(a) Flow equalization. - (i) Storage requirements. Where mechanical plants experience large diurnal variations in flow rate that will cause mechanical, hydraulic, or biological process upsets, flow equalization shall be provided.
- (ii) Location. Pretreatment facilities, such as bar screens, comminutors and grit chambers, and where possible, primary clarifiers should be located ahead of the equalization basin.
- (iii) Drainage and cleaning. Provisions shall be made to isolate, drain and clean the basin(s).
- (iv) Aeration and mixing. Aeration shall be sufficient to maintain a minimum of 2.0 mg/L of dissolved oxygen in the basin at all times. Air supply rates shall be a minimum of 10 cfm/ 1,000 cubic feet (10 m3/min/1000 m3) of volume for primary treated wastewater and 20 cfm/1,000 cubic feet (20 m3/min/1000 m3) of volume for raw or screened waste water.
- (v) Controls. Controls shall be provided to control the flow rate from the flow equalization basin. Flow measurement devices shall be provided.
(b) Screens. - (i) Location. Coarse screens shall be the first unit in the treatment process. Screens shall be housed. The housing shall be heated and ventilated. Access shall be separated from other enclosed spaces. Housing shall be designed for hazardous location (National Electrical Code, Class 1, Groups C and D, Division 1 locations).
- (ii) Capacity. The screen capacity shall be capable of handling the maximum anticipated peak hourly flow including inflow and infiltration.
- (iii) Types.
- (A) Mechanically cleaned. Bar screens shall be mechanically cleaned if the removal of the daily accumulation of screenings results in surging of the flow. Manually cleaned screens shall be provided in parallel channels to permit removal of the mechanically cleaned screen from service. Bars shall be between 45° and 90° measured from the horizontal.
- (B) Manually cleaned. Manually cleaned bar screens shall be used for bypass of a mechanically cleaned screen or for treatment installations having an average design capacity of less than 100,000 gpd (378 m3/day). Bars shall be between 30° to 45° from the vertical.
- (iv) Bar spacing. Clear spacing on mechanically cleaned bar screens shall range from 1/2 inch to 1 3/4 inches (1.27 cm to 4.45 cm). Manually cleaned screens shall have a range from one to 1 3/4 inches (2.54 cm to 4.45 cm) clear spacing. Coarse screens may have spacing greater than 1 3/4 inches (4.45 cm).
- (v) Velocities. Maximum approach velocity at average flows for a mechanically cleaned screen shall be 3.0 fps (0.91 mps). Maximum velocity for a manually cleaned bar screen shall be 1.5 fps (0.46 mps). Minimum velocities shall be 1.25 fps (0.38 mps).
- (vi) Channel. Channels shall be designed to eliminate deposition and permit draining. The channel shall contain a rock trap ahead of mechanically cleaned screens. Multiple channels shall be designed to allow uniform and equal flow to the screens. Slide gates shall be provided to permit isolating sections of channel containing screens.
- (vii) Controls. Cleaning operation shall be controlled by one or several of the following methods.
- (A) Timers. A timer to start the cleaning operation, and a device to stop the cleaning operation after one cycle.
- (B) Differential head. Cleaning device starts and stops on differential head across screen.
- (C) High level switch. Cleaning device starts on high level and runs for predetermined length of time.
All screens shall have manual override capability. All controls shall be suitable for use in hazardous location (National Electrical Code, Class 1, Groups C and D, Division 1 locations).
- (viii) Handling. Screenings receptacles shall be designed to contain a minimum of one day's screenings. Manually cleaned bar screens shall include an easily accessible and safe working platform. All handling areas should be well drained.
- (ix) Disposal. Screenings shall be disposed of in a manner approved by the Department of Environmental Quality, Solid Waste Management section. Grinding of screenings and return to the wastewater flow is not acceptable.
(c) Comminutors. - (i) Location. When used, comminutors shall be located downstream of a coarse screen. Where grit removal is provided, comminutors shall be located downstream.
- (ii) Capacity. Comminution or screening capacity shall be adequate with the largest comminutor out of service.
- (iii) Number of units. Wherever comminutors are used, a bypass, manually cleaned bar screen shall be installed.
- (iv) Channel. Provide stop plates or similar devices to permit isolating a comminutor for maintenance. Provide drainage and washdown facilities. Where grit removal is not provided upstream, provide a gravel trap upstream of each comminutor.
- (v) Bypass. An emergency bypass with a manually cleaned bar screen shall be provided. All flow exceeding the operating capacity of the comminutor(s) shall be automatically directed to the emergency bypass.
- (vi) Controls. The comminutor shall run continuously. All electrical controls shall be NEC Class 1, Groups C and D, Division 1 rated.
(d) Grit removal and disposal. - (i) Where required. Grit removal shall be provided either by providing for its accumulation in other process units or by removal in a specially designed basin. Where accumulation is provided in other process units, duplicate units shall be provided to permit removal of grit.
- (ii) Location. Grit removal shall be placed after bar screens or racks, but before comminutors and other treatment units. Where grit removal facilities can be located at grade, they shall be upstream of raw sewage pumping stations. Grit basins may be located outdoors with proper precautions against freezing, but all grit conveying, washing and handling facilities shall be located indoors.
- (iii) Capacity. Grit removal devices shall be designed to effectively remove grit at the peak instantaneous flow rate. The grit handling capacity shall be a minimum of fifteen (15) cubic feet per million gallons (1.12 m3/1,000,000 m3).
- (iv) Number of units. A minimum of one mechanically cleaned unit and a bypass pipe or channel shall be provided for plants serving separate sewers. Five hundred thousand gallons per day (500,000 gpd) (1892.7 m3/d) plants or smaller may have a manually cleaned unit and bypass. Plants larger than 1.0 mgd (3784 m3/d), shall have two mechanically cleaned units with capability to isolate each one.
- (v) Type.
- (A) Aerated.
- (I) Air requirements. Air supply must be controllable and capable of varying from 10 to 40 cfm/1,000 cubic feet (10 to 40 m3/m/1,000 m3) of basin. Air diffusers shall be located above the tank bottom and positioned for adequate mixing.
- (II) Equipment requirements. The tank shall be sized for a three (3) minute retention time at peak flows. Grit shall be collected to a hopper for removal by sixty (60) or greater sloped sides or mechanical equipment. The inlet and outlet shall be designed to avoid short-circuiting. Air diffusers shall be removable without taking the basin out of service.
- (B) Gravity chamber. Horizontal channel grit basins shall have an outlet control weir and specially shaped channel to maintain velocities from 0.8 to 1.3 fps (0.24 to 0.4 m/s) over the anticipated range of flows. Square basins shall be designed for an overflow rate of 30,000 gpd/sq ft (1220 m3/m2/d) at the peak instantaneous flow rate.
- (vii) Drains. Each unit in the grit facility shall be capable of being dewatered.
- (viii) Grit disposal. Grit disposal methods shall be approved by the Department of Environmental Quality, Solid Waste Management Office.
020-11 Wyo. Code R. § 11-12