(a) Site selection. In the selection of a site for
a lift station, consideration shall be given to accessibility and
potential nuisance aspects. The station shall be protected from the
100-year flood and shall be accessible during a 25-year flood. All
lift stations shall be intruder-resistant with a controlled access.
Lift stations should be located as remotely as possible from populated
areas.
(b) Design.
(1) Small lift stations. Lift stations designed for
a discharge capacity of less than 100 gallons per minute will be reviewed
on a case-by-case basis by the commission and shall be used only for
institutional use or other locations where it is necessary to pump
the sewage from a single building, school, or other measurable source
establishment into the sanitary sewer lines. If the location of the
discharge does not provide a positive head due to elevation, then
a positive pressure control valve shall be provided. Ejectors may
be used for this type of lift station. Whenever a lift station handles
waste from two or more residential housing units, or from any public
establishment, standby pumps shall be provided. In the case of ejectors
or eductors, two air compressors shall be provided. Grinder pumps
should be used for all small installations.
(2) Dry well sump pump. The following design considerations
shall be addressed in providing dry well sump pumps.
(A) Two separate sump pumps should be provided for
removal of leakage or water from the dry well floor.
(B) The discharge pipe level from the sump pumps shall
be above the maximum liquid level of the wet well. A check valve should
be installed on the discharge side of each sump pump.
(C) All floor and walkway surfaces shall have an adequate
slope to a point of drainage with sufficient measures taken to maximize
traction and safety.
(D) Motors to drive sump pumps shall be located above
the height of the maximum liquid level in the wet well. As an alternate,
sump pumps may be of the submersible type.
(3) Pump controls. All lift stations shall have automatically
operated pump control mechanisms. Pump control mechanisms shall be
located so that they will not be affected by flow currents in the
wet well. Provisions shall be made to prevent grease and other floating
materials and rags in the wet well from interfering with the operation
of the controls. When a float tube is located in the dry well, its
height shall be such as to prevent overflow of the sewage into the
dry well. Pump control mechanisms which depend on a bubbler in the
wet well shall be equipped with a backup air supply system. All connections
to level controls in the wet well shall be accessible at all times.
The circuit breakers, indicator lights, pump control switches, and
other electrical equipment should be located on a control panel at
least three feet above ground surface elevation. If controls are located
in a dry well, the dry well shall be protected from flooding.
(4) Wet wells.
(A) Wet wells and dry wells, including their superstructure,
shall be separated by at least a watertight and gastight wall with
separate lockable entrances provided to each. Equipment requiring
regular or routine inspection and maintenance shall not be located
in the wet well, unless the maintenance can be accomplished without
entering the wet well.
(B) Based on design flow, wet well capacity should
provide a pump cycle time of not less than six minutes for those lift
stations using submersible pumps and not less than 10 minutes for
other nonsubmersible pump lift stations.
(C) All influent gravity lines into a wet well shall
be located where the invert is above the "off" setting liquid level
of the pumps, and preferably should be located above the lead pump
"on" setting.
(5) Stairways. Stairways with non-slip steps shall
be provided in all underground dry wells. Removable ladders may be
provided in small stations where it is impractical to install stairways.
(6) Ventilation. Ventilation shall be provided for
lift stations, including both wet and dry wells.
(A) Passive ventilation such as gooseneck type or turbine
ventilators designed to prevent possible entry of insects or birds
shall be provided in all wet wells if mechanical ventilation is not
provided. All mechanical and electrical equipment in wet wells should
be explosion-proof and spark-proof construction if mechanical ventilation
is not provided.
(B) Mechanical ventilation shall be provided for all
dry wells below the ground surface. The ventilation equipment shall
have a minimum capacity of six air changes per hour under continuous
operations. At least a capacity of 30 air changes per hour shall be
required where the operation is intermittent. All intermittently operated
venting equipment shall be interconnected with the stations lighting
system.
(7) Wet well slopes. The bottom of wet wells shall
have a minimum slope of 10% to the pump intakes and shall have a smooth
finish. There shall be no projections in the wet well which will allow
deposition of solids under ordinary operating conditions. Antivortex
baffling should be considered for the pump suctions in all large sewage
pumping stations (greater than five million gallons per day (mgd)
firm pumping capacity).
(8) Hoisting equipment. Hoisting equipment or access
by hoisting equipment for the removal of pumps, motors, valves, etc.,
shall be incorporated in the station design.
(9) Dry wells and valve vault drains. Drains from dry
wells or valve vaults to the wet well shall be equipped with suitable
devices to prevent entry of potentially hazardous gases.
(c) Pumps.
(1) General. All raw sewage pumps shall be of a non-clog
design, capable of passing 2 1/2 inch diameter spheres, and shall
have no less than three-inch diameter suction and discharge openings.
Inspection and cleanout plates, located both on the suction and discharge
sides of each pumping unit, are suggested for all nonsubmersible pumps
so as to facilitate locating and removing blockage-causing materials.
Where such openings are not provided on the pumps, a hand hole in
the first fitting connected to the suction of each pump shall be provided.
All pumps shall be securely supported so as to prevent movement during
operation. For submersible pumps, rail-type pump support systems incorporating
manufacturer-approved mechanisms designed to allow the operator to
remove and replace any single pump without first entering or dewatering
the wet well should be provided.
(2) Lift station pumping capacity. The firm pumping
capacity of all lift stations shall be such that the expected peak
flow can be pumped to its desired destination. Firm pumping capacity
is defined as total station maximum pumping capacity with the largest
pumping unit out of service.
(3) Variable capacity pumps. Lift stations or transfer
pumping facilities at a wastewater treatment plant or those discharging
directly to the treatment plant where the plant's permitted daily
average flow is equal to or greater than 100,000 gallons per day shall
be provided with three or more pumps or with duplex automatically
controlled variable capacity pumps or other automatic flow control
devices. The pumps or other devices shall be adjusted for actual flow
conditions and controlled to operate so as to minimize surges in the
treatment units. No single pumping unit shall have a capacity greater
than the design peak flow of the wastewater treatment plant unless
flow splitting/equalization is provided.
(4) Pump head calculations. The engineering design
report accompanying the plans shall include system curves, pump curves,
and head calculations. Calculations and pump curves at both minimum
(all pumps off) and maximum (last normal operating pump on) static
heads and for a C value of both 100 and 140 must be provided for each
pump and for the combination of pumps (modified pump curves). Where
a suction lift is required, the report shall include a calculation
of the available net positive suction head (NPSH) and a comparison
of that value to the required NPSH for the pump as furnished by the
pump manufacturer.
(5) Self-priming pumps. Only self-priming pumps or
pumps with acceptable priming systems, as demonstrated by a reliable
record of satisfactory operation, shall be used where the suction
head is negative. All self-priming pumps shall include a means for
venting the air back to the wet well when the pump is priming.
(6) Pump positioning. All raw sewage pumps, other than
submersible pumps without "suction" piping and self-priming units
capable of satisfactory operation under any negative suction heads
anticipated for the lift station under consideration, shall be positioned
such that the pumps always experience, during their normal on-off
cycling, a positive static suction head.
(7) Grinder pumps. See §317.2(d) of this title
(relating to Sewage Collection System).
(d) Piping.
(1) Pump suctions. Each pump shall have a separate
suction pipe. Cavitation may be avoided by using eccentric reducers
in lieu of typical reducers in order to prevent air pockets from forming
in the suction line.
Cont'd... |