At the St. Louis Water Company, a study demonstrated that water receiving proper pretreatment can be successfully filtered at rates of 5‐6 gpm/sq ft, with either a rapid sand filter or a dual‐media filter. Pretreatment requirements will vary with the quality of the raw water and should be determined on this basis. The pretreatment facilities utilized for this study had detention times based on a 3 gpm/sq ft filtration rate. The water treated had widely varying characteristics. Statistics show the range of raw‐water characteristics, turbidity of water applied to dual‐media filters, and turbidity of the filter effluent during 1967. The unit dosage of chemicals to achieve these results is also presented.

This article discusses the use of a submerged filter at the St. Louis County Water Company's South County Plant. Treatment basin down time required for submerged filter backwashing coincides with the off‐peak operation hours at night. Because the filter beds are not readily accessible during normal backwashing, sufficient prefiltration water treatment should be provided. The amount of floc retained, as measured by initial filter head loss, is the same following each wash, and probably is beneficial in that it prevents floc penetration of the sandbed. Filter beds are skimmed semi‐annually as a preventive maintenance procedure. Backwashing the submerged filter does not cause floc destruction. Filtering to waste after backwashing and before returning the filter to service has not been necessary. Surface washing of the filter would probably cause destruction of the floc washed out of the filter and would thus interfere with this floc settling in the purification basin. Wash water requirements for filter backwashing are in excess of the requirements for conventional filter washing; however, all of the wash water is reclaimed in the purification basin. The submerged filter, serving as the treatment basin takeoff system, has improved treatment basin velocities and sedimentation, and has resulted in a very substantial savings in plant construction costs.

This article discusses an example of reducing magnesium by lime‐softening in the Meramec River water source in the presence of accumulated softening slurry, to an amount that will not precipitate in 140°F domestic hot water heaters and will not require recarbonation. The lime dose for this purpose should not exceed the raw‐water alkalinity equivalent plus 34.24 ppm. The slurry concentration for contacting should be about 5 per cent by weight and should be from softening products only, being virtually free of activated silica, coagulant, and turbidity. If large amounts of magnesium solids are accumulated in the slurry, and are not removed with the effluent water, the slurry becomes less effective as a softening aid. Mixing and contacting time for the softening reactions should be about 30 min.

Fresh-water sponges can clog raw-water conduits, and to remove them can be expensive. But modest amounts of chlorine can arrest their development.