Impacts of Thermal Hydrolysis on Biosolids Quality Parameters: Dewaterability, Odors, Indicators and Pathogens

The objectives of this research were to evaluate several different enhanced or advanced digestion processes for their ability to minimize the risk for a sustainable Class A biosolids program. The two key risks that were considered were odors and increases in indicator bacteria density after dewatering. The four processes evaluated included thermal hydrolysis pretreatment, dual digestion, enhanced enzymic hydrolysis, and multi-stage thermophilic digestion. All of the processes were able to reduce the culturable density of indicator bacteria, fecal coliform, below the Class A limit of 1000 MPN/g DS. However, the thermal hydrolysis process appears to be the process that best minimizes the risk of increases in fecal coliform densities after dewatering. Similar results were obtained for spiking experiments with Salmonella. The odorant production by the cake from the different processes were generally lower than conventional mesophilic anaerobic digestion processes with centrifuge dewatering, although were somewhat variable, and may present some risk of nuisance odors. The advantage of the thermal hydrolysis pretreatment is that high solids can be achieved with belt filter press dewatering that are equivalent to centrifuge dewatering. Belt filter press cakes have generally been shown to have lower odors and less likelihood of indicator bacteria increases in the cake which further helps the thermal hydrolysis process minimize risks related to odors and indicators.