Migration and changes of heavy metals during biotransformation of pig manure by black soldier fly

Abstract: A large amount of livestock manure has caused seriously ecological pollution, as the rapid development of livestock and poultry industry. The black soldier fly larvae (BSFL) are a good candidate for poultry and livestock manure treatment. The harvested insect bodies and feces can also be used as animal feed and organic fertilizer. The usage of BSFL can be efficient and environment-friendly to treat pig manure, but the heavy metals in pig manure have posed a great impact on the BSFL transformation process. In this study, the 7-day-old BSFL were used to transform pig manure at 30℃, in order to explore the BSFL growth and the transformation efficiency of manure, as well as the changes of heavy metals (Cu, Zn, Cr, Cd, As) in the BSFL bodies and feces. After 8 days of transformation, the larvae were about to enter the prepupal stage, where the transformation process terminated. The results showed that the dry weight of BSFL bodies increased with the prolongation of growth time during 8 days, and reached the highest value on the 8th day. The growth rate was the lowest from 0 to 2 d, whereas, the highest from 4 to 6 days. The conversion rate of pig manure increased with time, where the maximum value of 11.5% on the 6th day, and then decreased on the 8th day. The concentrations of Cu, Cr and As in BSFL bodies on the 8th day decreased by 24.5%, 21.7% and 33.0%, respectively, compared with those on the 2th day, indicating an obvious decrease in the concentrations of heavy metals with the larval growth. The content of Cd in BSFL increased with the larval growth, and the Cd concentration on the 8th day increased by 75.4%, compared with that on the 2th day. The changes of Cu, Cr, As and Cd concentrations in the BSFL feces were opposite to those of the bodies. The contents of Zn in the BSFL bodies and feces did not change significantly with the larval growth time. After 8 days of transformation, the Cd content in BSFL feces was significantly lower than that in pig manure, while, the contents of Cu, Cr, As and Zn were not significantly different from those in pig manure. The bioaccumulation factor of Cd by BSFL was the highest (reaching 3.8), and the values of other metals were lower than 1. The bioaccumulation of heavy metals was ranked in order Cd > Zn > Cu > Cr > As. After transformation, the heavy metals Cu, Cr, As and Zn in pig manure were mainly distributed in BSFL feces, accounting for 83.6%-92.7%; whereas, the distribution of Cd was 30.3%-50.2% in BSFL bodies and 49.8%-69.7% in feces. Compared with pig manure, the biological activities of Cu and As in BSFL feces increased, while those of Cr and Cd decreased. The biological activity of Zn did not change significantly after the transformation process. This finding can provide a potential data basis for the resource utilization of livestock manure by BSFL and the safety of transformed products.