An industrial reverse osmosis plant consists of three separate sections. The first section is the pretreatment section in which the feed water is treated to meet the requirements of reverse osmosis element manufacturers and the dictates of good engineering practice. Following pretreatment, the feed water is introduces into the reverse osmosis section where the feed water is pressurized and routed to the reverse osmosis elements, which are in pressure vessels. The feed water flows across the membrane surface where product water permeates through the membrane and a predetermined amount remains behind as reject. The reject is discharged to waste while the product water is routed to the post-treatment section. The third or post-treatment section treats the product water to remove carbon dioxide and adds chemicals as required for industrial use of the product water.