Study of LFG bubble accumulation and discontinuous flow in the highly saturated region of landfill below the leachate level.

Landfills in developing countries are typically characterized by high waste water content and elevated leachate levels. Despite the ongoing biodegradation of waste in the highly saturated regions of these landfills, which leads to gas accumulation and bubble formation, the associated gas pressure that poses a risk to landfill stability is often overlooked. This paper introduces a landfill gas (LFG) bubble generation model and a two-fluid model that considers bubble buoyancy and porous medium resistance. The entire process can be divided into two stages based on the force balance and velocity of bubbles: Bubble Development Stage and the Two-Fluid Flow Stage. The models were validated using a one-dimensional analytical solution of hydraulic distribution that considers bubble generation, as well as an experiment involving air injection into a saturated medium. The mechanisms of LFG accumulation and ascent, leachate level rise, and discontinuous leachate-gas flow were then investigated in conjunction with continuous flow in the unsaturated region. The results indicate that the generation of LFG bubbles below the leachate level can cause a rise in the level height of more than 20%. During the Bubble Development Stage, there is a critical height for bubble ascent, above which the buoyancy exceeds the combined forces of gravity and resistance, resulting in less than 10% of bubbles continuously flowing into the unsaturated zone for recovery. The developed model effectively captures the accumulation and flow of LFG bubbles below the leachate level and could be further utilized to study leachate-gas pumping in the future.

Application of advanced techniques for the assessment of bio-stability of biowaste-derived residues: A minireview.

Bio-stability is a key feature for the utilization and final disposal of biowaste-derived residues, such as aerobic compost or vermicompost of food waste, bio-dried waste, anaerobic digestate or landfilled waste. The present paper reviews conventional methods and advanced techniques used for the assessment of bio-stability. The conventional methods are reclassified into two categories. Advanced techniques, including spectroscopic (fluorescent, ultraviolet-visible, infrared, Raman, nuclear magnetic resonance), thermogravimetric and thermochemolysis analysis, are emphasized for their application in bio-stability assessment in recent years. Their principles, pros and cons are critically discussed. These advanced techniques are found to be convenient in sample preparation and to supply diversified information. However, the viability of these techniques as potential indicators for bio-stability assessment ultimately lies in the establishment of the relationship of advanced ones with the conventional methods, especially with the methods based on biotic response. Furthermore, some misuses in data explanation should be noted.