The safe and efficient disposal and utilization of sludge are major issues to be solved in solid waste treatment and environmental protection due to the complex characteristics of sludge and the low rate of innocuous treatments. Composting is a process of decomposing organic matter and transformed low-molecular organic acids into high-molecular humus substances under the action of microorganisms. Although land-use after composting has become an important direction for sludge treatment, heavy metal pollution is still the bottleneck problem restricting land use of sludge compost. Adding zeolite, hydroxyapatite, and other conditioning agents to the composting process affects the concentration or form of some heavy metals and effectively reduces the environmental risk. Lime and phosphorus modifiers change heavy metal speciation in samples, playing a role in decreasing biological availability and mobility. In this study, the effects of sludge composting treatment and conditioning agents on the concentrations and forms of heavy metals are reviewed. This review will provide a theoretical basis to treat heavy metals in sludge composting and lay the foundation for the land utilization and waste recycling of sludge.

Sludge compost; heavy metal; conditioning agents; remediation