NOVELTY - The method involves providing controlled flow hydrodynamic cavitation apparatus (2,16,26) with local constriction. The stream (4) of bio solids sludge (1) is passed through local constriction. A hydrodynamic cavitation zone is formed to downstream of local constriction by hydrodynamic cavitation apparatus. The hydrodynamic cavitation zone is comprised with cavitation bubbles. The cavitation bubbles are collapsed under static pressure of 30 psi to form treated bio solids sludge (3).    USE - Method of treating bio solids sludge for producing biogas and bio solid for use as fuel and fertilizer, from water work, industrial or wastewater treatment plant (claimed).    ADVANTAGE - Since the volatile solids in the bio solids sludge is reduced with the use of hydrodynamic cavitation, the biogas yield is increased. The efficient and cost-effective bio solids sludge treatment is achieved.    DETAILED DESCRIPTION - The method involves maintaining cavitation bubbles in hydrodynamic cavitation zone for less than 0.01 second. The localized flow constriction is orifice, baffle, bluff portion or nozzle. The bio solids sludge is passed through orifice to create pressure differential across orifice of 150 psi. The bio solids sludge is at pressure of 175 psi prior to passing through local constriction. The bio solids sludge is treated in absence of ultrasonic cavitation. The hydrodynamic cavitation apparatus is static cavitation apparatus. The bio solids sludge is dewatered bio solids sludge from water work or wastewater treatment plant. The dewatered sludge is subjected to thermal hydrolysis prior to passing through local constriction of controlled flow in hydrodynamic cavitation apparatus. The treated bio solids sludge is subjected to thermal hydrolysis, anaerobic digestion or combination of both. The treated bio solids sludge is subjected to anaerobic digestion to produce biogas comprising carbon dioxide and methane. 15 percent more biogas is produced by treated bio solids sludge as compared to untreated bio solids sludge subjected to the anaerobic digestion. The treated bio solids sludge is subjected to anaerobic digestion to reduce volatile solids in the treated bio solids sludge. 2 percent less volatile solids are produced by treated bio solids as compared to untreated bio solids sludge subjected to the anaerobic digestion. The static controlled flow hydrodynamic cavitation apparatus is provided with two local constrictions in series. The dewatered bio solids sludge is passed through two local constrictions. The dewatered bio solids sludge is passed through the local constriction at two times.    DESCRIPTION OF DRAWING(S) - The drawing shows a flow diagram illustrating the process of the bio solids sludge treatment system.    Bio solids sludge (1)    Controlled flow hydrodynamic cavitation apparatuses (2,16,26)    Treated bio solids sludge (3)    Stream (4)    Sludge mixture (6)