Organic matter transformations and kinetics during sewage sludge composting in a two-stage system

► Sewage sludge composting was conducted in a two-stage system. ► Concentration of total humic acids (HA) increased according to first-order kinetics. ► Labile humic acids (L-HA) content did not exceed 12% of total HA. ► HA belong to R-type, indicating a low degree of humification. ► Temperature was a crucial factor affecting polymerization of fulvic acids (FA) to HA.