(a) Secondary
settling. Secondary settling is required after suspended growth and attached
growth biological processes such as activated sludge, trickling filters and
RBC's.
(b) Configuration. The
largest dimension (either diameter or length) of a clarifier shall be eighty
(80) feet (24.4 m). Corner sweeps on circular equipment are not
acceptable.
(c) Flow distribution.
Positive flow splitting shall be provided ahead of multiple sedimentation
basins to ensure proportional hydraulic flows and solid loadings to each basin.
Flow splitting shall be achieved using positive means such as weirs or valves
and meters.
(d) Clarifier inlet and
outlet structures.
(i) Clarifier inlet
structures shall be designed to dissipate the:
(A) Inlet kinetic energy.
(B) Distribute the flow evenly into the
basin.
(C) Minimize hydraulic
turbulence.
(D) Prevent short
circuiting.
Inlet devices that promote flocculation are
encouraged.
The inlet structure for rectangular tanks shall be the full
width of the basin, for peripheral feed clarifiers it shall be the entire
periphery, and for center feed basins it shall be at least 20 percent of the
tank diameter. Baffled scum relief ports shall be provided between the inlet
structure and the clarifier.
(ii) Inlet conveyance pipe or channels shall
be designed to maintain a minimum velocity of 0.5 fps (0.15 mps) at the design
flow. Where channels provide less velocity, provide mixing, flushing, or other
means of resuspending solids.
(iii)
Clarifier outlet systems shall be designed to minimize vertical velocities and
reduce the effect of density currents at the effluent weir. Weir level shall be
adjustable.
(e)
Freeboard. The outer walls of settling tanks shall extend at least six (6)
inches (0.15 m) above the surrounding ground and provide at least twelve (12)
inches (0.3 m) of free board to the water surface. Where settling basin walls
are less than four (4) feet (1.22 m) above the surrounding ground, a fence or
other debris barrier shall be provided on the wall.
(f) Design parameters.
(i) Surface overflow rates.
(A) Activated sludge. Settling basins
following an activated sludge process shall be designed to both thicken the
sludge and clarify the liquid flow entering the tanks. The overflow rate shall
not exceed:
|
Design
Flow
|
Peak Hourly
Flow
|
Activated Sludge |
gpd/ft2
600
|
m3/m2/d
24.4
|
gpd/ft2
1,200
|
m3/m2/d
48.8
|
Separate Nitrification |
400 |
16.3
|
800 |
32.5 |
(B)
Attached growth biological reactors. Overflow rates for settling basins
following attached growth processes shall not exceed:
|
Design
Flow
|
Peak Hourly
Flow
|
|
gpd/ft2 |
m3/m2/d |
gpd/ft2 |
m3/m2/d |
Trickling Filters and RBC's |
800 |
32.5 |
1,200 |
48.8 |
(ii) Solids loadings. Solids loadings for
settling basins following an activated sludge process shall not exceed:
|
Design
Flow
|
Peak Hourly
Flow
|
|
lbs/d/ft2 |
kg/d/m2 |
lbs/d/ft2 |
kg/d/m2 |
All Activated Sludge Processes |
28
|
136.7 |
50 |
244.1 |
Separate Nitrification |
25 |
122.1
|
40 |
195.3 |
(iii) Side water depth. Settling basins shall
be deep enough to provide adequate distance between the sludge blanket and the
effluent weirs to avoid disturbance of settled sludge.
The volume of the settling basin shall provide a minimum
detention time of two (2) hours at peak hourly flow rate. The peak hourly flow
is the projected maximum flow over a one hour period during the design year.
Peak hourly flow shall include all recycle flows entering clarifier.
(iv) Weir overflow rates and
placement. Weir loading rates shall not exceed the following values:
|
Design
Flow
|
Peak Hourly
Flow
|
|
gpd/ft2 |
m3/m2/d |
gpd/ft2 |
m3/m2/d |
Launder and weir at outer wall Launder and weir at
|
12,000 |
489 |
20,000 |
815 |
3/4 point of radius or less |
18,000
|
733 |
36,000 |
1467 |
Where double weirs or serpentine type weirs are used, the
weir length shall be computed as the length of the centerline of the
launder.
(g)
Baffles. Baffles shall be located at the water surface and in such a position
as to intercept all floating materials (scum) prior to the weirs. Baffles shall
extend three (3) inches (7.6 cm) above the weir level and twelve (12) inches
(0.3 m) below the water surface. In circular basins, the baffle shall be a
minimum of six (6) inches (0.15 m) inside the weir plate. In rectangular
basins, the baffle shall extend across the width of the basin and upstream of
the effluent weirs.
(h) Basin and
equipment access. Walkways and access ways shall be provided to drive units,
effluent launders, and manual scum devices.
(i) Sludge removal. Sludge collection and
withdrawal equipment shall provide complete and continuous removal of settled
sludge. Rapid sludge removal pipes shall return sludge to a well at the surface
that enables visual observation of flow. Mechanical rakes shall move sludge to
a hopper at the floor. The tip speed for circular mechanisms shall not exceed 8
fpm (2.4 m/min) and straight line flight speed shall not exceed 1 fpm (0.3
m/min).
The return sludge removal pipes shall be at least four (4)
inches (10.2 cm) in diameter. The hydraulic differential between the clarifier
water level and the return sludge level shall be sufficient to maintain a 3-fps
(0.9 mps) velocity in each rapid return sludge withdrawal pipe. Each sludge
withdrawal pipe shall be accessible for rodding or backflushing when the
settling basin is in operation.
(ii) Scum removal. Provide effective baffling
and scum collection and removal facilities for all secondary settling basins.
Equipment shall include a mechanical, positive scum skimmer.
(iii) Sludge hopper. The minimum side slope
of the hopper shall be 1.7 vertical to 1.0 horizontal. Hopper bottoms shall
have a maximum dimension of two (2) feet (0.61 m). The sludge removal pipe
should be flush with hopper bottom, and have a minimum diameter of six (6)
inches (0.15 m).
(iv) Scum box.
Locate scum box outside settling tank and adjacent to the scum collection
point. Provide method for mixing contents of scum box, such as air jets or
surface wetting using waste sludge. Provide access and washwater for washing
the scum box. The scum box shall be located on the side of the tank opposite
the prevailing wind direction.