Current through Register Vol. 41, No. 3, September 23, 2024
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.
Statutory Authority
§ 62.1-44.19 of the Code of Virginia.
