This paper analyses sanitary landfills as biochemical reactors and particularly the significance of water balance and multi-phase flow in a landfill system. Field studies reveal a strong coupling between water content and landfill gas production. This study focuses on evaluation of field observations on water content, temperature, gas and leachate production in specially designed and operated landfills. Theoretical simulation of multiphase flow conditions - water and gas transport - has been carried out for different components of the landfill system. Top cover as barrier to gas exchange, 1-D heat transport and heat source distribution and water percolation reproducing mechanical dispersion due to presence of waste elements in the sanitary landfill are modelled to improve insight into the overall performance of sanitary landfills.