PURPOSE: The following criteria have been prepared as a guide for the design of settling tanks. This rule is to be used with rules 10 CSR 20-8.110–10 CSR 20-8.220 for the planning and design of the complete treatment facility. This rule reflects the minimum requirements of the Missouri Clean Water Commission as regards adequacy of design, submission of plans, approval of plans and approval of completed sewage works. Deviation from these minimum requirements will be allowed where sufficient documentation is presented to justify the deviation. These criteria are taken largely from Great Lakes-Upper Mississippi River Board of State Sanitary Engineers Recommended Standards for Sewage Works and are based on the best information presently available. These criteria were originally filed as 10 CSR 20-8.030. It is anticipated that they will be subject to review and revision periodically as additional information and methods appear. Addenda or supplements to this publication will be furnished to consulting engineers and city engineers. If others desire to receive addenda or supplements, please advise the Clean Water Commission so that names can be added to the mailing list. Editor’s Note: The secretary of state has determined that the publication of this rule in its entirety would be unduly cumbersome or expensive. The entire text of the material referenced has been filed with the secretary of state. This material may be found at the Office of the Secretary of State or at the headquarters of the agency and is available to any interested person at a cost established by state law.

1. (1) Definitions. Definitions as set forth in the Clean Water Law and 10 CSR 20-2.010 shall apply to those terms when used in this rule, unless the context clearly requires otherwise. Where the terms shall and must are used, they are to mean a mandatory requirement insofar as approval by the agency is concerned, unless justification is presented for deviation from the requirements. Other terms, such as should, recommend, referred and the like, indicate discretionary requirements on the part of the agency and deviations are subject to individual consideration.
2. (2) Exceptions. This rule shall not apply to facilities designed for twenty-two thousand five hundred gallons per day (22,500 gpd) (85.4m3) or less (see 10 CSR 20-8.020 for the requirements for those facilities).

3. (3) General Considerations.

   1. (A) Number of Units. Multiple units capable of independent operation are desirable and shall be provided in all plants where design flows exceed one hundred thousand (100,000) gpd (379m3/d). Plants not having multiple units shall include other provisions to assure continuity of treatment.
   2. (B) Arrangement. Settling tanks shall be arranged in accordance with subsection 10 CSR 20-8.140(5)(E).
   3. (C) Flow Distribution. Effective flow measurement devices and control appurtenances (that is, valves, gates, splitter boxes, etc.) shall be provided to permit proper proportion of flow to each unit.
   4. (D) Tank Configuration. Consideration should be given to the probable flow pattern in the selection of tank size and shape, and inlet and outlet type and location.

4. (4) Design Considerations.

   1. (A) Dimensions. The minimum length of flow from inlet to outlet should be ten feet (10') (3 m) unless special provisions are made to prevent short-circuiting. The sidewater depth for primary clarifiers shall be as shallow as practicable, but not less than seven feet (7') (2.1 m). Clarifiers following the activated sludge process shall have sidewater depths of at least twelve feet (12') (3.7 m) to provide adequate separation zone between the sludge blanket and the overflow weirs. Clarifiers following fixed film reactors shall have sidewater depth of at least seven feet (7') (2.1m).

   2. (B) Surface Settling Rates (Overflow Rates).

      1. 1. Primary settling tanks. Surface set-

tling rates for primary tanks should not exceed one thousand (1000) gpd per square foot (41m3/m2/day) at design average flows or one thousand five hundred (1500) gpd per square foot (61m3m2/day) for peak hourly flows. 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 thirty to fifty percent (30–50%) of the influent BOD. However, anticipated BOD removal for sewage containing appreciable quantities of industrial wastes (or chemical additions to be used) should be determined by laboratory tests and consideration of the quantity and character of the wastes.

1. 2. Intermediate settling tanks. Surface

settling rates for intermediate settling tanks following series units of fixed film reactor processes shall not exceed one thousand five hundred (1500) gpd per square foot (61m3/m2/day) based on peak hourly flow.

1. 3. Final settling tanks. Settling tests

should be conducted wherever pilot study of biological treatment is warranted by unusual waste characteristics or treatment requirements. Testing shall be done where proposed loadings go beyond the limits set forth in this section. Surface settling rates for settling tanks following trickling filters or rotating biological contractors shall not exceed one thousand two hundred (1200) gpd per square foot (49m3/m2/day) based on peak hourly flow. Final settling tanks following activated sludge processes must be designed to meet thickening as well as solids separation requirements. Since the rate of recirculation of return sludge from the final settling tanks to the aeration or re-aeration tanks is quite high in activated sludge processes, surface settling rate and weir overflow rate should be adjusted for the various processes to minimize the problems with sludge loadings, density currents, inlet hydraulic turbulence and occasional poor sludge settleability. The hydraulic design of intermediate and final settling tanks following activated sludge processes shall be based upon the anticipated peak hourly rate for the area downstream of the inlet baffle. The hydraulic loadings shall not exceed—one thousand two hundred (1200) gpd per square foot (49m3/m2/day) for conventional, step aeration, contact stabilization and the carbonaceous stage of separatestage nitrification; one thousand (1000) gpd per square foot (41m3/m2/day) for extended aeration; and eight hundred (800) gpd per square foot (33m3/m2/day) for the separate nitrification stage. The solids loading for all activated sludge processes shall not exceed fifty pounds (50 lbs.) solids per day per square foot (244 kg/m2/day) at the peak rate. Consideration should be given to flow equalization.

1. (C) Inlet Structures. Inlets should be designed to dissipate the inlet velocity, to distribute the flow equally both horizontally and vertically and to prevent short-circuiting. Channels should be designed to maintain a velocity of at least one foot (1') per second (0.3m/s) at one-half (1/2) the design flow. Corner pockets and dead ends should be eliminated and corner fillets or channeling used where necessary. Provisions shall be made for elimination or removal of floating materials in inlet structures.

2. (D) Weirs.

   1. 1. General. Overflow weirs shall be

adjustable for leveling.

1. 2. Location. Overflow weirs shall be

located to optimize actual hydraulic detention time, and minimize short-circuiting.

1. 3. Design rates. Weir loadings should

not exceed ten thousand (10,000) gpd per lineal foot (124m3/m/day) for plants designed for average flows of 1.0 mgd (3,785m3/day) or less. Higher weir loadings may be used for plants designed for larger average flows but should not exceed fifteen thousand (15,000) gpd per lineal foot (186m3/m/day). If pumping is required, weir loadings should be related to pump delivery rates to avoid short-circuiting.

1. 4. Weir troughs. Weir troughs shall be

designed to prevent submergence at maximum design flow and to maintain a velocity of at least one foot (1') per second (0.3m/s) at one-half (1/2) the design flow.

1. (E) Submerged Surfaces. The tops of troughs, beams and similar submerged construction elements shall have a minimum slope of 1.4:1; the underside of the elements should have a slope of one to one (1:1) to prevent the accumulation of scum and solids.
2. (F) Unit De-watering. Unit de-watering features shall conform to the provisions outlined in 10 CSR 20-8.140(6). The unit isolation design should also provide for redistribution of the plant flow to the remaining units.
3. (G) Freeboard. Walls of settling tanks shall extend at least six inches (6") (15 cm) above the surrounding ground surface and shall provide not less than twelve inches (12") (30 cm) 10 CSR 20-8

freeboard. Additional freeboard or the use of wind screens is recommended where larger settling tanks are subject to high velocity wind currents that would cause tank surface waves and inhibit effective scum removal.

1. (5) Sludge and Scum Removal.

   1. (A) Scum Removal. Effective scum collection and removal facilities, including baffling, shall be provided for all settling tanks. The unusual characteristics of scum which may adversely affect pumping, piping, sludge handling and disposal should be recognized in design. Provisions may be made for the discharge of scum with the sludge; however, other special provisions for disposal may be necessary.

   2. (B) Sludge Removal. Sludge collection and withdrawal facilities shall be so designed as to assure rapid removal of the sludge. Suction withdrawal should be provided for activated sludge plants designed for reduction of the nitrogenous oxygen demand and is encouraged for those plants designed for carbonaceous oxygen demand reduction.

      1. 1. Sludge hopper. The minimum slope

of the side walls shall be 1.7:1. Hopper wall surfaces should be made smooth with rounded corners to aid in sludge removal. Hopper bottoms shall have a maximum dimension of two feet (2') (.6m). Extra depth sludge hoppers for sludge thickening are not acceptable.

1. 2. Cross-collectors. Cross-collectors

serving one (1) or more settling tanks may be useful in place of multiple sludge hoppers.

1. 3. Sludge removal piping. Each hopper

shall have an individually-valved sludge withdrawal line at least six inches (6") (15 cm) in diameter. The static head available for withdrawal of sludge shall be thirty inches (30") (76 cm) or greater as necessary to maintain a three-foot (3') per second (0.9m/s) velocity in the withdrawal pipe. Clearance between the end of the withdrawal line and the hopper walls shall be sufficient to prevent bridging of the sludge. Adequate provisions shall be made for rodding or back-flushing individual pipe runs. Piping shall also be provided to return waste sludge to primary clarifiers.

1. 4. Sludge removal control. Sludge wells

equipped with telescoping valves or other appropriate equipment shall be provided for viewing, sampling and controlling the rate of sludge withdrawal. The use of easily maintained sight glass and sampling valves may be appropriate. A means of measuring the sludge removal rate shall be provided. Air lift type of sludge removal will not be approved for removal of primary sludges. Sludge pump motor control system shall include time clocks and valve activators for regulating the duration and sequencing of sludge removal. (6) Protective and Service Facilities.

1. (A) Operator Protection. All settling tanks shall be equipped to enhance safety for operators. These features shall appropriately include machinery covers, life lines, stairways, walkways, hand rails and slip-resistant surfaces.
2. (B) Mechanical Maintenance Access. The design shall provide for convenient and safe access to routine maintenance items such as gear boxes, scum removal, mechanism and baffles, weirs, inlet stilling baffle area and effluent channels.
3. (C) Electrical Fixtures and Controls. Electrical fixtures and controls in enclosed settling basins shall be suitable for hazardous locations (National Electrical Code for Class I, Group D, Division 1 location). The fixtures and controls shall be located so as to provide convenient and safe access for operation and maintenance. Adequate area lighting shall be provided.

AUTHORITY: section 644.026, RSMo Supp. 1988.* Original rule filed Aug. 10, 1978, effective March 11, 1979. *Original authority 1972, amended 1973, 1987, 1993.