Abstract: A system for treating water has a plurality of concatenated reverse osmosis (RO) units, each having a membrane. Water to be treated is fed to one side of the membrane at a pressure to overcome osmotic pressure across the membrane and to force permeate to the other side of the membrane. The water on the one side of the membrane retains dissolved solids to become concentrate. Concentrate from a downstream RO unit is directed to the other side of the membrane of the upstream RO unit to mix the directed concentrate from the downstream RO unit with the permeate of the upstream RO unit. The mixed concentrate and permeate from the upstream unit is then treated at the downstream RO unit.

Abstract: A process and apparatus is described for filtering water with immersed membranes. In a batch process, permeate is withdrawn while the flow of feed is reduced or stopped at the end of a permeation cycle. The water level is reduced to a level where a portion of the membranes are exposed to air before draining the tank. In this or another process, the level of liquid is reduced to correspond with an area of the membrane fibers having an accumulation of solids. Aeration is provided for a period of time with the liquid at this level to dislodge at least a portion of the solids from the membranes. In these or other processes, the tank is partially drained between cycles to deconcentrate the tank, aeration is provided during backwashing and intermittently while permeating, and/or retentate is withdrawn from the tank during a portion of a permeation step.

Abstract: A membrane batch filtration process has a step of reducing the water level in the tank by permeation prior to emptying the tank to reduce the volume of water drained after each batch. Permeation may continue even after a portion of the membranes is exposed to air to further lower the water level. The membranes may be backwashed after the water level has been lowered. The water level may be lowered again after the backwash. The tank drain may begin with a portion of the membranes exposed to air.