(a) Sedimentation. - (i) Number of basins. For plants having an average design capacity greater than 100,000 gpd (378.4 m3/d) and where primary settling is provided, multiple units capable of independent operation shall be provided.
- (ii) Design parameters.
- (A) Performance. Unless full-scale data is available, primary settling shall be assumed to remove one third of the influent BOD and 55 percent of the influent suspended solids. It is unacceptable to return waste activated sludge to the primary clarifier.
- (B) Water depth. The minimum side water depth shall be seven (7) feet (2.1 m).
- (C) Surface overflow rates. Surface overflow rates shall not exceed 1,000 gpd/sq ft (41 m3/m²d) of surface area at the average design flow nor 1,500 gpd/sq ft (61 m3/m²d) of surface area at the maximum day flow rate. Maximum day flow is the highest flow over a 24 hour period that is projected to occur during the design year.
- (D) Weir loading rates. Circular basins (or basins with center inlets) shall be provided with a full periphery weir. Rectangular basins shall be provided with end weirs that provide less than 80,000 gpd/ft (9,920 m3/m d) weir hydraulic loading at peak instantaneous flow rates.
- (iii) Clarifier inlet and outlet.
- (A) General. Clarifier inlet structures shall be designed to achieve the following:
- (I) Dissipate the inlet kinetic energy.
- (II) Distribute the flow evenly into the tank.
- (III) Prevent short circuiting.
Inlet channels or piping shall be designed for minimum velocities of one (1) fps (0.3 mps). Where minimum velocities are less, mixing, flushing or other means of resuspending solids shall be provided.
Circular basins shall be provided with symmetrical baffling to distribute flow equally in all radial directions.
Rectangular basins shall be provided with inlet parts uniformly distributed along the entire end of the basin and shall be provided with baffles.
- (B) Weirs. Weir plates shall be adjustable for leveling and sealed against the effluent channel.
- (C) Baffles. Provide scum baffles at the water surface to intercept all floating materials and scum prior to the weir. Baffles should extend three (3) inches (7.6 cm) above the weir plate elevation and eight (8) inches (20.3 cm) below the water surface.
- (D) Clarifier effluent channel.
- (I) Size. The effluent channel shall be sized to prevent weir submergence at the peak hourly flow.
- (E) Freeboard. The outer walls of sedimentation tanks shall extend at least six (6) inches (0.15 m) above the surrounding ground and shall provide at least twelve (12) inches (0.3 m) of freeboard to the water surface. Where basin walls do not extend four (4) feet (1.2 m) above the surrounding ground, a fence or suitable barrier to prevent debris from entering the basin shall be provided.
- (F) Basin equipment and access. Provide walkways and accessways to collector drive units, effluent launders and manual skimmer. Handrail shall be provided.
(b) Fine screens. - (i) Number of units. A minimum of two (2) units shall be provided. Multiple units shall be capable of independent operation. With the largest unit out of service, the remaining units shall be capable of passing the peak flow rate.
- (ii) Flow distribution. Positive means of flow distribution shall be provided ahead of the screens to ensure even loading and hydraulic flows.
- (iii) Design parameters.
- (A) Performance. In the absence of pilot plant data, the removal efficiency of fine screens shall be assumed to be zero percent removal of BOD5 and 15 percent removal of suspended solids.
- (B) Preliminary treatment requirement. Prior to the fine screens, removal of large debris shall be provided by coarse screens. Comminution shall not be provided ahead of screens.
- (iv) Screenings storage and disposal. Screens with openings of 0.10 inch (2.5 mm) or more shall be disposed of directly to landfill in accordance with the requirements of the Department of Environmental Quality, Solid Waste Management Office. Screens with openings less than 0.10 inch (2.5 mm) shall discharge the screenings (primary sludge) to sludge handling system for organic stabilization.
- (v) Cleaning and maintenance. Provide facilities to permit regular cleaning of screens with a high pressure, hot water or steam system.
- (vi) Controls. For rotating screens, each screen or series of screens shall be provided with an overflow. An alarm shall be provided when overflowing.
(c) Sludge handling. - (i) Sludge removal. Mechanical sludge collection equipment is required for all primary settling basins. The sludge collection rake arms or flights and the drive assembly shall be designed to withstand the maximum anticipated loads and move sludge to the hopper.
- (ii) Scum removal. Provide scum collection and removal facilities for all primary settling basins. Scum shall be removed from the liquid process and not returned.
- (iii) Sludge hopper. The minimum side slope of the hopper shall be 1.7 vertical to 1.0 horizontal. Hopper bottoms shall have a maximum dimension of two feet (0.61 m). The sludge removal pipe shall be flush with the hopper bottom, and have a minimum diameter of six inches (15.2 cm).
- (iv) Scum box. The scum box shall be located outside and immediately adjacent to the scum collection point (beaching plate). The beaching plate shall be located on the opposite side of the basin from the prevailing wind. Provide for mixing the contents of the scum box, such as a mechanical mixer or air diffusion. Provide access and wash water for washing the scum box.
- (v) Controls.
- (B) Primary screen sludge facilities. Where sludge pumping is provided, include a means to shut off the pump when insufficient material is being supplied to the pump suction. The controls for the pump shall be designed to match the pumping rate to quantity of sludge. Where conveyors are used, they shall run continuously and alarm when off.
020-11 Wyo. Code R. § 11-13