A wastewater treating system comprising: one or more module/s for sequential nitrification/denitrification process, each SBR module comprising a first tank-A and a second tank-B, and - one or more membrane bioreactor module/s, wherein - the SBR module/s and the MBR module/s are fluidly connected, and wherein the wastewater is fed from the nitrification process of the SBR module/s to the MBR module/s; wherein - the SBR is arranged in hydraulic connection with a wastewater inlet pipe through an inlet housing, wherein the first tank-A is arranged in hydraulic connection with the inlet housing through an opening and the second tank-B is arranged in hydraulic connection with the inlet housing though an opening, wherein a controlling mechanism is arranged to control the flow from the inlet pipe through either the opening or opening into the SBR, and wherein both the first tank-A and the second tank-B are arranged in hydraulic connection with a MBR channel through respective weirs (11, 10), which can be closed by a respective closing mechanism (11a, 10a); and where - the MBR includes at least one chamber (12a, 12b, 12c), with an opening (13a, 13b and 13c), and the MBR is in hydraulic connection to the MBR channel through the openings (13a, 13b, 13c), and the MBR has an outlet in hydraulic connection with the at least one chamber (12a, 12b, 12c), and with an outlet pipe 15, and wherein the MBR channel is arranged between the SBR comprising the first tank-A and the second tank-B and the at least one chamber (12a, 12b, 12c) of the MBR, and wherein the system is further provided with a recirculation pipe, which connects both the first tank-A and the second tank-B to the at least one chamber (12a, 12b, 12c) of the MBR, and wherein - in the SBR there is arranged an opening between the first tank-A and the second tank-B through which a flow equivalent to the feed flow is arranged to flow from a tank receiving the feed flow to a tank provided with a closed opening (7; 8) connected with the inlet housing during at least some phases of a sequential nitrification/denitrification process of a wastewater treatment process. - one or more membrane bioreactor module/s, wherein - the SBR module/s and the MBR module/s are fluidly connected, and wherein the wastewater is fed from the nitrification process of the SBR module/s to the MBR module/s; wherein - the SBR is arranged in hydraulic connection with a wastewater inlet pipe through an inlet housing, wherein the first tank-A is arranged in hydraulic connection with the inlet housing through an opening and the second tank-B is arranged in hydraulic connection with the inlet housing though an opening, wherein a controlling mechanism is arranged to control the flow from the inlet pipe through either the opening or opening into the SBR, and wherein both the first tank-A and the second tank-B are arranged in hydraulic connection with a MBR channel through respective weirs (11, 10), which can be closed by a respective closing mechanism (11a, 10a); and where - the MBR includes at least one chamber (12a, 12b, 12c), with an opening (13a, 13b and 13c), and the MBR is in hydraulic connection to the MBR channel through the openings (13a, 13b, 13c), and the MBR has an outlet in hydraulic connection with the at least one chamber (12a, 12b, 12c), and with an outlet pipe 15, and wherein the MBR channel is arranged between the SBR comprising the first tank-A and the second tank-B and the at least one chamber (12a, 12b, 12c) of the MBR, and wherein the system is further provided with a recirculation pipe, which connects both the first tank-A and the second tank-B to the at least one chamber (12a, 12b, 12c) of the MBR, and wherein - in the SBR there is arranged an opening between the first tank-A and the second tank-B through which a flow equivalent to the feed flow is arranged to flow from a tank receiving the feed flow to a tank provided with a closed opening (7; 8) connected with the inlet housing during at least some phases of a sequential nitrification/denitrification process of a wastewater treatment process.