(a) Sedimentation.
(i) Number of basins. For plants having an
average design capacity greater than 100,000 gpd (378.4
m3/d) and where primary settling is provided,
multiple units capable of independent operation shall be provided.
(ii) Design parameters.
(A) Performance. Unless full-scale data is
available, primary settling shall be assumed to remove one third of the
influent BOD and 55 percent of the influent suspended solids. It is
unacceptable to return waste activated sludge to the primary
clarifier.
(B) Water depth. The
minimum side water depth shall be seven (7) feet (2.1 m).
(C) Surface overflow rates. Surface overflow
rates shall not exceed 1,000 gpd/sq ft (41
m3/m²d) of surface area at the average design
flow nor 1,500 gpd/sq ft (61 m3/m²d) of surface
area at the maximum day flow rate. Maximum day flow is the highest flow over a
24 hour period that is projected to occur during the design year.
(D) Weir loading rates. Circular basins (or
basins with center inlets) shall be provided with a full periphery weir.
Rectangular basins shall be provided with end weirs that provide less than
80,000 gpd/ft (9,920 m3/m d) weir hydraulic loading
at peak instantaneous flow rates.
(iii) Clarifier inlet and outlet.
(A) General. Clarifier inlet structures shall
be designed to achieve the following:
(I)
Dissipate the inlet kinetic energy.
(II) Distribute the flow evenly into the
tank.
(III) Prevent short
circuiting.
Inlet channels or piping shall be designed for minimum
velocities of one (1) fps (0.3 mps). Where minimum velocities are less, mixing,
flushing or other means of resuspending solids shall be provided.
Circular basins shall be provided with symmetrical baffling
to distribute flow equally in all radial directions.
Rectangular basins shall be provided with inlet parts
uniformly distributed along the entire end of the basin and shall be provided
with baffles.
(B) Weirs. Weir plates shall be adjustable
for leveling and sealed against the effluent channel.
(C) Baffles. Provide scum baffles at the
water surface to intercept all floating materials and scum prior to the weir.
Baffles should extend three (3) inches (7.6 cm) above the weir plate elevation
and eight (8) inches (20.3 cm) below the water surface.
(D) Clarifier effluent channel.
(I) Size. The effluent channel shall be sized
to prevent weir submergence at the peak hourly flow.
(E) Freeboard. The outer walls of
sedimentation tanks shall extend at least six (6) inches (0.15 m) above the
surrounding ground and shall provide at least twelve (12) inches (0.3 m) of
freeboard to the water surface. Where basin walls do not extend four (4) feet
(1.2 m) above the surrounding ground, a fence or suitable barrier to prevent
debris from entering the basin shall be provided.
(F) Basin equipment and access. Provide
walkways and accessways to collector drive units, effluent launders and manual
skimmer. Handrail shall be provided.
(b) Fine screens.
(i) Number of units. A minimum of two (2)
units shall be provided. Multiple units shall be capable of independent
operation. With the largest unit out of service, the remaining units shall be
capable of passing the peak flow rate.
(ii) Flow distribution. Positive means of
flow distribution shall be provided ahead of the screens to ensure even loading
and hydraulic flows.
(iii) Design
parameters.
(A) Performance. In the absence
of pilot plant data, the removal efficiency of fine screens shall be assumed to
be zero percent removal of BOD5 and 15 percent removal
of suspended solids.
(B)
Preliminary treatment requirement. Prior to the fine screens, removal of large
debris shall be provided by coarse screens. Comminution shall not be provided
ahead of screens.
(iv)
Screenings storage and disposal. Screens with openings of 0.10 inch (2.5 mm) or
more shall be disposed of directly to landfill in accordance with the
requirements of the Department of Environmental Quality, Solid Waste Management
Office. Screens with openings less than 0.10 inch (2.5 mm) shall discharge the
screenings (primary sludge) to sludge handling system for organic
stabilization.
(v) Cleaning and
maintenance. Provide facilities to permit regular cleaning of screens with a
high pressure, hot water or steam system.
(vi) Controls. For rotating screens, each
screen or series of screens shall be provided with an overflow. An alarm shall
be provided when overflowing.
(c) Sludge handling.
(i) Sludge removal. Mechanical sludge
collection equipment is required for all primary settling basins. The sludge
collection rake arms or flights and the drive assembly shall be designed to
withstand the maximum anticipated loads and move sludge to the
hopper.
(ii) Scum removal. Provide
scum collection and removal facilities for all primary settling basins. Scum
shall be removed from the liquid process and not returned.
(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 feet (0.61 m). The sludge removal pipe shall be
flush with the hopper bottom, and have a minimum diameter of six inches (15.2
cm).
(iv) Scum box. The scum box
shall be located outside and immediately adjacent to the scum collection point
(beaching plate). The beaching plate shall be located on the opposite side of
the basin from the prevailing wind. Provide for mixing the contents of the scum
box, such as a mechanical mixer or air diffusion. Provide access and wash water
for washing the scum box.
(v)
Controls.
(A) Primary settling sludge
facilities. Primary sludge and scum shall be removed using positive
displacement pumps. Each basin shall have a separately activated and controlled
pump. (The standby pumps may be shared by more than one basin.) Pumps shall be
on timers and the pumps should be designed to initiate sludge removal two (2)
or more times per hour.
Include devices on the primary sludge piping for sampling
the primary sludge flow.
(B) Primary screen sludge facilities. Where
sludge pumping is provided, include a means to shut off the pump when
insufficient material is being supplied to the pump suction. The controls for
the pump shall be designed to match the pumping rate to quantity of sludge.
Where conveyors are used, they shall run continuously and alarm when
off.