Theoretical and experimental studies on the effects of aeration strategies on the composting process

Forced ventilation in composting systems creates gradients of temperature, moisture, and oxygen concentrations in the composting matrix. Aeration strategies such as intermittent aeration, air recirculation, reversed direction airflow, and air recirculation with reversed direction airflow systems were investigated both from a theoretical as well as experimental point of view to determine the impacts on decomposition rates of a paper mill sludge/broiler litter mix with a carbon/nitrogen of 25.; Four different pilot scale-composting systems were built to investigate these aeration strategies and provide data for validation of a numerical model. A two-dimensional finite difference numerical model of composting was developed, based on a two component first order kinetic model and heat and mass balance equations. Results showed good correlation for layers 2, 3, and 4 (inner layers) in the system but indicated a need to strengthen modeling of boundary conditions effects. Condensation of moisture was an important factor affecting the top layer in all the aeration studies but it was not considered in the present model.; Results of both experimental and theoretical studies showed that intermittent aeration yielded the highest temperature gradients during composting, air recirculation and air recirculation with reversed direction airflow had the smallest temperature gradients. Reversing the direction of airflow with both reversed direction airflow and air recirculation with reversed direction airflow reduced moisture gradients. Moisture retention was increased with reversed direction airflow. As a result of gradients in the process variables, decomposition gradients existed. The decomposition gradients were highly variable in the experimental studies and were not highly correlated with simulation results.; The validated numerical model was used to simulate the four different systems at different rates of airflow, recirculation ratios, reversal times, and fan on-off times to optimize and compare the systems in terms of gradients and energy usage of the systems. Low airflow, regardless of system, seemed to be the main factor to minimize energy usage and yet to achieve a specific rate of decomposition.