Temperature and physical modelling studies of open windrow composting

• Main Author: Irvine, Richard John
• Other Authors: Collier, Phillip
• Published: Abertay University 2002
• Subjects: 631; Sustainable waste management
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.247300

There is a growing demand for sustainable forms of waste management due to both legislative pressures (e.g. the European Union Landfill Directive (99/31/EC)) and increasing public awareness of environmental issues. Composting of biodegradable waste materials to produce a stabilized beneficial multi-functional product (compost) is being widely promoted. Currently, the most popular method of composting is the open windrow method, whereby the mixed and shredded feedstocks are arranged in long rows termed windrows and turned on a regular basis. During the process the waste material is subject to aerobic exothermic microbial decomposition. Commercial composting operations suffer from the problem that much of the processing is performed using empirical approaches. Improved understanding of the composting process based on scientific methodology is required to allow composting to develop its potential as an economic, safe and reliable method of sustainable waste management. This study used a series of large scale windrow-based seasonal field trials employing urban green waste as a feedstock, to investigate in unprecedented detail the temperature trends and patterns of behaviour within such structures. In addition physical-chemical profiling was undertaken. It demonstrated that windrow temperature development is not uniform in either a spatial or temporal sense. Temperature variation is a key feature of composting. All regions of typical windrows exhibited thermophilic and mesophilic temperature zones throughout the composting process. There was little seasonal variation. Sustained high temperatures were most widespread in the core regions at a height of around lm. Thus, these areas should not be favoured during temperature assessment to avoid bias results. It was demonstrated that current methods of temperature assessment are inadequate. It is suggested that greater numbers of data points are collected at varying positions and instead of simply calculating overall mean temperature that individual trends are plotted. The use of temperature frequency distribution histograms and cumulative temperature plots is additionally advised. Increased windspeed (greater than approximately 15MPH) was demonstrated to be a major factor preventing the development and sustaining of thermophilic temperatures within windrows. It is recommended that commercial composters routinely assess windspeed and direction. Changes in organic matter content, bulk density and pH provided an indication of the composting process in the long term but lacked the sensitivity of temperature measurement. Surveying using electronic tacheometry allowed changes in windrow shape and volume to be assessed. The field trial data allowed a novel physical compost model to be developed, based upon open windrow composting of urban green waste. Existing models are based on in-vessel composting systems and are technically flawed in certain key aspects. The model successfully simulated the initial stages of windrow composting, which was proven by experimentation and comparison with field trial data. The use of low level internal feedstock heating was demonstrated to be a viable method of stimulating natural enhanced microbial activity. A non- insulated model windrow and the employment of an environmental simulation system allowed the natural relationship between windrow and external environment to be modelled. The importance of such a model to the compost scientist and waste manager is highlighted. The study showed that it is possible to successfully physically model the open windrow composting process.