Physical modelling of the composting environment: a review. Part 2: Simulation performance

This paper reviews previously published heat balance data for experimental and full-scale composting reactors, and then presents an evaluation of the simulation performance of laboratory and pilot-scale reactors, using both quantitative and qualitative temperature profile characteristics. The review indicates that laboratory-scale reactors have typically demonstrated markedly different heat balance behaviour in comparison to full-scale systems, with ventilative heat losses of 36-67%, and 70-95% of the total flux, respectively. Similarly, conductive/convective/radiative (CCR) heat losses from laboratory reactors have been reported at 33-62% of the total flux, whereas CCR losses from full-scale composting systems have ranged from 3% to 15% of the total. Full-scale windrow temperature-time profiles from the literature were characterised by the present authors. Areas bounded by the curve and a 40 degrees C baseline (A(40)) exceeded 624 degrees C. days, areas bounded by the curve and a 55 degrees C baseline (A(55)) exceeded 60 degrees C days, and times at 40 and 55 degrees C were >46 days and >24 days, respectively, over periods of 50-74 days. For forced aeration systems at full scale, values of A(40) exceeded 224 degrees C days, values of A(55) exceeded 26 degrees C days, and times at 40 and 55 degrees C were >14 days and >10 days, respectively, over periods of 15-35 days. Values of these four parameters for laboratory-scale reactors were typically considerably lower than for the full-scale systems, although temperature shape characteristics were often similar to those in full-scale profiles. Evaluation of laboratory-, pilot- and full-scale profiles from systems treating the same substrate showed that a laboratory-scale reactor and two pilot-scale reactors operated at comparatively high aeration rates poorly simulated full-scale temperature profiles. However, the curves from two moderately insulated, self-heating, pilot-scale reactors operated at relatively low aeration rates appeared to closely replicate full-scale temperature profiles. The importance of controlling aeration rates and CCR losses is discussed and further work suggested in order to investigate the links between simulation of the composting environment and process performance.