A gravity pressure vessel (GPV) for processing a fluid stream containing organic materials provides several improvements to the art. In one embodiment, a GPV of the present invention sustains the heat energy requirements of a hydrolysis GPV by including therein, a first reaction chamber for conducting an exothermic reaction, and subsequently providing a second reaction chamber for hydrolysis of certain organic materials refractory to the exothermic reaction. Another embodiment provides for the perpetual cleaning of the heat transfer surfaces of the GPV. Still another embodiment provides for selective alteration of a hydrolysis reaction within a GPV independent of the flow of the fluid stream through the GPV.