Chemical and spectroscopic analysis of organic matter transformation during composting of sewage sludge and green plant waste

When a mixture of thick sewage sludge (10 m3) and green waste (5 m3) was composted, the thermogenic phase (72 °C) improved the hygiene of the final product. After 135 days of composting, overall decomposition reached 60%, with percentage decomposition of hemicellulose, cellulose and lignin 70%, 61% and 37%, respectively. The C/N ratio was 12 and the NH4+/NO3− ratio was 0.24, illustrating the maturity of the final product. During composting, the humification process resulted in an increase in humic acids from 26 to 39.5 mg g−1, and a decrease in fulvic acids from 38 to 18 mg g−1. The different humification indices and the Fourrier transform infra-red (FTIR) spectroscopic analysis revealed an increase in aromaticity and degree of polycondensation, with a decrease in aliphatic groups and a reduction in the easily assimilated peptide and carbohydrate components. The germination index for both lettuce (Lactuca sativa) and turnip seed (Brassica rapa) was 98% after 135 days of composting, showing that the final compost was not phytotoxic.