Current through Register Vol. 41, No. 3, September 23, 2024
A.
Features. Sewage pump stations should be located as far as practicable from
present or proposed built-up residential areas, and an all-weather road shall
be provided. Stations should have a proper zone of controlled or limited use
surrounding them. Within such zones, residential uses or high density human
activities or activities involving food preparation should be prevented.
Provisions for noise control and odor control, and station architectural design
should conform to site requirements. Sites for stations shall be of sufficient
size for future expansion or addition, if applicable. All mechanical and
electrical equipment which could be damaged or inactivated by contact with or
submergence in water (motors, control equipment, blowers, switch gear,
bearings, etc.) shall be physically located above the 100-year flood/wave
action or otherwise protected against the 100-year flood/wave action damage.
All stations shall be designed to remain fully operational during the 25-year
flood/wave action.
1. Where it may be
necessary to pump raw (untreated) or unsettled sewage prior to grit removal,
the design of the wet well shall receive special attention. The discharge
piping shall be designed to prevent grit settling in the discharge lines when
pumps are not operating.
2. At
least two pumping units shall be provided. Where two units are provided, each
shall be capable of handling flows in excess of the expected maximum flow or a
minimum of 2-1/2 times the average design flow, whichever is greater. Where
three or more units are provided, they shall be designed to fit actual flow
conditions and must be of such capacity that, with any one unit out of service,
the remaining units will have capacity to handle the maximum sewage flow or a
minimum of 2-1/2 times the average design flow, whichever is greater. When the
station is expected to operate at a flow rate less than one-half times the
average design flow for an extended period of time, the design shall address
measures taken to prevent septicity due to long holding times of untreated
sewage in the wet well.
3.
Treatment works pump stations should be designed so that sewage will be
delivered to the treatment works at approximately the same rate it is received
at the pump station. At least two pumping units shall be provided. Treatment
works pump stations are those stations which discharge to sewage treatment
works without dissipation of flow through a gravity collection system. Where
only two pumping units are to be utilized, they shall be variable speeded and
sized so that the pumps deliver from 1/2 to 2-1/2 times the average design flow
or the maximum flow, whichever is greater, except where flow equalization is
utilized in accordance with this chapter. Where constant speed pumps are to be
utilized without equalization, either (i) at least three pumps, each having a
capacity of approximately 1-1/4 times the average design flow, or (ii) two
pumps, each having a capacity of approximately 1-1/4 times the average design
flow, with the third pump having a capacity of 2-1/2 times the average design
flow, shall be provided as needed to transfer the maximum flow. Multiple-speed
pumps in lieu of variable speed pumps may be considered for specific
applications. These criteria for influent flows will not apply to such
treatment works where several days' holding capacity is provided, such as in
stabilization ponds or in aerated lagoons.
4. Pumps handling raw sewage should be
preceded by readily accessible bar racks with clear openings not exceeding
2-1/2 inches, unless pneumatic ejectors are used or special devices are
installed to protect the pumps from clogging or damage. Where the size of the
installation warrants, a mechanically cleaned bar screen with either a grinder
or comminution device is recommended. Where screens are located below ground,
convenient facilities must be provided for handling screenings. For the larger
or deeper stations, duplicate protection units of proper capacity are
preferred. Interceptor or separation basins may be necessary prior to pumps
handling raw sewage.
5. Pumps in
which the solids pass through the impeller(s) shall be capable of passing
spheres of at least three inches in diameter. Pumping equipment having integral
screens for preventing solids from passing through the impeller shall be
capable of passing spheres of at least two inches in diameter. Pumping
equipment preceded by grinding equipment shall be capable of passing the solids
discharged from the grinding mechanism.
6. Pumps shall be so placed that under normal
start conditions they will start with a positive suction head, except as
specified for suction lift pumps. Each pump shall have an individual intake and
suction line. Wet well design should be such as to avoid turbulence near the
intake. Pump suction and discharge piping shall not be less than four inches in
diameter except where design of special equipment allows. The design velocity
in pump piping should not exceed (i) six feet per second in the suction piping,
and (ii) in the discharge piping, eight feet per second. All pumps should be
provided with an air relief line on the pump discharge piping.
7. Control float cages shall be so located as
not to be affected by the flows entering the wet well or by the suction of the
pumps. Float tubes will not be permitted in either the wet or dry well.
Air-operated pneumatic controls are preferred for all sewage pump stations.
Provisions shall be made to automatically alternate the pumps in use (which is
referred to as lead-lag operation) unless adequate operation and maintenance is
to be provided to protect against pump failure.
8. For the purpose of designating liquid
levels for alarm requirements, high liquid level in the wet well is defined as
a level of sewage in the wet well above normal operating levels such that
either:
(i) a backup of sewage in the
incoming sewer may occur, or
(ii)
an overflow may occur, or
(iii)
standby pump(s) may be required to be activated. In the case of a duplex pump
station with limited wet well volume, the alarm design should include
activation at the time of simultaneous operation of both pumps, initiating when
the second alternating pump starts (referred to as the lag pump).
9. Suitable shut-off valves shall
be placed on each suction and each discharge line of each pump for normal pump
isolation. A check valve is to be placed on each discharge line, between the
shut-off valve and the pump. No shut-off valve need be placed on the suction
side of suction lift or submersible pumps. Periodic exercising of valves should
be specified within the routine maintenance programs.
10. System pump stations should have the
provision for installing flow measuring devices when necessary. Consideration
should be given to installation of such devices in system pump stations whose
flow rate can affect the proper operation of the treatment works.
11. Adequate lighting for the entire pump
station shall be provided in accordance with VOSH and other applicable codes
and standards.
12. Pump stations
shall be designed in accordance with the statewide building code and so as to
minimize the adverse effects of vandalism. Pump stations shall be equipped with
a secure external disconnect switch located above grade where
possible.
B. Ventilation
shall be provided in accordance with VOSH requirements and shall comply with
this chapter for enclosed spaces within pump stations during all periods when
the station is manned. Where the pump is permanently mounted below the ground,
mechanical ventilation is required and shall be arranged so as to independently
ventilate the dry well.
1. As a minimum,
ventilation of the wet well shall be accomplished by the provision of a
properly screened vent, with the end either turned downward or provided with a
"mushroom" cap. The vent shall be at least four inches in diameter. If screens
or mechanical equipment, which might require periodic maintenance and
inspection, are located in the wet well, then it shall be mechanically
ventilated at the time of access by maintenance personnel.
2. There shall be no interconnection between
the wet well exhaust flow and the dry well ventilation systems. In pits over 15
feet deep, multiple inlets and outlets are desirable. Dampers shall not be used
on exhaust or fresh air ducts, and fine screens or other obstructions in air
ducts shall be avoided to prevent clogging. In climates where excessive
moisture or low temperature are problems, consideration should be given to
installation of automatic heating and dehumidification equipment.
3. Switches for operation of ventilation
equipment shall be marked and conveniently located above grade and near the
pump station entrance. Consideration should be given also to automatic controls
where intermittent operation is used. The fan drive shall be fabricated from
nonsparking material in accordance with applicable codes and
standards.
4. Where heat buildup
from pump motors may be a problem, consideration should be given to automatic
cooling and ventilation to dissipate motor heat.
5. Ventilation of wet wells in accordance
with VOSH requirements may be either continuous or intermittent. Ventilation,
if continuous, shall provide at least 12 complete air changes per hour; if
intermittent, at least 30 complete air changes per hour. Such ventilation shall
be accomplished by mechanical means.
C. Water supply. There shall be no cross
connection between any potable water supply and a sewage pump station which
under any conditions might cause contamination of the potable water supply. Any
potable water supply brought to the station shall comply with conditions
stipulated in the Virginia Waterworks Regulations (12VAC5-590). Where
conditions do not warrant the installation of an approved reduced pressure zone
backflow prevention device on the water supply line to the pump stations, other
approved devices may be considered on a case-by-case basis.
D. Service. Provisions shall be made to
facilitate removing pumps, motors, and other equipment without interruption of
system service while providing all necessary worker safety features.
1. In accordance with VOSH requirements,
suitable and safe means of access shall be provided to dry wells and wet wells
containing equipment requiring inspection or maintenance. Compliance with all
applicable VOSH and Uniform Statewide Building Code requirements is
recommended. All ladders shall have slip-resistant rungs.
2. If the dry well or wet well floor is more
than 10 feet below the entrance, special consideration shall be given to safety
features such as harness lifts, ladder cages, spiral stairways, or intermediate
landings. Intermediate landings should not exceed 10 foot vertical
intervals.
E. Wet wells.
Proper design of wet wells is essential to effective pump station operation.
1. The wet wells at major pumping stations
and in those located in critical areas should be divided into two sections
properly interconnected to facilitate repairs and cleaning.
2. The wet well size and control settings
shall be designed and operated so as to avoid heat buildup in the pump motor
due to frequent starting and to avoid septic conditions due to excessive
detention time.
3. Provisions shall
be made to prevent solids deposition. Where used, wet well fillets shall have a
minimum slope of one-to-one to the hopper bottom. The horizontal area of the
hopper bottom shall be no greater than necessary for proper installation and
function of the inlet.
