Fig. 2. Evolution of humic acid (a), fulvic acid (b) and humic/fulvic acid ratio (c) during composting of DFSS þ WS: Dewatered fresh sewage sludge þ wheat straw (Control); DFSS þ WS þ L: Dewatered fresh sewage sludge þ wheat straw þ lime; DFSS þ WS þ L þ B: Dewatered fresh sewage sludge þ wheat straw þ lime þ biochar; Results are the mean of three replicates and error bars indicates standard deviation