| (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.
(2) Valves. Full closing valves shall be installed
on the discharge piping of each pump and on the suction of all dry
pit pumps. A check valve shall be installed on the discharge side
of each pump, preceding the full closing valve. Check valves should
be of a swing check type with external levers. Rubberball check valves
may be used for grinder pump installations in lieu of the swing check
type. Butterfly valves, tilting disc check valves, or other valves
with a pivoted disc in the flow line are not allowed. The design shall
consider surge effects and provide protection where necessary. Surge
relief shall be contained in the system.
(3) Valve position indicators. Gate valves should be
rising-stem valves. If other than rising-stem gate valves and check
valves with external levers are used, the valves shall include a position
indicator to show their open and closed positions.
(4) Lift station piping. Flanged pipe and fitting or
welded pipe shall be used for exposed piping inside of lift stations.
A flexible or flanged connection shall be installed in the piping
to each pump so that the pump may be removed easily for repairs. Provisions
shall be made in the design to permit flexure where pipes pass through
walls of the station. Piping should normally be sized so that the
maximum suction velocity does not exceed five feet per second and
the maximum discharge velocity does not exceed eight feet per second.
(5) Force main pipe selection. Force mains shall be
a minimum of four inches in diameter, unless justified, as with the
use of grinder pumps. In no case shall the velocity be less than two
feet per second with only the smallest pump operating, unless special
facilities are provided for cleaning the line at specified intervals
or it can be shown that a flushing velocity of five feet per second
or greater will occur one or more times per day. Pipe specified for
force mains shall be of a type having an expected life at least as
long as that of the lift station and shall be suitable for the material
being pumped and the operating pressures to which it will be subjected.
All pipe shall be identified in the technical specifications with
appropriate American Society for Testing and Materials (ASTM), American
National Standards Institute (ANSI), or American Water Works Association
(AWWA) specifications numbers for both quality control (dimensions,
tolerances, etc.) and installation (bedding, backfill, etc.). All
pipe and fittings shall have a minimum working pressure rating of
150 pounds per square inch.
(6) Force main tests. Final plans and specifications
shall describe and require pressure testing for all installed force
mains. Minimum test pressure shall be 1.5 times the maximum design
pressure.
(7) Air release valves. Air release valves or combination
air release/vacuum valves suitable for sewage service shall be provided
at all peaks in elevation. The final engineering drawings must depict
all proposed force mains in both plan and profile.
(e) Emergency provisions. Lift stations shall be designed
such that there is not a substantial hazard of stream pollution from
overflow or surcharge onto public or private property with sewage
from the lift station. Options for a reliable power source may include
the following.
(1) Power supply. The commission will determine the
reliability of the existing commercial power service. Such determinations
shall be based on power outage records obtained from the appropriate
power company and presented to the commission. When requesting outage
records for submittal to the commission, it is important to note that
the records be in writing, bear the signature of an authorized utility
employee, identify the location of the wastewater facilities being
served, list the total number of outages that have occurred during
the past 24 months, and indicate the duration of each recorded outage.
The facility will be deemed reliable if the demonstrated wastewater
retention capacity, in the station's wet well, spill retention facility,
and incoming gravity sewer lines, is sufficient to insure that no
discharge of untreated wastewater will occur for a length of time
equal to the longest electrical outage recorded in the past 24 months.
If records for the service area cannot be obtained, a 120 minute worst
case outage duration will be assumed. Provisions for a minimum wastewater
retention period of 20 minutes should be considered even in those
cases where power company records indicate no actual outages of more
than 20 minutes occurred during the past 24 months.
Cont'd... |
