Municipal yard waste composting: process parameters, windrow gases, and leachate quality

A Municipal Yard Waste Composting Research and Demonstration Project was conducted to examine the process, quality of leachate, and viability of this technology for large-scale, low-technology application. Project research objectives were: 1) to characterize C/N, temperature, and moisture relationships of yard waste during composting and the elemental composition of the finished compost; 2) to investigate the oxygen (O₂), carbon dioxide (CO₂) and methane (CH₄) relationships in the composting yard waste; and 3) to quantify water quality parameters of the leachate from yard waste composting mixes. Mixes of 3 parts leaves/1 part grass, 2 parts woodchips/1 part grass, and leaves alone were composted in windrows. Process control was by a front-end loader/back-hoe according to a 60°C maximum temperature set-point, initially, and by temperature plateaus or moisture conditions following most active composting. Composting was more efficient with the presence of grass clippings. Some CH₄ was observed within windrows microsites, even under overall aerobic conditions. Occasional methane production not in excess of approximately 4x10⁻² mmol/l reflects inconsequential anaerobiosis. Leachate from individual windrows was minimal but contained high concentrations of total Kjehldahl nitrogen (TKN), total phosphorus, and biological oxygen demand. Soil degradation processes should ameliorate these. The leaf/grass mix generated the greatest quantities of carbon dioxide, strongly correlated oxygen and carbon dioxide concentrations, steepest temperature profile, and the highest concentrations of plant nutrients in material composted for between 8 and 24 weeks. Low-technology composting of yard wastes can be an effective, low-cost alternative to landfilling, with substantial societal and environmental benefits.