SECTION 252:656-17-2. Clarifier design considerations

(a)   Flow distribution. Effective flow splitting devices and control appurtenances (i.e. gates, splitter boxes, etc.) shall be provided to permit proper proportioning of flow and solids loading to each unit throughout the expected range of flows.

(b)   Primary clarifier design criteria. Primary clarifiers shall be placed downstream of flow distribution devices. Surface settling rates for primary tanks shall not exceed 1,000 gal/ft/day at design average flows or 1,500 gal/ft/day for peak hourly flows. Peak hourly flow is based upon a 2-hour sustained peak, as defined by Wastewater Engineering: Treatment, Disposal & Reuse, Metcalf & Eddy, Inc. 4^th Edition (2003). The primary clarifier must have a minimum side water depth of twelve feet (12'). Clarifier sizing shall be calculated for both flow conditions and the larger surface area determined shall be used. Primary settling of normal domestic sewage can be expected to remove 30 to 35% of the influent BOD. However, anticipated BOD removal for sewage containing appreciable quantities of industrial wastes (or chemical additions to be used) shall be determined by laboratory tests and consideration of the quantity and character of the wastes.

(c)   Secondary clarifier design criteria. See Appendix B.

(d)   Inlet structures. Design inlets to prevent short-circuiting, to dissipate velocity and diffuse flow equally across the entire cross-section of the settling chamber. Design channels to maintain a velocity of at least 1 fps at one-half design flow. When scum ports in the inlet diffusion well baffle are provided, the elevation of the bottom edge of the ports shall be no lower than 0.10 feet below the elevation of the crest of the overflow weirs.

(e)   Weirs. Overflow weirs shall be adjustable and level.

(1)   Location. Locate overflow weirs to optimize hydraulic retention time and minimize short-circuiting.

(2)   Design rates. Weir loadings shall not exceed 10,000 gal/linear foot/day for plants designed for average flows of 1.0 mgd or less. Higher weir loadings may be used for plants designed for larger average flows, but shall not exceed 15,000 gal/linear foot/day. Where the flow is pumped to the clarifier, the weir length shall be based on the average pump delivery rates to avoid short-circuiting.

(3)   Weir troughs. Design weir troughs to prevent submergence at maximum design flow, and to maintain a velocity of at least 1 fps at one-half design flow.

(4)   Dewatering. Provide the necessary piping and equipment to permit complete dewatering to the floor for the bypassing of individual units for maintenance and repair.

(5)   Freeboard. Walls shall extend at least 6 inches above the surrounding ground surface and provide at least 12 inches of freeboard. Provide additional freeboard or wind screens for larger clarifiers subject to high velocity wind currents that would cause tank surface waves and inhibit scum removal.

[Source: Added at 17 Ok Reg 1140, eff 6-1-00; Revoked at 23 Ok Reg 937, eff 6-15-06; Added at 28 Ok Reg 1282, eff 7-1-11]