anaerobic waste degradation, occurred simultaneously during landfill aeration. The maximum O₂ consumption rate was 0.013 mol day^-1 kg^-1 dry waste. CH₄ production was stimulated after the O₂ content was insufficient to sustain the aerobic environment. Higher CH₄ production was likely attributed to the remaining substrate and biomass from dead aerobic microorganisms decomposed by growing anaerobic microorganisms. Based on the biochemical reaction and principle of mass conservation, a gas balance model during waste aeration was established to analyze the proportions of aerobic waste degradation, CH₄ oxidation, and anaerobic waste degradation. The CH₄ oxidation reaction was more advantageous than the aerobic waste degradation reaction during aeration. With an increase in gas injection times, the anaerobic reaction gradually weakened. The maximum proportion of CH₄ oxidation reaction could achieve at 21.4 % during aeration, which is of great significance for the waste degradation reaction. The maximum proportion of aerobic waste degradation and the minimum proportion of anaerobic waste degradation were approximately 16.0 % and 74.2 %, respectively. The results show that landfill gas should be considered in the progress of landfill aeration. This study provides a novel approach for calculating the proportion of reactions during landfill aeration, which deepens the understanding of the reaction process and contributes to the design of aerobic landfill projects.

A systematic assessment of aeration rate effect on aerobic degradation of municipal solid waste based on leachate chemical oxygen demand removal

Aeration is one mainstream technique to accelerate municipal solid waste (MSW) degradation in landfills. The determination of an appropriate aeration rate is critical to the design and operation of a landfill aeration system. In this study, we analyze 132 waste degradation tests reported in forty one studies in the literature. We use L min^-1 kg^-1 dry organic matter (L min^-1 kg^-1 DOM) as the uniform unit to quantify the aeration rates in all tests. The first order rate coefficient for chemical oxygen demand (COD) removal in leachate (k_COD) is selected as the parameter to characterize MSW degradation process. We further divide aerobic tests into five aerobic groups base on the respective aeration rates, i.e., <0.02, 0.02-0.1, 0.1-0.3, 0.3-1, and >1 L min^-1 kg^-1 DOM. With an increase in the aeration rate, the k_COD increases first and then decreases. The aeration rate between 0.1 and 0.3 L min^-1 kg^-1 DOM has the best enhancement on the k_COD. The k_COD values are not much higher than the anaerobic and semi-aerobic tests when the aeration rates are <0.1 L min^-1 kg^-1 DOM, because such aeration rates may be lower than the actual oxygen consumption rates. An aeration rate >0.3 L min^-1 kg^-1 DOM reduces the k_COD likely due to excess water evaporation and ventilation cooling. Among the analyzed results, the aeration rate is the most related to the k_COD in principal component analysis than the other factors, including liquid recirculation and addition, waste total density, waste degradation level, and waste initial temperature.

A review of mathematical models for composting

Composting is a valuable method to treat and valorize organic waste. However, the process is defined by its dynamic nature and governed by a multitude of operating parameters. As such, mathematical modelling of the process offers a powerful tool to simulate and predict the variable outcomes of the process, allowing for its optimization. This can include improving efficiency, lowering costs and reducing environmental impact. To aid with the development of future models, we provide an up to date review and assessment on the state of the art of composting modelling. By reviewing 40 years of literature, this review paints the most complete picture of the field to date. This includes an analysis of trends in composting modelling: looking at the type of systems that are targeted, the aim of the models and the approaches to kinetics and mass and heat transfer. Regarding modelling approaches, we explore the fractionation of both substrates and microorganisms, the biological processes that can be included (disintegration, hydrolysis, uptake and death) and their kinetics (first-order, Monod-type), energy balances (biological generation, convection, conduction) and mass balances. We also provide a review of the results of sensitivity analyses performed on composting models, finding that models are most sensitive to microbial growth and death rates, as well as consumption rates and product yields. In the final portion of the review, we identify, explore, and provide guiding recommendations for work on emerging areas and areas requiring development in composting modelling (volume change, pH, maturation, artificial intelligence, etc.).

Exploring the mechanisms of organic matter degradation and methane emission during sewage sludge composting with added vesuvianite: Insights into the prediction of microbial metabolic function and enzymatic activity

Effect mechanisms of organic matter (OM) degradation and methane (CH₄) emission during sewage sludge (SS) composting with added vesuvianite (V) were studied by high-throughput sequencing (HTS) and phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt). Results show that the addition of V accelerated the OM degradation and decreased the cumulative CH₄ emissions by 33.6% relative to the control. In addition, V significantly decreased the mcrA gene abundance and the methanogen community richness at the genus level. PICRUSt also indicated that V strengthens the microbial metabolic function and enzymatic activity related to OM degradation, and reduced the enzymatic activity related to CH₄ production. Methanogens community variation analysis proved the ratio of carbon and nitrogen and moisture content are the significant variables affecting CH₄ emissions. Thus, optimizing the ratio of carbon and nitrogen and moisture content will decrease CH₄ emission during SS composting.

Long-term risk assessments comparing environmental performance of different types of sanitary landfills

Landfilling of non-pretreated, mixed municipal solid and similar waste remains a major method of waste management in many parts of the world today, particularly in developing countries. Modern dry-type and bioreactor landfills are considered to be relatively safe facilities in contrast to less engineered sanitary landfills. However, highly-engineered landfills are often not applicable in low-income countries because of their high cost. Performance of low-cost types of sanitary landfills deserve further exploration, because uncontrollable dumpsites pose real long-term threats to aquifers and the most reliable means of improving the situation is by upgrading these sites into technically similar, but environmentally safe facilities. Risk assessments can provide some insight into differences in environmental performance between major types of low-cost and high-cost landfills simulating the essential landfill processes. The risk of contamination at a presumed aquifer that resides directly below a presumed landfill was quantitatively assessed in this study comparing long-term environmental performances of four different types of facilities. A risk assessment software tool which explicitly enables inclusion of uncertainty present in the estimates to generate results with a wide range of possible outcomes was used to fulfill this purpose. It was demonstrated that contaminants derived from a modern, dry-type landfill would likely leech into an aquifer with the greatest impact occurring several decades after landfill closure. However, a specifically designed and properly operated, passive semiaerobic above-ground landfill was shown to be a comparatively safe facility.

Improving understanding of carbon storage in wood in landfills: Evidence from reactor studies

Approximately 1.5 million tonnes (Mt) of wood waste are disposed of in Australian landfills annually. Recent studies have suggested that anaerobic decay levels of wood in landfills are low, although knowledge of the decay of individual wood species is limited. The objective of this study was to establish the extent of carbon loss for wood species of commercial importance in Australia including radiata pine, blackbutt, spotted gum and mountain ash. Experiments were conducted under laboratory conditions designed to simulate optimal anaerobic biodegradation in a landfill. Bacterial degradation, identified by both light microscopy and electron microscopy, occurred to a varying degree in mountain ash and spotted gum wood. Fungal decay was not observed in any wood samples. Mountain ash, the species with the highest methane yield (20.9 mL CH₄/g) also had the highest holocellulose content and the lowest acid-insoluble lignin and extractive content. As the decay levels for untreated radiata pine were very low, it was not possible to determine whether impregnation of radiata pine with chemical preservatives had any impact on decay. Carbon losses estimated from gas generation were below 5% for all species tested. Carbon losses as estimated by gas generation were lower than those derived by mass balance in most reactors, suggesting that mass loss does not necessarily equate to carbon emissions. There was no statistical difference between decay of blackbutt derived from plantation and older, natural forests. Addition of paper as an easily digestible feedstock did not increase carbon loss for the two wood species tested and the presence of radiata pine had an inhibitory effect on copy paper decay. Although differences between some of the wood types were found to be statistically significant, these differences were detected for wood with carbon losses that did not exceed 5%. The suggested factor for carbon loss for wood in landfills in Australia is 1.4%. This study confirms that disposal of wood in landfills in Australia results in long-term storage of carbon, with only minimal conversion of carbon to gaseous end products.

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.