Physico-chemical and Spectroscopy Assessment of Sludge Biodegradation During Semi-industrial Composting Under Semi-arid Climate

The purpose of this study is to assess the maturity and stability of the lagooning sludge (LS) with green waste (GW) during semi-industrial composting. Physico-chemical parameters, spectroscopy analysis and germination test were monitored. After 105 days, the decomposition rate of about 33.6; 39 and 36%, a stabilization of pH around neutrality; NH4+/NO3− ratio decreased to 0.922, 0.974 and 1.038; the C/N ratio decreased to 9.47, 9.24 and 10.10 respectively for mixture 1 (M1), mixture 2 (M2) and mixture 3 (M3). This reduction is explained by the bio-oxidation of organic matter. The intense microbial activity was characterized by the rise of temperature (between 40 and 50 °C) during the first weeks. Humification process was characterized by the increase of humification index (HI), which was approximately 62.1%; 73.6%; 61.5% respectively for M1, M2 and M3. The progress of humification process was determined by the decrease of aliphatic absorbance bands and the increase of aromatic absorbance bands. The increase in germination index (GI) (> 50%) for the four seeds: cress, alfalfa, turnip and radish, at the end of composting indicated that the composts are phytotoxic-free and are rich in stable organic compounds and nutrients. The results show that all three composts are mature, which opens the way for their application in agriculture without risk for the soil–plant system.