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) wastewater sewage sludge (CPWW), iii) and 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 (O2%), pH, volatile solids (VS), total organic carbon (TOC), and total Kjeldahl nitrogen (TKN). Beside the main parameters the dissolved organic carbon (DOC), the inorganic nitrogen (N-min) and the Oxygen Uptake Rate (OUR) were monitored. All the mixtures easily reached a peak temperature around 70°C, related to the lowest O2%. After 90 days mixtures showed an intense organic matter mineralization: 74.8% (CPFP), 53.8% (CPFP+WW) and 49.6% (CPWW). The Norg recovery on TS basis was -4.75% (CPFP); -5.71% (CPFP+WW) and +11.6% (CPWW). The DOC strongly decreased during the composting process but did not successfully fit the adopted exponential model. The N-min did not follow the typical pattern with the NH4 +-N disappearance and NO3 --N accumulation. More specifically strong NO3 --N losses were evident in all blends, while NH4 +-N accumulation were detectable in CPFP, and CPFP+WW. The NH4 +/NO3 - ratio did not satisfactorily describe the composting process in time. The OUR was the only parameter which successfully fitted the adopted exponential model and well described the stabilization process in time, independently by mixture origin.