US Patent 7771601 Treatment of waste sludge with recycling

A process for treating a sludge of biological solids has the steps of mixing an oxide-containing chemical with a pozzolanic material, reacting the mixture with the sludge so as to elevate a temperature of the sludge to between 40° C. and 140° C., pressurizing the blended sludge to a pressure of greater than 14.7 p.s.i.a., recycling a portion of the reacted sludge so as to increase a solids content of the sludge, and discharging at least a portion of the pressurized mixed sludge. The step of discharging includes flashing the pressurized mixed sludge across a restricting orifice so as to cause the liquid component to be evaporated.