Improving Composting by Control of the Solid Matrix Structure

Food processing is a major industry that is rapidly growing because of a demand for packaged foods in urban areas. In Atlanta, Georgia, several food processors are in the business of packaging fresh salads for grocery stores. These companies have large processing facilities where vegetables, such as lettuce, cabbage, carrots, onions etc. are cleaned, chopped, mixed and packaged. In a typical operation, the amount of wastes generated equal in quantity (by weight) to the amount of product shipped. Presently, these wastes are land disposed or landfilled. Vegetable wastes do not provide any known concerns relating to pathogens or human health issues, however, they are prone to potential odors during decomposition and are expensive to dispose because of of their high moisture content leading to high landfill tip fee and transportation cost. Composting vegetable wastes, McGuckin et al. (1999) reported that the sulfur content of lettuce and onion wastes were 0.2 and 0.7 %, respectively. Discarded components of lettuce and onions have an effective carbon to sulfur ratio of 215 and 63, moisture contents of 96.2 and 91.1% and carbon to nitrogen ratios of 10.3 and 11.5, respectively. The low C/S ratio of onions indicates that mixes with high fractions of onions can result in release of odorous sulfur compounds. High water content, most of which is bound within the vegetable fiber, results in significant leachate formation during composting and collapse of the composting matrix from initial height of e.g. 1.5 m to a lower value of e.g. 0.5 m resulting in reduction in air space and oxygen availability within the pile. Each of the phenomena discussed above potentially result in increasing the amount of odors released and therefore negatively affecting the composting process.