NOVELTY - Treating wastewater comprises: receiving an influent of wastewater into a wastewater treatment system having at least one anoxic or anaerobic section and at least one aerobic section; processing the wastewater in a bioreactor of the wastewater treatment system to produce sludge containing biosolids; diverting a portion of the sludge to a plug flow type reactor (30); introducing ozone to the sludge in the plug flow type reactor to cause lysis of the biosolids in the sludge; and providing the ozonated sludge to the anoxic or anaerobic section of the wastewater treatment system.    USE - The method is useful for treating wastewater.    ADVANTAGE - The method: reduces the oxygen content in the ozonated sludge to prescribed level and recycle the gases removed from the ozonated sludge to the aerobic basin or other sections within the wastewater treatment system (claimed); exhibits improved sludge conditioning; manages nutrient levels; alters the sludge floc for enhanced settling or digester operation; exhibits enhanced cell lysis, and thus effectively converts non-biodegradable material to biodegradable form; is cost effective; achieves a high selectivity for the lysis reaction; limits exposure of the lysed cells to additional or excess ozone within the reactor; and maintains a very narrow but optimized range of residence time distributions for the sludge and bacterial cells within the reactor.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) a wastewater treatment system comprising an anaerobic basin or anoxic basin (16) arranged to receive a wastewater stream for anaerobic or anoxic microbial breakdown of material in the wastewater stream, a plug flow type reactor configured to receive a sludge stream containing biosolids and introduce ozone to the sludge to induce lysis of the biosolids in the sludge, and a conduit arranged to provide the ozonated sludge from the plug flow type reactor to the anaerobic basin or anoxic basin; and    (2) treating waste activated sludge comprising producing a stream of waste activated sludge containing biosolids in a wastewater treatment system, diverting a portion of the waste activated sludge to a plug flow type reactor, introducing ozone to the waste activated sludge in the plug flow type reactor to cause lysis of the biosolids in the sludge, and providing the ozonated sludge to a digester (25) of the wastewater treatment system.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic representation of the wastewater treatment system incorporating a sludge ozonation process for providing ozonated sludge to the anoxic or other anaerobic section of the wastewater treatment system.    Intake conduit (14)    Anaerobic basin or anoxic basin (16)    Activated sludge basin (20)    Digester (25)    Plug flow type reactor (30)