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

settling basins shall be not less than 0.5 feet per second nor greater than 1.5feet per second. Allowances must be made to minimize turbulence atbends and changes in direction.g. Other designs. Baffling may be used to provide for flocculation in smallplants only after consultation with the department. The design should besuch that the velocities and flows noted above will be maintained.h. Superstructure. A superstructure over the flocculation basins may berequired.4.1.4. Sedimentation.Sedimentation, a process for removal of solids before filtration by gravityseparation, shall follow flocculation. The detention time for effectiveclarification is dependent upon a number of factors related to basin designand the nature of the raw water. The following criteria apply toconventional sedimentation units:a. Conventional rectangular settling units shall have a minimum length towidth ratio of three to one (3:1) or shall have baffles that will provide aflow path that gives the same ratio;b. Detention Time. Settling units shall provide a minimum of four hours ofsettling time. This may be reduced to two hours for lime-soda softeningfacilities treating only groundwater. The volume that will be used indetermining the detention time shall be calculated using the effectivedimensions of the basin. The effective length is measured from the insideedge of the last influent weir or launder to the inside edge of the firsteffluent weir or launder. The effective side water depth is measured fromthe effluent level of the launders or submerged orifices to the bottom of thebasin. The volume above the submerged orifices should not be includedwhen calculating for the detention time. Where a mechanical residualsscraper is not provided and residual storage volume is allocated at thebottom of the basin, the effective side water depth shall be measured abovethe allocated residual storage. Reduced sedimentation time may beapproved when equivalent effective settling is demonstrated;c. Inlet Devices. Inlets shall be designed to distribute the water equally and atuniform velocities. Open ports, submerged ports, and similar entrancearrangements are required. A baffle should be constructed across the basinclose to the inlet and should project several feet below the water surface todissipate inlet velocities and provide uniform flows across the basin;d. Outlet Devices. Outlet weirs or submerged orifices shall be designed tomaintain velocities suitable for settling in the basin and to minimizeshort-circuiting. The use of submerged orifices is recommended in order toprovide a volume above the orifices for storage when there are fluctuationsin flow. Outlet weirs and submerged orifices shall be designed as follows:1. The rate of flow over the weirs shall not exceed 20,000 gallons per dayper foot of the outlet launder;Aug. 29, 2003Page 49