In most countries, controlled landfills are currently the most common disposal method for municipal solid wastes (MSWs). Despite many benefits, the generation of heavily contaminated leachate with substantial differences in both chemical composition and flow rate represents a major drawback. The realization of this has compelled authorities to adopt ever more stringent establishment of pollution control requirements. Landfill leachate is a serious problem in the treatments of municipal solid wastes using landfills methods. This leachate is usually heavily contaminated, but little attention is paid to its treatment. Optimal treatment of leachates to completely decrease the harmful environmental impacts is today's challenge. However, the complex composition of leachates makes it challenging to formulate general recommendations. Leachate variability, particularly over time and within sites, suggests that the most appropriate treatments are adaptable, universal, and simple. Landfill leachate is characterized by high biological and chemical oxygen demand and is usually composed of undesirable ingredients such as inorganic and organic pollutants. Landfill leachate varies with landfill age and content, decomposition methods, hydrological and climate conditions. Recirculation of leachate does not affect alterations in the degree of biodegradability of the waste. During landfill operations, recirculation of leachate is often applied to help stabilize landfill degradation. Leachate recirculation is widely used in practical engineering, and recirculation using vertical wells is one of the more effective methods.

The aim of the current study is to determine the impacts of leachate recirculation on the degradation of three compositions of municipal solid waste in anaerobic bioreactors. The study was completed by using six columns with equal volume (0.042m3) containing different densities and compositions of solid waste, in order to follow waste degradation over a limited time. Three compositions of waste were studied: simulated fresh waste of standard composition, simulated fresh waste of fermentable composition, and actual aged waste. Measurements of the significant parameters including pH, leachate conductivity, Chemical Oxygen Demand (COD), and waste settlement, were carried out. The quantity of oxidized organic matter was increased by the leachate recirculation, and the degradation period was reduced by using leachate recirculation. After 300, 150, and 480 days, waste stabilization seemed to be reached for fresh, aged, and fermentable waste, whereas the organic content decreased to 650, 480, and 4000mg COD/L, respectively.