Section 500. Screening

Article 4. Preliminary Processes

A. Conventional preliminary treatment shall include adequate screening to remove solids and debris that could interfere with the performance of downstream unit operations. The process design shall address the means of proper waste management for screenings.

B. Requirements. Protection for pumps and other equipment shall be provided by installing large openings, one inch or more, coarse screens, or bar racks, and smaller openings screens. All screen equipment and facilities shall be readily accessible for maintenance. Small openings, fine screening, or comminution should follow grit removal, which should be preceded by coarse screening. Screen locations are to be provided as follows:

1. Manually cleaned screens shall be provided at all sewage treatment works unless adequate redundant mechanical screens are provided and manually cleaned screens shall be located in open areas with easy access. Mechanical screens that can be manually cleaned upon mechanical failure may be used to meet this requirement.

2. Manually cleaned screens located in deep pits shall be provided with stairway access, adequate lighting and ventilation, and convenient and adequate means for removing screenings.

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

C. Design. Clear openings between the bars of coarse screens should be from one to 1-3/4 inches. Other size openings will be considered on a case-by-case basis. Coarse screen design shall provide for installation such that the screening equipment can be conveniently and safely accessed for maintenance and management of screenings.

1. Where a single mechanically cleaned screen that cannot be manually cleaned upon mechanical failure is used, an auxiliary manually cleaned screen shall be provided.

2. Where two or more mechanically cleaned screens are used, the design shall provide for taking any unit out of service without sacrificing the capability to handle the peak design flow.

3. Manually cleaned screens, except those for emergency use, shall be placed on a slope of 30 to 60 degrees with the horizontal.

4. All mechanical units that are operated by timing devices shall be provided with auxiliary controls which will set the cleaning mechanism in operation at predetermined high water levels.

5. The design or electrical fixtures and controls in enclosed places where gas may accumulate will be evaluated in accordance with the National Electrical Code specifications for hazardous conditions.

6. The use of fine mesh static or mechanical screens as an adjunct to or in lieu of sedimentation will be considered on a case-by-case basis.

D. Flow control. At the normal operating flow conditions, approach velocities should be no less than 1.25 feet per second, to prevent settling, and no greater than 3.0 feet per second, to prevent forcing materials through the openings.

1. The approach flow velocity 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.

2. The screen channel invert shall be three to six inches below the invert of the incoming sewers. To prevent jetting action, the length and construction of the screen channel shall be adequate to reestablish hydraulic flow pattern following the drop in elevation.

3. Multiple channels, where provided, shall be equipped with the necessary gates to isolate flow from any one screening unit operation. Provisions shall also be made to facilitate dewatering each unit. The channel preceding and following the screen shall be shaped to eliminate settling and accumulation of solids. Fillets may be necessary.

E. Screening management. Properly sized facilities shall be provided for removal, storage, and disposal of screenings as required by the approved operation and maintenance manual or sludge management plan. Manually cleaned screening facilities shall include an accessible platform, in accordance with VOSH requirements, from which the operator may remove screenings easily and safely. Suitable drainage facilities shall be provided both for the platform and for storage areas, with all drain water returned to the raw or primary influent flow.

F. Comminution. Comminution should be provided in treatment works that do not provide primary sedimentation, unless other means of protecting downstream processes and equipment are provided. The term "comminutors" shall be understood to also include barminutors, or other shredding/grinding equipment.

1. Comminutors should be located downstream of any grit removal equipment. Areas containing comminution devices shall be provided with stairway access, adequate lighting and ventilation in accordance with VOSH requirements and convenient and adequate means for maintenance and device removal. Comminutors installed in a building where other equipment or offices are located should be accessible only through a separate outside entrance. Comminutor capacity shall be adequate to handle expected peak flows.

2. A bypass channel with appropriate screening shall be provided. Gates to isolate flow from the comminutor channel shall be installed. Each comminutor not preceded by grit removal shall be protected by a screen, trap, or other means to remove potentially harmful gravel.

3. Electrical equipment in comminutor chambers is to be designed in accordance with the applicable requirements of the National Electrical Code. Protection against accumulation of hazardous gases and accidental submergence shall be provided as required by state and federal regulations.

Historical Notes

Former 12VAC5-581-560 derived from Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-500, Virginia Register Volume 20, Issue 09, eff. February 12, 2004.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.