On July 3rd, 2008 Mr. Roberto Valencia Vázquez successfully defended his PhD thesis and propositions on Enhanced stabilisation of municipal solid waste in bioreactor landfills. Professor Huub Gijzen was his promotor, and Henk Lubberding, PhD, his co-promotor.
In order to minimise the negative impacts of landfilling of municipal solid waste, the bioreactor landfill has been developed. 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. The main purpose is to reach so-called 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.
The main objective of this thesis was to achieve a FSQ status of waste through laboratory and pilot-scale experiments. 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.
Laboratory (1 L and 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 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 standards 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. 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 they may play an important role in the total removal of nitrogen from the system.