The function of "Cambi® thermal hydrolysis plus anaerobic digestion" on heavy metal behavior and risks in a full-scale sludge treatment plant based on four seasons investigation

The environmental risk of heavy metals in sewage sludge from a full-scale "Cambi (R) thermal hydrolysis + anaerobic digestion" sludge treatment plant was discussed based on four seasons' data. Results showed that the order of heavy metal concentration in sludge was Zn > Cu > Cr > Ni > As > Pb > Hg > Cd, which all increased significantly due to the "enrichment effect" caused by the degradation of organics. Nevertheless, the mass of heavy metals except for Cd decreased. Chemical fractions of different heavy metals in raw sludge varied greatly. The proportion of their residual fraction all increased slightly after treatment. Thermal hydrolysis and anaerobic digestion led to the transformation of some heavy metal fractions. Deep dehydration process reduced the mass of heavy metals from sludge (less than 10%). Potential ecological risk of heavy metals was low (RI <150) when sludge is applied 0.75 kg/m2 to soil according to GB 4284-2018, in which the risk of Hg and Cd was highest. Furthermore, the accumulation amounts of heavy metals in test soil and rural soil with the annual sludge application amount of 0.75 kg/m2 for 15 years were calculated, which did not exceed GB 36600-2018 and GB 15618-2018 respectively.