Composting of Municipal Sewage Sludge and Lignocellulosic Waste: Nitrogen Transformations and Humic Substances Molecular Weight

As increasing soil organic matter is considered one of the main strategies for reducing desertification in Europe, the production and use of high-quality composts has gained importance during the last decade. It is undisputed that the quantity and quality of humic substances (HS) and their fractions, i.e. fulvic acids (FA) and humic acids (HA) in compost are considered important indicators of compost maturity and chemical stability. Other important aspects are concentrations of macro- and micronutrients and heavy metals (HMs) that are introduced to the soil with mature compost. In this light, this study assessed the detailed characteristic of compost from municipal sewage sludge. Moreover, kinetic of organic matter (OM), and nitrogen transformations, therein nitrogen loss, were analysed. OM degradation proceeded according to first-order kinetics. In the bioreactor, the rate constant of OM removal and the rate of OM removal were 0.134 d−1 and 12.6 mg/(g d.m.d), respectively. In the windrow, these constants were 5.2-fold and 16.7-fold lower, respectively. In mature compost, the concentration of HS equaled 240.3 mg C/g OM (1.65-fold higher than in the feedstock) and the concentrations of HA and FA were 120.7 mg C/g OM and 119.6 mg C/g OM, respectively. In FA predominated those with a molecular weight in the range of 10–30 kDa (47.2%), FA with a molecular weight >100 kDa accounted for only 14.4%. In HA, however, fraction with the highest molecular weight (>100 kDa) accounted for more than half (51.2%), while the share of HA with a molecular weight <10 kDa was only 6.8%. During composting, nitrogen loss was observed, which resulted from NH3 rather than N2O emission. In mature compost, organic nitrogen predominated (17.82 g/kg d.m.; ca. 92% of the overall nitrogen). The final concentrations of ammonia nitrogen and nitrate nitrogen were 0.23 and 1.12 g/kg d.m., respectively. The compost met the Polish requirements for the content of HMs (the HMs concentrations were as follows: Cd 1.85 mg/kg d.m., Pb 12.16 mg/kg d.m., Ni 11.05 mg/kg d.m., Cr 24.14 mg/kg d.m., Cu 104.24 mg/kg d.m., Zn 854 mg/kg d.m., Hg 0.12 g/kg d.m.).