The technology of municipal solid waste biological treatment in Sequence-Anaerobic-Bioreactors consists an enhanced, and controlled fermentation occurring in waste heap having status of bioreactor. Comparing to traditional landfills, where also as an effect of biological processes biogas is generated, the organic waste stabilization is accomplished after 5 years. According to construction, and operation regime, the reactor is a sequence bioreactor having self-heating character. Waste, substrate, are delivered into bioreactor once time, bioreactor is closed, gas extraction and leachate recirculation system, and monitoring devices are installed. Finally mineral sealing is placed. From that moment optimization of conditions of fermentation process by gathering, treatment, and recirculation of leachate starts. Sequence-Anaerobic-Bioreactor has been implemented by USKOM Ltd. Enterprise in location of Kosiny Bartosowe near Mlawa. Bioreacto has total capacity of municipal solid waste 200000 Mg. The research on the influence of recirculation of leachate on Sequence-Anaerobic-Bioreactor, with hydraulic loading rate ranging from 0 to 4 mm/d, on fermentation conditions and biogas composition were done. The measurements of temperature inside the bioreactor, and gas composition in gas wells were executed. The implementation of leachate recirculation with hydraulic loading rate from 1 to 4 mm/d improved the fermentation conditions and quality of generated biogas. The hydraulic loading rate of leachate 1 mm/d has been proposed for application.