Abstract :
[en] Chapter 1: INTRODUCTION
The introductory chapter established the global and national context of landfill leachate management, highlighting the pressing challenges faced by developing countries such as Vietnam. Landfill leachate is characterized by extremely high pollutant concentrations, including ammonium levels often exceeding 800 – 1,200 mgN.l-1 and chemical oxygen demand (COD) values ranging between 2,500 – 5,000 mg.l-1, with a substantial fraction composed of refractory organics. The persistence of these pollutants results in significant environmental risks, particularly when untreated or insufficiently treated leachate infiltrates surface and groundwater.
The objective of the research was to explore advanced biological treatment strategies capable of achieving reliable nitrogen and COD removal from landfill leachate, with a focus on combining Membrane Bioreactor (MBR) technology and Activated Sludge Models (ASM1 and ASM3). This integrated approach was designed to optimize biological nitrogen removal, characterize refractory COD behavior, and provide a robust modeling framework for system design and operation.
The methodology outlined the rationale for adopting MBR systems, which offer stable biomass retention, high effluent quality, and flexibility under variable leachate conditions. Furthermore, the incorporation of ASM models allowed for systematic parameter estimation and simulation, essential for predicting treatment performance.
The introduction concluded by identifying the knowledge gap: limited data existed on applying ASM1 and ASM3 to landfill leachate in tropical regions, and little was known about the dynamics of partial nitrification and refractory COD removal in MBRs under Vietnamese conditions. This research therefore sought to fill this gap through combined experimental and modeling approaches, providing both scientific and practical contributions to sustainable leachate management.
Chapter 2: LITERATURE REVIEW
The literature review systematically analyzed previous studies on landfill leachate characteristics, treatment technologies, and modeling approaches. It emphasized the distinction between young and old leachates: young leachates with high BOD/COD ratios and rapidly biodegradable organic matter, and old leachates with BOD/COD ratios below 0.1, dominated by refractory humic substances and inert COD. In Vietnam, most leachates correspond to intermediate to old stages, making treatment particularly challenging.
The review highlighted conventional biological processes such as activated sludge, sequencing batch reactors, and constructed wetlands. These systems often failed to consistently meet effluent standards due to inhibitory compounds and high ammonium loads. Advanced options such as MBRs have shown superior performance, with reported nitrogen removal efficiencies of 70 – 90% and COD removal of 60 – 80%, but their application to tropical landfill leachate remains underexplored.
In terms of nitrogen removal, the review underscored the importance of partial nitrification–denitrification pathways as more energy-efficient alternatives to full nitrification. Factors such as dissolved oxygen, pH, and free ammonia concentrations were recognized as critical control parameters. For COD, the persistence of soluble inert fractions (SI), often comprising 20 – 30% of total COD, was consistently reported as a limiting factor.
The review also analyzed activated sludge models, particularly ASM1 and ASM3, as tools for simulating biological treatment. ASM1 is effective in representing nitrogen transformations, whereas ASM3 offers improved description of COD fractions and storage processes. However, few studies have attempted parameter calibration for landfill leachate.
The chapter concluded that while MBR technology combined with ASM modelling presents a promising approach, research is needed to calibrate models for leachate conditions, optimize partial nitrification, and quantify refractory COD behavior—objectives directly addressed in the present thesis.
Chapter 3: LANDFILL LEACHATE CHARACTERIZATION FOR SIMULATION OF BIOLOGICAL TREATMENT WITH ACTIVATED SLUDGE MODEL No.1 (ASM1) AND No.3 (ASM3)
Chapter 3 provided a detailed analysis of the composition and seasonal variability of landfill leachate in Vietnam, forming the experimental basis for subsequent modelling and treatment studies. Composite samples were collected over both dry and rainy seasons. Influent COD concentrations averaged 2,800 – 3,200 mg.l-1, with slowly biodegradable COD (XS) accounting for 45 – 50% and inert soluble COD (SI) contributing 20 – 25%. Biodegradable soluble COD (SS) rarely exceeded 30%, confirming the refractory nature of the leachate.
Nitrogen species were dominated by ammonium, with concentrations consistently between 850 – 1,100 mgN.l-1, while nitrate and nitrite remained negligible (< 5 mgN.l-1), indicating the absence of significant nitrification in the landfill environment.
Alkalinity levels of 2,000 – 3,000 mg.l-1, CaCO3 were sufficient to buffer nitrification, though the presence of possible inhibitory compounds was noted. Seasonal variations revealed slightly higher COD and ammonium during the rainy season due to increased leaching of fresh waste layers.
Respirometric assays confirmed slow biodegradation rates. Oxygen uptake tests showed that only 50 – 55% of COD was biodegradable within 5 days, with endogenous respiration dominating thereafter. Biodegradation kinetics indicated reduced maximum heterotrophic growth rates (µH ≈ 2.5 – 2.8 d⁻¹) compared to municipal wastewater values, highlighting stress conditions for biomass.
Chapter 4: CHARACTERIZATION OF SLUDGE FOR STOICHIOMETRIC AND KINETIC PARAMETERS FOR MODEL No.1 AND No.3 FOR LEACHATE TREATMENT
This chapter focused on the estimation and calibration of kinetic and stoichiometric parameters for Activated Sludge Models ASM1 and ASM3 to enable their application to landfill leachate treatment. Since leachate differs significantly from municipal wastewater, with high ammonium and refractory COD fractions, the default model parameters were unsuitable. The aim was therefore to determine accurate values for parameters governing ammonium oxidation, heterotrophic COD degradation, and decay processes, and to compare the predictive capacity of ASM1 and ASM3 under leachate conditions.
Respirometric batch tests were carried out using acclimated activated sludge exposed to different substrates, measuring oxygen uptake rates (OUR) under controlled conditions. Data analysis showed that the maximum autotrophic growth rate (µA) was approximately 0.25 d⁻¹, considerably lower than the typical value for municipal wastewater (~0.8 d⁻¹). The half-saturation constant for ammonium (KNH4) averaged 2.3 – 2.6 mgN.l-1, nearly double the ASM default, reflecting the reduced affinity of nitrifiers in the inhibitory leachate environment. For heterotrophic biomass, the maximum growth rate (µH) was around 2.8 d⁻¹, while the yield coefficient was reduced to 0.55 gCOD/gCOD compared with the standard 0.67, indicating stress conditions. The fraction of inert COD was confirmed at 20 – 25% of total COD, with slowly biodegradable COD making up almost half of the COD load.
Model calibration demonstrated that ASM1 could adequately represent nitrogen transformations but underestimated COD removal, as it lacks explicit storage and hydrolysis mechanisms. In contrast, ASM3 achieved a better fit, particularly in describing slowly biodegradable COD fractions and endogenous storage, with correlation coefficients exceeding 0.9 for OUR curves. Sensitivity analysis.
highlighted the strong influence of µA, KNH4, and heterotrophic yield on model predictions.
Chapter 5: MODELLING OF PARTIAL NITRIFICATION AND DENITRIFICATION
This chapter investigated the feasibility of achieving stable partial nitrification in landfill leachate treatment, with the dual aim of suppressing nitrite oxidation and promoting ammonium conversion to nitrite as a precursor for subsequent denitrification. The background highlighted that conventional nitrification in leachate often fails due to high ammonium concentrations, inhibitory compounds, and the accumulation of inert COD, making controlled partial nitrification a more energy-efficient and sustainable option.
The experimental approach employed controlled batch and sequencing batch reactor (SBR) runs under varying dissolved oxygen (DO), pH, and temperature conditions. Respirometric assays were also used to quantify nitrification rates and inhibition levels. The principal objective was to define the operating window that suppresses nitrite oxidizing bacteria (NOB) while maintaining the activity of ammonium oxidizing bacteria (AOB).
Results demonstrated that ammonium concentrations in the raw leachate averaged 800 – 1,200 mgN.l-1, with alkalinity consistently above 2,500 mg.l-1, CaCO₃, favoring nitrification potential. Under controlled DO at 0.5 – 1.0 mg.l-1, nitrite accumulation ratios exceeded 60 – 70%, while higher DO (> 2 mg.l-1) led to rapid nitrite oxidation and nitrate formation. Temperature was another key factor: at 30 – 32°C, AOB exhibited maximum activity (µA ≈ 0.28 d⁻¹), whereas NOB growth was strongly inhibited, resulting in stable nitrite accumulation. pH between 7.5 – 8.0 supported sustained partial nitrification, while lower pH caused inhibition of both AOB and NOB. Batch tests showed nitrite accumulation up to 450 mgN.l-1, confirming effective NOB suppression.
Model simulations using calibrated ASM1 and ASM3 parameters aligned well with experimental data, with ASM3 providing more accurate predictions of nitrite accumulation dynamics. Sensitivity analyses confirmed that DO and free ammonia concentrations were the most influential factors governing the balance between AOB and NOB activity.
In conclusion, Chapter 5 demonstrated that partial nitrification of landfill leachate is achievable and stable under optimized operating conditions of low DO (0.5 – 1.0 mg.l-1, moderate pH (7.5 – 8.0), and mesophilic temperature (~30°C). This strategy not only reduces oxygen demand but also establishes a suitable feed for subsequent anoxic denitrification, thereby improving the overall nitrogen removal efficiency.
Chapter 6: NITROGEN REMOVAL IN LANDFILL LEACHATE TREATMENT WITH MEMBRANE BIOREACTOR IN VIETNAM
This chapter examined the performance of a pilot-scale Membrane Bioreactor (MBR) treating landfill leachate, with a particular focus on nitrogen removal efficiency under tropical operational conditions. The motivation stemmed from the limitations of conventional activated sludge systems, which struggle with high ammonium and refractory COD levels in leachate. The pilot MBR was designed to integrate controlled partial nitrification with subsequent denitrification, ensuring stable effluent quality.
The pilot system had a treatment capacity of 5 m³.d-1, operated under continuous flow, with submerged hollow-fiber membranes providing solid–liquid separation. Influent leachate contained COD ranging from 2,500 – 3,200 mg.l-1, and ammonium 800 – 1,100 mgN.l-1. Operating conditions maintained a hydraulic retention time (HRT) of 24 – 36 hours, sludge retention time (SRT) > 40 days, and controlled dissolved oxygen at 0.5 – 1.0 mg.l-1 in the aerobic zone to facilitate partial nitrification.
Results indicated excellent nitrogen removal. Ammonium removal consistently exceeded 90 – 95%, with effluent ammonium reduced to < 50 mgN.l-1. Total nitrogen removal reached 75 – 82%, depending on influent load and temperature, with effluent TN concentrations typically between 120 – 180 mgN.l-1. Nitrite accumulation was observed in intermediate stages, confirming the effectiveness of partial nitrification–denitrification. COD removal ranged from 70 – 78%, limited by the presence of slowly biodegradable and inert COD fractions identified earlier. Transmembrane pressure (TMP) increased gradually but remained manageable, with membrane fouling controlled through periodic backwashing and air scouring.
Model validation using ASM1 and ASM3 showed that ASM3 provided closer agreement with measured data, particularly in simulating COD fractions and nitrogen transformations. The calibrated model predicted effluent concentrations with an R² above 0.9, confirming its applicability for leachate MBR design.
In conclusion, the pilot-scale MBR demonstrated robust performance for nitrogen removal from landfill leachate, achieving > 90% ammonium removal and > 75% total nitrogen reduction under optimized low-DO conditions. While COD removal was moderate due to the persistence of refractory organics, the system consistently produced stable effluent suitable for discharge or further polishing. These findings confirm that MBR technology, supported by calibrated ASM modeling, is a viable solution for leachate treatment in tropical developing countries such as Vietnam.
Chapter 7: REFRACTORY COD REMOVAL IN LANDFILL LEACHATE TREATMENT WITH MEMBRANE BIOREACTOR IN VIETNAM
This chapter focused on the capacity of the pilot Membrane Bioreactor (MBR) to remove refractory organic matter, a major challenge in landfill leachate treatment. While biological processes are effective for nitrogen removal, the persistence of inert or slowly biodegradable COD typically limits effluent quality. The objective of this chapter was to quantify COD removal efficiency, identify the refractory fraction, and evaluate strategies to enhance performance.
Influent COD concentrations during pilot operation averaged 2,500 – 3,200 mg.l-1, with fractionation showing that 45 – 50% of the COD was slowly biodegradable (XS), while 20 – 25% was inert soluble COD (SI) resistant to biological degradation. Standard operation at a hydraulic retention time of 24 – 36 hours and sludge retention time > 40 days achieved overall COD removal of 72 – 78%, with effluent COD typically 550 – 700 mg.l-1. Despite the relatively high efficiency, effluent COD remained above discharge standards, primarily due to the persistence of SI.
Batch assays confirmed that only 5 – 10% of SI could be further degraded even under extended aeration, reinforcing its refractory nature. Adsorption onto biomass contributed minimally, with less than 7% COD reduction attributable to biosorption. Model simulations with ASM1 and ASM3, using the calibrated parameters from Chapter 4, accurately reproduced COD removal trends, and sensitivity analyses confirmed that effluent COD was strongly controlled by the proportion of inert fractions rather than kinetic parameters.
In conclusion, the MBR demonstrated consistent removal of biodegradable COD but could not completely eliminate refractory COD, which persisted in effluents at levels of several hundred mg.l-1. The findings emphasize the need for complementary polishing steps — such as advanced oxidation processes (ozonation, Fenton, or activated carbon adsorption) — to achieve regulatory standards. Importantly, the study provided quantitative evidence that inert COD accounts for up to one-quarter of total COD in Vietnamese landfill leachate, limiting the achievable performance of biological treatment alone. This insight is critical for future leachate treatment design and highlights the role of integrated treatment trains combining MBR with advanced physicochemical processes.
