Settling

PURPOSE: This rule specifies the minimum standards for the design of settling operations that are part of wastewater collection and treatment systems. This rule is to be used with rules 10 CSR 20-8.110 through 10 CSR 20-8.210. It does not address all aspects of design, and the design engineer may refer to other appropriate reference materials so long as these minimum standards set forth in this rule are met. 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) Applicability. Wastewater systems that utilize settling shall be designed based on criteria contained in this rule, published standards, applicable federal and state requirements, standard textbooks, current technical literature, and applicable safety standards. In the event of any conflict between the above criteria, the requirement in this rule shall prevail. (A) This rule shall not apply to treatment units covered in 10 CSR 20-8.300. (B) This rule shall not apply to treatment units covered in 10 CSR 20-8.500. (2) General Considerations. (A) Number of Units. Multiple settling units capable of independent operation are desirable and shall be provided in all wastewater treatment facilities where design flows exceed one hundred thousand (100,000) gallons per day (gpd). Wastewater treatment facilities without multiple settling units shall be designed to include other provisions to assure continuity of treatment. (B) Flow Distribution. Effective flow splitting devices and control appurtenances (e.g. gates and splitter boxes) shall be provided to permit proper proportioning of flow and solids loading to each settling unit, throughout the expected range of flows. (3) Design. (A) Side Water Depth. The minimum side water depth shall be as follows in Table 160-1 below: (B) Surface Overflow Rates. 1. Primary settling tanks. Calculate the surface overflow rates for both design average flow and design peak hourly flow from Table 160-2 below. The larger area shall determine the size of the settling tank. 2. Final settling tanks – attached growth biological reactors. Surface overflow rates for settling tanks following attached growth biological reactors shall not exceed one thousand two hundred gallons per day per square foot (1,200 gpd/ ft2) based on the design peak hourly flow. 3. Final settling tanks – activated sludge. The following design criteria in Table 160-3, included herein, shall not be exceeded: (C) Weirs. 1. General. Overflow weirs shall be readily adjustable over the life of the structure to correct for differential settlement of the tank. 2. Design rates. The following weir loadings in Table 160-4, below, shall not be exceeded: (D) Submerged Surfaces. The underside and the tops of troughs, beams, and similar submerged construction elements shall have a minimum slope of one vertical to one horizontal (1:1) to prevent the accumulation of scum and solids. (E) Freeboard. Walls of settling tanks shall extend at least six inches (6") above the surrounding ground surface and shall provide not less than twelve inches (12") of freeboard. (4) Sludge Removal. Table 160-1. Minimum Side Water Depth. Type of Settling Tank Minimum Side Water Depth (ft) Primary (>100,000 gpd) 10 Primary (<100,000 gpd) 7 Final following activated sludge process 12 Final following attached growth biological reactor (>100,000 gpd) 10 Table 160-2. Maximum Primary Settling Tank Surface Overflow Rates. Type of Primary Settling Tank Surface Overflow Rates1: At Design Average Flow (gpd/ft2) At Design Peak Hourly Flow (gpd/ft2) Tanks not receiving waste activated sludge 1,000 3,000 Tanks receiving waste activated sludge 700 1,700 Chemically enhanced 1,400 1,500 1 Calculate surface overflow rates with all flows received at the settling tanks. Table 160-4. Maximum Weir Loading Rates. Average Wastewater Treatment Facility Capacity (million gallons per day or MGD) Loading Rate at Design Peak Hourly Flow (gpd/lf) Less than 0.1 10,000 0.1 through 1.0 20,000 Greater than 1.0 30,000 Table 160-3. Maximum Activated Sludge Final Settling Tank Rates. Treatment Process Surface Overflow Rate at Design Peak Hourly Flow1 (gpd/ft2) Peak Solids Loading Rate2 (lb/day/ft2) With diurnal flow equalization3 1,000 35 Without diurnal flow equalization3 150 x Peaking Factor4 35 Conventional, Step Aeration, Complete Mix, Contact Stabilization, Carbonaceous Stage of Separate Stage Nitrification 1,200 5 40 Extended Aeration Single-Stage Nitrification 1,000 35 Multi-Stage Nitrification 800 35 Activated Sludge with Chemical addition to Mixed Liquor for Phosphorus Removal 900 35 1 Based on influent flow only. 2 Calculate the peak solids loading rate based on the design maximum day flow rate plus the design maximum return sludge rate requirement and the design mixed liquor suspended solids under aeration. 3 Applicable to wastewater treatment facilities with a design average flow of less than one hundred thousand gallons per day (100,000 gpd). 4 To determine the peaking factor use 10 CSR 20-8.110(3) Equation 110-1. 5 Wastewater treatment facilities needing to meet twenty milligrams per liter (20 mg/L) suspended solids or less should reduce the surface overflow rate to one thousand gallons per day per square foot (1,000 gpd/ft2). (A) Settling floor. The minimum slope of the settling floor shall be one vertical to twelve horizontal (1:12) for conventional settling tanks and one vertical to one hundred ninety-two horizontal (1:192) for suction style settling tanks. (B) Sludge hopper. The minimum slope of the sludge hopper side walls shall be one and seven tenths vertical to one horizontal (1.7:1) (i.e., sixty degrees (60°) above the horizontal). (C) When used, dual sludge hoppers shall provide a minimum water depth of two feet (2') over the connecting wall that is between hoppers. (5) Protective and Service Facilities. (A) Operator Protection. Safety features shall appropriately include machinery covers, life lines, handrails on all stairways and walkways, and slip resistant surfaces. For additional safety follow the provisions listed in 10 CSR 20-8.140(8). (B) Mechanical Maintenance Access. The design shall provide for convenient and safe access to routine maintenance items such as gear boxes, scum removal mechanism, baffles, weirs, inlet stilling baffle areas, and effluent channels. (C) Electrical Equipment, Fixtures, and Controls. For electrical equipment, fixtures, and controls in enclosed settling basins and scum tanks, where hazardous concentrations of flammable gases or vapors may accumulate, follow the provisions in 10 CSR 20-8.140(6)(B). The fixtures and controls shall be conveniently located and safely accessible for operation and maintenance. AUTHORITY: section 644.026, RSMo 2016.* Original rule filed Aug. 10, 1978, effective March 11, 1979. Amended: Filed June 15, 2018, effective Feb. 28, 2019. *Original authority: 644.026, RSMo 1972, amended 1973, 1987, 1993.