insofar as possible. Therefore. elongated designs should be avoided, and the inlet and outlet configurationally should be arranged to minimize short circuiting Discharging the influent near the mixing equipment usually minimizes short circuiting. If the geometry of the basins is controlled by the available land area and an elongated geometry must be used, it may be necessary to use multiple inlets and outlets. Provisions should be included in the basin design for access by cleaning equipment such as front-end loaders. Multiple compartments are also desirable to reduce cleaning costs and for odor control Basin Construction. New basins may be of earthen concrete or steel construction earthen basins are generally the least expensive Depending on local conditions, the interior side slopes may vary between 3: 1 and 2: 1. A section through a typical earthen basin is shown on Fig. 5-10. In most installations, a liner ground-water level, and topography if a liner is used in areas of high groundwater, the effects of hydraulic required depends on the Lining and local wind conditions If a floating aerator is used prevent septicity and odor formation. a minimum operating level is needed aerator Fig. 5-10 Typical open tpe flow Typically, the minimum water depth can vary fron With float through a lined earthen basin; (b shallow erators, a concrete pad in;(e/deep concrete basin should be provided below e aerators to minimize erosion wind-induced erosion in the upper portions of the to protect the slopes with riprap. soil cement or a partial concrete laver. Fencing should also be provided to prevent public access to the basins. In areas of high groundwater, drainage facilities should be provided to prevent embankment failure. To further ensure a stable embankment, the tops of the dikes should be of adequate width. The use of an dequate dike width will facilitate the use of mechanical equipment for maintenance and will also reduce construction costs, especially where mechanical compaction equipment is used Mixing and Air Requirements. The proper operation of both in-line and off-line equalization basins generally requires proper mixing and aeration. Mixing equipment should be sized to blend the contents of the tank and to prevent deposition of solids in the basin. To minimize mixing requirements. grit-removal facilities should precede equalization basins where possible. Mixing requirements for blending a medium-strength municipal wastewater having a suspended solids concentration of approximately 210 mg/L. range from 0.004 to 0.008 kW/mof storage Aeration is required to prevent the wastewater from becoming septic and odorous. To maintain aerobic conditions, air should be supplied at a rate of 0.01 to 015 m/m. min In equalization basins that follow primary sedimentation and have short detention times (less than 2 h). aeration may not be required. Where mechanical aerators are used, baffling may be necessary to ensure proper mixing, particularly with a circular tank configuration To protect the aerators in the event of excessive level drawdown, low-level shutoff controls should be provided. Because it may be necessary to dewater the equalization basins periodically, the aerators should be equipped with legs or draft tubes that allow them to come to rest on the bottom of the basin without damage. Various types of diffused air systems may also be used for mixing Operational Appurtenances. Among the appurtenances that should be included in the design of equalization basins are(D) facilities for flushing any solids and grease that may tend o accumulate on the rial and foar on of foam on the sides of the basin and to ai odor control facilities where covered equalization basins must be used. Solids removed from equalization 5-115-11 insofar as possible. Therefore, elongated designs should be avoided, and the inlet and outlet configurationally should be arranged to minimize short circuiting. Discharging the influent near the mixing equipment usually minimizes short circuiting. If the geometry of the basins is controlled by the available land area and an elongated geometry must be used, it may be necessary to use multiple inlets and outlets. Provisions should be included in the basin design for access by cleaning equipment such as front-end loaders. Multiple compartments are also desirable to reduce cleaning costs and for odor control. Basin Construction. New basins may be of earthen, concrete, or steel construction; earthen basins are generally the least expensive. Depending on local conditions, the interior side slopes may vary between 3:1 and 2:1. A section through a typical earthen basin is shown on Fig. 5-10. In most installations, a liner is required to prevent ground-water contamination. Basin depths will vary depending on land availability, ground-water level, and topography, if a liner is used in areas of high groundwater, the effects of hydraulic uplift on the liner must be considered. The freeboard required depends on the surface area of the basin and local wind conditions. If a floating aerator is used to provide mixing and prevent septicity and odor formation, a minimum operating level is needed to protect the aerator. Typically, the minimum water depth can vary from 1.5 to 2 m. With floating aerators, a concrete pad should be provided below the aerators to minimize erosion. To prevent wind-induced erosion in the upper portions of the basin, it may be necessary to protect the slopes with riprap, soil cement, or a partial concrete layer. Fencing should also be provided to prevent public access to the basins. In areas of high groundwater, drainage facilities should be provided to prevent embankment failure. To further ensure a stable embankment, the tops of the dikes should be of adequate width. The use of an adequate dike width will facilitate the use of mechanical equipment for maintenance and will also reduce construction costs, especially where mechanical compaction equipment is used. Mixing and Air Requirements. The proper operation of both in-line and off-line equalization basins generally requires proper mixing and aeration. Mixing equipment should be sized to blend the contents of the tank and to prevent deposition of solids in the basin. To minimize mixing requirements, grit-removal facilities should precede equalization basins where possible. Mixing requirements for blending a medium-strength municipal wastewater, having a suspended solids concentration of approximately 210 mg/L, range from 0.004 to 0.008 kW/m3 of storage. Aeration is required to prevent the wastewater from becoming septic and odorous. To maintain aerobic conditions, air should be supplied at a rate of 0.01 to 0.015 m3 /m3 .min. In equalization basins that follow primary sedimentation and have short detention times (less than 2 h), aeration may not be required. Where mechanical aerators are used, baffling may be necessary to ensure proper mixing, particularly with a circular tank configuration. To protect the aerators in the event of excessive level drawdown, low-level shutoff controls should be provided. Because it may be necessary to dewater the equalization basins periodically, the aerators should be equipped with legs or draft tubes that allow them to come to rest on the bottom of the basin without damage. Various types of diffused air systems may also be used for mixing and aeration including static tube, jet, and aspirating aerators. Operational Appurtenances. Among the appurtenances that should be included in the design of equalization basins are (1) facilities for flushing any solids and grease that may tend o accumulate on the basin walls; (2) a high water takeoff for the removal of floating material and foam; (3) water sprays to prevent the accumulation of foam on the sides of the basin and to aid in scum removal; and (4) separate odor control facilities where covered equalization basins must be used. Solids removed from equalization Fig. 5-10 Typical open type flow equalization basins: (a)typical section through a lined earthen basin;(b)shallow concrete basin; (c)deep concrete basin Minimum allowable operating level to protect aerator freeboard