The permeate is recycled to the make-up of the cooling water system. This reduces the amount of fresh water make-up required to maintain the TDS in the water that enters the cooling tower below 1500 mg/l. Draw a flow diagram of the system, draw up a water- and TDS mass balance for the cooling system incorporating the permeate recycle and calculate the volume of make-up required to maintain the TDS below 1500 mg/l in the system assuming that conditions of evaporation and windage losses remain unchanged.  The TDS of the make-up water is 250 mg/l.