Sewage sludge, a kind of organic waste from wastewater treatment plants, will continue global growth because of the rising number of households connected to the central treatment plant. Untreated sewage sludge has a tendency to deteriorate the surrounding environment since they are general composited with degradable organic compounds, non-essential trace metals, microorganisms, and pathogens (Joo et al., 2019). Thermal treatments like catalytic pyrolysis (Xie et al., 2014), gasification (Freda et al., 2018) and combustion (Wang et al., 2019), can reduce the volume of sludge, diminish pathogenic bacteria and achieve energy recovery in a short time. However, undesirable aromatics would inevitably be formed during the thermal process (Dai et al., 2014).
The black solider fly (BSF), Hermetia illucens receive substantial attention as an ecologically sustainable method in the field of organic waste treatment. It can promote resource conversion efficiency and value-added product as pullulating new economic growth points for kitchen waste disposal industry and livestock and poultry manure treatment industry in developing countries (Anshika et al., 2019). By the biodegradation of organic matters, BSF larvae can grow on different substrates, varying from agricultural by-products to sewage sludge (Dennis et al., 2020; Dennis et al., 2021; Jonathan et al., 2021;Sarpong et al.,2019). BSF meal is expected to be a substitute of soybean and fish meal to reduce the cost of protein source in feed and food production (Onsongo et al., 2018; Wang Yu-Shiang et al., 2017; Thomas et al., 2018; Marianna et al., 2021; Guido et al., 2014).
Current research has shown that high-quantify substrate can provide efficient nutrient substance for the rapid growth of BSF larvae development, like chicken manure, pig manure and so on. On the contrary, growth of BSF larvae is very slow on low-quantify as sewage sludge from anaerobic digesters which is rich in proteins and other constituents of nutritional value (Lalander et al., 2019). Laganaro elucidated the feasibility of combined treatment in degassed sludge and chicken feed. When degassed sludge was included in substrate mixtures, the BSF larvae utilized more feed for respiration purposes at high rates of maintenance metabolism (Growth and metabolic performance of black soldier fly larvae grown on low and high-quality substrates). Although BSF larvae can convert different substrates into new biomass, specific flora in the bacterial biodiversity of BSF larvae production residues is needed to be identified for the further use.
Metagenomics is a direct genetic analysis of environment sample without cultivating clonal cultures (Anastasis et al., 2015). Metagenomics analysis can facilitate revealing the variation trend of bacterial communities to reply to the dynamic environment ( Jithin et al., 2020). The changes of microbial community in chicken manure treated with black solider fly have showed that Firmicutes(97.72% in bacteria), Penicillium(46.82% in fungi) and Aspergillus(45.22% in fungi) dominated in manure after treatment (Zhang Xiaoxiao et al., 2020). Protebacteria(9.52–13.50%), Bacteroidetes(7.18%-10.65%) and Firmicutes(7.53%-9.46%) were the dominant bacterium group in anaerobic digestion sludge (Jeroen et al., 2020). Hence, metagenomics analysis is an efficiency measure for investigating changes of bacterial biodiversity during the disposal of combined organic waste.
In our study, livestock manure and surplus sludge were chosen as the target organic waste due to their wide produced in social development. The mixture of chicken manure and municipal sewage sludge have been proved to be treated by BSF lavae (Cai Minmin et al., 2017). Differences in the bacterium communities between chicken manure, sludge and their mixture were demonstrated through the metagenomic analysis of various substrates. We focused on the changes of bacterium community in combined treatment of chicken manure and surplus sludge by BSF larvae.