Modelling of the kinetics of municipal solid waste composting in full-scale mechanical-biological treatment plants

A thesis submitted in fulfilment of the requirements for the Degree of Doctor of Philosophy in Sanitary Engineering in the Faculty of Sciences and Technology of the New University of Lisbon

This work focused on the study of the kinetics of municipal solid waste composting in four full-scale mechanical-biological treatment (MBT) plants. We investigated how well the existent plants in Portugal were being operated, and estimated their performance at optimum operation. To achieve this, volatile solids (VS) content and several environmental conditions, namely temperature (T), moisture content (MC), oxygen concentration ([O2]), and free air space (FAS), were monitored throughout the composting process. Experimental data was fitted with a first-order kinetic model, and a rate constant (k) that corrects for T, MC, [O2] and FAS conditions was obtained, i.e., k is characteristic of composting under optimum environmental conditions. The kinetic model satisfactorily described the experimental data from three MBT plants. k values ranged from 0.043 d-1 to 0.082 d-1. Modelling the fourth plant was less successful,probably due to sampling errors on the VS determinations. This is the first time that a kinetic composting model has been applied to full-scale MBT plants. We also concluded that two of the MBT plants were poorly operated. Optimization of process management with measures of simple practical implementation was estimated to be highly significant in these poorly managed plants, increasing performance by 103% in MBT1 and 53% in MBT2. In conclusion, this work highlights the importance of having process performance monitoring and optimization programs in full-scale composting systems. It is proposed that the procedures developed here are applied for this purpose. The composting model was further tested by applying it to data from lab- and full-scale studies collected from the literature. This is the first time that rate constants from a large set of data sources are corrected for T, MC, [O2] and FAS. k values from full-scale studies varied little, whereas those from lab-scale studies varied widely with k reaching much higher values. These observations indicate that: (i) factors other than those included in the model have a significant effect on the composting rate, as previously suggested; (ii) there is a large margin for improvement in the performance of real-scale systems; (iii) extrapolation of data from laboratory- to full-scale can be misleading and should be cautiously applied; and (iv) more studies of full-scale systems should be conducted, because these constitute an important control over laboratory-scale studies, especially when the aim is the optimization of the design, or operation, of full-scale systems.

European Project “AWAST; 5th Framework Programme (project n. EVK4-2000-00514)