020-11 Wyo. Code R. §§ 11-12 - Pretreatment
(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.
(vi) Method of grit removal. Grit removal
facilities located in pits six (6) feet (1.8 m) or deeper and for plants larger
than 500,000 gpd (1892.7 m3/d) shall be provided
with mechanical equipment for moving grit to ground level.
Plants having an average design capacity less than 100,000 gpd (378 m3/d) may be provided with manually cleaned grit basins.
(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.
Notes
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