| (a) Settling Compartment.
(1) The minimum length-to-width ratio of a settling
compartment is 2.0 to 1.0.
(2) A tank inlet must provide uniform flow distribution
across the width of a settling compartment.
(3) The septum walls must slope to the center of a
compartment at an angle of at least 50 degrees but not more than
60 degrees from horizontal. The septum walls must create an overlap
with a continuous slot that is at least 8.0 inches wide between the
walls to allow solids to be dispersed into the digestion compartment.
To prevent gases from escaping through the slot, one of the septum
walls must continue past the slot to create a slot overhang of at
least 8.0 inches.
(4) The depth between the water level at design flow
and the plane of a slot must not be more than 9.0 feet.
(5) At least 18 inches of freeboard must be provided
above the normal water level.
(b) Surface Loading.
(1) The settling compartment surface loading rate
must not exceed 800 gallons per day per square foot of settling compartment
area under design flow conditions.
(2) The longitudinal velocity of wastewater through
a settling compartment must not exceed 1.0 foot per second under
peak flow conditions.
(c) Scum Baffles. An inlet and an outlet of an Imhoff
tank must include scum baffles with a height that meets the water
levels at all flows from minimum flow to peak flow.
(d) Gas Vents.
(1) An Imhoff tank must include gas vents with a total
area not less than 20% of the total Imhoff tank surface area. The
total Imhoff tank surface area must be included in the engineering
report.
(2) At least one vent opening must be large enough
to allow equipment into the digestion compartment for maintenance.
(e) Digestion Compartment Loading. The digestion compartment
minimum volume must be 3.5 cubic feet per capita or 20.5 cubic feet
per pound of influent five-day biochemical oxygen demand (BOD5 ) per day, whichever is greater.
(f) Imhoff Tank Dimensions. The total depth of an
Imhoff tank must not be less than 16.5 feet from the water surface
to the bottom of a digestion compartment at design flow.
(g) Sludge Removal.
(1) The digestion compartment of an Imhoff tank must
have a sludge withdrawal pipe.
(2) A sludge withdrawal pipe must have a minimum diameter
of 8.0 inches and include a provision for regular cleaning.
(3) A digestion compartment design must allow a portable
pump to remove accumulated sludge.
(h) Odor Management.
(1) The design of an Imhoff tank must minimize the
effect of odor from the gas vents.
(2) The executive director may require a bio-filter,
a carbon filter, or other odor control device to minimize odor.
(i) Treatment Efficiency.
(1) An Imhoff tank must be followed by at least one
subsequent treatment unit.
(2) A design may assume that an Imhoff tank removes
35% of the influent BOD5 .
(j) Material and Construction.
(1) An Imhoff tank must be constructed of reinforced
and sealed concrete.
(2) Each component of an Imhoff tank must be resistant
to the corrosive effects of a wastewater environment.
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