NOVELTY - The system comprises: a reactor comprising a first absorber (118) comprising a first liquid waste stream inlet (120), a first exhaust inlet (119) and a first exhaust outlet (121), and a second absorber comprising a second liquid waste stream inlet, a second exhaust inlet and a second exhaust outlet; a biogas production unit (12) that converts waste to a biogas stream (16) and a liquid waste stream, and comprises a biogas stream outlet and a liquid waste stream outlet; and a device constructed to utilize the biogas stream to produce energy and an exhaust comprising a gas.    USE - The system is useful for treating an exhaust such as nitrogen oxide (NOx), sulfur oxide (SOx), or carbon monoxide (COx) (all claimed) from a biogas fired internal combustion engine.    ADVANTAGE - The system enables the use of the low cost and highly efficient internal combustion engine to produce both electrical and thermal energy without the necessity of resorting to inferior and expensive alternatives such as fuel cells and micro-turbines, upgrading biogas for the pipeline and gasification, and economically reduces NOx and SOx emissions from the biogas combustion using the liquid waste stream from the anaerobic digester in contact with the biogas engine exhaust.    DETAILED DESCRIPTION - The system comprises: a reactor comprising a first absorber (118) comprising a first liquid waste stream inlet (120), a first exhaust inlet (119) and a first exhaust outlet (121), and a second absorber comprising a second liquid waste stream inlet, a second exhaust inlet and a second exhaust outlet; a biogas production unit (12) that converts waste to a biogas stream (16) and a liquid waste stream, and comprises a biogas stream outlet and a liquid waste stream outlet, where the liquid waste stream outlet is operatively connected to the first liquid waste stream inlet of the first absorber and operatively connected to the second liquid waste stream inlet of the second absorber; and a device constructed to utilize the biogas stream to produce energy and an exhaust comprising a gas. The device comprises: a biogas stream inlet operatively connected to the biogas stream outlet of the biogas production unit; and a device exhaust outlet, provided that the device exhaust outlet is operatively connected to the first exhaust inlet of the first absorber, and the first exhaust outlet of the first absorber is operatively connected to the second exhaust inlet of the second absorber, or provided that the device exhaust outlet is operatively connected to the first exhaust inlet of the first absorber and the second exhaust inlet of the second absorber. The reactor is constructed to treat exhaust so as to decrease or reduce emissions or pollutants by contacting the exhaust with the liquid waste stream from the biogas production unit. The device comprises one or more of a biogas engine, an internal combustion engine, a gas fired boiler, a turbine, or a microturbine. The device further comprises a natural gas stream inlet. The device is constructed so that the natural gas stream and the biogas stream are utilized to produce energy. The system further comprises: a quencher operatively connected to the exhaust outlet of the device; a cooler operatively connected to the quencher; a fan operatively connected to one or more of the quencher, the cooler, the first absorber, and the second absorber; a mist eliminator operatively connected to the cooler so as to access the exhaust; and an oxidant injection site that is operatively connected to the system so as to access to the exhaust, and comprises one or more inlets. The reactor further comprises a mist eliminator operatively connected to the first or second absorber so as to access the exhaust. The oxidant is provided to the system so as to access the exhaust. INDEPENDENT CLAIMS are included for:    (1) a reactor; and    (2) a method for treating exhaust.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a system for treating an exhaust.    Biogas production unit (12)    Biogas stream (16)    First absorber (118)    First exhaust inlet (119)    First liquid waste stream inlet (120)    First exhaust outlet. (121)