Reduction of Ammonia Emission and Waste Gas Volume by Composting at Low Oxygen Pressure
This chapter reports first results on the performance of a composting reactor with off-gas oxygen levels ≤10%. The low oxygen level is the result of uncoupling the two functions of the airflow: heat removal and oxygen supply. Uncoupling these functions is
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Introduction In The Netherlands, there is a local excess of manure caused by an intensification of pig production. This means manure has to be transported over considerable distances, leading to higher production costs for the farmer. Export could be part of the solution, but due to EC regulations concerning animal manure, it cannot be exported unprocessed. Composting can be an option for manure management. If composted properly, the product complies with EC sanitation rules and can be exported. In addition, compost weighs only half as much as the fresh manure and
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Wageningen University, Subdepartment of Environmental Technology, PO Box 8129, 6700 EV Wageningen, The Netherlands e-mail: dale.ruc!rum(i:z:algemeen.ml.wag-ur.nl. tel.:+31(317)484993 fax:+ 31 (317)4821 08 H. Insam, N. Riddech, S. Klammer (Eds.) Microbiology of Composting © Springer-Verlag Berlin Heidelberg 2002
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transport is consequently cheaper. Composting can expand the market for excess manure. Traditional low-technology composting methods take a long time and require a lot of space. As space is scarce and expensive in The Netherlands, a high-rate compo sting process must be used. These high-rate processes emit a considerable amount of ammonia when composting low C/N materials. Therefore, Dutch law demands treatment of the waste gas emitted by high-rate composting. The costs of this treatment can outweigh the economic advantage of composting. According to Finstein et al. (1983), the ventilation in temperature controlled high-rate composting processes has two functions: supply of oxygen and removal of excess heat. Veeken et al. (1999) showed that these functions can be uncoupled by introduction of gas cooling. In the uncoupled process fresh air is added to supply oxygen, and recirculated gas is cooled for heat removal. A part of the ammonia emitted from the compost is trapped in the condense that forms when cooling the recirculated gas. As a result, both the off-gas amount and the ammonia emission to the environment are strongly reduced. Optimised to minimise emissions this system may be able to comply with Dutch legislation. In the uncoupled system, the oxygen level and temperature can be independently controlled. There is a considerable literature on the choice of optimal temperature, see Richard and Walker (1998) for a review. It is generally accepted that the best compo sting results are obtained in the range from 55 to 60°C. The major question regarding the uncoupled system is the optimal oxygen concentration of the off-gas. Model calculations showed that the lower this concentration is set the less air is required for compo sting and the less ammonia is emitted (Veeken et al. 1999). However, Richard et al. (1999) have shown that under low oxygen levels the composting rate can be limited. Any limitation of the process rate leads to a slower process and consequently higher land usage, thus higher costs. The objective of this study is to investigate changes in the performance of the compo sting process at low oxygen levels an
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