R317-3-6. Settling

6.1. General Considerations

A. Number of Units. Multiple units capable of independent operation shall be provided in all plants where the design rate of flow exceed 1 million gallons per day (3,785 cubic meters per day). Plants where the design rate of flow is less than one (1) million gallons per day (3,785 cubic meters per day), shall include other provisions to assure continuity of treatment.

B. Arrangement. Settling tanks shall be arranged for optimum site utilization, and shall be consistent with the hydraulic head requirements for other ancillary units.

C. Flow Distribution. Effective flow measurement devices and control appurtenances (e.g. valves, gates, splitter, boxes, etc.) should be provided to permit proper proportioning of flow to each unit.

D. Tank Configuration. The selection of tank size and shape, and inlet and outlet type and location shall be based on the site and flow patterns.

6.2. Design Considerations

A. Dimensions.

1. The minimum length of flow from inlet to outlet should not be less than be 10 feet (3 meters) unless special provisions are made to prevent short circuiting. The sidewater depth for primary clarifiers shall be not less than 8 feet (2.4 meters).

2. Clarifiers following an activated sludge process shall have sidewater depths of at least 12 feet (3.7 meters) to provide adequate separation zone between the sludge blanket and the overflow weirs.

3. Clarifiers following fixed film reactors shall have sidewater depth of at least 8 feet (2.4 meters).

B. Surface Loading (Overflow) Rates

1. Primary Settling Tanks

a. Surface loading or overflow rates at the average design rate of flow for primary tanks shall not exceed:

(1) 600 gallons per day per square foot (24 cubic meters per square meter per day) for plants treating at the rate of flow less than 1 million gallons per day (3,785 cubic meter per day), or

(2) 1,000 gallons per day per square foot (41 cubic meters per square meter per day) for plants treating at the rate of flow more than 1 million gallons per day (3,785 cubic meter per day).

b. For primary settling, expected influent BOD₅ removal and surface loading is as shown by the relationship: E = (41.5 - (0.01 x Surface loading at average design Q)) where, E = efficiency, percent, and surface loading less than or equal to 2,000 gallons per day per square foot (82 cubic meters per square meter per day). However, anticipated higher BOD₅ removal than the one predicted using above relationship for sewage or sewage containing appreciable quantities of industrial wastes (or chemical additions to be used), shall be validated by plant performance data.

2. Intermediate Settling Tanks. Surface loading or overflow rates for intermediate settling tanks following fixed film reactor processes shall not exceed 1,000 gallons per day per square foot (41 cubic meters per square meter per day) at the average design rate of flow.

3. Final Settling Tanks

a. Settling tests should be conducted wherever a pilot study of biological treatment is warranted by unusual waste characteristics or treatment requirements.

b. The applicant will conduct pilot testing where proposed loadings go beyond the limits set forth in this section.

c. Surface loading or overflow rates for settling tanks following fixed film processes shall not exceed 800 gallons per day per square foot (33 cubic meters per square meter per day) at the average design rate of flow.

d. Settling tanks following activated sludge processes must be designed to meet thickening as well as solids separation requirements. Surface loading or overflow, and weir overflow rates must be adjusted for the various processes to minimize the problems with sludge loadings, density currents, inlet hydraulic turbulence, and occasional poor sludge settleability. The high rate of recirculation of return sludge from the final settling tanks to the aeration or reaeration tanks requires careful consideration of above factors. The hydraulic design of intermediate and final settling tanks following the activated sludge process shall be based upon the average design rate of flow excluding activated sludge return flow as shown in Table R317-3-6.2(B)(3)(d).

C. Inlet Structures. Inlets should be designed to dissipate the inlet velocity and 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 per second (0.3 meter per second) at the minimum 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.

D. Effluent Overflow Weirs

1. General. Effluent overflow weirs shall be adjustable for leveling.

2. Location. Effluent overflow weirs shall be located to optimize actual hydraulic detention time, and minimize short circuiting.

3. Design Rates. Weir loadings shall not exceed 10,000 gallons per day per lineal foot (124 cubic meters per meter per day) for plants treating the average design rate of flow of one (1) million gallons per day (3,785 cubic meters per day) or less. Higher weir loadings may be used for plants designed for larger average flows, but shall not exceed 15,000 gallons per day per lineal foot (186 cubic meters per meter per day). If pumping is required, weir loadings must be related to pump delivery rates to avoid short circuiting.

4. Weir Troughs. Weir troughs shall be designed to prevent submergence at the maximum design rate of flow (peak daily flow), and to maintain a velocity of at least one foot per second (0.3 meter per second) at one-half of the average design rate of flow. Submergence may be permitted at the maximum design rate of flow (peak daily flow) with one unit out of service.

E. Submerged Surfaces. The tops of troughs, beams, and similar submerged construction elements shall have a minimum slope of 1.4 vertical to 1 horizontal; the underside of such elements should have a slope of 1 to 1 to prevent the accumulation of scum and solids.

F. Unit Dewatering. The bypass design shall provide for redistribution of the plant flow to the remaining units in operation.

G. Freeboard. Walls of settling tanks shall extend at least 6 inches (15 centimeters) above the surrounding ground surface and shall provide not less than 12 inches (30 centimeters) freeboard. Additional freeboard or the use of wind screens should be provided where larger settling tanks are subject to high velocity wind currents that would cause tank surface waves and inhibit effective scum removal.

6.3. Sludge and Scum Removal

A. Scum Removal. Effective scum collection and removal facilities, including baffling, shall be provided for primary, intermediate and secondary 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.

B. Sludge Removal. Sludge collection and withdrawal facilities shall be designed to assure rapid removal of the sludge. Suction withdrawal of sludge from the tank floor should be provided for activated sludge plants designed for reduction of the nitrogenous oxygen demand.

1. Sludge Hopper. When scrapers are used to move sludge into a discharge hopper, the minimum slope of the side walls shall be 1.7 vertical to 1 horizontal. 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 (0.6 meter). Deep sludge hoppers for sludge thickening are not acceptable.

2. Sludge Removal Piping. Each hopper shall have an individually valved sludge withdrawal line at least six inches (15 centimeters) in diameter. The static head available for withdrawal of sludge shall be 30 inches (76 centimeters) or greater, as necessary to maintain a three foot per second (0.91 meter per second) 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 for activated sludge secondary clarifiers except for oxidation ditch clarifiers. Piping shall also be provided to return waste sludge to primary clarifiers.

3. Sludge Removal Control. Sludge wells shall be provided with telescoping valves or other equipment for viewing, sampling and controlling the rate of sludge withdrawal. The use of 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 must not be used for removal of primary sludges. Sludge pump motor control systems shall include time clocks and valve controls for regulating the duration and sequencing of sludge removal.

6.4. Protective and Service Facilities

A. Operator Protection. All settling tanks shall be equipped to provide safe working conditions for operators. Such features shall include machinery covers, life lines, stairways, walkways, handrails and slip resistant surfaces.

B. Mechanical Maintenance Access. The design shall provide for convenient and safe access to routine maintenance items such as gear boxes, scum removal mechanisms, baffles, weirs, inlet stilling baffle area, sludge and scum pumps, and effluent channels.

C. Electrical Fixtures and Controls. Electrical fixtures and controls in enclosed settling basins shall meet the requirements of the National Electrical Code for Class 1, Group D, Division 1 locations. The fixtures and controls shall be located so as to provide convenient and safe access for operation and maintenance. Walkways, bridge area and area around settling tanks shall be illuminated with area lighting for operating personnel safety.