Statutes, codes, and regulations
Wyoming Administrative Code
Agency 020 - Environmental Quality, Dept. ofSubagency 0011 - Water QualityChapter 11 - DESIGN AND CONSTRUCTION STANDARDS FOR SEWERAGE SYSTEMS, TREATMENT WORKS, DISPOSAL SYSTEMS OR OTHER FACILITIES CAPABLE OF CAUSING OR CONTRIBUTING TO POLLUTION AND MOBILE HOME PARK AND CAMPGROUND SEWERAGE AND PUBLIC WATER SUPPLY DISTRIBUTION SYSTEMS
Part B - MUNICIPAL AND DOMESTIC SEWERAGE SYSTEMS, TREATMENT WORKS, AND DISPOSAL SYSTEM
Section 11-17 - Secondary settling

020-11 Wyo. Code R. § 11-17

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Current through April 27, 2019
Section 11-17 - Secondary settling

(a) Secondary settling. Secondary settling is required after suspended growth and attached growth biological processes such as activated sludge, trickling filters and RBC's.

(b) Configuration. The largest dimension (either diameter or length) of a clarifier shall be eighty (80) feet (24.4 m). Corner sweeps on circular equipment are not acceptable.

(c) Flow distribution. Positive flow splitting shall be provided ahead of multiple sedimentation basins to ensure proportional hydraulic flows and solid loadings to each basin. Flow splitting shall be achieved using positive means such as weirs or valves and meters.

(d) Clarifier inlet and outlet structures.

  • (i) Clarifier inlet structures shall be designed to dissipate the:
    • (A) Inlet kinetic energy.
    • (B) Distribute the flow evenly into the basin.
    • (C) Minimize hydraulic turbulence.
    • (D) Prevent short circuiting.

      Inlet devices that promote flocculation are encouraged.

      The inlet structure for rectangular tanks shall be the full width of the basin, for peripheral feed clarifiers it shall be the entire periphery, and for center feed basins it shall be at least 20 percent of the tank diameter. Baffled scum relief ports shall be provided between the inlet structure and the clarifier.
  • (ii) Inlet conveyance pipe or channels shall be designed to maintain a minimum velocity of 0.5 fps (0.15 mps) at the design flow. Where channels provide less velocity, provide mixing, flushing, or other means of resuspending solids.
  • (iii) Clarifier outlet systems shall be designed to minimize vertical velocities and reduce the effect of density currents at the effluent weir. Weir level shall be adjustable.

(e) Freeboard. The outer walls of settling tanks shall extend at least six (6) inches (0.15 m) above the surrounding ground and provide at least twelve (12) inches (0.3 m) of free board to the water surface. Where settling basin walls are less than four (4) feet (1.22 m) above the surrounding ground, a fence or other debris barrier shall be provided on the wall.

(f) Design parameters.

  • (i) Surface overflow rates.
    • (A) Activated sludge. Settling basins following an activated sludge process shall be designed to both thicken the sludge and clarify the liquid flow entering the tanks. The overflow rate shall not exceed:

      Design Flow Peak Hourly Flow
      Activated Sludge

      gpd/ft2

      600

      m3/m2/d

      24.4

      gpd/ft2

      1,200

      m3/m2/d

      48.8
      Separate Nitrification 400 16.3 800 32.5

    • (B) Attached growth biological reactors. Overflow rates for settling basins following attached growth processes shall not exceed:

      Design Flow Peak Hourly Flow
      gpd/ft2 m3/m2/d gpd/ft2 m3/m2/d
      Trickling Filters and RBC's 800 32.5 1,200 48.8

  • (ii) Solids loadings. Solids loadings for settling basins following an activated sludge process shall not exceed:

    Design Flow Peak Hourly Flow
    lbs/d/ft2 kg/d/m2 lbs/d/ft2 kg/d/m2
    All Activated Sludge Processes 28 136.7 50 244.1
    Separate Nitrification 25 122.1 40 195.3

  • (iii) Side water depth. Settling basins shall be deep enough to provide adequate distance between the sludge blanket and the effluent weirs to avoid disturbance of settled sludge.

    The volume of the settling basin shall provide a minimum detention time of two (2) hours at peak hourly flow rate. The peak hourly flow is the projected maximum flow over a one hour period during the design year. Peak hourly flow shall include all recycle flows entering clarifier.
  • (iv) Weir overflow rates and placement. Weir loading rates shall not exceed the following values:

    Design Flow Peak Hourly Flow
    gpd/ft2 m3/m2/d gpd/ft2 m3/m2/d
    Launder and weir at outer wall Launder and weir at 12,000 489 20,000 815
    3/4 point of radius or less 18,000 733 36,000 1467

    Where double weirs or serpentine type weirs are used, the weir length shall be computed as the length of the centerline of the launder.

(g) Baffles. Baffles shall be located at the water surface and in such a position as to intercept all floating materials (scum) prior to the weirs. Baffles shall extend three (3) inches (7.6 cm) above the weir level and twelve (12) inches (0.3 m) below the water surface. In circular basins, the baffle shall be a minimum of six (6) inches (0.15 m) inside the weir plate. In rectangular basins, the baffle shall extend across the width of the basin and upstream of the effluent weirs.

(h) Basin and equipment access. Walkways and access ways shall be provided to drive units, effluent launders, and manual scum devices.

  • (i) Sludge removal. Sludge collection and withdrawal equipment shall provide complete and continuous removal of settled sludge. Rapid sludge removal pipes shall return sludge to a well at the surface that enables visual observation of flow. Mechanical rakes shall move sludge to a hopper at the floor. The tip speed for circular mechanisms shall not exceed 8 fpm (2.4 m/min) and straight line flight speed shall not exceed 1 fpm (0.3 m/min).

    The return sludge removal pipes shall be at least four (4) inches (10.2 cm) in diameter. The hydraulic differential between the clarifier water level and the return sludge level shall be sufficient to maintain a 3-fps (0.9 mps) velocity in each rapid return sludge withdrawal pipe. Each sludge withdrawal pipe shall be accessible for rodding or backflushing when the settling basin is in operation.

  • (ii) Scum removal. Provide effective baffling and scum collection and removal facilities for all secondary settling basins. Equipment shall include a mechanical, positive scum skimmer.
  • (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 (2) feet (0.61 m). The sludge removal pipe should be flush with hopper bottom, and have a minimum diameter of six (6) inches (0.15 m).
  • (iv) Scum box. Locate scum box outside settling tank and adjacent to the scum collection point. Provide method for mixing contents of scum box, such as air jets or surface wetting using waste sludge. Provide access and washwater for washing the scum box. The scum box shall be located on the side of the tank opposite the prevailing wind direction.

020-11 Wyo. Code R. § 11-17

Amended, Eff. 6/29/2018.