Design Guide for Community Water Systems - The Water, Sanitation ...

emoval processes shall be included as a part of the ozone facilitydesign.4.5. Softening.The softening process selected must be based upon the mineral quality of the rawwater and the desired finished water quality in conjunction with requirements fordisposal of residuals or brine waste, cost of plant, cost of chemicals and plant location.Applicability of the process chosen shall, be demonstrated.4.5.1 Lime or lime-soda process.Design standards for rapid mix, flocculation, and sedimentation are insubsection 4.2. Additional consideration must be given to the followingprocess elements:a. Hydraulics. When split treatment is used, the bypass line should besized to carry total plant flow, and an accurate means of measuring andsplitting the flow must be provided;b. Aeration. Determinations should be made for the carbon dioxide contentof the raw water. When concentrations exceed 10 milligrams perliter, the economics of removal by aeration as opposed to removal withlime should be considered if it has been determined that dissolvedoxygen in the finished water will not cause corrosion problems in thedistribution system. When split treatment is utilized, the split should beprior to aeration, and the re-blending prior to filtration;c. Chemical feed point. Lime should be fed directly into the rapid mixbasin;d. Rapid mix. Rapid mix basins must provide 30 seconds detention timewith adequate velocity gradients to keep the lime particles dispersed;e. Stabilization. Equipment for stabilization of water softened by the limeor lime-soda process is required;f. Residuals collection and disposal.1. Mechanical residuals removal equipment shall be provided in thesedimentation basin; and2. Provisions must be included for proper disposal of softeningresiduals in accordance with regulations in 10 CSR 20;g. Disinfection. The use of excess lime is no substitute for disinfection;andh. Plant start-up. The plant processes must be manually started followingshutdown, unless approved by the department where automaticmonitoring controls are provided.4.5.2. Cation exchange process.a. Alternative methods of hardness reduction should be investigated whenthe sodium content and dissolved solids concentration are of concern.Aug. 29, 2003Page 73