Albert Einstein (1879-1955) Abstract Biogas production, pH and ammonium evolution of seven bioreactor landfills was simulated using the LDAT (Landfill Degradation and Transport) model, developed by the University of Southampton, UK. The LDAT model was able to describe relatively accurately the processes occurring in Compression Anaerobic Reactors (CARs). However, LDAT was not able to simulate the processes occurring in the bioreactor landfill simulators. Main reasons for the discrepancy of the simulations are related to problems in the model construction, namely the biological and chemical reactions and the fact that LDAT operates at 20ºC. In addition inhibition factors such as sulphate (competition and its end-product H2S) and ammonium inhibition were not included in LDAT. A literature review on mathematical models for waste degradation revealed that ionic balances used were not complete, which has a significant influence on the buffer capacity of the landfill system. These findings suggest that in order to increase accuracy and applicability of these models, complete leachate chemistry (ionic balance) should be used as an important factor determining the rate at which biological and chemical reactions occur in the (bioreactor) landfills. Moreover, these findings highlight the importance of inorganic chemistry measurements in order to complement available data sets and reduce the

number of uncertainties during mathematical simulation of waste degradation in landfills. 1. Introduction

Operating landfill sites as bioreactors, the so-called bioreactor landfill

approach, is gaining popularity among landfill operators due to the enhanced landfill gas production that can be used for energy purposes if properly captured (Pohland, 1980, 1996; Reinhart and Townsend, 1998). Additional benefits are: the gain of landfill volume and the reduction of the leachate chemical and biological strength (Morris et al., 2003; Benson et al., 2007). Substantial research has been conducted during the last three decades using laboratory and pilot-scale experiments from which it has been possible to identify the optimal ranges of the key process parameters namely pH, temperature, moisture content, inhibitory content, diverse microbial kinetics, etc. (Gurijala and Suflita, 1993; Barlaz and Ham, 1993). Several municipalities, especially in United States of America, have adopted the bioreactor approach at full-scale. However, despite all positive feedback, no concrete conclusions about their effectiveness can be drawn at this point since most of these sites are still under scrutiny and monitoring periods are expecting to be completed in a period of five years from 2007 (Morris et al., 2003; Benson et al., 2007). This physical experimentation is essential and valuable to acquire knowledge but requires time and intensive labour input, which is reflected in the large amounts of financial resources being expended on it, especially when full-scale testing is required. Mathematical modelling, a less expensive and time consuming activity, of different scenarios could provide valuable insight of the diverse processes occurring within the landfill, and the possible effects of alterations in operation and/or environmental conditions can be processed quickly. Moreover, the results of modelling could help to improve designs and during the decision-making processes for a full-scale implementation of the best-case scenario.