Temperature feedback and control via aeration rate regulation in biological composting systems.

摘要

Biological composting systems are heterogeneous, aerobic, high solids degradation systems which decompose and stabilize organic materials. Previous studies of the composting process have focused on system dynamics and empirical control. While controllability of biological systems has been investigated, there have been few comprehensive studies on composting control. Numerical simulations, laboratory experiments and field studies were designed and implemented to examine composting dynamics and process control.; A numerical model was constructed using four primary variables: substrate, oxygen, moisture and energy (temperature). Effects of changes in variables on system dynamics were explored. Open and closed loop simulations were performed with the primary focus on using aeration rate to control temperature. Aeration rates were varied between 0.02 and 0.50 kg{dollar}rmsb{lcub}air{rcub}/kgsb{lcub}substrate{rcub}{dollar}/hour, with a constant flow of 0.04 kg{dollar}rmsb{lcub}air{rcub}/kgsb{lcub}substrate{rcub}{dollar}/hour providing the quickest heating profile. These simulations helped define conditions for experimental and field studies.; Bench-scale laboratory experiments involved fifteen liter static bed, forced aeration reactors, using simulated municipal solid waste (SMSW) or separated dairy manure (SDM) as substrates. Aeration-temperature interactions, studied by perturbing aeration rates (using impulses or step changes) confirmed and quantified some of the nonlinear, time varying system dynamics. Comparison of the experimental results with simulations showed excellent qualitative agreement. Introduction of a conductive loss term in the model improved the quantitative agreement. Other improvements to the model are suggested.; Several temperature control strategies were implemented in a field-scale system ({dollar}sim{dollar}200,000 liter) using separated dairy manure (SDM). It was not possible to control temperature during the initial temperature rise due to limits in available equipment, but temperature was controlled between 55-65{dollar}spcirc{dollar}C for greater than 300 hours. Excellent quantitative agreement with simulations were found for parts of the process. The temperature control implemented in this field study strongly suggests that substrate degradation, drying and pathogen reduction can be influenced by temperature control. Economic and environmental benefits of improved process control include energy conservation, increased pathogen destruction and reclamation of nutrients and organic matter.