The evaluation of stability during the composting of different starting materials: Comparison of chemical and biological parameters

Three blends formed by: (i) food processing waste (CPFP), (ii) waste water sewage sludge (CPWW), and (iii) their mixture (CPFP+WW), blended with tree pruning as bulking agent, were composted over 3 months. During composting the blends were monitored for the main physical-chemical characteristics: temperature, oxygen saturation level (02%), pH, total and volatile solids, total organic carbon, and organic nitrogen (Nord. In addition to the main parameters, the dissolved organic carbon (DOC), the inorganic nitrogen and the Oxygen Uptake Rate (OUR) were monitored. All the mixtures easily reached a peak temperature around 70 degrees C, related to the lowest O-2%. After 90d, CPFP, CPFP+WW, and CPWW showed an organic matter mineralization of 43%, 35% and 33%, respectively; CPFP fitted an exponential model while both CPFP+WW, and CPWW fitted a logistic model. During composting an OUR reduction of 79%, 78% and 73% was registered in CPFP, CPFP+WW, and CPWW, respectively; the OUR successfully fitted the adopted exponential model and well reflected the stabilization process in time. The N-org recovery at the end of the process was positive only in CPWW (11.6%). The DOC significantly decreased during the composting process but did not successfully fit any model. The mineral nitrogen did not follow the typical pattern with NH4+ disappearance and NO; accumulation. Strong NO3- losses were evident in all blends, while NH4+ accumulations were detectable only in CPFP, and CPFP+WW. The NH4+/NO3-; ratio did not satisfactorily reflect the composting process over time. The comparison of the first order (exponential) and logistic (sigmoidal) models applied to the OUR and OM course highlights the role of mineral nitrogen as limiting factor during composting of the more stabilized sludge. (C) 2011 Elsevier Ltd. All rights reserved.