(569g) Integrating Supercritical Salt Precipitation into Hydrothermal Processing of Wastewater for Critical Resource Recovery

The United States produces more than 24 billion gallons of wastewater daily. Treating this volume requires more than 30 TW-hr of energy and are burdened by the production of 6 million dry tons of biosolids – an organic byproduct containing 5 times the needed energy to operate treatment facilities. A high-water content limits energy recovery to 15% of biosolids via incineration. Coupling hydrothermal processes offers a promising alternative to incineration for recovering the energy content of wastewater solids, thereby making wastewater treatment energy-neutral or even net energy-producing. Hydrothermal processes such as hydrothermal liquefaction (HTL) operate under temperatures ranging from 250-600C and can be coupled to convert wet solid wastes into a variety of industry ready fuels.

Wastes, such as biosolids, contain high ash fractions (21 wt%) such as sulfur which are known to deactivate precious metal catalysts needed for fuel upgrading. This work aims to develop sulfur pathways through the HTL process and prevent it from continuing to deactivate downstream catalysts. Currently, 75% of the sulfur has been identified in the HTL products, evenly distributed through the char, oil, and aqueous phases. With the addition of an oxidant (H₂O₂) the inorganic sulfur (SO₄^2-) concentration increased 5-fold. Here, we demonstrated the potential of supercritical salt precipitation (SCSP). This process occurs beyond the critical point of water (374C, 217 atm). Under these conditions, the dielectric point of water drops severely, preventing water from hydrogen bonding with dissolved ions. This phenomenon causes a density gradient as dissolved inorganics are expelled from solution. Results show that at the critical point, there is a 99% expulsion of inorganics. Due to the high-temperature and high-pressure nature of hydrothermal processes, SCSP provides a viable ash recovery that would not require a drop in operating temperatures or pressures.