| transfer, the mechanical aeration devices shall
also be required to provide sufficient mixing to prevent deposition
of mixed liquor suspended solids under any flow condition. A minimum
of 100 horsepower per million gallons of aeration basin volume shall
be furnished.
(h) Nutrient removal.
(1) Nitrogen removal. Biological systems designed for
nitrification and denitrification may be utilized for the conversion/removal
of nitrogen. Various physical/chemical processes may be considered
on a case-by-case basis.
(2) Phosphorus removal.
(A) Chemical treatment. Addition of lime or the salts
of aluminum, or iron may be used for the chemical removal of soluble
phosphorus. The phosphorus reacts with the calcium, aluminum, or iron
ions to form insoluble compounds. These insoluble compounds may be
flocculated with or without the addition of a coagulant aid such as
a polyelectrolyte to facilitate separation by sedimentation. When
adding salts of aluminum or iron, the designer should evaluate the
wastewater to ensure sufficient alkalinity is available to prevent
excessive depression of the wastewater or effluent pH. This is of
particular importance when the system will also be required to achieve
nitrification. The designer is referred to Nutrient Control, Manual
of Practice FD-7 Facilities Design, published by the Water Pollution
Control Federation and the Process Design Manual for Phosphorus Removal,
published by the Environmental Protection Agency, for additional information.
(B) Biological phosphorus removal. Biological phosphorus
removal systems will be considered on a case-by-case basis for systems
which can produce operating data which demonstrate the capability
to remove phosphorus to the required levels. All biological systems
which are required to meet a 1.0 mg/liter effluent phosphorus concentration
shall make provision for standby chemical treatment to ensure the
1.0 mg/liter is achieved.
(i) Aerated lagoon.
(1) Horsepower. Mechanical aeration units in aerated
lagoons shall have sufficient power to provide a minimum of 1.6 pounds
of oxygen per pound of BOD5 applied with
the largest unit out of service. If oxygen requirements control the
amount of horsepower needed, proposed oxygen transfer rates in excess
of two pounds per horsepower-hour must be justified by actual performance
data. The amount of oxygen supplied or the pounds of BOD5 per hour that may be applied per horsepower-hour
may be calculated by the use of any acceptable formula. The combined
horsepower rating of the aeration units shall not be less than 30
horsepower per million gallons of aerated lagoon volume.
(2) Construction. Earthen ponds shall have large sections
of concrete slabs or equivalent protection under each aeration unit
to prevent scouring of the earth. Concrete scour pads shall be used
in all areas where the velocity exceeds one foot per second. Earthen
ponds shall have protection on the slopes of the embankment at the
water line to prevent erosion of the slopes from the turbulence in
the lagoon. Where the horsepower level is more than 200 horsepower
per million gallons of lagoon volume, the pond embankment at the water
line shall be protected from erosion with riprap which may be concrete,
gunite, a six-inch thick layer of asphalt-saturated or cement-stabilized
earth rolled and compacted into place, or suitable rock riprap. The
crest and dry slopes of embankments shall be protected from erosion
by planting of grass.
(3) Subsequent treatment, discharge systems. Aerated
lagoon effluent will normally be routed to additional ponds for secondary
treatment and to provide sufficient detention time for disinfection.
The secondary ponds system shall consist of two or more ponds. Secondary
pond sizing shall not exceed 35 pounds of BOD5 per
acre per day. Hydraulic detention time in a combined aerated lagoon
and secondary pond system shall be a minimum of 21 days (based on
design flow) in order to provide adequate disinfection. In designing
the secondary ponds, BOD5 removal efficiency
in the aerated lagoon(s) may be calculated using the following formula.
Attached Graphic
(j) Wastewater stabilization ponds (secondary treatment
ponds).
(1) Pretreatment. Wastewater stabilization ponds shall
be preceded by facilities for primary sedimentation of the raw sewage.
Aerated lagoons or facultative lagoons may be utilized in place of
conventional primary treatment facilities.
(2) Imperviousness. All earthen structures proposed
for use in domestic wastewater treatment or storage shall be constructed
to protect groundwater resources. Where linings are necessary, the
following methods are acceptable:
(A) in-situ or placed clay soils having the following
qualities may be utilized for pond lining:
(i) more than 30% passing a 200-mesh sieve;
(ii) liquid limit greater than 30%;
(iii) plasticity index greater than 15; and
(iv) a minimum thickness of two feet;
(B) membrane lining with a minimum thickness of 20
mils, and an underdrain leak detection system;
(C) other methods with commission approval.
(3) Distribution of flow. Stabilization ponds shall
be of such shape and size to insure even distribution of the wastewater
flow throughout the entire pond. While the shapes of ponds may be
dictated to some extent by the topography of the location, long narrow
ponds are preferable and they should be oriented in the direction
of the prevailing wind such that debris is blown toward the inlet.
Ponds with narrow inlets or sloughs should be avoided.
(4) Access area. Storm water drainage shall be excluded
from all ponds. All vegetation shall be removed from within the pond
area during construction. Access areas shall be cleared and maintained
for a distance of at least 20 feet from the outside toes of the pond
embankment walls.
(5) Multiple ponds. The use of multiple ponds in pond
systems is required. The operation of the ponds shall be flexible,
enabling one or more ponds to be taken out of service without affecting
the operation of the remaining ponds. The ponds shall be operated
in series during routine operation periods.
(6) Organic loading. The organic loading on the stabilization
ponds, based on the total surface area of the ponds, shall not exceed
35 pounds of BOD5 per acre per day. The
loading on the initial stabilization pond shall not exceed 75 pounds
of BOD5 per acre per day.
(7) Depth. The stabilization ponds or cells shall have
a normal water depth of three to five feet.
(8) Inlets and outlets. Multiple inlets and multiple
outlets are required. The inlets and outlets shall be arranged to
prevent short circuiting within the pond so that the flow of wastewater
is distributed evenly throughout the pond. Multiple inlets and outlets
shall be spaced evenly. All outlets shall be baffled with removable
baffles to prevent floating material from being discharged, and shall
be constructed so that the level of the pond surface may be varied
under normal operating conditions. Submerged outlets shall be used
to prevent the discharge of algae.
(9) Embankment walls. The embankment walls should be
compacted thoroughly and compaction details shall be covered in the
specifications. Soil used in the embankment shall be free of foreign
material such as paper, brush, and fallen trees. The embankment walls
shall have a top width of at least 10 feet. Interior and exterior
slope of the embankment wall should be one foot vertical to three
feet horizontal. There shall be a freeboard of not less than two feet
nor more than three feet based on the normal operating depth. All
embankment walls shall be protected by planting grass or riprapping.
Where embankment walls are subject to wave action, riprapping should
be installed. Erosion stops and water seals shall be installed on
all piping penetrating the embankments. Provisions should be made
to change the operating level of the pond so the pond surface can
be raised or lowered at least six inches.
(10) Partially mixed aerated lagoons.
(A) Horsepower. With partially mixed aerated lagoons,
no attempt is made to keep all pond solids in suspension. Mechanical
or diffused aeration equipment should be sized to provide a minimum
of 1.6 pounds of oxygen per pound of BOD5 applied
with the largest unit out of service. Where multiple ponds are used
in series, the power input may be reduced as the influent BOD5 to each pond decreases. proposed oxygen transfer
rates in excess of two pounds per horsepower-hour must be justified
by actual performance data.
(B) Pond sizing. Partially mixed aerated lagoons should
be sized in accordance with the formula in subsection (i)(3) of this
section using K-0.28. Pond length to width ratios should be three
to one or four to one.
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