1. (a) Settling Compartment.

   1. (1) The minimum length-to-width ratio of a settling compartment is 2.0 to 1.0.
   2. (2) A tank inlet must provide uniform flow distribution across the width of a settling compartment.
   3. (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 at least 8.0 inches wide provided between the walls to allow solids to be dispersed into a digestion compartment.
   4. (4) The maximum depth between the normal water level and the plane of a slot is 9.0 feet.
   5. (5) The minimum freeboard above the normal water level is 18 inches.
   6. (6) One of the septum walls must continue past the slot to create a minimum slot overhang of 8.0 inches.

2. (b) Surface Loading.

   1. (1) The settling compartment overflow loading rate must not exceed 800 gallons per day per square foot of settling compartment area under design flow conditions.
   2. (2) The longitudinal velocity of wastewater through a settling compartment must not exceed 1.0 foot per second under peak flow conditions.
3. (c) Scum Baffles. An inlet and an outlet of a tank must include scum baffles with a height that meet the water levels at all flows from minimum flow to peak flow.

4. (d) Gas Vents.

   1. (1) An Imhoff tank must include gas vents with a total area not less than 20% of the total tank surface area.
   2. (2) The width of at least one vent opening must allow maintenance access into a digestion compartment.
5. (e) Digestion Compartment Loading. The digestion compartment minimum volume must be the greater of 3.5 cubic feet per capita or 20.5 cubic feet per pound of influent five-day biochemical oxygen demand (BOD₅) per day.
6. (f) Imhoff Tank Dimensions. The total depth of an Imhoff tank must not be less than 15 feet from the water surface at design flow to the bottom of a digestion compartment, not including the first 18 inches of tank depth below the plane of a slot for design digestion volume.

7. (g) Sludge Removal.

   1. (1) An Imhoff tank must have a sludge withdrawal pipe in a digestion compartment.
   2. (2) A sludge withdrawal pipe must have a minimum diameter of 8.0 inches and include a provision for regular cleaning.
   3. (3) A digestion compartment design must allow a portable pump to remove accumulated sludge.

8. (h) Odor Management.

   1. (1) The design of an Imhoff tank must minimize the effect of odor from the gas vents.
   2. (2) The executive director may require a bio-filter, a carbon filter, or other odor control device to minimize odor.

9. (i) Treatment Efficiency.

   1. (1) An Imhoff tank must be followed by at least one subsequent treatment unit.
   2. (2) A design may assume that an Imhoff tank removes 35% of the influent BOD₅.
   3. (3) A design may assume that subsequent treatment units remove 60% of influent total suspended solids.

10. (j) Material and Construction.

    1. (1) An Imhoff tank must be constructed of reinforced and sealed concrete.
    2. (2) Each component of an Imhoff tank must be resistant to the corrosive effects of a wastewater environment.

Source Note:The provisions of this §217.204 adopted to be effective August 28, 2008, 33 TexReg 6843.