Potential Pollutants from Farm, Food and Yard Waste Composts at Differing Ages: Part I. Physical and Chemical Properties

There is little information about the changes in physical and chemical properties of compost as a function of compost feedstocks and age of the compost mixture. As a consequence of this gap, the potential of compost to serve as a pollutant source relative to compost age (from initial mixing of raw feedstocks to mature compost) and feedstocks (i.e. % of food, manure, bedding, leaves, etc.) during the composting operation is not well known. This study was conducted to assess the variation of physical and chemical properties of compost under different mixes of source material and at differing ages (from freshly mixed to mature compost). The change in compost C:N ratio with age during composting was represented well (R² ≥ 0.81) by the equation Y = CNf + (CNi-CNf)e^(−X/r), where Y is the C:N ratio at any time X. CNf is the value of the C:N ratio as the composting time approaches infinity. CNi is the initial C:N ratio at the beginning of the composting process and 1/r is the rate of decrease of the C:N ratio. The total (tot) N concentration of the mature composts was higher than the initial tot N concentration for all mixes investigated. There was a net decrease from the initial NH4-N concentrations at mixing to the final NH4-N concentrations at maturity for all compost mixtures. However, the variations of compost tot N and NH⁴4-N concentrations with compost age were different between compost mixtures and years of experiment. The phosphorus (P) concentrations increased linearly with compost age. These results indicated that the potential of the compost as a source for N and P pollution changes during the composting process. The results also showed that the combination of C:N ratio, compost temperature, and compost NH4-N concentration could be used as simple but reliable indices for compost stability and maturity.