Influence of operations on leachate characteristics in the Aerobic-Anaerobic Landfill Method

Characteristics and key driving factors of nitrous oxide emissions from a full-scale landfill leachate treatment system

The characteristics of N2O emission from a full-scale landfill leachate treatment system were investigated by in-situ monitoring over 1.4 years and driving factors responsible for these emissions were identified by statistical analysis of multidimensional environmental variables. The results showed that the maximum N2O emission flux of 2.21 × 107 mg N·h-1 occurred in the nitrification tanks, where 98.5 % of the total N2O was released, with only 1.5 % of the total N2O emitted from the denitrification tanks. Limited oxygen in nitrification tank was responsible for N2O hotspot. The N2O emissions from the parallel lines A and B (both comprising the primary biochemical system) accounted for 52.6 % and 46.6 %, respectively, while the secondary biochemical system contributed only 0.8 % to the total emissions. Higher nitrite concentration in line A and lower nitrogen loading in the secondary biochemical system caused these discrepancies. We found that during the steady state of leachate treatment, intensive N2O emissions of 253.4-1270.5 kg N·d-1 were measured. The corresponding N2O emission factor (EF) ranged from 8.86 to 49.6 %, much higher than those of municipal wastewater treatment. But N2O EF was inconceivably as low as 0.42 % averagely after system maintenance. Influent with low salinity was the key reason, followed by the high MLSS and varying microbial community after maintenance. The dominant genus shifted from Lentimicrobium and Thauera to Norank-F-Anaerolineaceae and Unclassified-F-Rhodocyclaceae. This study underscores the significance of landfill leachate treatment in urban nitrogen management and provides valuable insights into the characteristics and driving factors of N2O emissions from such systems. The findings offer important references for greenhouse gas emission inventories and strategies for N2O control in full-scale wastewater treatment plants.

Effect of temperature on fungal nitrification in simulated in-situ aeration of aged MSW landfill

Fungal nitrification is one kind of heterotrophic nitrification that involves certain species of fungi promoting the transformation of organic nitrogen and ammonia nitrogen to nitrite/nitrate. In this study, simulated aerated landfill reactors (SALRs) were constructed to investigate fungal nitrification in aged municipal solid refuse, with a focus on understanding the effect of temperature on the performance of fungal nitrification as well as fungal contribution to ammonia nitrogen transformation. Different nitrogen metabolism patterns have been observed in the system with fungi only (SALRF) and complete microbial consortium, i.e., bacteria + fungi (SALRC). At a temperature of 35 °C, autotrophic nitrification dominated the ammonia nitrogen transformation, while fungal nitrification did not significantly contribute to ammonia removal. However, at elevated temperatures (i.e., 45 °C and 55 °C), fungi played a crucial role in ammonia transformation through fungal assimilation and fungal nitrification, with bacterial function suppressed. Furthermore, 45 °C was found to be the optimal temperature for fungal nitrification, exhibiting the highest nitrification rate (13.98 mg L-1 d-1) which accounted for 49.80% of total nitrification rate in the aerated landfill. High throughput sequencing revealed reshaping of fungal community in response to temperature variation. The abundance of Aspergillus fumigatus, with a relative abundance ranging from 67.13% to 92.71% at elevated temperatures, suggested its significant potential for fungal nitrification. These findings have implications for the promotion of nitrogen cycle through strengthening fungal nitrification in aerated landfill sites which often operate at high temperatures.

Prediction and control of elevated temperatures within landfills under aeration and recirculation based on the thermal non-equilibrium model

Aeration is an effective approach to sustainable landfilling but may lead to elevated temperatures within landfills, resulting in landfill fires or explosions. Therefore, aeration is usually combined with leachate recirculation to control the elevated temperatures within landfills. To predict landfill temperatures during aeration and recirculation, a local thermal non-equilibrium model is developed considering the heat generation of biodegradation, the heat removal due to evaporation and leachate-gas flow, and the effects of the capillary. The solver is implemented in OpenFOAM based on the finite volume method and validated against a waste-column experiment and an in-situ aeration test. The simulation results demonstrate that the assumption of local thermal equilibrium will distinctly overestimate the temperature, maximally by 15 °C in the studied case. The model is then used to simulate a typical aerobic landfill unit to investigate the formation of explosive gas mixtures and elevated temperatures under different operating conditions. The simulation results of gas composition suggest that aeration will not result in explosive gas within landfills. A reasonable recirculation method for temperature control with corresponding operating parameters under a group of values of aeration pressure (2000-4000 Pa) and recirculation rate (0.0001-0.0008 m/s) are proposed, which can provide some guides for the design of an aeration and recirculation combined system. For a given total volume of added leachate, a higher recirculation rate does not always mean better cooling, and the cooling effect of continuous recirculation is better than that of intermittent recirculation.

Increasing recovery opportunities of metal(loid)s from municipal solid waste via landfill leachate recirculation

The recovery of 12 critical raw materials (CRM) from municipal solid wastes (MSW) via leachate recirculation was evaluated using a 4 L semi-pilot scale column percolation. The results showed that the recovery of the metal(loid)s was mainly influenced by order of importance: pH > organic content > type of metal(loid)s > age of the waste > redox potential. Among the CRM, Cd and Ni were the most mobile elements, while As and Cr were the least mobile. A comparison of leachate from the leachate recirculated columns before and after the initiation of recirculation indicates an increase in the concentrations of certain CRM and metalloids. The first recirculation cycle supported achieving 100 % recovery. CRM and metalloids in leachate can be recovered; however, the concentrations of CRM and metalloids are usually below 1 mg/L. In this regard, leachate recirculation may enhance the increasing concentration of CRM in landfill leachate. For example, after first recirculation cycle, Ni concentration increased from 0.05 mg/L to 0.11 mg/L. The results obtained from this study can develop further methodologies for the potential recovery of CRM and help foster further research into overcoming limitations for recovering CRM in landfill leachate.

A hybrid process for leachate wastewater treatment: Evaporation and reverse osmosis/sequencing batch reactor

In this study, a hybrid process for leachate wastewater treatment including evaporation and reverse osmosis (RO) membrane or biological treatment systems was suggested. Experiments were performed on a real landfill leachate wastewater. The leachate was subjected to evaporation; as a result, a distillate was obtained containing less organic matter and less substantial amounts of other pollutants, as ammonium salts and total phenols were removed. Tests were carried out at different evaporation temperatures and times. The initial leachate pH was adjusted and optimized. For optimum conditions, each of chemical oxygen demand (COD), total phenol, and ammonium salt concentrations were reduced to 99.99%, 95.00%, and 83.00%, respectively. The distillate of the first stage of the proposed process was then exposed to RO membrane system, as a first study, under different transmembrane pressure of 20, 30, and 40 bar and at different pH values of 7, 8, and 9. As a second suggested treatment system, the distillate was subjected to a biological treatment process for 30 days as a retention time, pH = 6, and room temperature 25°C ± 1°C. At the end of the research study, a comparison was conducted between results obtained with RO membrane separation and biological treatment system as two distinct treatment systems proposed for leachate landfill wastewater treatment. Although both systems were effective for landfill leachate wastewater treatment, however, with the RO membrane separation system, COD removal efficiency reached 99.99%. In the other hand, with biological treatment process, COD elimination was as much as 90.00%. Certainly, evaporation and RO are not novel ways of landfill leachate treatment; however, few studies have attempted to use similar combined system for landfill leachate wastewater treatment and attained effective results of treated water. PRACTITIONER POINTS: A hybrid process of evaporation and RO membrane or biological treatment systems was suggested for leachate wastewater treatment. For optimum conditions, COD, total phenols, and ammonium salt reductions were achieved to 99.99%, 95%, and 83%, respectively, after the first evaporation stage. The distillate of the first stage of the proposed process was then exposed to RO membrane system and biological treatment system. Different transmembrane pressure and different pH values were optimized for RO.

Intermittent aeration reducing N2O emissions from bioreactor landfills with gas-water joint regulation

Landfills are important emission sources of atmospheric N₂O, especially bioreactor landfills with leachate recirculation. In this study, N₂O emissions were characterized in four bioreactor landfills with different ventilation methods, including intermittent (2-h aeration per 12 h or 4 h/d in continuous) and continuous aeration (20 h/d), in comparison to a traditional landfill without aeration. During the experiment, the N₂O emissions from the landfill reactors with intermittent aeration were 7.48 and 7.15 mg, accounting for only 20.8% and 19.9% of those with continuous aeration, respectively. Continuous aeration was more favorable for the biodegradation of organic matter than intermittent aeration in the landfilled waste and leachate. Intermittent and continuous aeration could both effectively remove total nitrogen (TN) and NH₄⁺-N with removal efficiencies above 64% in the leachate. In the experimental landfill reactors with gas-water joint regulation, the proportion of N₂O-N to TN loss ranged from 0.02% to 0.75%. Luteimonas, Pseudomonas, Thauera, Pusillimonas and Comamonas were the dominant denitrifying bacteria in the landfill reactors. The denitrifying bacterial community in the landfilled waste was closely related to its degree of stabilization and nitrogenous compound concentrations in the landfilled waste and leachate. The NO₃^--N and NO₂^--N concentrations of leachate were the most important environmental factors affecting the succession of nirS-type and nirK-type denitrifying microbial communities in the landfilled waste. These findings indicated that intermittent aeration was an economical and effective way to accelerate the stabilization of landfilled waste and reduce the pollutants in leachate and N₂O emissions during landfill mining and reclamation.

Effect of cigarette butt on concentration of heavy metals in landfill leachate: health and ecological risk assessment

Cigarette butt is known as hazardous waste with numerous toxic and carcinogenic pollutants which impose serious concern for both the environment and human. Heavy metals are recognized as the most common pollutant in the cigarette butts. The concentration of some heavy metals (cadmium, chromium, nickel, lead and zinc) in leachate obtained from the pilot landfill with commingled waste and freshly smoked cigarettes butts were analyzed. The results showed that the addition of 0.76% (in weight) freshly smoked cigarette butts in landfilled waste increased total heavy metal concentration by 4.8%, while addition of 1.3% (in weight) freshly smoked cigarette butts leads to increased 3.72% of total heavy metals concentrations. An increased 10.52% and 3.43% health risk values were found from the leachate of the landfill pilot, where 1% freshly smoked cigarette butt and a littered cigarette were added, respectively. Overall, it can be concluded that cigarette butt landfilling is not recommended for management of this type of waste and is necessary to be replaced with less hazardous ways such as recycling.

Removal of Pollution by Intensive Aeration Technology for Landfill Leachate Treatment

BACKGROUND

Landfill leachate is a source of environmental pollution and a major concern for human health because it contains high concentrations of organic and inorganic contaminants.

OBJECTIVES

The objective of the present study is to validate the efficiency of the forced aeration treatment technique, which consists of intensively injecting a continuous oxygen flow of 16.75 kg/m³/h for 30 days using a bubble air diffuser in a relatively small volume of 1 m³. The principle of the technique is essentially based on acceleration of the degradation of the effluent as well as reduction of the organic and nitrogenous matter contents.

METHODS

Forced aeration technology was used for the treatment of leachate from the Mohammedia-Benslimane landfill. The sample was treated by injecting a flow of oxygen to accelerate the biodegradability of the pollutants by the microorganisms.

RESULTS

The physicochemical characterization of the raw leachate at the inlet of the aeration tank showed high values for chemical oxygen demand (COD) (38,600 mg O₂/l), biological oxygen demand (BOD5) (24,000 mg O₂/l), and total Kjeldahl nitrogen (TKN) (5,932.45 mg/L). The proposed treatment technique allowed relatively high purification yields to be achieved, with abatement rates for the major elements COD, BOD5 and TKN of 73%, 98%, and 85%, respectively.

CONCLUSIONS

The treatment of leachate by intensive aeration technology reduces considerably the pollutant load and achieves a high purification yield.

COMPETING INTERESTS

The authors declare no competing financial interests.

Effect of waste compaction density on stabilization of aerobic bioreactor landfills

Landfill stabilization contributes to the safe operation and maintenance of landfills. This study used a simulated aerobic bioreactor landfill to investigate the impact of different compaction densities on its stabilization to provide a basis for optimal parameter selection during landfill design. Samples of municipal solid waste were tested with compaction densities of 450, 500, 550, 600, and 650 kg/m³ during the experiment. The optimum compaction density was obtained by periodically monitoring the temperature of the waste pile, the water quality of leachate, and the composition of the waste. The impacts of waste compaction density on waste pile temperature and leachate were investigated and coupled with the analysis of waste composition to discuss the possible reaction mechanism. Results showed that the most complete waste degradation occurred at 550 kg/m³ compaction density, which was effective at accelerating stabilization of the simulated aerobic bioreactor landfill. Limitations of the experiment are given to lay foundations for further study.

Reutilization of high COD leachate via recirculation strategy for methane production in anaerobic digestion of municipal solid waste: Performance and dynamic of methanogen community

The influences of reutilization of high COD leachate via recirculation strategy on methane production and dynamic of methanogen community in anaerobic digestion of Municipal Solid Waste (MSW) were revealed. With a COD concentration of 6000 mg·L^-1 recirculation, the efficiency of hydrolytic acidification process was improved and alleviated the pH reduction during acidification, while the highest COD removal efficiency was achieved. The maximum methane production rate and accumulated CH₄ production by the 6000 mg·L^-1 group increased by 90.7% and 156.0%, respectively. Whereas the performance of the 9000 mg·L^-1 group was actually below the control group. According to high-throughput sequencing, the superiority of acetotrophic Methanothrix was replaced by hydrogenotrophic Methanobacterium in the 3000- and 6000-mg·L^-1 systems. Methanoculleus predominated in the 9000-mg·L^-1 system, while Methanoregula, Methanolinea, and Methanospirillum suffered intensive inhibition effects. Canonical correspondence analysis verified a positive correlation between the dominant methanogens Methanobacterium and CH₄ production, and a negative correlation with Methanoculleus.

Numerical model of aerobic bioreactor landfill considering aerobic-anaerobic condition and bio-stable zone development

Aeration by airflow technology is a reliable method to accelerate waste biodegradation and stabilization and hence shorten the aftercare period of a landfill. To simulate hydro-biochemical behaviors in this type of landfills, this study develops a model coupling multi-phase flow, multi-component transport and aerobic-anaerobic biodegradation using a computational fluid dynamics (CFD) method. The uniqueness of the model is that it can well describe the evolution of aerobic zone, anaerobic zone, and temperature during aeration and evaluate aeration efficiency considering aerobic and anaerobic biodegradation processes. After being verified using existing in situ and laboratory test results, the model is then employed to reveal the bio-stable zone development, aerobic biochemical reactions around vertical well (VW), and anaerobic reactions away from VW. With an increase in the initial organic matter content (0.1 to 0.4), the bio-stable zone expands at a decreasing speed but with all the horizontal ranges larger than 17 m after an intermittent aeration for 1000 days. When waste intrinsic permeability is equal or greater than 10^-11 m², aeration using a low pressure between 4 and 8 kPa is appropriate. The aeration efficiency would be underestimated if anaerobic biodegradation is neglected because products of anaerobic biodegradation would be oxidized more easily. A horizontal spacing of 17 m is suggested for aeration VWs with a vertical spacing of 10 m for screens. Since a lower aeration frequency can give greater aeration efficiency, a 20-day aeration/20-day leachate recirculation scenario is recommended considering the maximum temperature over a reasonable range. For wet landfills with low temperature, the proportion of aeration can be increased to 0.67 (20-day aeration/10-day leachate recirculation) or an even higher value.