Recommended Design Criteria for Preliminary Treatment Processes...
Screening Devices (Bar Racks and Screens)...
1. Purpose : Coarse bar racks and screens shall be provided to remove coarse materials that would damage pumps and other equipment or interfere with plant operability.

2. Location :

a. Coarse bar racks or bar screens shall precede communitors and mechanically cleaned grit chambers.
b. Fine screens, if used, should follow grit removal and have additional provisions for the removal of floatable greases and oils. Comminuting devices shall not be used ahead of fine screens.
c. Screening devices shall be installed in a building that has adequate heat, ventilation and moisture control. Screening devices shall be separated from other equipment or offices and be provided with separate outside entrances.

3. Design and Installation :

a. Manually Cleaned Screens : Manually cleaned screens shall be provided for bypass of a mechanically cleaned screen, comminutor, or for mechanical treatment plants having an average design capacity of 0.1 MGD or less. The screen shall consist of vertical or inclined bars with clear openings spaced at equal intervals of 1 to 1 3/4 inches across a channel. Manually cleaned screens, except those for emergency use, should be placed on a slope of 30 to 45 degrees from the vertical. Approach velocities, at minimum flow, shall not be less than 1.0 foot per second and not more than 2.0 feet per second at peak hourly flow. Velocities for bar screens shall be calculated from a vertical projection of the screen openings on the cross-sectional area between the invert of the channel and the flow line.
b. Mechanically Cleaned Screens : The mechanically cleaned screen shall consist of vertical or inclined bars with clear openings spaced at equal intervals of 5/8 to 1 3/4 inches across a channel. Mechanically cleaned bar screens should be placed on a slope of 0 to 45 degrees from the vertical. Approach velocities, at minimum flow, shall not be less than 1.25 feet per second and not more than 3.0 feet per second at peak hourly flow. When grit chambers follow bar screens, the minimum velocity should be greater than 1.3 feet per second. Velocities shall be calculated as mentioned above for manual screens.

4. Channels :

a. Entrance channels shall be designed to allow equal flows and uniform distribution to the screens.
b. The invert of the screen channel should be from 3.0 to 6.0 inches below the invert of the incoming sewer.
c. The channel preceding and immediately following the screen shall be shaped and sloped to eliminate the deposition of solids and permit draining of the channel.
d. Multiple parallel channels shall be provided and equipped with the necessary gates to isolate or divert the flows from sections of the channels containing screens or comminuting units.
e. Capacity shall be provided to treat peak hourly flow with one unit out of service.

5. Control Systems :

a. Mechanically cleaned bar screens shall be properly controlled by differential water level and timing devices or continuous operation. Mechanical units which are operated by timing devices shall be provided with auxiliary controls which will start the cleaning operation at a preset, high water elevation and a device to stop the cleaning operation after a predetermined length of time.
b. Flow measurement devices should be located and selected for reliability and accuracy.
c. Automatic controls shall have manual override capability.
d. The maximum allowable head loss through clogged racks and bar screens is generally limited to 2.5 feet. Generally, the absolute minimum head loss allowance through a manually cleaned bar screen is 6 inches, assuming frequent attention by operating personnel. Assurance should be provided that the head loss through the screening unit at peak hourly flows will not surcharge the incoming sewer.

6. Safety and Servicing :

a. The drive mechanism of a mechanically cleaned screen shall be enclosed.
b. A positive power lockout device for each mechanical device shall be provided.
c. Screen channels shall be provided with guard railings and/or deck gratings where necessary. Temporary access should be considered to facilitate maintenance and repair or removal.
d. Manually cleaned screening facilities shall include an accessible platform from which the operator may rake screenings easily and safely. Suitable drainage facilities back to the raw wastewater wet well shall be provided for both the platform and the storage area.
e. Screening areas shall be provided with adequate stairway access, lighting, ventilation, and a convenient means for removing the screenings.
f. Electrical fixtures and controls in screen areas shall comply with the National Electrical Code, Class 1, Groups C and D, Division 1 locations.
g. Treatment plant operators should be supplied with a complete set of operational instructions, including maintenance schedules, tools and spare parts as may be necessary.
h. Due consideration shall be given to the selection of materials because of corrosive conditions present.

7. Handling and Disposal :

a. Screening receptacles shall be designed to contain a minimum of one day's screenings.
b. No screenings shall be returned to the wastewater flow.
c. Facilities shall provide for removal, handling, storage, and disposal of screenings in a sanitary manner.
d. Screenings shall be disposed of in an approved sanitary landfill or by incineration.
e. Pressurized wash water facilities shall be provided to facilitate cleaning. Potable water lines may be used but shall be protected with backflow prevention devices or vacuum breakers. If wash water is non-potable, all outlets shall be permanently posted to indicate water is not safe.
Comminuting Devices...
1. General :

a. Provisions for location shall be in accordance with those for screening devices.
b. Comminutors shall be provided in plants that do not have primary clarifiers, fine screens, or mechanically cleaned bar screens.
c. Provisions for safety and servicing shall be in accordance with those for screening devices.

2. Design Considerations :

a. Comminutors should be located downstream of grit removal equipment. If no grit removal equipment is provided; then the comminutor should be protected with at least a 6.0-inch deep gravel trap.
b. If a comminutor is to be used, the manufacturers should be consulted to select the unit size on the basis of the largest particle that may be allowed to pass. Generally, comminuting devices are installed in the wastewater flow to shred the material retained in the flow to a size of 1/4 inch to 3/4 inch.
c. Comminution or screening capacity shall be adequate to handle peak hourly flows with the largest comminutor out of service.
d. Channels in which comminutors are installed shall be provided with an emergency bypass to a bar screen. Flow exceeding the operating capacity of the comminutor(s) shall be automatically diverted to the emergency bypass. The channels shall be parallel with provision for slide gates or similar devices to permit isolating and draining of the channels.
e. The comminutor shall run continuously and provision for another comminutor to be ready for service is desirable.
Grit Removal Devices...
1. General :

a. Grit chambers should be provided for all mechanical wastewater treatment plants and some oxidation ditches. Mechanical plants receiving wastewater from combined sewers or from collection systems receiving substantial amounts of wastes from garbage disposals and/or grit shall be provided with grit removal devices. Oxidation ditches receiving wastewater from combined sewers or from collection systems receiving substantial amounts of grit shall be provided with grid removal devices.
b. Mechanical grit removal devices will be required where substantial amounts of grit to plant processes or equipment will interfere with the operation and maintenance; cause deterioration and subsequent replacement of equipment; or will substantially increase the need for additional storage capacity in other treatment units where grit is likely to accumulate.
c. If a treatment facility, is designed without grit removal devices; the design shall include provision for future installation. This condition may be required on a case-by-case basis for oxidation ditches.
d. A minimum of one mechanically cleaned unit and a bypass pipe or channel shall be provided for plants serving separate sewers and receiving flows between 0.5 and 1.0 MGD. Plants smaller than 0.5 MGD may have a manually cleaned unit with a bypass. Plants larger than 1.0 MGD or serving combined sewers, shall have two mechanically cleaned grit removal units designed for peak hourly flow with the capability to isolate each one.
e. Grit removal facilities other than channel- type are acceptable if provided with adequate and flexible controls for agitation and/or air supply devices and with grit removal equipment.

2. Location :

a. Grit chambers may be located outdoors with proper protection from freezing, but all grit conveying, washing and handling facilities shall be installed in a building that has adequate heat, ventilation and moisture control. Ventilation, if continuous, should provide at least 12 complete air changes per hour; if intermittent, at least 30 complete air changes per hour.
b. Grit removal devices should be located after coarse bar racks or coarse bar screens, but ahead of comminutors, pumps and other treatment units. Where grit chambers are mechanized, it may be desirable to locate the bar screens and/or comminuting devices ahead of grit chambers to reduce the effect of rags and other gross particles. However, whether the screening and/or comminuting devices are located ahead of or after the grit chamber, it shall be readily accessible for inspection, maintenance and handling of materials.

3. Design Considerations :

a. Inlet and Outlet : Turbulence should be minimized by design considerations for transition sections, such as approach channels, and inlet and outlet devices. The inlet and outlet shall be designed to avoid short circuiting. In some cases, the installation of longitudinal and traverse baffles improve grit removal.
b. Velocity and Detention : ( 1 ) Channel-type grit chambers shall be designed to provide controlled velocities within the range of 0.75 and 1.25 feet per second. The velocity should be maintained as close as possible to 1.0 feet per second for all anticipated volumes of flow. ( 2 ) Detention time of a channel-type grit chamber shall be based on the size and specific weight of the particle to be removed. The chamber must be designed with a sufficient detention period to allow a particle of 0.21 mm in size to settle from the wastewater surface to the bottom before the flow leaves the chamber. A particle of this size having a specific gravity of 2.65 will settle at a rate of about 4.0 feet per minute in wastewater at a temperature of 15.5 degrees C. In terms of overflow rates, this is slightly less than 43,000 gpd per square foot. Overflow rates may govern the basis of design whether the chamber is round or rectangular. ( 3 ) For aerated or vortex type grit chambers, the minimum detention time shall be between 1 and 3 minutes at peak hourly flow. Generally, a detention time of 3 minutes is necessary for good removal. ( 4 ) All grit removal facilities should be provided with adequate automatic control devices to regulate detention time, agitation, and air supply. The units shall have manual override capability.
c. Drains : Provisions shall be made for isolating and dewatering each unit.
d. Capacity : Since grit storage capacity is essential but cannot be easily determined due to the extreme variations in grit quantities of different collection systems, the following typical design values are suggested :

Combined systems - 10 to 30 cu.ft./mil gal.
Separate systems - 2 to 10 cu.ft./mil gal.

In general, 1.0 to 4.0 cubic feet of grit per million gallons of domestic wastewater flow from well constructed sewers can be expected. A generous safety factor is recommended in calculations involving the actual storage, handling, or disposal of grit.
e. Aerated Type Grit Chamber : ( 1 ) The air supply must be controllable and capable of varying from 10 to 40 cfm / 1,000 cubic feet. The minimum air requirement is three cubic feet per minute per foot of chamber length when the total depth of the basin is approximately 10 to 12 feet. Higher air rates are required for wider and deeper installations. ( 2 ) Chambers should have a depth to width ratio of 2 to 1 or larger but not less than 1.5 to 1. Length to width ratios of 3:1 to 5:1 are desirable. ( 3 ) Grit shall be directed to a grit hopper with steep sloped sides for removal in manually or mechanically cleaned grit basins. ( 4 ) The location of the air diffusion tubes should be approximately 1.5 to 2.0 feet from the bottom of the chamber and along its side, parallel to the wastewater flow, provided that the depth of basin is approximately 10 to 12 feet. ( 5 ) Air diffusers shall be removable without taking the basin out of service. Swing type diffusers are most desirable.

4. Safety and Servicing :

a. Safe access shall be provided to the grit chambers and, where mechanical equipment is involved, all moving parts shall be enclosed.
b. Adequate stairway access with handrails and landings or approved type elevator or manlift to above-or- below-grade facilities shall be provided. Due consideration shall be given to the selection of materials because of corrosive conditions present.
c. Consideration should be given to locating drain valves, weir plates and stop gates for ease of operation and safety.
d. Treatment plant operators should be supplied with a complete set of operational instructions, including maintenance schedules, tools and spare parts as may be necessary.
e. Electrical equipment and controls in enclosed grit removal areas where hazardous gases may accumulate shall meet the requirements of the National Electrical Code for Class 1, Groups C and D, Division 1 locations.

5. Grit Washing, Handling, and Disposal :

a. The need for grit washing to separate organic and inorganic materials should be determined by the method of final grit disposal.
b. Grit removal facilities located in pits 6 feet or deeper and plants larger than 0.5 MGD shall be provided with mechanical or air lift systems to hoist or transport grit to ground level. Impervious, non-slip, working surfaces with drains shall be provided for grit handling areas.
c. Grit conveying and transporting equipment shall be provided with protection against freezing and loss of material. Consideration should be given in large plants for loading facilities to discharge grit directly by mechanical means to a transport vehicle.
d. An adequate supply of water under pressure shall be provided for cleanup. If the source is a potable water line, it shall be protected with backflow prevention devices or vacuum breakers. Suitable drainage facilities back to the raw wastewater wet well shall be provided for cleanup.
e. Grit shall be disposed of in an approved sanitary landfill or a satisfactory method approved by this Department.
Pre-Aeration and Flocculation...
General :

a. Pre-aeration and flocculation of wastewater to reduce septicity, high organic strength or suspended solids may be required in special cases.
b. Generally, it is recommended that pre- aeration be combined with aerated grit chambers. c. The units should be designed so that removal from service will not interfere with normal operation of the remainder of the plant.
Deviations from Design Criteria...
The Department may consider and allow deviations where adequate documentation is provided to prove the need for such deviation.