Study of the microbial dynamics and diversity after inoculation of sterile digestates with a complex community

Sludge from wastewater treatment systems is normally stabilized by anaerobic digestion, and the remaining digestate can be used to fertilize crops. Before utilized as biofertilizer, the digestate needs to be sterilize to avoid the spread of pathogens. This sterilization has traditionally been achieved by thermophilic digestion, or partial sterilization before or after anaerobic digestion, and more recently by Thermal Hydrolysis Processing (THP), which effectively sterilize the material. THP prior to anaerobic digestion enhance the biogas production, while THP after anaerobic digestion also results in a sterile digestate. Although both may seem attractive from a health perspective, there is a risk for regrowth of pathogens due to lack of competitors: it is practically impossible to avoid microbial recontamination (including pathogens) during handling and storage, and these organisms will rapidly colonize the sterile material.The goal of this project was to test if the growth of pathogens in sterile digestates can be eliminated by inoculation with a complex microbial community of harmless bacteria. This was tested this in a laboratory incubation of THP treated digestates, using the growth/survival of E. coli as a pathogen model, introduced by contaminating the material with wastewater. Compost bacteria were introduced 2 days prior to the wastewater contamination. During the 600 hours of aerobic incubation, we monitored respiration, abundance of E. coli as viable counts and gene abundance. To study the dynamic and diversity of the microbial community established from the re-contamination, we performed 16S rRNA amplicon sequencing on a Illumina MiSeq platform of samples taken over the time period.