Each reverse osmosis unit has a device known as a flow restrictor that controls the quantity of water that runs to drain. In overall, the flow restrictor should be sized in rapport with membrane production size. The flow to drain is typically about (3) three times higher than membrane production, while membrane becomes larger the drain flow ratio decreases. For some membranes, the ratio could be decreased from 3 to 2.5 or high production to 1. Production to Drain Ratio (PDR).
To calculate this drain flow is difficult because the membrane production is usually presented in gallons per day (GPD), while flow restrictors are normally sized in milliliters per minute (ml/m).