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S~S9 38 METROPOLITAN WATER DISTRICT siliceous exchange materials, the remaining 18 units having poly- styrene cation exchange resins in service. The resinous exchange material in continuous service in soft- ener unit No. 12 since April 19,J9 has softened more than 2,900,000 gallons of water per cubic foot of exchanger without evidence of deterioration or loss of efficiency, The experimental unit contain- ing similar material has been in essentially uninterrupted service since February 1947. After more than 5 years of operation and a throughput of 6,800,000 gallons of water per cubic foot, no signifi- cant degradation in physical structure or operating efficiency is apparent. The continued excellent performance of the polystyrene exchangers in plant operation and pilot plant tests fully confirms the optimistic reports on these materials which followed the earlier experimental work by the District staff in this field. Until the end of October 1951 the usual practice was continued of adding sodium silicate to the softener influent water in order to raise the silica content to such a level that the dissolution of this constituent from the siliceous zeolite, then in service, would be reduced to a minimum. Inasmuch as the organic exchangers con- tain no silica and, therefore, are in no way benefited by the addi- tion of sodium "i1icate to the suftener influent water, this practice was discontinued \""hen sufficient polystyrene resin was in service to warrant this action from an economic standpoint. Research U'ork Flocculation and coagulation studies have been continued to improve coagulation during lime treatment and to evolve a ",ti8- factory method of flocculation for use without lime treatment. Proper adjustment of flocculator paddle speed has proved benefi- cial when lime is used, resulting in a reduction of 40 percent in coagulant dosage. Althongh activated silica has proved very effective as a coagulation aid during lime treatment, it has not been satisfactory when used alone, without lime or a primary coagulant. Recent tests involving the use of activated silica in conjunction with low dosages of alum have proved quite promising hath in the lahoratory and in the plant. This procedure has reduced appreciably the need for lime treatment during those periods when conditioning of the water prior to liltration was required. Because of this, not only has it been possible to maintain a more uniform quality of linished water, hut this has been achieved at a lower unit cost for treatment. Tests involving the II