[Review on the Safety of Heavy Metals During Land Utilization of Municipal Sludge after Composting].

The leaching and release of heavy metals in municipal sludge pose critical constraints on the land utilization of municipal sludge after composting. To comprehensively understand the safety of heavy metals during land utilization of municipal sludge after composting， in this work， the heavy metal content levels of typical municipal sludge in China were summarized； the changes in the total amount， valence， and speciation forms of different types of heavy metals before and after composting were analyzed； and the short-term and long-term environmental behavior patterns of heavy metals after the application of municipal sludge composting products were reviewed. The mean concentrations of As， Cd， Cr， Cu， Hg， Ni， Pb， and Zn in municipal sludge in China were 21.78， 2.41， 108.57， 231.09， 2.35， 67.57， 55.03， and 630.24 mg·kg， respectively， which remained within the permissible limits outlined in the China national standard GB 4284-2018 for Class A sludge products and exhibited a declining trend over the past three decades. During the composting process， the heavy metal content in the non-screened municipal sludge compost products decreased due to mixing auxiliary materials. Moreover， the humification of organic matter served to enhance heavy metal chelation， facilitating their conversion into more stable forms and thereby curtailing their mobility， leachability， and biological availability. The land utilization of municipal sludge compost products can gradually elevate the heavy metal content in soil， necessitating a judicious consideration of both application volumes and the duration of continuous usage to uphold soil quality and agricultural product safety. The heavy metals in municipal sludge compost products may undergo leaching and migration during land utilization， while literatures suggest minimal risks of this process to environmental quality， human health， crop yields， and soil fertility. Owing to variations in municipal sludge characteristics， the long-term safety implications of heavy metals in the land utilization of municipal sludge compost products warrant ongoing scrutiny； therefore， adjustments to the duration of continuous application should be mandated in accordance with specific sludge properties and land utilization conditions.