Advanced thermal hydrolysis pretreatment boosts biogas production

Thermal hydrolysis of wastewater sludge before it enters anaerobic digestion units can increase the rate of residuals-to-biogas conversion (biogas production) and increase the feed concentration, the combination of which significantly reduces anaerobic-digester reactor volume. An advanced thermal hydrolysis process (THP) is being installed as part of the Washington Suburban Sanitary Commission’s Piscataway Water Resource Recovery Facility bio-energy project in Prince George’s county, Maryland. The facility will be among the first in the nation with such an advanced THP system.

“THP pretreatment changes the nature of the substrate going into the digester, and it increases the reaction rates, so the digesters can be loaded at higher concentrations, leading to biogas produced at a faster rate,” explains Dru Whitlock, global practice leader for biosolids and energy recovery with Stantec (www.stantec.com), the engineering firm designing the project as part of a design-build team with PC Construction and Hazen and Sawyer.

With THP pretreatment, unstabilized wastewater residuals are fed into a series of batch reactors in parallel, where the sludge is heated to 165°C at elevated pressures (90–100 psi). The treatment improves the rate kinetics of the sludge, thus boosting the subsequent performance of the anaerobic digester. In addition, the dewaterability, reduced odor content, and significantly reduced pathogen content allow it to be safely reused. In some cases, the amount of residuals converted to biogas increases, thus allowing more energy recovery.

Phase One of the project is now underway, which includes design and early construction involving demolition of existing on-site facilities and relocation of existing utilities. The facility is expected to be operational by spring 2024.