Section R317-3-5 - Screening and Grit Removal5.1. Screening Devices. Coarse bar racks or screens shall be used to protect pumps, comminutors, flow measurement devices and other equipment.5.2. Bar Racks and Screens A. Location 1. Indoor. Screening devices, installed in a building where other equipment or offices are located, shall be accessible only through a separate outside entrance to protect the operating personnel and the equipment from damage and nuisance caused by gases, odors and potential flooding.2. Outdoors. Screening devices not installed in enclosures or buildings shall be protected from freezing or other adverse environmental conditions.B. Access. Screening areas shall be provided with proper work and safe access and egress, proper and emergency lighting, ventilation, and a convenient and safe means for removing the screenings.C. Design and Installation 1. Bar Spacing. Clear openings between bars should be: a. not more than 1 inch (2.54 centimeters) for manually cleaned screens; andb. less than 5/8 of an inch (1.59 centimeters) for mechanically cleaned screens.2. Bar Slope. Manually cleaned screens, except those for emergency use, should be placed on a slope of 30 to 45 degrees from the horizontal.3. Approach Velocities. At average design flow conditions, approach velocities should be no less than 1.25 feet per second (38 centimeters per second), to prevent settling; and no greater than three (3) feet per second (91 centimeters per second) to prevent forcing material through the openings.4. Channels. Dual channels shall be provided and equipped with the necessary gates to isolate flow from any screening unit. Provisions shall also be made to facilitate dewatering each unit. The channel preceding and following the screen shall be shaped to eliminate stranding and settling of solids. Entrance channels should be designed to provide equal and uniform distribution of flow to the screens.5. Reliability. A minimum of two screens shall be provided. Each screen shall be designed to handle the peak design rate of flow. Where more than two screens are provided, the peak design rate of flow shall be handled with one of the largest units out of service. Where a single mechanical screen handles the peak design rate of flow, then other unit can be a manually cleaned screen.6. Flow Measurement. The types and locations of flow measurement devices should be selected for reliability and accuracy. The effect of changes in backwater elevations, due to intermittent blinding and cleaning of screens, should be considered in the selection of the locations for flow measurement equipment.7. Invert. The screen channel invert should be 3.0 to 6.0 inches (7.6-15.2 centimeters) below the invert of the incoming sewer.D. Safety 1. Railings and Gratings. a. All screening installations shall be equipped with guard rails and deck grating to insure operator safety.b. The manually cleaned bar rack shall be accessible for cleaning insuring operator safety.c. Proper guard rails and enclosures shall be used to protect the operator from moving parts of mechanically operated and cleaned screens. These guard rails and enclosures shall be removable for safe access to maintain and repair mechanically operated and cleaned screens. Catchments shall be provided to prevent dripping of liquids in multi- level installations.2. Equipment Deactivation and Lockout. Each piece of electrical power mechanical equipment shall be equipped with a positive means of deactivating or locking out or isolating from its power source. Such device shall be located in close proximity to the equipment.3. Removal of Screenings. The design shall provide for mechanical conveying or lifting systems for safe transport of screenings from a subgrade installation to a collection point on grade.E. Power Control Systems 1. Timing Devices. All mechanical units which are operated by timing devices shall be provided with auxiliary override controls which will set the cleaning mechanism in operation at a preset high water elevation or water differential across the screen.2. Electrical Fixtures and Controls. Electrical fixtures and controls in screening areas where hazardous gases may accumulate shall meet the requirements of the National Electrical Code for Class I, Group D, Division 1 locations.3. Manual Override. Automatic controls shall be supplemented with a manual override.F. Disposal of Screenings 1. Facilities shall be provided for removal, handling, storage, and disposal of screenings in a sanitary manner. Separate grinding of screenings and return to the sewage flow is unacceptable. Manually cleaned screening facilities should include an accessible platform from which the operator may rake screenings easily and safely. Suitable drainage facilities shall be provided for both the platform and the storage areas.2. Screenings may be landfilled. The ultimate disposal of screenings shall conform to and comply with the requirements for the ultimate disposal of residues or sludge management plan. 5.3. Comminutors A. General. Comminutors may be used in plants, excepting aerated or facultative or total containment lagoons, where mechanically cleaned bar screens are not used.B. Design Considerations 1. Location. Comminutors should be located downstream of bar screen and any grit removal equipment.2. Size. Comminutor capacity shall be adequate to handle the peak design rate of flow.3. Installation. a. A comminutor bypass channel, with manually cleaned bar screen, shall be provided. The use of the bypass channel should be automatic at depths of flow exceeding the design capacity of the comminutor. The bypass channel should be able to pass the peak design rate of flow when the comminutor channel is out of service.b. Each comminutor that is not preceded by grit removal equipment should be protected by a 6-inch (15.2 centimeters) deep easily cleaned gravel trap. PC Maintenance. Gates shall be provided for isolation of comminutor, comminutor channel including bypass channel for draining, repairs and maintenance. Provisions shall be made to facilitate servicing of units in place and removing units from their location for servicing.
5. Electrical Power Controls and Motors. Electrical equipment in comminutor chambers where hazardous gases may accumulate shall meet the requirements of the National Electrical Code for Class 1, Group D, Division 1 locations. Motors in areas not governed by this requirement may need protection against accidental submergence. 5.4. Grit Removal Facilities A. General. Grit removal facilities shall be provided for all mechanical treatment plants. Pumps, comminutors, and other mechanical equipment preceding grit removal, shall be protected from the damaging effects of grit. Storage capacity shall be provided in treatment units where grit is likely to accumulate.B. Location. Grit removal facilities should be located ahead of pumps and comminuting devices. Coarse bar racks should be placed ahead of grit removal facilities.C. Enclosed Facilities 1. Ventilation. Uncontaminated air shall be introduced continuously at a minimum rate of 12 air changes per hour, or intermittently at a minimum rate of 30 air changes per hour. Odor control facilities are recommended.2. Access. Grit removal facilities shall be provided with proper and safe access, and egress from equipment and facilities.3. Electrical Work. All electrical work in enclosed grit removal areas where hazardous gases may accumulate shall meet the requirements of the National Electrical Code for Class 1, Group D, Division 1 locations.D. Outdoor Facilities. Grit removal facilities located outside the buildings shall be protected from freezing, and other adverse environmental conditions.E. Type and Number of Units 1. Number of Units. For plants treating: a. more than 1 million gallons per day rate of flow (3,785 cubic meters per day), two mechanically cleaned grit removal units shall be installed in a parallel configuration. Each grit channel shall be designed to handle the peak design rate of flow.b. less than 1 million gallons per day rate of flow (3,785 cubic meters per day), a single manually cleaned or mechanically cleaned grit chamber with a bypass channel shall be provided.2. Other types. When arrangements other than channel-type of grit removal is considered, equipment for agitation, air supply, grit collection, grit removal, and grit washing shall be provided with controls for handling variations in rates of flow, and providing operating flexibility.F. Design Factors 1. General. The designed effectiveness of a grit removal system shall be commensurate with the requirements of the subsequent process units.2. Inlet Configuration. Inlet turbulence shall be minimized. The inlet flow direction must be parallel to the induced roll direction within aerated grit chambers.3. Velocity and Detention Time. a. Horizontal Channel-type Grit Chambers. (1) Velocity of flow through a channel-type chamber shall be controlled such that it is not less than one foot per second (30 centimeters per second) during normal variations in flow.(2) The detention time shall be based on the size of particle to be removed but not less than 20 seconds at the maximum design flow. Velocity and detention time in the channel shall be regulated by installation of control devices such as proportional flow, Sutro weirs, etc.b. Aerated grit chambers. (1) The velocity of flow through an aerated grit chamber shall not be less than 1 foot per second (30 centimeters per second) during normal variations in flow, in the direction of induced roll.(2) A minimum detention time of two to five minutes at the maximum design flow shall be provided. Rate of aeration shall not be less than 4 cubic feet per minute per lineal foot (1.5 liters per second per meter). Outlet weir shall be provided parallel to the direction of induced roll.c. Square grit chambers. Detention time and overflow rate for square grit chambers shall be based on the size of particles intended to be removed. Overflow rate should not exceed 40,000 gallons per day per square foot of the chamber area (1,600 cubic meters per day per square meter).4. Grit Washing. Grit should be washed before the disposal.5. Drains. Provision shall be made for to adequately bypass, isolate and dewater each grit removal unit for maintenance.6. Water. An adequate supply of service or non-potable plant water under pressure shall be provided for cleanup.G. Grit Handling. 1. Mechanical equipment for hoisting or transporting grit to ground level shall be provided in grit removal facilities located in deep pits. Impervious, non-slip, working surfaces with adequate drainage shall be provided for grit handling areas. Grit transporting facilities shall be provided with protection against freezing and loss of material.2. Grit may be landfilled. The ultimate disposal of grit shall conform to and comply with the requirements for the ultimate disposal of residues or sludge management plan.Utah Admin. Code R317-3-5