Reverse osmosis uses pressure and a semipermeable membrane to remove inorganics, organics, bacteria and particulates efficiently and cost-effectively from feed water. Pressure in the module overcomes the natural osmosis pressure, allowing water molecules to be pushed through the pores in the membrane. Most of the impurities (95-98%) are retained and then drained with the reject water. The rejection rate is adjustable with some systems optimizing the ratio of rejection water and product water automatically based on feed water quality. The standard ratio is 40 % product water and 60 % rejection water or vice versa, depending on water quality. If the reject water percentage is higher, the conductivity is lower, with a lower water flow rate. Reverse osmosis is used for producing RO water to be used as such or as a pretreatment step for producing ultrapure water. RO modules are also common as an integrated step in pure water systems.