Leachate formation and characteristics from gasification and grate incineration bottom ash under landfill conditions

Comparison of the PFAS and physical-chemical parameter fluctuations between an ash landfill and a MSW landfill

Studies of per- and polyfluoroalkyl substances (PFAS) fluctuations at landfills have focused on municipal solid waste (MSW) leachate. Few studies exist that evaluate fluctuations (defined by the coefficient of variation, CV) in MSW incinerator ash (MSWA) landfill leachate and that evaluate PFAS fluctuations in stormwater, groundwater, and treated liquids on-site. In this study, aqueous landfill samples (leachate, treated leachate, stormwater, gas condensate, ambient groundwater, and effluent from a groundwater remediation system) were collected from a MSW and an MSWA landfill geographically located within close proximity (less than 40 km). The objective of this study was to compare the leachate compositions between these two landfill types and to evaluate temporal variations. Results indicated that the CV of total detected PFAS concentrations in leachate was higher for the MSW landfill (CV = 43 %) compared to the MSWA landfill (CV = 16 %). The total detected PFAS concentration in MSW leachate samples (mean: 9641 ng/L) was higher than in MSWA leachate samples (mean: 2621 ng/L) (p < 0.05). Within a landfill, PFAS concentrations were correlated (rs > 0.6, p < 0.05) with alkalinity, total organic carbon (TOC), and ammonia. Results from the on-site leachate treatment system at the MSW landfill indicated reductions in COD, TOC, and ammonia; however, the ∑26PFAS concentration increased 3 % after the treatment. Overall, results demonstrated that differences between landfill types and fluctuations in PFAS within landfills should be considered when designing landfill leachate collection and treatment systems to remove PFAS. The comparative analysis in this study can provide insights into optimizing leachate management for MSW and MSWA landfills.

Eco-friendly cultivation of microalgae using a horizontal twin layer system for treatment of real solid waste leachate

Solid waste leachate (SWL) requires dilution with water to offset the negative effects of high nutrient concentration and organic compounds for its microalgae-based treatment. Among attached cultivation systems, twin layer is a technology in which limited information is available on treatment of high strength wastewater using microalgae. Moreover, widespread application of twin layer technology is limited due to cost of substrate and source layer used. In the present study, potential of Scenedesmus sp. for the treatment of SWL was assessed on horizontal twin layer system (HTLS). Novel and cost-effective substrate layers were tested as attachment material. Wetland treated municipal wastewater (WMW) was used to prepare SWL dilutions viz, 5%, 10%, 15%, 20% and 25% SWL. Recycled printing paper showed maximum biomass productivity of 5.19 g m-2 d-1. Among all the SWL dilutions, Scenedesmus sp. achieved maximum growth of 103.05 g m-2 in 5% SWL which was 16% higher than WMW alone. The maximum removal rate of NH4+ -N, TKN, and PO43- P was obtained in 20% SWL which was 1371, 1588 and 153 mg m-2 d-1 respectively. Varying concentrations of nutrients in different SWL dilutions significantly affected lipid biosynthesis, with enhanced productivity of 2.28 g m-2 d-1 achieved in 5% SWL compared to 0.97 g m-2 d-1 in 20% SWL. Hence, it can be concluded that 5% SWL dilution was good for biomass and lipid production, while the highest nutrient removal rates were obtained at 20% SWL mainly attributed to biotic and abiotic processes. Based on these results HTLS can be a promising technology for pilot scale to explore industrialized application of wastewater treatment and algal production.

Relationships between per- and polyfluoroalkyl substances (PFAS) and physical-chemical parameters in aqueous landfill samples

Variable chemistries of liquids from landfills can potentially impact levels of per- and polyfluoroalkyl substances (PFAS). The objective of the current study was to evaluate relationships between physical-chemical properties (bulk measurements, oxygen demand components, and metals) and PFAS concentrations in different types of aqueous landfill samples. Aqueous landfill samples were collected from 39 landfill facilities in Florida, United States. These samples included leachates from landfills that receive different waste types, such as municipal solid waste incineration ash (MSWA), construction and demolition debris (C&D), and municipal solid waste (MSW). Additional aqueous landfill samples were sourced from treated landfill leachate, gas condensate, stormwater, and groundwater from within and near the landfill boundaries. Results showed significant correlations (p < 0.05) between ∑26PFAS and alkalinity (rs = 0.83), total organic carbon (TOC) (rs = 0.84), and ammonia (rs = 0.79) for all leachate types. Other physical-chemical parameters that were significantly correlated (rs > 0.60, p < 0.05) with PFAS included specific conductivity, chemical oxygen demand (COD), and to a lesser extent, total dissolved solids (TDS) and total solids (TS). For gas condensates, PFAS was significantly correlated with TOC. Stormwater and groundwater, within and near the landfill boundaries, had considerably lower levels of PFAS and had a minimal correlation between PFAS and physical-chemical parameters. Although PFAS concentrations and physical-chemical parameters and their correlations varied between different types of aqueous landfill samples, results suggest that physical-chemical properties can be useful indicators of relative PFAS concentrations within a leachate type. More research is needed to validate the mechanisms that relate physical-chemical parameters to PFAS concentrations in landfill leachates.

Impact of municipal solid waste landfill leachate on biogas production rate

Landfills/open dump sites are the final disposal facilities for municipal solid waste (MSW). These sites undergo continuous process of biochemical reactions and anaerobic degradation, which make them prone to generation of landfill gas (LFG) and leachate. Worldwide, the quantitative and qualitative assessment for leachate treatment and management has been a growing concern. The present study investigated the physico-chemical characteristics and heavy metal parameters for fresh, 3-month, 6-month and 3-year old landfill leachate samples. The total dissolved solids (13280 mg/l), alkalinity (13000 mg/l), chemical oxygen demand (42000 mg/l) and total organic carbon (16500 mg/l) was found to be maximum in 3-year old leachate sample. While, the 3 and 6-month old leachate samples had maximum heavy metal concentration. The attempt was also made to identify the key parameters responsible to enhance biogas production yield from different ages of MSW. The substrate combinations of MSW and 3-year old leachate samples was prepared at varying proportion. The study was performed in three cycles and the volume of leachate diffused in each cycle was kept constant. The control samples with no leachate diffusion was also prepared to compare the percentage increase in biogas production rate. It was found that the cumulative methane (CH₄) production from fresh (358 ml/g) and 3-month old MSW (273 ml/g) was maximum, and the overall percentage increase was 43% and 32%. It was also conclusive that the excess leachate diffusion of >15 ml results in low calcination behaviour and CH₄ production rate. The response surface methodology was used to correlate and validate independent input variables (volatile solids, C/N ratio and leachate concentration) responsible for maximum CH₄ yield.

Characteristics of leachate from refuse transfer stations in rural China

The properties of leachate from refuse transfer stations (RTSs) in rural China were indefinite. In this study, a total of 14 leachate samples from RTSs in nine provinces of China were characterized for their pH, electric conductivity, chromaticity, concentration of organic substances, nitrogen distribution, volatile organic compounds (VOCs), organic phosphorous pesticide, and heavy metals. The structural composition of fluorescent dissolved organic matter (FDOM) was also determined. To evaluate the leachate pollution potential in this study, a leachate pollution index was derived and used. Chromium (Cr) was the most polluting heavy metal present in rural leachate. Ethanol and ethyl acetate were the most frequently detected VOCs at high concentrations. Three-dimensional fluorescence excitation-emission matrix spectra were used to characterize the FDOM. Three components, tryptophan (C₁), tyrosine-like (C₂), and humic acid- and fulvic acid-like (C₃) substances, were identified from all 14 samples. Tryptophan was the major component of FDOM and present in 45.7% of the samples by calculating the fluorescence intensity percentage, on average. Pearson correlations revealed that the fluorescence intensity of C₁ and C₃ was strongly related to soluble chemical oxygen demand and dissolved oxygen carbon, while C₂ had significant positive correlations with ammonia nitrogen and total phosphorus of the solid waste. This study provided detailed data and findings that could serve as a preliminary basis for broadening options for the treatment and management of leachate from rural RTSs in China.

Do PFAS changes in landfill leachate treatment systems correlate with changes in physical chemical parameters?

Per- and polyfluoroalkyl substances (PFAS) have been found at relatively elevated concentrations in landfill leachates. Some landfill facilities treat physical-chemical parameters of their leachates using on-site leachate treatment systems before discharge. The objective of this study was to evaluate whether changes in physical-chemical parameters of leachate at on-site treatment systems (including bulk measurements, oxygen demanding components, and metals) were associated with concentration changes in PFAS. Leachates were evaluated at 15 on-site treatment facilities which included pond systems, aeration tanks, powdered activated carbon (PAC), sand filtration, reverse osmosis (RO) and combination treatment processes. Results show that most physical-chemical parameters and PFAS were significantly reduced in RO systems (over 90 %). For pond systems, statistically significant correlations (r_s > 0.6, p < 0.05) were observed between ∑₂₆PFAS changes and the changes in pH, alkalinity, ammonia, and some metals. Significant correlations were also found between ∑₈PFAAs precursors changes and specific conductivity (SPC), pH, alkalinity, ammonia, and metals changes. For aeration tank systems, significant correlations (r_s > 0.6, p < 0.05) were observed between ∑₂₆PFAS changes and changes in total dissolved solids and zinc, and between the changes of ∑₈PFAAs precursors and field pH. These correlations are believed to be associated with rainfall dilution and precipitation of calcium carbonate and other metals as leachate is introduced to the atmosphere.

Potential of medium chain fatty acids production from municipal solid waste leachate: Effect of age and external electron donors

A large quantity of leachate is generated during municipal solid waste collection operation and in landfills due to the large amount of organic waste and high humidity. The content of medium chain fatty acids (MCFAs) in the leachate is a low cost feedstock for bio-based chemical and fuel production processes. The aim of this study is to investigate the MCFA production potential of three leachate ages through chain elongation process under uncontrolled pH batch test. Moreover, the effect of using different external electron donors (ethanol, methanol and a mixture of both) is studied. The experiment consists of characterizing the samples then adding external electron donors with a specific ratio to leachate samples under mesophilic temperature. For this investigation, also a statistical analysis is done, which shows the production of MCFAs is highly influenced by leachate age. The results indicate that the production of even-numbered acids increase from 600 to 1,000 mg/L by the end of the ethanol chain elongation experiment for young leachate. However, a higher MCFA production of more than 1,000 mg/L is achieved by using the mixture of methanol and ethanol as electron donor. Furthermore, all methanol chain elongation experiments lead to an odd-numbered production of MCFAs, such as pentanoic and heptanoic acids. These results confirm the potential improvement of MCFA production from leachate through choosing the optimal leachate age and electron donor. Overall, producing MCFAs from leachate is a good example of circular bio-economy because waste is used to produce biochemicals, which closes the material cycle.

Performance of structural concrete using Waste-to-Energy (WTE) combined ash

In the U.S., about 27 million metric tons of municipal solid waste are used as fuel in Waste-to-Energy (WTE) power plants, generating about seven million tons of mixed bottom ash and fly ash (combined ash) annually, which are disposed of in landfills after metal separation. This study assessed the effect of using combined ash as a substitute of mined stone aggregates on the mechanical properties and leachability of cement mortar and concrete. The as-received combined ash was separated into three fractions: fine (<2 mm), medium (2-9.5 mm), and coarse (9.5-25 mm). The substitution of up to 100% of stone aggregate by the coarse and medium fractions of combined ash produced concrete with compressive strength exceeding 28 MPa after 28 days of curing. Similar results were obtained when the fine combined ash was used as a sand substitute, at 10 wt%, in mortar. The concrete specimens were subjected to several days of curing and mechanical testing. The results were comparable to the properties of commercial concrete products. The mechanical test results were supplemented by XRD and SEM analysis, and leachability tests by EPA Method 1313 showed that the optimal concrete products effectively immobilized the heavy metals in the combined ash.

Laboratory Scale Investigations on Heap (Bio)leaching of Municipal Solid Waste Incineration Bottom Ash

Municipal solid waste incineration bottom ash (MSWI BA) is the main output of the municipal solid waste incineration process, both in mass and volume. It contains some heavy metals that possess market value, but may also limit the utilization of the material. This study illustrates a robust and simple heap leaching method for recovering zinc and copper from MSWI BA. Moreover, the effect of autotrophic and acidophilic bioleaching microorganisms in the system was studied. Leaching yields for zinc and copper varied between 18–53% and 6–44%, respectively. For intensified copper dissolution, aeration and possibly iron oxidizing bacteria caused clear benefits. The MSWI BA was challenging to treat. The main components, iron and aluminum, dissolved easily and unwantedly, decreasing the quality of pregnant leach solution. Moreover, the physical nature and the extreme heterogeneity of the material caused operative requirements for the heap leaching. Nevertheless, with optimized parameters, heap leaching may offer a proper solution for MSWI BA treatment.

Leaching behaviour of incineration bottom ash in a reuse scenario: 12years-field data vs. lab test results

Several types of standardized laboratory leaching tests have been developed during the past few decades to evaluate the leaching behaviour of waste materials as a function of different parameters, such as the pH of the eluate and the liquid to solid ratio. However, the link between the results of these tests and leaching data collected from the field (e.g. in disposal or reuse scenarios) is not always straightforward. In this work, we compare data obtained from an on-going large scale field trial, in which municipal solid waste incineration bottom ash is being tested as road sub-base material, with the results obtained from percolation column and pH-dependence laboratory leaching tests carried out on the bottom ash at the beginning of the test. The comparisons reported in this paper show that for soluble substances (e.g. Cl, K and SO₄), percolation column tests can provide a good indication of the release expected in the field with deviations usually within a factor of 3. For metals characterized by a solubility-controlled release, i.e. that depends more on eluate pH than the liquid to solid ratio applied, the results of pH-dependence tests describe more accurately the eluate concentration trends observed in the field with deviations that in most cases (around 80%) are within one order of magnitude (see e.g. Al and Cd). The differences between field and lab-scale data might be in part ascribed to the occurrence in the field of weathering reactions (e.g. carbonation) but also to microbial decomposition of organic matter that modifying leachate pH affect the solubility of several constituents (e.g. Ca, Ba and Cr). Besides, weathering reactions can result in enhanced adsorption of fulvic acids to iron/aluminum (hydr)oxides, leading to a decrease in the leaching of fulvic acids and hence of elements such as Cu, Ni and Pb that strongly depend on DOC leaching. Overall, this comparison shows that percolation column tests and pH-dependence tests can represent a reliable screening tool to derive data that could be employed in risk-based analysis or life cycle assessment (LCA) frameworks for evaluating potential environmental impacts deriving from specific disposal/reuse options for waste materials.

Inhibitory effect of high calcium concentration on municipal solid waste leachate treatment by the activated sludge process

This research focused on the inhibitory effects of Ca on the aerobic biological treatment of landfill leachate containing extremely high Ca concentrations. When the Ca concentration in leachate to be treated was more than 4500 mg l^-1, the total organic carbon removal rate was significantly reduced and the processing time to achieve the same removal efficiency was 1.4 times that in the control treatment without added Ca. In contrast, the total nitrogen and ammonia nitrogen (NH₄⁺-N) removal efficiencies were positively related to the Ca concentration, increasing from 65.2% to 81.2% and from 69.2% to 83.7%, respectively, when the dosage of added Ca increased from zero to 8000 mg l^-1. During aerobic treatment, the reductions of solution Ca concentration were in the range of 1003-2274 mg l^-1 and were matched with increases in the Ca content in the residual sludge. The inhibition threshold of Ca in the leachate treated by the activated sludge process appeared to be 4500 mg l^-1, which could be realized by controlling the influent Ca concentration and using an appropriate sludge return ratio in the activated sludge process.

Co-gasification of sewage sludge and woody biomass in a fixed-bed downdraft gasifier: toxicity assessment of solid residues

As the demand for fossil fuels and biofuels increases, the volume of ash generated will correspondingly increase. Even though ash disposal is now strictly regulated in many countries, the increasing volume of ash puts pressure on landfill sites with regard to cost, capacity and maintenance. In addition, the probability of environmental pollution from leakage of bottom ash leachate also increases. The main aim of this research is to investigate the toxicity of bottom ash, which is an unavoidable solid residue arising from biomass gasification, on human cells in vitro. Two human cell lines i.e. HepG2 (liver cell) and MRC-5 (lung fibroblast) were used to study the toxicity of the bottom ash as the toxins in the bottom ash may enter blood circulation by drinking the contaminated water or eating the food grown in bottom ash-contaminated water/soil and the toxic compounds may be carried all over the human body including to important organs such as lung, liver, kidney, and heart. It was found that the bottom ash extract has a high basicity (pH = 9.8-12.2) and a high ionic strength, due to the presence of alkali and alkaline earth metals e.g. K, Na, Ca and Mg. Moreover, it also contains concentrations of heavy metals (e.g. Zn, Co, Cu, Fe, Mn, Ni and Mo) and non-toxic organic compounds. Although human beings require these trace elements, excessive levels can be damaging to the body. From the analyses of cell viability (using MTS assay) and morphology (using fluorescence microscope), the high toxicity of the gasification bottom ash extract could be related to effects of high ionic strength, heavy metals or a combination of these two effects. Therefore, our results suggest that the improper disposal of the bottom ash wastes arising from gasification can create potential risks to human health and, thus, it has become a matter of urgency to find alternative options for the disposal of bottom ash wastes.

Toxicity of waste gasification bottom ash leachate

Toxicity of waste gasification bottom ash leachate from landfill lysimeters (112 m(3)) was studied over three years. The leachate of grate incineration bottom ash from a parallel setup was used as reference material. Three aquatic organisms (bioluminescent bacteria, green algae and water flea) were used to study acute toxicity. In addition, an ethoxyresorufin-O-deethylase (EROD) assay was performed with mouse hepatoma cells to indicate the presence of organic contaminants. Concentrations of 14 elements and 15 PAH compounds were determined to characterise leachate. Gasification ash leachate had a high pH (9.2-12.4) and assays with and without pH adjustment to neutral were used. Gasification ash leachate was acutely toxic (EC(50) 0.09-62 vol-%) in all assays except in the algae assay with pH adjustment. The gasification ash toxicity lasted the entire study period and was at maximum after two years of disposal both in water flea (EC(50) 0.09 vol-%) and in algae assays (EC(50) 7.5 vol-%). The grate ash leachate showed decreasing toxicity during the first two years of disposal in water flea and algae assays, which then tapered off. Both in the grate ash and in the gasification ash leachates EROD-activity increased during the first two years of disposal and then tapered off, the highest inductions were observed with the gasification ash leachate. The higher toxicity of the gasification ash leachate was probably related to direct and indirect effects of high pH and to lower levels of TOC and DOC compared to the grate ash leachate. The grate ash leachate toxicity was similar to that previously reported in literature, therefore, confirming that used setup was both comparable and reliable.