Physical modelling of the composting environment: a review. Part 1: Reactor systems

In this paper, laboratory- and pilot-scale reactors used for investigation of the composting process are described and their characteristics and application reviewed. Reactor types were categorised by the present authors as fixed-temperature, self-heating, controlled temperature difference and controlled heat flux, depending upon the means of management of heat flux through vessel walls. The review indicated that fixed-temperature reactors have significant applications in studying reaction rates and other phenomena, but may self-heat to higher temperatures during the process. Self-heating laboratory-scale reactors, although inexpensive and uncomplicated, were shown to typically suffer from disproportionately large losses through the walls, even with substantial insulation present. At pilot scale, however, even moderately insulated self-heating reactors are able to reproduce wall losses similar to those reported for full-scale systems, and a simple technique for estimation of insulation requirements for self-heating reactors is presented. In contrast, controlled temperature difference and controlled heat flux laboratory reactors can provide spatial temperature differentials similar to those in full-scale systems, and can simulate full-scale wall losses. Surface area to volume ratios, a significant factor in terms of heat loss through vessel walls, were estimated by the present authors at 5.0-88.0m(2)/m(3) for experimental composting reactors and 0.4-3.8m(2)/m(3) for full-scale systems. Non-thermodynamic factors such as compression, sidewall airflow effects, channelling and mixing may affect simulation performance and are discussed. Further work to investigate wall effects in composting reactors, to obtain more data on horizontal temperature profiles and rates of biological heat production, to incorporate compressive effects into experimental reactors and to investigate experimental systems employing natural ventilation is suggested.     

Behavior of potentially toxic elements from stoker-boiler fly ash in Interior Alaska: paired batch leaching and solid-phase characterization

Environmental Science and Pollution Research - Despite significant investigation of fly ash spills and mineralogical controls on the release of potentially toxic elements (PTEs) from fly ash,...     

Describing variability of MSW composition data with the log-logistic distribution

Variations in solid waste composition data are necessary as inputs to solid waste planning, yet uncertainty exists regarding which probability distributions might be generally valuable to describe the variability. Twenty-two detailed analyses of solid waste from British Columbia, Canada, were fitted to distributions using the BestFit software. Alternative distributions were ranked based on three goodness-of-fit parameters and twelve waste fractions. The log-logistic distribution was found to be the most able to fit over the wide range of composition types considered. The results were demonstrated to be insensitive to the number of waste components or to the choice of a two- or three-parameter distribution. Although other distributions were able to better match the waste composition for individual waste types, the log-logistic distribution was demonstrated to fit, overall, a wide variety of waste composition types.     

VFA and ammonia from residential food waste as indicators of odor potential

Research was conducted to determine suitable chemical parameters as indicators of odor from decomposing food wastes. Prepared food scraps were stored in 18 l plastic buckets (2 kg wet weight each) at 20 °C and 8 °C to reproduce high and low temperature conditions. After 1, 3, 7, 10 and 14 days of storage, the odor from the buckets were marked to an intensity scale of 0 (no odor) to 5 (intense) and the corresponding leachate analyzed for volatile fatty acids, ammonia and total organic carbon. A linear relationship between odor intensity and the measured parameter indicates a suitable odor indicator. Odor intensified with longer storage period and warmer surroundings. The study found ammonia and isovaleric acid to be promising odor indicators. For this food waste mixture, offensive odors were emitted if the ammonia and isovaleric acid contents exceeded 360 mg/l and 940 mg/l, respectively.     

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.