The Effects of Process Control Strategies on Composting Rate and Odor Emission

Poor compost quality and odor emission are often significant problems in the composting industry. Composting process control can potentially help reduce both of these problems. In spite of the recent development of a number of process control strategies, very few direct comparisons have been made between these, particularly in terms of compost quality and odor emission. To help address this need, a series of experiments were conducted to evaluate the effects of several in-vessel process control strategies on organic matter conversion, nitrogen transformation and pH, and odor emission. The strategies focussed on aeration control. Fixed aeration, temperature feedback, oxygen feedback, and combined temperature/oxygen feedback algorithms were tested. A modified algorithm called linear temperature feedback was also developed and tested. Results showed that the compost temperature profiles were quite similar for the various feedback control algorithms, whereas fixed rate aeration led to significantly higher temperature, as expected. Compost properties such as C:N ratio and organic matter loss were also similar between process control methods. However, oxygen content was maintained more consistently using oxygen feedback or linear temperature feedback algorithms. Linear temperature feedback is preferable to oxygen feedback in that it does not require oxygen sensors to operate. Mass emission rates of odorous gas (methyl mercaptan and dimethyl sulfide) were typically found to increase with higher aeration rates, such as those used to limit temperature, though the gas concentration was lower. For maximum retention of nitrogen, adequate supply of readily biodegradable carbon in the feedstock is vital.