(a) Pretreatment.
Where primary clarification is not provided, screening of the raw sewage to
remove debris larger than 3/4 inch (1.9 cm) shall be provided. The screened
material shall not be returned to the plant process. Where primary clarifiers
are not provided, cleanouts, grinders, or other similar provisions shall be
made in the return sludge piping.
(b) Loading rates. Activated sludge systems
shall be designed to accommodate peak day loadings at the design year.
Permissible loadings are presented in the following table. Where raw sewage
BOD
5 is less than 200 mg/L,
detention times may be reduced.
(i)
Conventional, including complete mix, plug flow, step aeration
|
|
Average Day
|
Detention (*) hrs,
|
Following primary clarifiers |
6 minimum |
|
Without primary clarifiers |
9
minimum |
Organic Loading:
|
lb/1,000 cu ft/day (kg/1000 m3d) |
35
maximum (560 |
MLSS, mg/L
|
|
1,000 - 3,000 |
(ii) Contact stabilization.
Detention (*) hrs, Contact Zone |
|
0.5 - 3 |
Sludge Stabilization Zone |
|
6
minimum |
|
|
Average Day
|
Organic Loading (**)
|
lb/1,000 cu ft/day |
50 |
|
(kg/1000 m3d)
|
(800) |
MLSS, mg/L Contact Zone
|
|
1,000 - 3,000 |
Sludge Stabilization Zone |
|
5,000 -
10,000 |
(iii) Extended aeration, including oxidation
ditch.
Detention (*) hrs, Organic
Loading, (kg/1000 m3d) |
lb/1,000 cu ft/day |
16 minimum
15 maximum (240)
|
MLSS, mg/L
|
|
1,000 - 3,000 |
(*) Based on average day raw sewage flow rate exclusive of
recirculation flow.
(**) Based on contact zone and sludge stabilization zone
combined.
(c)
Number of basins. For all design average flows in excess of 0.1 mgd (378
m3/d), two or more aeration basins shall be
provided. For flows less than 0.1 mgd (378 m3/d),
one aeration basin may be provided if the aeration devices can be readily
removed while the basin is in operation.
(d) Configuration. The basin configuration
shall promote mixing, transfer of oxygen, and minimize stagnant
zones.
(e) Freeboard. The walls of
the aeration shall extend above the normal water surface to provide a minimum
freeboard as follows:
|
Minimum inches |
Freeboard (*) cm
|
Diffused air |
18 |
45.7 |
Surface aeration |
48 |
121.9 |
Submerged turbine |
18 |
45.7 |
Brush aeration, less than 10 feet from aeration
device |
48 |
121.9 |
Brush aeration, 10 feet or more from aeration device
|
18 |
45.7 |
Surface aeration, where aeration |
36
|
91.40 |
is 30 or more feet from basin wall
|
|
|
(*) Vertical walls. For sloped walls, the runup effect
shall be considered.
(f)
Inlet and outlet conditions. Inlets may be submerged and shall be baffled or
directed away from the outlet to minimize shortcircuiting. Outlets shall be of
the overflow type to discourage buildup of foam and floatables on the aeration
basins. Pipe and channels shall provide a minimum velocity of 0.5 fps (0.15
m/s).
(g) Aeration requirements.
(i) Carbonaceous BOD. When it can be shown
that nitrification will not occur in the activated sludge process, the aeration
devices may be sized to meet only the carbonaceous oxygen demand. The oxygen
provided by the aeration device shall be selected to be adequate for the
projected maximum day loading. In the absence of other data, an oxygen
requirement of two (2) times the average design day BOD5
to the aeration basin shall be used.
(ii) Nitrification. Where nitrification is
required to meet the effluent requirements or where the process cannot be
operated to prevent nitrification, the aeration requirements will be selected
to provide oxygen for both carbonaceous BOD and nitrification on the projected
maximum day loading. In the absence of other data, an oxygen requirement of two
times the average design day BOD5 plus 7.5 times the
average day ammonia nitrogen to the aeration basin shall be used.
(iii) Minimum dissolved oxygen. Oxygen supply
shall be selected to transfer the design quantity during the maximum day
loading while maintaining an aeration basin dissolved oxygen of 2.0 mg/L. The
oxygen supply shall be designed for the specific site considering all factors
that affect oxygen transfer efficiency.
(h) Mechanical aeration. Mechanical surface
aerators shall be designed to maintain all organics in suspension, enhance the
oxygen transfer capability of the unit, and minimize mist and spray that escape
the basin. Drive units shall be protected from freezing mist and
spray.
(i) Diffused aeration.
(i) Diffuser requirements. The number and
location of diffusers shall be selected to distribute the design air quantity
for efficient aeration and mixing. Diffusers in a basin shall be grouped on
control valves to permit varying the air supply to different parts of the
basin. Oxygen transfer efficiencies used for design purposes shall be
conservatively selected, based on experimentally determined transfer rates of
generically similar diffusers. The effect of transferring oxygen to wastewater,
in lieu of water, and the effect of altitude shall be considered. The aeration
basin mid-depth shall be used to determine the oxygen saturation concentration.
Differential head loss to individual diffuser inlets shall not be more than 0.2
psi (14 gm/cm2).
(ii) Blower requirements. Blowers shall be
sized to provide the air requirements for the aeration basins and other plant
uses of low-pressure air. The inlet air to the blowers shall be filtered or
otherwise conditioned to effectively remove dust and other particulate
material. Removal of particulate material for fine bubble diffusers shall be
designed for 95 percent of 0.3 micron. Filters designed for blowers shall be
easily replaceable. Blower intakes shall be located to avoid clogging from
drifting snow. Blowers shall be housed. The housing shall be ventilated to
prevent more than a 15° F (8° C) temperature rise with all blowers
operating, excepting the standby blower. The housing, blowers, and blower
piping shall be arranged to permit removal of individual blowers while all
other blowers are operating. Noise attenuating materials shall be used in the
building interior. Blower systems shall be designed to permit varying the
volume of air delivered. Blower motors shall be of a size to operate the blower
throughout the range of ambient air temperatures experienced at the plant site.
(j) Sludge
recirculation and waste.
(i) Rates. Sludge
recirculation from the secondary settling basin to the aeration basin shall be
variable within 25 to 100 percent of the average design flow. Sludge wasting
from the activated sludge process may be from the mixed liquor or the return
sludge. Sludge wasting shall be variable to enable wasting ½ of the
total system solids in one day to zero wasting.
(k) Equipment requirements.
(i) Return sludge. Return sludge pumping
shall be variable. The return sludge rate from each secondary settling unit and
the rate to each aeration basin shall be controllable. Pumps shall be housed in
heated, ventilated space. The pump floor shall be sloped and drained. Valves
shall permit isolating each pump. Pumps and piping shall be arranged to allow
ready removal of each pump. Check valves shall be provided where backflow
through the pump could occur. Check valves shall be located in the horizontal.
Pump suction and discharge shall be three (3) inches (7.6
cm) minimum diameter. Sludge piping diameter shall be four (4) inches (10.2 cm)
or larger. Cleanouts and couplings shall be provided in sludge piping to enable
cleaning the pipe or to remove pumping equipment. All pipe high points shall be
provided with air releases. All sludge piping shall be metallic material.
Should air lift pumps be used, the units shall be designed with a minimum of 80
percent static submergence.
(ii) Waste sludge. If separate waste sludge
pumps are provided, the rate shall be controlled by timers or variable speed
devices. Pumping units shall be housed in heated, ventilated space, with sloped
and drained floors. Pump suction and discharge piping shall be three (3) inches
(7.6 cm) minimum diameter. Sludge piping shall be four (4) inches diameter
(10.2 cm) or larger, except short, easily removable sections that may be
required to maintain velocities above one fps (0.3 mps), or for use in
conjunction with meters.
(l) Metering.
(i) Return sludge. For treatment plants
having an average day design capacity greater than 100,000 gpd (378
m3/d) the return sludge flow rate from each
secondary settling unit and to each aeration basin shall be metered to indicate
flow rate. Return sludge metering devices shall be suitable for liquids
carrying grease and solids, and shall be accurate to within ±5 percent
of the actual flow rate. Meters shall be readily field calibrated by plant
personnel. Meters shall be arranged to avoid trapping air.
(ii) Waste sludge. For treatment plants
having an average day design capacity greater than 100,000 gpd (378
m3/d), waste sludge flows shall be metered to
indicate and totalize. Waste sludge meters shall meet the requirements
described for return sludge meters.
(iii) Air flow. Low-pressure air used for
basin aeration and other plant uses shall be metered. Separate meters shall be
used to indicate the flow rate to each aeration basin and to the ancillary uses
made of the low-pressure air. Indicators shall be located near the device used
to control the air flow rate. Pressure gages shall be provided immediately
downstream from each blower and immediately upstream of each aeration
basin.
(m) Controls.
Facilities for control shall be provided for:
(i) Control of flow split between parallel
process units.
(ii) Control of
return sludge flow rate to each aeration basin.
(iii) Control of waste sludge
quantity.
(iv) Control of air flow
rate to each aeration basin.
(v)
Control of air distribution to different zones in aeration basin.
(vi) Control of energy imparted with
mechanical aeration. Facilities for control shall include a meter or device to
measure rate and a device to change the rate such as a valve or adjustable
weir.
(n) Prefabricated
treatment units. Prefabricated activated sludge units shall conform to the
applicable requirements described.
(o
) Ancillary facilities. Adequate nonpotable washdown water shall
be provided around the aeration basins sludge pumping area and secondary
settling basins. Sampling ports, pipes or other access shall be provided on
aeration basin inlets, return sludge piping, waste sludge piping and secondary
settling basins. Hoisting or other means of equipment removal shall be
provided. All subgrade floors shall be drained.