Section 110.16. Screening devices.

 

(1)   General design considerations.

(a) Applicability. All wastewater treatment plants shall be provided with protection for pumps and other equipment by installing coarse screens, bar racks, mechanically cleaned bar racks or comminutors.

(b) Location.

1. Screening devices installed in a building where other equipment or offices are located shall be separated from the rest of the building and provided with separate outside entrances.

2. Screening devices shall be provided with convenient access.

3. Screening devices may not be located such that changes in backwater elevations will interfere with the accuracy of upstream flow measuring equipment.

(c) Ventilation. Screening areas shall be ventilated.

(d) Channels.

1. The channel preceding and following the screen shall be shaped to minimize settling of solids. Fillets shall be installed as necessary.

2. The screen channel invert must be at least 8 centimeters (3 inches) below the invert of the incoming sewer.

3. Where multiple screening units are installed the channels shall be equipped with the necessary gates to direct flow from any one screening unit. Methods for dewatering each channel shall be provided.

4. Entrance channels shall be designed to distribute flow uniformly to the screening units.

(e) Handling screenings. Adequate facilities must be provided for removal, handling and storage of screenings in a sanitary manner. Hand-cleaned screening facilities must include an accessible platform from which the operator may rake screenings. Suitable drainage facilities must be provided for both the platform and the storage areas.

(2)   Design requirements for screening devices.

(a) Bar screens.

1. Clear spacing between bars may not be less than 2.5 centimeters (1 inch), nor more than 5 centimeters (2 inches).

2. Bar screens must be placed on a slope of 30 to 45 degrees with the horizontal with the exception of those installed for emergency use.

3. Approach velocities may be no less than 38 centimeters per second (1.25 feet per second) at design average flow conditions to prevent settling, and no greater than 91 centimeters per second (3 feet per second) at maximum design daily flow to prevent forcing material through the openings.

(b) Mechanical screens.

1. Maximum clean spacing between bars may not exceed 5 centimeters (2 inches).

2. Approach velocities may be no less than 38 centimeters per second (1.25 feet per second) at average design flow conditions to prevent settling, and no greater than 91 centimeters per second (3 feet per second) at maximum daily flow to prevent forcing material through the openings.

3. All mechanical units which are operated by timing devices shall be provided with auxiliary controls which will set the cleaning mechanism in operation at a preset high water elevation.

4. Automatic controls shall be supplemented by a manual override. Manual overrides shall be located in view of the equipment.

5. Electrical fixtures and controls in screening areas where hazardous gases may accumulate shall meet the requirements of the national electrical code for class 1, group D, division 1 locations.

(c) Comminutors.

1. Comminutors shall be designed to comminute the maximum design flow rate.

2. Comminutor channels shall be designed to contain the upstream water depth associated with the head loss which occurs at maximum design flow without surcharging the incoming sewer or other treatment processes. The expected head loss shall take into account the effects of clogging during operation.

3. Comminutor channels shall be equipped with drains.

4. A screened bypass channel shall be provided so that the comminutor may be removed from service for maintenance.

5. Bypass channels will not be required where 2 comminutors are installed. Each comminutor shall be capable of comminuting the maximum design flow.

6. Control switches or a disconnecting device for the comminutor shall be located in view of the comminutor.

7. Electrical fixtures and controls in comminutors areas where hazardous gases may accumulate shall meet the requirements of the national electrical code for class 1, group D, division 1 locations.

History: Cr. Register, November, 1974, No. 227 , eff. 12-1-74; r. and recr. Register, February, 1983, No. 326 , eff. 3-1-83.