Systems and methods for producing processed organic-carbon-containing feedstock from an unprocessed carbon-containing feedstock are described. Unprocessed feedstock is introduced into and transported through at least one reaction chamber. The reaction chamber is configured for each feedstock to produce processed feedstock having a water-soluble intracellular salt reduction of at least 50 percent from that of unprocessed organic-carbon-containing feedstock and a water content of less than 40 percent.