The process of biotransformation can produce insect protein and promote the effective inactivation of heavy metals.

An experiment was performed to study the inactivation effect of aerobic composting on heavy metals in maggot, pig and chicken manures. After composting, Cu mainly occurred in the oxidizable (OXI) fraction with a percentage distribution above 54%. Zn and Cd mainly existed in the bioavailable factor (BF), which has strong activity, with percentage distributions greater than 88.3% and 82.7%, respectively. Cr and Pb mainly existed in the stable residual (RES) fraction with a percentage distribution of approximately 50%. The aerobic composting process had a clear inactivation effect on heavy metals. For maggot manure compost in particular, the inactivation effects of Cu, Cr, Zn, Cd, and Pb were very good throughout the composting process, and the inactivation effect of Pb reached 54.42%. In addition, the process of biotransformation by housefly maggots promoted the conversion of fulvic acid (FA) to humic acid (HA) in pig manure, and the final increase in HA/FA after maggot manure composting was the largest among the different types of manure and beneficial to the inactivation of heavy metals. Compounds containing -CH and -CH groups were reduced, and aromatic structures were enhanced. Moreover, a maggot yield equivalent to 13.2% of the fresh pig manure was achieved during the process of biotransformation. The correlation analysis results showed that moisture content was an important factor affecting the inactivation rates of heavy metals in the three manure composts. Our results highlight that the process of biotransformation by housefly maggots can promote composting maturity and the inactivation of heavy metals, and produce a large amount of insect protein, yielding beneficial ecological and economic benefits.