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CLARIFICATION 7.5

Basin Dimensions. Rectangular basins are generally designed to be long and narrow,
with width-to-length ratios of 3:1 to 5:1. This shape is least susceptible to short-
circuiting--the hydraulic condition in a basin when the actual flow time of water through
the basin is less than the computed time. Short-circuiting is primarily caused by uneven
flow distribution and density or wind currents that create zones of near-stagnant water in
corners and other areas.
Basin widths are most often selected to match the requirements of the chosen me-
chanical sludge collection equipment. Chain-and-flight collectors, for example, are lim-
ited to about a 20-ft (6-m) width for a single pass, but it is possible to cover a wider basin
in multiple passes. Traveling bridge collectors can be up to 100 ft (30 m) wide, limited
only by the economics of bridge design and alignment.
Basin depths may be selected to provide a required detention time (although detention
time is not a good design parameter) or may be selected to limit flow-through velocities
and the potential for resuspension of settled floc. Basins with mechanical sludge removal
are usually between 10 and 14 ft (3.0 and 4.3 m) deep.
Because settling is primarily based on area, multiple-tray basins have been developed
as shown in Figure 7.2. A depth of about 7 ft (2 m) is typically provided between trays
to allow access for cleaning and maintaining equipment.
Inlet Zone. A basin's effectiveness at any overflow rate can be greatly changed by short-
circuiting. Short-circuiting reduces the actual area traversed by the flow, increasing the
apparent overflow rate and reducing solids removal efficiency.
Many publications provide testimony to the importance of proper design of the basin
inlet (Yee and Babb, 1985; Monk and Willis, 1987; Hudson, 1981; Kawamura, 1991).
Good design of the inlet zone establishes uniform distribution into the basin and mini-
mizes short-circuiting potential.
For long, narrow basins being fed directly from a flocculation basin, slots or a few in-
dividual inlets may suffice. To obtain uniform flow distribution through wider basins, per-
forated baffle walls should be provided; a typical arrangement is shown in Figure 7.3. For
best results, flow from the flocculation basin should be in line with the basin axis.
Following hydraulic principles to ensure equal flow distribution, head loss through the
perforations should be 4 to 5 times the velocity head of the approaching flow. The ve-
locity gradient G should be equal to or less than that in the last flocculation compartment
to minimize floc breakup (Hudson, 1981). The number of ports should be the maximum
practical that will provide the required head loss. Port velocities typically must be about
0.7 to 1.0 ft/s (21 to 30 cm/s) for sufficient head loss. Ports should be arranged to cover
as much of the basin's cross section as possible without creating high velocities in the
sludge collection zone that might cause scouring action. Thus the lowest port should be
about 2 ft (0.6 m) above the basin floor. Port spacing is typically 10 to 24 in. (25 to 61
cm) with a port diameter of 4 to 8 in. (10 to 20 cm).
Introducing flow across the entire inlet end of the basin reduces short-circuiting caused
by density currents---created when water entering the top of the basin is colder and heav-
ier than the water below. The cold influent settles quickly to the bottom, causing flow to
move along the bottom of the tank and back across the top. When influent is introduced
uniformly across the tank from top to bottom and side-to-side, water temperature remains
more uniform, and density currents are less likely to form.
Outlet Design. Outlet design is also critical in reducing short-circuiting and scouring of
settled solids. Outlet designs have undergone a number of transformations. Basins were
originally designed with end weirs. This type of outlet causes an increase in horizontal
and vertical velocity as flow is forced up the end wall to the weir, and the increased ve-