The method of detection and intensivity estimation of anaerobic processes accompanied by formation of gases (CO2, CH4, H2, N2 etc.) is proposed. This method is realized in such a way: the PET-bottle is filled by 4/5 of its volume with the nutrients solution or wastewater, anaerobic bacteria, activated sludge or other anaerobic biocenoses. Thereafter by simply squeezing of a PET-bottle the air is removed from it and the bottle is tightly corked. The bottle is then placed into a thermostat and the levels of gas generated and accumulated in this anaerobic bioreactor are periodically measured. whenever necessary gas composition and concentration change of substances in the water (medium) are analyzed. Thanks to the method for the first time in Ukraine there has been identified the presence of ANAMMOx (Anaerobic Ammonium Oxidation) bacteria in activated sludge of biological treatment facilities. By this method the possibility of anaerobic treatment of beer-brewing wastewater with increased concentration of chloride-ion (up 2500 mg/l) was proved.