type plants.
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[Settling tank shall have a hopper bottom [when mechanical sludge
collection equipment is not provided]]. The settling tank shall have a
freeboard of not less than [_____]. The settling tank [, except in
circular plants with concentric inner tank,] shall be [in a single unit or]
in multiple units [each of which is capable of independent operation and]
whose combined capacity equals the design flow. [Hopper bottom of the
settling tank shall have slide slopes of not less than 1.05 rad 60 degrees
from the horizontal; horizontal area of hopper bottom shall be commensurate
with capability of sludge pump for efficient sludge withdrawal]. A
stilling well, baffle or other approved means of velocity control shall be
provided at the settling tank inlet. Scum baffle or other suitable means
shall be provided to prevent scum from passing out with settling tank
effluent.
2.7.4
Aerobic Digester
The aerobic digester shall be rectangular, circular or annular segment
shape. This tank shall be provided with a compartment so placed as to
produce supernatant. When supernatant return is by hydraulic displacement,
the supernatant transfer port or pipe connection to the aeration tank shall
have its invert elevation or overflow level at not less than 75 mm 3 inches
above the design liquid level in the aeration tank. An inlet pipe
connection or sludge weir shall be provided for transfer of sludge from the
settling tank to the aerobic digester. Pipe connection(s) shall be
provided in this tank for waste sludge draw-off. This tank shall have a
freeboard of not less than [_____].
2.7.5
Chlorine Contact Tank
Chlorine contact tank shall be rectangular, circular, or annular segment
and shall have all necessary rim angles and anchor plates. Tank may be an
integral part of the plant or an adjacent detached tank. A 50 mm 2 inch
drain with plug shall be provided in tank bottom for flushing and draining
sediment from the tank. Bottom of tank shall slope to the drain. Tank
shall have a freeboard of not less than 450 mm 18 inches. The tank shall
contain not less than three internal baffles which shall be positioned so
as to provide thorough mixing within the tank. A flange conforming to AWWA
C207, Class B, 200 mm 8 inch size minimum, shall be on the outlet of the
tank.
2.7.6
Structural Requirements for Steel Tanks
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NOTE: The paragraph when concrete plant only is
required. This specification is structured for
steel plant only, concrete plant only, or steel or
concrete plant at the Contractor's option. Among
the factors to be considered are initial cost,
maintenance costs, life expectancy, and possible
need for relocation during period of useful life.
Steel plant is generally more economical in first
cost than concrete plant; however, maintenance costs
may be higher. Concrete plant would have a higher
life expectancy in highly corrosive soils. Small
steel plants are suitable for relocation. Concrete
plant is considered as plant having outer walls of
SECTION 11375N
Page 31
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