Composting as a reactor design problem

Passively aerated compost piles are built on a base of porous material such as wood chips in which perforated air supply pipes are distributed. The piles are not turned during composting, nor is forced aeration equipment used, which significantly reduces the operating and capital expenses associated with these piles. Currently, pile configurations and materials are worked out by trial and error. Fundamentally based design procedures are difficult to develop because the natural convection air flow rate is not explicitly known, but rather is closely coupled with the pile temperature. In this paper, we report on a mathematical model to help design the base to assure adequate air distribution. This model has one dimensionless number dependent on the pile and base properties which characterizes the velocity distribution between the pipes. We also report on winter-time field trials of passively aerated composting used to collect data for a more complex model of reaction and transport in the pile itself. This more complex model is briefly described, including preliminary results.