This is determined by the characteristics of reverse osmosis membrane, raw water put into the membrane, in the role of external pressure greater than the counterweight, the part of the water molecules through the membrane into pure water, the other part of the purified water into the water attached to particles, colloidal particles the larger particles such as impurities, as well as cover the hole in the membrane surface or film of sediment out of film surface, to extend the service life of membranes, resulting in the waste water, we can see that there must be water reverse osmosis membrane is generated, if the waste water emission is too small, can easily result in reverse osmosis membrane block to shorten the life of film, so that reverse osmosis membrane is impossible not to discharge wastewater.