NOVELTY - Operating an immersed membrane water treatment system comprising calculating membrane resistance value, comparing the calculated value to a predetermined set point, and adjusting system operating parameter or process to alter the membrane resistance to provide a more desired resistance, is new.    USE - For operating an immersed membrane water treatment system serving as part of a membrane bioreactor for treating industrial, municipal, or other wastewater.    ADVANTAGE - The inventive process considers resistance values and adjusts operational parameters such as membrane aeration frequency factor or backwash flow and duration to reduce the operational costs related to membrane fouling removal.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) treatment of water by:    (a) providing a water treatment system having filtering membranes for withdrawing filtered permeate from water being treated and operable according to at least two discrete modes of operation differing in their effect on the fouling of the membranes;    (b) operating the system in one of the modes of operation in each time;    (c) determining value(s) of parameter(s) related to resistance of the membranes from time to time while operating the system; and    (d) considering the values of parameters in choosing the mode of operation for specified time;    (2) provision of scouring bubbles to a membrane unit by:    (a) providing a flow of gas in the form of bubbles which rise past the membrane unit;    (b) alternating the rate of gas flow between a value in a first range of flow rates and a value in a second range of flow rates ranging from no flow to a flow rate below the first range of flow rates; and    (c) altering the length of time that the gas flows at the second flow rate compared to the length of time that the gas flows at the first flow rate considering a parameter related to a resistance of the membrane unit; and    (3) a water treatment system comprising:    (a) a membrane unit immersed in a tank;    (b) a sub-system for performing a function related to the operation of membrane unit;    (c) data collector(s) for collecting data related to resistance of the membranes; and    (d) a controller linked to the sub-system and data collectors to automatically switch the mode of operation of the sub-system in consideration of the data.    DESCRIPTION OF DRAWING(S) - The figure is a schematic representation of an immersed membrane treatment system with a control system.