Engineered passive barrier systems for mitigating landfill gas emissions

Abstract

The objective of this study was to provide an alternative methanotrophic system aimed at filtering out methane and other gases emitting from landfills and to reduce their release into the atmosphere in arid land, in a passive way. Methane and other landfill gases are oxidised and converted into carbon dioxide and other less harmful by-products through microorganisms present in soils (methanotrophic bacteria) and in the presence of oxygen. The inherent low air diffusion in the top cover layer of landfills, compounded by the low permeability soils existing in desert countries, such as Kuwait, renders the use of conventional passive mitigation systems unusable in these countries. Based upon these facts, a comprehensive literature review revealed that the abundance of the research was dedicated to finding better mitigation systems to deliver oxygen to the landfills’ covers through the top surface. Even by designing a structure or experimenting with cover materials, the air delivery of these passive filter systems is still dependent upon the natural atmospheric and molar diffusion into the soil, which, even insufficient and inadequate delivery mechanism in normal circumstances, could not be used in landfills located in arid land environment. Through a series of batch experiments, oxygen was found to be the most important and limiting factor that surpassed the other factors needed by the methanotrophic bacteria for any landfill and particularly for landfills located in arid lands. The batch test also highlighted the importance of supplying oxygen in an adequate and continuous flow, and revealed that a time lag of three to four days could occur before the methanotrophic bacteria could regenerate and commence converting methane gas. In addition, a continuous flow reactor experiments (CFR) confirmed that poor nutrient desert sand and soil cover could not support efficient methane elimination, making it necessary to supplement these covers with nutrient-rich amendments. The CFR experiments also showed that a supply of oxygen in levels deeper within the soil layers has a much higher methane oxidation rates than the conventional surface oxygen supply, reaching more than 65% average rate of oxidation. Based on these findings, and on the particular circumstances in arid lands, particularly the dustladen environment of Kuwait, a passive concept system was proposed and a modelled numerically to compare with results of field a trial data, which showed advantage of this passive system over conventional systems. A mitigation scheme was also recommended to control environmentally harmful gases from release into the atmosphere from landfills located in the arid land of Kuwait.