California Institute for Energy and Environment (CIEE)

The elemental composition of fresh and digested food and green waste were measured. The suitability of these materials as soil amendments was evaluated. Stability of digested and composted food and green wastes were also experimentally determined in terms of respiratory quotient (RQ) and maximum carbon dioxide evolution index (MCI). Results showed that the concentration of many elements in the food and grass waste increased, as percentage of total solid contents of materials, during digestion but the nutrient content within the system did not change. The heavy metal content of both green and food wastes increased during digestion. The digested food waste had ammonia to nitrate ratio of 52.5 and the digested grass waste had a ratio of 65.0. The C/N ratio of the digested residues was less than 25. Based on the ammonia/nitrate ratio, the fresh and digested food and green wastes were classified as immature materials to be used as soil conditioners. However, based on the C/N ratio, using these digestates would not negatively affect the plant growth. The RQ was approximately 0.6 for the digested grass and food wastes and the composted food waste. The RQ of the composted green waste was 0.39 and the potting soil was 0.12. The results of the MCI showed that the digested food waste was unstable. While composted food wastes and the digested grass waste were stable and the composted green wastes and potting soil were very stable.