Studies on the microbial ecology of open windrow composting

  • Peter Stenbro-Olsen

Student thesis: Doctoral Thesis


Due to the pressure of recent legislative changes (eg: the EU Landfill Directive and the imposition of a Landfill Tax), composting as a waste disposal technique is now being viewed as the preferred alternative to the landfilling of organic waste. However, while composting has been practised in one form or another for 2500 years, the underlying principles behind the microbial ecology of composting, is poorly understood. In order to obtain an insight into the ecology and hence, the critical nature of the composting process, a number of low cost open-windrows containing urban botanical wastes were established. These windrows were subjected to microbial and physico-chemical analysis over the initial period of exothermically active composting (25 days). This study demonstrated that, whilst average temperatures within open windrows can reach in excess of 65 °C, the sustainability and range of these temperatures depended upon the windrow bulk density. Windrows with bulk densities of 600kgm*3 had a larger insulation factor and thus, were able to sustain high temperatures for longer periods. However, these windrows were more susceptible to the development of areas of low temperature (cold spots) at depths below 20cm. Windrows with bulk densities of 400kgm'3 had smaller insulation factors and therefore, lost heat at a faster rate than windrows with higher bulk densities. This loss of heat was observed to be the case with the windrow surface layers, but they exhibited fewer cool spots at lower levels. This study found that the average microbial population of windrow material was 2.29x1013 CFU kg*1 and that each microbial cell could generate between 6.33 and 8.56xl0*13 Mjkg*1. This resulted in the generation of between 1.13 and 1.70 Mjkg*1 °C*1 of heat energy. Contrary to the published literature, this study observed that temperatures above 65 °C did not result in the significant loss of ammonia from the windrow. However, high levels of ammonia did suppress the formation of nitrate within the windrows. Experiments investigating microbial population kinetics within the windrows indicated that observed changes were proportional to temperature up to 60 °C, when a reduction in population numbers was observed between 60 °C and 65 °C. However, between 60 °C and 70 °C population levels increased once again. It was also noted that at the start of the composting process, 13 different microbial species or genera could be identified. However, after 17 days of exothermic composting, this had been reduced to 2 genera, including a novel large bacterial species belonging to the genus Bacillus. This study also showed that samples of windrows exposed to temperatures above 55 °C for 48 hours did not eliminate mesophilic or psycrotrophic microbial populations as previously assumed by other workers, but only suppressed their metabolism during the high temperature period.
Date of AwardAug 1998
Original languageEnglish
SupervisorPhillip J. Collier (Supervisor)

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