NOVELTY - System comprises a reactor (2) that is operatively connected to a biogas production unit (3) for converting waste to a biogas stream; and an engine that utilizes the biogas stream from the biogas producing unit to produce energy and an engine exhaust, where the reactor decreases nitrogen oxides (NOx) emission from the engine exhaust by contacting the engine exhaust with a liquid waste stream (4) provided from the biogas production unit.    USE - The system is useful for converting waste to a biogas stream by decreasing NOx emissions, Sox emissions and Cox (all claimed).    ADVANTAGE - The reactor: utilizes the spent liquid waste from the on-site anaerobi digester to cheaply scrub NOx from an engine or cogeneration unit's exhaust stream; and enhances the liquid/vapor NOx extraction and subsequent chemical conversion processes by the unique liquid/vapor continuous-flow design, which ensures a large excess of chemical reductant and limited contact time to enhance NOx removal while minimizing unwanted reverse reaction due to by-product saturation.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) a reactor that is operatively connected to a biogas production unit for converting waste to a biogas stream; and an engine that utilizes the biogas stream from the biogas producing unit to produce energy and an engine exhaust, where the engine exhaust is coperatively connected to the reactor and exhaust from the reactor contains an amount less than or equal to 1 pound of NOx per MWh of energy produced by the engine;    (2) a method comprising the step of decreasing NOx emissions in a reactor that is operatively connected to a biogas production unit for converting waste to a biogas stream; and an engine that utilizes the biogas stream from the biogas producing unit to produce energy and an engine exhaust, where the step comprises decreasing NOx emissions from the engine exhaust by contacting the engine exhaust with a liquid waste stream provided from the biogas production unit.    DESCRIPTION OF DRAWING(S) - The figure shows schematic illustration of a reactor demonstrating nitrogen oxides removal for biogas engines placed at an anaerobic digestion facility placed at a concentrated animal feeding operation comprising dairy farm or other waste processing facility e.g. waste water treatment plant.    Anaerobic digester (1)    Reactor tank (2)    Biogas engine (3)    Liquid waste stream (4)    Internal combustion engine exhaust (5)