NOVELTY - Catalytic hydrothermal process for treating wet biomass feedstock involves heating under pressure wet biomass feedstock to pre-treatment temperature for organic constituents in feedstock to decompose; reacting soluble sulfate contaminants with cations present in feedstock; separating precipitates of inorganic wastes and sulfate-containing precipitates out of wet biomass feedstock to yield a liquid of wet biomass feedstock having decreased sulfate content; and exposing liquid of wet biomass feedstock to heterogeneous metal catalyst and gasifying organic constituents after separating.    USE - For treating wet biomass feedstock comprising biomass, inorganic contaminants, soluble sulfate contaminants, sub-critical liquid water (claimed), where biomass includes high-moisture biomass slurries, biosludge from wastewater treatment systems, sewage sludge from municipal treatment systems, wet byproducts from biorefinery operations, wet byproducts/residues from food processing, animal waste and waste from centralized animal raising facilities, organic chemical manufacturing wastewater streams, and/or industrial wastewater contaminated with organics.    ADVANTAGE - The present method involves operations at temperatures and pressures that maintain the wet biomass feedstock in the liquid phase without forming a critical or supercritical fluid; and overcomes the problem of poisoning and fouling of the catalyst, especially by sulfate contaminants that are soluble in the liquid portion of the wet biomass feedstock. The solubility of the sulfate-containing precipitate is low enough at the operating conditions described, and the cations can facilitate removal of the otherwise soluble sulfate contaminants by reaction to form precipitates. Previously, it had been assumed that compounds, which result from reactions between the soluble sulfate contaminants and the cations, were too soluble to be useful in removing soluble sulfate contaminants. The cations and the soluble sulfate contaminants were thus soluble in the feedstock at conventional temperatures and pressures, but reacted to form insoluble precipitates at elevated temperatures (i.e. the pre-treatment temperatures and/or hydrothermal gasification temperatures). This change in solubility facilitates removal of the sulfate contaminants from the feedstock prior to gasification.    DETAILED DESCRIPTION - Catalytic hydrothermal process for treating a wet biomass feedstock comprising biomass, inorganic contaminants, soluble sulfate contaminants, and sub-critical liquid water involves: a) heating under pressure the wet biomass feedstock to a pre-treatment temperature sufficient for organic constituents in the feedstock to decompose, for precipitates of inorganic wastes to form, and for preheating the wet feedstock in preparation for removal of the soluble sulfate contaminants; b) reacting the soluble sulfate contaminants with cations present in the feedstock to yield a sulfate-containing precipitate; c) separating the precipitates of inorganic wastes and the sulfate-containing precipitates out the wet biomass feedstock to yield a liquid of the wet biomass feedstock having a decreased sulfate content; and d) exposing the liquid of the wet biomass feedstock to a heterogeneous metal catalyst and gasifying at least a portion of the organic constituents after the separating, where the process is operated at temperatures and pressures that maintain the wet biomass feedstock in liquid phase without forming a supercritical fluid.