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The effects of aeration strategies (one-directional airflow, one-directional airflow with recirculation, reversed-direction airflow, and reversed-direction airflow with recirculation) on the composting process under intermittent aeration with a 15 min on and a 45 min off cycle for all tests were experimentally investigated. Intermittent aeration was used in these studies because it has been shown to have higher efficacy (more energy efficient and less odor emission) than continuous aeration for the one-directional airflow case. Paper mill sludge and broiler litter were composted using four 208 L stainless steel reactor vessels modified to allow studies on four different aeration strategies with a C:N ratio of 25:1. Compost mass was divided into five layers by polypropylene screens. Detailed analysis of the experimental data showed that aeration strategies yielded different temperature, moisture, CO2, O-2, and decomposition profiles. One-directional airflow yielded the highest temperature, moisture, and decomposition gradient across the composting bed. The experimental data for one-directional airflow with recirculation showed that the compost temperature profiles were smaller than for the no-recirculation case and were similar to what was reported in the literature. Reversed-direction airflow had the smallest moisture gradient and maximum moisture retention. Reversed-direction airflow with recirculation showed temperature and moisture profiles that were a superimposition of the results for the two independent aeration strategies. Oxygen levels, due to the on/off cycle pattern and decomposition rates, limited performance and it was concluded that each system was not operated at the optimum point. Results of these studies will be used to validate finite-difference computer models, which can then be used to find the optimum operating points for each system. |
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