separation in the air flotation tank 24 the ozone treatment in the ozone contact tower 30 in addition to removing hydrocarbons by oxidation also removes by oxidation any sulphur compounds present in the aqueous phase entering the tower 30 by line 29 the aqueous phase which results from the ozone contact tower 30 is substantially free from hydrocarbons but still contains all the minerals of the brackish water this hydrocarbon free aqueous phase is forwarded by line 20 to the heat exchange operation in heat exchanger 18 after passage through the heat exchanger 18 the heated hydrocarbon free stream in line 38 is mixed with fresh make up water in line 40 to provide an aqueous stream in line 42 which is passed to an electrodialysis reversal unit 44 of any convenient construction where the make up water is substantially mineral free it may be added after the demineralization steps if desired the electrodialysis reversal unit 44 removes in known manner all the mineral species as a concentrated waste stream 46 with the exception of silica which remains in the purified water in line 48 silica removal is effected in a silica removal unit 50 which may take the form of an ion exchange unit an alumina adsorption column or other suitable silica removal means so that a substantially mineral free and hydrocarbon free aqueous stream results in line 52 the silica waste removed by the silica removal unit 50 is passed by line 54 to mix with the concentrated waste stream 46 removed from the electrodialysis removal unit 44 to form a combined concentrated mineral waste stream 58 for disposal the concentrated waste stream 58 is passed through a heat exchanger 60 to recover the heat value thereof and to provide heat to the.