Page 403 - Water and wastewater engineering

10-20 WATER AND WASTEWATER ENGINEERING

be uniform across the cross-sectional area of the tank as it enters the settling zone. The inlet
pipe carrying solids to the clarifier often is designed to have velocities high enough to keep the
particles from settling in the pipe. This high velocity must be reduced sufficiently to prevent jet
effects in the basin. The design solution is to provide a diffuser wall and, perhaps, an inlet baffle
(also known as a target baffle ).

Density Currents. Short-circuiting is the term used to describe the effect of density currents
on settling tank performance. Short-circuiting occurs when the flow through the tank is not uni-
form and a current carries the particulate matter to the effluent launders before the particles can
settle. Temperature differentials and changes in solids concentration are major causes of density
currents.
The addition of warm water to a sedimentation basin, or the warming of the surface water in
a basin containing cooler water, leads to short circuiting because the warmer water rises to the
surface and reaches the launders in a fraction of the theoretical detention time. Conversely, the
cooler water tends to dive down, flow along the bottom, and rise at the tank outlet. Temperature
density currents are commonly caused by exposure to sunlight, changing the mixing ratio of two
or more water sources, switching from one source to another, and shifting the reservoir intake
elevation.
A rapid increase in the influent solids concentration from floods or high winds on lakes and
reservoirs will cause a higher density in the influent than in the basin. This will cause it to plunge
as it enters the basin, flow along the bottom, and rise at the tank outlet. Intermediate diffuser
walls have been used to counteract density current effects.

Wind Effects. Large, open tanks are susceptible to induced currents and, in sufficiently strong
winds, waves along the top of the tank. An underflow current in the opposite direction to the sur-
face current is also created. In addition to short circuiting, this may lead to scouring of the already
settled particulate matter from the sludge zone. The design solutions include limiting the length
of the tank and placing wave breakers along the tank surface.

10-3 SEDIMENTATION PRACTICE

Alternatives
Typical sedimentation tanks used in water treatment are listed in Table 10-1 . Of those listed, the
recommended order of preference for settling coagulation/flocculation floc is (1) a rectangular
tank containing high-rate settler modules, (2) a long rectangular tank, and (3) a high-speed mic-
rosand clarifier (also known as ballasted sand sedimentation ). For the lime-soda softening pro-
cess, the upflow solids contact unit (also known as a reactor clarifier or sludge blanket clarifier)
is preferred.
The upflow and upflow, solids-contact clarifiers are proprietary units that have their basic size
and blueprints preestablished by the equipment manufacturers. They are not preferred for remov-
ing alum floc for the following reasons: (1) temperature fluctuations as small as 0.5 C can cause
severe density flow short circuiting, and (2) there is a rapid loss of efficiency if there is hydraulic or
solids overloading. There are circumstances when they may be appropriate. These are discussed in
detail by Kawamura (2000). Horizontal flow with center feed, peripheral feed, and simple upflow
clarifiers are not recommended because of their hydraulic instability (Kawamura, 2000).