The increasing development and urbanization of the society has led to an increase per-capita production of municipal solid waste (MSW) materials. These MSW materials are of organic and inorganic nature that can be of rapidly, moderately and slowly biodegradable or inert characteristics. With regard to these waste streams a wide variety of treatments exist: reuse and recycling, composting, anaerobic digestion, incineration, and land disposal are the most common ones, pyrolysis and gasification are in use to a lesser extent or on a smaller scale. Regardless of the method chosen for treatment all these methods produce residues, which will be eventually disposed at open dumps or sanitary landfills. Sanitary landfills are engineered facilities that make use of barriers to isolate the waste from the biosphere in order to protect human health and the environment. However, these barriers will fail in the long-term allowing the intrusion of moisture into the waste mass, which will trigger restrained physical, chemical and biological processes causing pollution in the form of leachate and landfill gas. In order to minimise the negative impacts of landfilling of waste, researchers conducted experiments, which resulted today in the so-called bioreactor landfill approach. The bioreactor landfill is a system that is operationally influenced to promote synergy between the inherent microbial consortia, and is controlled to accelerate the sequential phases of waste stabilisation through the addition of liquids and/or air. These past investigations have allowed the determination of optimal ranges for the key process parameters and the implementation of alternative operational conditions, the so-called enhancement techniques. Two main perspectives can be identified: 1) the American perspective, which attempts to apply enhancement techniques in order to maximise landfill gas production; 2) the European perspective that focus on the achievement of the Final Storage Quality (FSQ) status of residues within a generation timeframe (30 years). The term FSQ suggests that the potential of a waste material to produce pollution is reduced to nearly zero in the long-term perspective, similar to the characteristics for inert waste laid in the Waste Acceptance Criteria (WAC) of the European Landfill Directive. Until now, the main technical problem faced by landfill operators is homogenous liquid addition and distribution (key enhancement technique) within the waste mass. The main objective of this thesis was to achieve a FSQ status of waste, through laboratory and pilot-scale experiments, that complies with the WAC of the Landfill Directive for Inert waste, which had been considered as the worstcase scenario due to the stringent criteria established. Therefore, this thesis focused on the interaction and modification of the factors controlling the waste stabilisation process in order to have a better understanding of the physical, chemical and biological processes occurring in a bioreactor landfill.Bench (1 L), laboratory (50 L) and pilot-scale (800 L) simulators were used to apply different combinations of enhancement techniques (shredding. buffer addition, septage addition, and forced air intrusion) in order to achieve FSQ of residues. In addition, coarse materials (as layers or homogenous mixtures) were used in order to improve the hydraulic conditions of the simulators. The results of these experiments revealed that it was possible to achieve biological stabilisation within 1 year, but not FSQ status. Achievement of FSQ status depends strongly on the initial solid waste composition. Nevertheless, the residues were close to comply with the WAC of the Landfill Directive for inert waste. Buffer and septage addition proved to have a positive impact on the waste stabilisation process, reducing the biogas production lag-phase. Additionally, the risks associated with septage disposal were practically eliminated as no faecal coliforms were detected after 1 year of operation. Also the use of coarse materials had a positive impact on the waste stabilisation process, especially as homogenous mixtures and layers to a lesser extent since they were prone to clogging. Nitrogen compounds, especially ammonium, have been identified as a main parameter that will jeopardise the achievement of FSQ status, hence the safe closure of landfill sites. Therefore, evolution and fate of nitrogen compounds were also investigated in this thesis. The experiments showed that about 40% of the total ammonium was released by physical processes within 24 hours; the other 60% was produced by biological degradation of proteins contained in the MSW. Anammox bacteria were found for the first time in bioreactor landfills and it was suspected to have an important contribution to the total removal of nitrogen from the system, beside other nitrogen removal processes. Nevertheless, it was not clear how or where the intermediate products (i.e. nitrite) necessary for Anammox metabolism were produced.The Landfill Degradation and Transport (LDAT) model was used to simulate the evolution of carbon and nitrogen compounds. The LDAT model was not suitable to represent accurately the processes occurring in the simulators mainly because the model operated at a fixed (20ºC) process temperature and the waste chemistry equations need to be improved. Other models found in literature could be more appropriate to describe these processes; however, it was noticed that these models lack a complete ionic balance which has great influence on the pH of the system. The experimental research emphasised the importance of increasing pH values to neutral pH values, which “triggered” most of the processes in the simulators. This thesis highlighted the need to focus future modelling efforts on the integration of this complete ionic balance and its influence on the development of neutral (even alkaline) pH levels.In conclusion, this research reduced our current gaps-in-knowledge and offered feasible technical alternatives to control and steer the processes occurring in a bioreactor landfill aiming to achieve FSQ status of residues