Analysis and modeling of metals release from MBT wastes through batch and up-flow column tests

Influence of non-uniform flow on toxic elements transport in soil column percolation test

The influence of flow channels on the leaching behavior of toxic elements in contaminated soil cannot be neglected in a column percolation test. This study presents a visualization of the flow channel formed in the soil and evaluated the relationship between the leaching behavior of soil components and flow. We conducted column percolation tests with two types of filling methods (Compaction and No compaction) and used X-ray computed tomography to visualize the soil structure and non-uniform flow. Additionally, the variations of flow in a cross section of water were evaluated using hydraulic conductivity based on differential pressure. Under No compaction, a flow occurred throughout the soil column at the beginning of the water passage, but a non-uniform flow emerged as the liquid–solid ratio increased; under Compaction, a non-uniform flow was formed from the beginning of water passage. The leaching behavior of the major components and toxic elements from soil with high adsorptive properties was significantly affected by the filling method up to a liquid–solid ratio of 2. These results suggest that the non-uniform flow formed in the column percolation test has a significant impact on the leaching of soil components.

Effects of cellulose-based bio-plastics on the aerobic biological stabilization treatment of mixed municipal solid waste: A lab-scale assessment

The aim of this work is to assess how the presence of cellulose-based bio-plastics influence the biological stabilization of mixed Municipal Solid Waste (MSW). For the scope, two cellulose acetate bio-plastics have been mixed with a synthetic mixed waste to create samples with and without bio-plastics. A self-induced biostabilization has been carried out for 7 and 14 days where temperature and off-gas have been monitored continuously. Results about temperature evolution, O₂ consumption, CO₂ production and respiratory quotient did not show a substantial difference regarding both the duration of the process and the presence of cellulose-based bio-plastics on the mixture. On the average, the temperature peak and the maximum daily O₂ consumption and CO₂ production were 52.2 °C, 35.81 g O₂/kg DM *d and 48.95 g CO₂/kg DM *d respectively. Disintegration of bio-plastics samples after 7 and 14 days were comparable (on the average 23.13%). The self-induced biostabilization gave its main contribution after 4 days and resulted almost finished at the end of the day 7 of the process. Results showed that cellulose-based bio-plastics did not give a negative effect on mixed MSW biological stabilization and suggest a possible management, aiming at energy recovery of the outputs.

Environmental Comparison of Different Mechanical–Biological Treatment Plants by Combining Life Cycle Assessment and Material Flow Analysis

The role of Mechanical–Biological Treatment (MBT) is still of the utmost importance in the management of residual Municipal Solid Waste (MSW). These plants can cover a wide range of objectives, combining several types of processes and elements. The aim of this work is to assess and compare, from an environmental point of view, the performance of seven selected MBT plants currently operating in different countries, which represent the main MBT layout and processes. For the scope, a combined Life Cycle Assessment (LCA) and Material Flow Analysis (MFA) approach has been adopted to assess plant-specific efficiencies in materials and energy recovery. Metals recovery was a common and high-efficiency practice in MBT; further recovery of other types of waste was often performed. Each assessed MBT plant achieved environmental benefits: among them, the highest environmental benefit was achieved when the highest amount of waste was recovered (not only with material recycling). Environmental results were strongly affected by the recycling processes and the energy production, with a little contribution from the energy requirement. The impacts achieved by the MBT process were, on average, 14% of the total one. The main condition for a suitable MBT process is a combination of materials recovery for the production of new raw materials, avoiding disposal in landfill, and refuse-derived fuel production for energy recovery. This work can be of help to operators and planners when they are asked to define MBT schemes.

Total organic carbon as a proxy for metal release from biostabilized wastes

In this study, we introduce a simple screening method to predict the metal release from biostabilized wastes as a function of the total organic carbon (TOC) content of the sample. The method relies on a model that simulates the release of dissolved organic carbon (DOC) as a function of the applied liquid to solid (L/S) ratio. The metal release is then estimated using generic empirical DOC to metal correlation coefficients (K_DOC,Me) extrapolated from the literature. Specifically, the results of leaching tests carried out on different types of biowastes and biostabilized wastes were used to calculate the upper and lower K_DOC,Me values that can be expected for common metals of concern (Al, Ba, Cr, Cu, Mo, Ni, Pb, V, and Zn). The statistical analysis of the estimated empirical coefficients highlighted that for most of the investigated metals, the adoption of generic DOC to metal correlation coefficients introduces relatively low uncertainties. The quartiles ratio (QR calculated as the ratio of third and first quartile) of the K_DOC,Me coefficients extrapolated from the literature was indeed below 3 for Cu, Ni, and Zn and below 5 for Al, Ba, Cr, Pb, and V. Only for Mo, the QR was around 10 indicating that for this element, DOC can be a poor indicator of the release expected as a function of the applied L/S. Furthermore, by performing a sensitivity analysis, we found that the experimental conditions only slightly influence the metals release predicted by the model. Based on this evidence, simple nomograms that estimate the cumulative metal release in percolation column tests as a function of the applied L/S are provided. Furthermore, a simple equation that predicts the cumulative metal release expected at L/S of 10 L/kg is presented. The application of the latter to the results of percolation column tests carried out on 8 mechanical biological treatment (MBT) waste samples highlighted that the predicted metals release was close to the measured values with deviations within a factor of 5 for all the investigated metals.

The influence of moisture enhancement on solid waste biodegradation

The objective of this study was to assess the influence of moisture enhancement strategies on biodegradation of municipal solid waste (MSW) in laboratory-scale reactors. Moisture enhancement strategies were varied with respect to dose volume (40, 80, 160, and 320 L/Mg-MSW) and dose frequency (dosing every ½, 1, 2, and 4 weeks). Biodegradation was evaluated based on methane generation to assess (i) the lag-time between the start of liquid dosing and onset of methane generation and (ii) the first-order decay rate for methane generation. In general, the decay rate increased with an increase in dose volume for a given dose frequency. In addition, trends of increasing decay rate and decreasing lag-time were observed for an increase in dose frequency for reactors operated with dose volumes of 40, 80, and 160 L/Mg-MSW. A key conclusion from this study was that reactors with more aggressive moisture enhancement attained more rapid methane generation that initiated at shorter elapsed times following the onset of dosing. An assessment of liquid dosing per month indicated that there were more pronounced impacts of increasing decay rate and decreasing lag-time as moisture enhancement increased from 40 L/Mg-MSW/month to 320 L/Mg-MSW/month as compared to the impact on both variables for an increase in liquid dosing above 320 L/Mg-MSW/month.

Compression behaviors of mechanically biologically treated wastes of Tianziling landfill in Hangzhou, China

This study aimed to investigate the compression behaviors of mechanically biologically treated (MBT) wastes. For this purpose, the short-term compression-rebound and long-term compression tests were conducted on MBT wastes collected from Hangzhou Tianziling landfill in China. The results showed that the duration of immediate compression was obtained as 15.17-36.67 h and was comparable to municipal solid waste (MSW). The immediate compression ratio was 0.233-0.247, and it was comparable to the aged high food waste content (HFWC)-MSW, fresh and aged low food waste content (LFWC)-MSW, but much lower than the fresh HFWC-MSW. The mechanical creep ratio (C'_αc) was 0.012-0.018, being close to the fresh and aged MSWs. The bio-induced compression ratio (C'_αb) was 0.143-0.174. The compression ratio rose exponentially with temperature (5-42 °C) in both mechanical creep stage and bio-induced compression stage, and it increased much faster in the bio-induced compression stage. The resilient strains was only 2.1-3.3% of the compression strain at the same stress interval, suggesting that the compression strain consisted of mostly plastic deformation and negligible elastic deformation. The above findings can provide a reference for settlement prediction and storage capacity estimation of an MBT waste landfill.

Release of pollutants in MBT landfills: Laboratory versus field

The evolution of liquid pollutants over time remains one of the main issues in a landfill for decades. However, although the literature contains several works that attempt to model the release of pollutants from waste, there is still scant information on the matter, especially for MBT waste. In this study, contaminant washing laboratory tests and field trials have been carried out on different waste particle sizes and fractions to estimate to what extent the former provide adequate information about the evolution of the leachate in a real MBT waste landfill. The results show significant differences between the test results. The majority of inorganic contaminants (Ca, Cu, Mg, Na, Ni, Zn), however, complex with organic matter in all percolation column tests. The use of other materials, the early beginning of the methanogenic phase, and the increase in pH and temperature in the field alter the reactions and, importantly, the leaching of some of the components that precipitate, such as Ca, Fe and Mg, or NH₄-N, which increases its presence. It is therefore necessary to adapt the models and the current test standards to assess the fate of biodegradable waste such as MBT under field conditions.

Optimization of a German short term percolation test to determine the leaching of granular materials

Different percolation tests were developed worldwide to characterize the leaching and to evaluate the environmental compatibility of granular materials. The German standard up-flow percolation test has a short testing time and can be used for both coarse and fine-grain materials. Some very fine-grain materials are difficult to percolate. According to the standard, admixture of 80% quartz sand (20% material) can be used for cohesive materials. It is assumed that equilibrium concentrations are reached and therefore the sand does not cause any interfering processes. However, the 80% sand admixture cannot be used for coarse materials due to dilution. A standardized sand admixture for both coarse and fine-grain materials is beneficial for the routine of laboratories. The sand admixture has the further advantage that it reduces the testing time. The experimental and the analysis procedures of the German standard were checked, specified, and optimized. An admixture of 50% sand is a good compromise for cohesive and coarse materials. The statistical variations of heavy metal and polycyclic aromatic hydrocarbons from the optimized test with and without sand admixture were determined with an 8-fold intralaboratory and an interlaboratory test. Then the sand admixture was validated for 16 materials (soils, demolition wastes, ashes and other industrial wastes).

Groundwater pollution assessment in landfill areas: Is it only about the leachate?

Groundwater is the environmental compartment very often involved in the broader issues related to contaminated sites characterization and reclamation. It is not uncommon to find areas in which some substances directly linked to the petrographic composition of the aquifer reach high values exceeding the limits set by the regulations. These concentrations are defined as Natural Background Levels (NBL) and the need to quantify their real contribution, in areas subject to strong anthropogenic pressures, represents an emerging problem. Global statistical analyses and laboratory testing are proposed here to distinguish between the impacts of different forcing influencing water quality in hydrogeological systems. The study focus on the application of a methodology based on the Component Separation analysis for the NBL estimation of selected chemical species in potentially contaminated aquifers flowing in the proximity of landfill areas, and on the subsequent validation of the results through experimental studies of field samples. A site located in Calabria, Italy, and constituting a typical example of an aquifer which has been subjected to possible contact with the leachate produced by waste degradation is considered. The work is keyed to NBLs characterization of aluminum, iron and manganese and to the identification of their natural component for a proper environmental assessment of the site. Estimated NBLs are consistent with the geochemical composition of site samples. The adopted methodology can represent a useful instrument to distinguish effective anthropogenic contamination from natural conditions and to define realistic environmental clean-up goals.

Long-term dynamics of leachate production, leakage from hazardous waste landfill sites and the impact on groundwater quality and human health

The long-term dynamics of leachate leakage from hazardous waste landfills (HWLs) and its influence on the surrounding groundwater and human health urgently requires decision-making processes for long-term HWL management and risk control. This study, based on the DMFU model, which is described in the literature as simulating the performance degradation of a landfill's main functional units, constructs a comprehensive model by coupling the HELP, EPACMTP, and Dose-Effect modules to investigate the long-term emissions of leachate from HWLs and their potential influence on groundwater quality and human health. Our results showed that the leakage rate over a long time period (50-1000 years) is 10 times higher than that of either a short (0-10 years) or medium (10-50 years) period. Due to the substantial increase in leakage rate, the negative influence on regional groundwater quality and human health changes from "insignificant" in the short term to "slight but acceptable" in the medium term, and finally to "substantial and unacceptable" in the long term. Studies also reveal that the uncertainty of risk increases over time. The information gained from this research provides useful insights into the long-term dynamics of leachate leakage, its risk consequences, and associated uncertainty, which can help landfill owners or risk managers make better decisions regarding the after-closure management of landfills.

Alternating pure oxygen and air cycles for the biostabilization of unsorted fraction of municipal solid waste

Biostabilisation is a process of treating the unsorted fraction of municipal solid waste (UFMSW) mechanically pre-treated. Although concepts such as circular economy would seem to limit biostabilization, several authors have recently described the advantages of biostabilization in regions where recycling systems are inadequate. In this perspective, the development of new MBT technologies is of considerable importance. The objective of the study was to evaluate the effects of the use of alternating air and oxygen cycles on the treated waste stability as well as on the quality of leachate and process gaseous emissions. Two Herhof biocells were prepared for this purpose. One implemented the conventional process and the other the "Air + O₂" process. The biostabilization of the inlet UFMSW (3965 ± 1965 mgO₂/kgVS/h) resulted in a final product with a dynamic respirometric index almost equal in both processes. The mass balance indicated that of the 400 tons representing the input waste, 37.57% were biostabilized waste, 0.29% leachate and 62.14% CO₂ and odours. However, the biostabilized waste was lower than that of the conventional process (equal to 40.18%). The Air + O₂ system resulted in a shorter duration, increased production of leachate (although characterized by higher quality) and process gaseous emissions quality. The energy balance (20.3 kJ/kg per input waste) and cost analysis (80.0 €/ton per input waste) showed values equal or better to those of the conventional system. By contrast, weakness was in the O₂ diffusion system. Although a life cycle analysis is necessary, the results highlighted the feasibility of the proposal especially for emergency situations.

Release of heavy metals during long-term land application of sewage sludge compost: Percolation leaching tests with repeated additions of compost

Leaching assessment procedures have been used to determine the leachability of heavy metals as input for evaluating the risk from sewage sludge compost land application. However, relatively little attention has been paid to understanding leaching from soils with repeated application of sewage sludge compost with elevated levels of heavy metals. In this paper, leaching assessment is extended to evaluate the potential leaching of heavy metals during repetitive application of composted sewage sludge to soils. Four cycling of compost additions and percolation leaching were conducted to investigate how leaching behavior of heavy metals changed with repeated additions of compost. Results showed that repetitive additions of compost to soil significantly increased the content of organic matter, which favored the formation of reducing condition due to improved microbial activities and oxygen consumption. Establishment of reducing conditions can enhance the leaching concentrations of As by approximately 1 order of magnitude, especially for the soil rich in organic matter. For Cd, Cr, Cu, and Pb, repeated additions of compost will cause accumulation in total contents but not enhancement in leaching concentrations. The infiltration following compost additions will leach out the mobile fraction and the residual fraction might not release in the next cycling of compost addition and infiltration. The cumulative release of Cd, Cr, Cu, and Pb accounted for less than 5% of the total contents during four times of compost applications.

Analysis and interpretation of the leaching behaviour of waste thermal treatment bottom ash by batch and column tests

This paper investigates the leaching behaviour of specific types of waste thermal treatment bottom ash (BA) as a function of both pH and the liquid-to-solid ratio (L/S). Specifically, column percolation tests and different types of batch tests (including pH-dependence) were applied to BA produced by hospital waste incineration (HW-I), Refuse Derived Fuel (RDF) gasification (RDF-G) and RDF incineration (RDF-I). The results of these tests were interpreted applying an integrated graphical and modelling approach aimed at identifying the main mechanisms (solubility, availability or time-controlled dissolution and diffusion) governing the release of specific constituents from each type of BA. The final aim of this work was in fact to gain insight on the information that can be provided by the leaching tests applied, and hence on which ones may be more suitable to apply for assessing the leaching concentrations expected in the field. The results of the leaching tests showed that the three samples of analysed BA presented differences of orders of magnitude in their leaching behaviour, especially as a function of pH, but also in terms of the L/S. These were mainly related to the differences in mineralogy of the samples. In addition, for the same type of bottom ash, the comparison between the results of batch and percolation column tests, expressed in terms of cumulative release, showed that for some constituents (e.g. Mg for HW-I BA and Cu for RDF-G BA) differences of over one order of magnitude were obtained due to variations in pH and DOC release. Similarly, the eluate concentrations observed in the percolation tests, for most of the investigated elements, were not directly comparable with the results of the pH-dependence tests. In particular, in some cases the percolation test results showed eluate concentrations of some constituents (e.g. K and Ca in HW-I BA) of up to one order of magnitude higher than the values obtained from the pH-dependence experiments at the same pH value. This was attributed to a rapid washout from the column of the soluble phases present in the BA. In contrast, for other constituents (e.g. Mg and Ba for the RDF-G BA), especially at high L/S ratios, the concentrations in the column tests were of up to one order of magnitude lower than the solubility value, indicating release under non-equilibrium conditions. In these cases, batch pH-dependence tests should be preferred, since column tests results could underestimate the concentrations expected in the field.

An easy-to-use tool for the evaluation of leachate production at landfill sites

A simulation program for the evaluation of leachate generation at landfill sites is herein presented. The developed tool is based on a water balance model that accounts for all the key processes influencing leachate generation through analytical and empirical equations. After a short description of the tool, different simulations on four Italian landfill sites are shown. The obtained results revealed that when literature values were assumed for the unknown input parameters, the model provided a rough estimation of the leachate production measured in the field. In this case, indeed, the deviations between observed and predicted data appeared, in some cases, significant. Conversely, by performing a preliminary calibration for some of the unknown input parameters (e.g. initial moisture content of wastes, compression index), in nearly all cases the model performances significantly improved. These results although showed the potential capability of a water balance model to estimate the leachate production at landfill sites also highlighted the intrinsic limitation of a deterministic approach to accurately forecast the leachate production over time. Indeed, parameters such as the initial water content of incoming waste and the compression index, that have a great influence on the leachate production, may exhibit temporal variation due to seasonal changing of weather conditions (e.g. rainfall, air humidity) as well as to seasonal variability in the amount and type of specific waste fractions produced (e.g. yard waste, food, plastics) that make their prediction quite complicated. In this sense, we believe that a tool such as the one proposed in this work that requires a limited number of unknown parameters, can be easier handled to quantify the uncertainties.

Pre-treatment of tannery sludge for sustainable landfilling

The wastewater produced during tanning activities are commonly conveyed to centralised industrial wastewater treatment plants. Sludge from physical-chemical treatments (i.e. primary sedimentation) and waste activated sludge from biological treatment units are called tannery sludge. Tannery sludge is a solid waste that needs to be carefully managed and its disposal represents one of the major problems in tannery industry. Conventional treatment and disposal of tannery sludge are based mainly on incineration and landfilling. The aim of this study was to evaluate the effects of a pre-treatment process composed of aerobic stabilisation, compaction and drying, for a sustainable landfilling of tannery sludge. The process produced a reduction of volume, mass and biodegradability of treated sludge. Results also demonstrated a reduced leachability of organic and inorganic compounds from treated sludge. The pre-treatment process could allow to extend landfill life time due to lower amounts of tannery sludge to be disposed off, minimise long terms landfill emissions and obtain a state of carbon sink for tannery sludge landfilling.

Characterization of phosphorus leaching from phosphate waste rock in the Xiangxi River watershed, Three Gorges Reservoir, China

Phosphate mining waste rocks dumped in the Xiangxi River (XXR) bay, which is the largest backwater zone of the Three Gorges Reservoir (TGR), are treated as Type I industry solid wastes by the Chinese government. To evaluate the potential pollution risk of phosphorus leaching from phosphate waste rocks, the phosphorus leaching behaviors of six phosphate waste rock samples with different weathering degrees under both neutral and acidic conditions were investigated using a series of column leaching experiments, following the Method 1314 standard of the US EPA. The results indicate that the phosphorus release mechanism is solubility-controlled. Phosphorus release from waste rocks increases as pH decreases. The phosphorus leaching concentration and cumulative phosphorus released in acidic leaching conditions were found to be one order of magnitude greater than that in neutral leaching conditions. In addition, the phosphorus was released faster during the period when environmental pH turned from weak alkalinity to slight acidity, with this accelerated release period appearing when L/S was in the range of 0.5-2.0 mL/g. In both neutral and acidic conditions, the average values of Total Phosphorus (TP), including orthophosphates, polyphosphates and organic phosphate, leaching concentration exceed the availability by regulatory (0.5 mg/L) in the whole L/S range, suggesting that the phosphate waste rocks stacked within the XXR watershed should be considered as Type II industry solid wastes. Therefore, the phosphate waste rocks deposited within the study area should be considered as phosphorus point pollution sources, which could threaten the adjacent surface-water environment.

Assessment of biogas production from MBT waste under different operating conditions

In this work, the influence of different operating conditions on the biogas production from mechanically-biologically treated (MBT) wastes is investigated. Specifically, different lab-scale anaerobic tests varying the water content (26-43% w/w up to 75% w/w), the temperature (from 20 to 25°C up to 55°C) and the amount of inoculum have been performed on waste samples collected from a full-scale Italian MBT plant. For each test, the gas generation yield and, where applicable, the first-order gas generation rates were determined. Nearly all tests were characterised by a quite long lag-phase. This result was mainly ascribed to the inhibition effects resulting from the high concentrations of volatile fatty acids (VFAs) and ammonia detected in the different stages of the experiments. Furthermore, water content was found as one of the key factor limiting the anaerobic biological process. Indeed, the experimental results showed that when the moisture was lower than 32% w/w, the methanogenic microbial activity was completely inhibited. For the higher water content tested (75% w/w), high values of accumulated gas volume (up to 150Nl/kgTS) and a relatively short time period to deplete the MBT waste gas generation capacity were observed. At these test conditions, the effect of temperature became evident, leading to gas generation rates of 0.007d(-1) at room temperature that increased to 0.03-0.05d(-1) at 37°C and to 0.04-0.11d(-1) at 55°C. Overall, the obtained results highlighted that the operative conditions can drastically affect the gas production from MBT wastes. This suggests that particular caution should be paid when using the results of lab-scale tests for the evaluation of long-term behaviour expected in the field where the boundary conditions change continuously and vary significantly depending on the climate, the landfill operative management strategies in place (e.g. leachate recirculation, waste disposal methods), the hydraulic characteristics of disposed waste, the presence and type of temporary and final cover systems.