An influent, non-pressurized wastewater injection and mixing device for treatment of unsettled sewage under sub-mesophilic temperatures in UASB reactors, characterized in that it comprises: - a wastewater feeding tank, located outside the reactor and above the reactor water level, having an effective wastewater volume for generating pulses by gravity ranging from 0.5% to 1.5% of the total reactor volume, said effective volume inside the tank being at a height between a minimum wastewater level H min equal or higher than 0.5 m and a maximum wastewater level H max equal or lower than 2.5 m above the water level of the reactor, H max being always higher than H min for producing the hydraulic energy of the pulse; and the feeding tank having an upper inlet as well as an outlet at the bottom for the wastewater, and being connected to the reactor through - an inlet manifold, that at one end connects downwards the outlet of the feeding tank with - one or more wastewater distribution strings at a second end, each distribution string having a valve automatically controlled by the effective wastewater level inside the feeding tank, the string or strings being buried in the sludge bed inside the reactor for distributing the wastewater pulses to the bottom of the reactor through - injection nozzles having a downwards discharge point located at a height of between 200 and 300 mm from the bottom of the reactor to avoid dead zones and energy losses, and - a water level sensor for measuring the minimum effective wastewater level H min and the maximum effective wastewater level H max inside the feeding tank, capable of sending a signal to control means that act closing or opening the valve. - a wastewater feeding tank, located outside the reactor and above the reactor water level, having an effective wastewater volume for generating pulses by gravity ranging from 0.5% to 1.5% of the total reactor volume, said effective volume inside the tank being at a height between a minimum wastewater level H min equal or higher than 0.5 m and a maximum wastewater level H max equal or lower than 2.5 m above the water level of the reactor, H max being always higher than H min for producing the hydraulic energy of the pulse; and the feeding tank having an upper inlet as well as an outlet at the bottom for the wastewater, and being connected to the reactor through - an inlet manifold, that at one end connects downwards the outlet of the feeding tank with - one or more wastewater distribution strings at a second end, each distribution string having a valve automatically controlled by the effective wastewater level inside the feeding tank, the string or strings being buried in the sludge bed inside the reactor for distributing the wastewater pulses to the bottom of the reactor through - injection nozzles having a downwards discharge point located at a height of between 200 and 300 mm from the bottom of the reactor to avoid dead zones and energy losses, and - a water level sensor for measuring the minimum effective wastewater level H min and the maximum effective wastewater level H max inside the feeding tank, capable of sending a signal to control means that act closing or opening the valve.