Recommended Design Criteria for Sewage Pumping Stations,
Force Mains and Pressure Sewers...
Engineer's Report, Plans and Specifications...
Information for sewage pumping stations must be submitted as required by Chapter 10, Recommended Standards for Sewage Works, 1978 or latest Edition. In addition, the following information is requested :
1. Contributary Area : A description of the extent of the existing and proposed contributary area with reference to a general system map as well as a description of probable future expansion of the contributary area is requested.
2. Location : The proposed pumping station, force main and point of discharge into the existing sewer system shall be shown on a map. In addition, the report shall discuss the capacity available in the existing downstream sewer, its ability
to convey the additional flow and the effect it will have on the treatment facility.
3. Design Data : The design data for the proposed project should include the following :
a. Design period.
b. Population densities per acre and total population served.
c. Area served in acres.
d. Per capita sewage contribution expressed as an average daily and as a peak hourly value.
e. Infiltration and inflow and method used to establish.
f. Industrial wastes contribution.
g. Design flow rates expressed as design average daily and peak hourly values.
General...
1. Flooding : Sewage pumping stations shall not be subject to flooding. It is important that the stations be readily accessible and fully operational during a twenty-five (25) year flood. A one hundred (100) year flood recurrence interval should be considered in the design for protection of structure and electrical and mechanical equipment from physical damage. The stations shall be readily accessible; preferably located off the traffic ways of streets and alleys.
2. Grit : Where it may be necessary to pump sewage prior to grit removal, the design of the wet well should receive special attention and the discharge piping shall be designed to prevent grit settling in the discharge lines of pumps not operating.
3. Security : Pumping stations shall be designed with security and lighting to discourage vandalism.
Design...
The following items should be given consideration in the design of sewage pumping stations :
1. Capacity of Pumps : The pumps shall have sufficient capacity to pump at least four times the design average flow rates for laterals and sub-main sewers; either two and one-half times the design average flow rate or peak hourly flow rate, whichever is greater, for main, trunk and interceptor sewers. Each pump shall be capable of handling peak sustained flow rates. If no peak hourly flow data is available, refer to Figure 1 in Chapter 1.
2. Type : Sewage pumping stations that are of the dry well type are the most desirable; however, circumstances will generally justify the use of submersible- type pumps.
3. Structures :
a. Wet and dry wells including their superstructures shall be completely separated.
b. Provision shall be made to facilitate and easily remove pumps and motors.
c. Suitable and safe means of access shall be provided to dry wells of pump stations and shall be provided to wet wells containing either bar screens or mechanical equipment requiring inspection or maintenance. Stairways should be provided for installations deeper than 15 feet and should be installed with rest landings not to exceed 10 foot vertical intervals. A manlift or elevator may be used in lieu of landings, provided emergency access is included in the design.
d. Due consideration shall be given to the selection of materials because of the presence of hydrogen sulphide and
other corrosive gases, greases, oils and other constituents frequently present in sewage. Structure materials may require cathodic and anodic protection.
e. Where a bar rack is provided, a mechanical hoist shall also be provided. The hoist and accessories shall be rated
for not less than 1.5 times the heaviest anticipated load.
f. The pumping station compartments shall resist hydrostatic uplift pressures.
Pumps and Pneumatic Ejectors...
1. At least two pumps or pneumatic ejectors shall be provided. Where the pumping installation will serve not more than 50 homes, a single unit will be permitted, provided that the station is designed to permit the installation of a future duplicate unit with no structural changes. A minimum of 3 pumps should be provided for stations handling flows greater than one (1) million gallons per day (mgd). If only two units are provided, they should have the same capacity. Each shall be capable of handling flows in excess of the expected peak hourly flow. Where three or more units are provided, they should
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 peak sewage flows. It is preferable that a standby pump or ejector be provided and available for service at all times.
2. 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, such as, grinders, are installed to protect the pumps from clogging or damage. Where the size of installation warrants, a mechanically cleaned bar screen with 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.
3. Pumps or ejectors shall be capable of passing spheres of at least 3 inches in diameter. Pump suction and discharge openings shall be at least 4 inches in diameter. Turned-down bellmouth inlets are preferred. The pumps and ejectors should have non-corrosive materials for inner parts. Where turned-down bellmouth inlets are used, the bell should be not more than D/2 and not less than D/3 above the floor of the wet well. It is recommended that sump and approach channel dimensions be provided as suggested by the pump manufacturer.
4. The pump shall be so placed that under the normal operating conditions it will operate under a positive suction
head, except as specified in Section K-1.
5. Electrical systems and components (e.g. motors, lights, cables, conduits, switchboxes, control circuits, etc.) in enclosed or partially enclosed spaces where flammable mixtures occasionally may be present (including raw sewage wet wells) shall comply with the National Electrical Code requirements for Class I, Division I, Groups C and D locations. When such equipment is exposed to weather it shall meet the requirements of weatherproof equipment (Nema 3R).
6. Each pump should have an individual intake. Wet well design should be such as to avoid turbulence near the intake. Intake piping should be as straight and short as possible.
7. A separate sump pump equipped with dual check valves shall be provided in the dry wells to remove leakage or
drainage with the discharge above the overflow level of the wet well. A connection to the pump suction is also recommended
as an auxiliary feature. Water ejectors connected to a potable water supply will not be approved. All floor and walkway surfaces should have an adequate slope to a point of drainage.
8. The pumps and controls of main pumping stations, and especially pumping stations operated as part of treatment works, should be selected to operate at varying delivery rates to permit the discharge of sewage from the station to the treatment works at approximately the sewage flow rate to the pump station. Design pumping rates should be established in accordance with Chapter 1, Section C.
Controls...
Control float tubes should be so located as not to be unduly affected by flows entering the wet well or by the suction of
the pumps. Float tubes in dry wells shall extend high enough to prevent overflow. In small stations with duplicate units, provision should be made to automatically alternate the pumps in use.
Valves...
Suitable shutoff valves shall be placed on suction and discharge lines of each pump with the exception of the suction line
on submersible and vacuum-primed pumps. A check valve shall be mounted in a horizontal position on each discharge line between the shutoff valve and the pump. Valves shall not be located in the wet well. Valves shall be capable of withstanding normal pressure and water hammer.
Wet Wells...
1. Where continuity of pumping operation is important, consideration should be given to dividing the wet well into two sections, properly interconnected, to facilitate repairs and cleaning.
2. The wet well size and control setting shall be appropriate to avoid heat buildup in the pump motor due to frequent starting.
3. The effective capacity of the wet well, except for large capacity stations, should be such that one pump will continuously run at least five (5) minutes of every 30 minute period at the minimum flow. The volume of a wet well between start and stop elevations for pump(s) and speed(s) can be determined by using the formula :
V = ( T ) x ( q / 4 )
Where :
V= Required capacity in gallons including storage of influent line.
T= Minimum time of one pumping cycle between successive starts, or speed increases of the control range in minutes.
q= Pump capacity in gallons per minute, for one pump, or the incremental pumping capacity for an additional pump, or pump speed.
Where large pumping units (3,000 gpm) are involved, they should be operated continuously insofar as is practical, (T) should not be less than 20 minutes. For smaller pumps, (T) can be reduced to 10 minutes with 15 minutes being the more desirable.
4. The wet well floor shall have a minimum slope of one to one (1:1) to the hopper bottom. The horizontal area of the hopper bottom shall be no greater than necessary for proper installation and function of the pump inlet pipe.
Ventilation...
Adequate ventilation shall be provided for all pump stations. Where the pump pit is below the ground surface, mechanical ventilation is required, so arranged as to independently ventilate the dry well and the wet well if screens or mechanical equipment requiring maintenance or inspection are located in the wet well. There shall be no interconnection between the
wet well and dry well ventilation systems. In pits over 15 feet deep, multiple inlet and outlets are desirable. Dampers should not be used on exhaust or fresh air ducts and fine screens or other obstructions in air ducts should be avoided to prevent clogging. Switches for operation of ventilation equipment should be marked and located conveniently. All intermittently operated ventilating equipment shall be interconnected with the respective pit lighting system. Consideration should be given also to automatic controls where intermittent operation is used. The fan wheel should be fabricated from non-sparking material. In climates where excessive moisture or low temperatures are a problem, consideration should be given to installation of automatic heating and/or dehumidification equipment.
1. Wet well ventilation may be either continuous or intermittent. Ventilation, if continuous, should 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 introduction of fresh air into the wet well by mechanical means.
2. Dry well ventilation may be either continuous or intermittent. Ventilation, if continuous, should provide at least six complete air changes per hour; if intermittent, at least 30 complete air changes per hour.
3. The outside vent shall terminate in an inverted "U" construction with the opening at least 24 inches above the finished ground elevation.
Flow Measurement...
Suitable devices for measuring sewage flow should be provided at all pumping stations.
Water Supply...
There shall be no physical connection between any potable water supply and a sewage pumping station which under any conditions might cause contamination of the potable water supply. If a potable water supply is brought to the station, it shall be provided with the proper air-gaps and backflow prevention devices. If a non-potable water supply is provided, all outlets shall be permanently posted to indicate water is not safe for drinking.
Suction-Lift Pump Stations...
Suction-lift pump station installations shall meet the applicable requirements under Sections C through J.
1. Priming - Suction-lift pumps shall be of the self-priming type as demonstrated by a reliable record of satisfactory operation.
2. Capacity - Approval will be restricted to installations where the capacity does not exceed 200 gpm per pump and the total suction-lift does not exceed 15 feet.
Submersible Pump Stations...
Submersible pump stations shall meet the applicable requirements under Sections C through J.
1. Pump Removal - Submersible pumps shall be readily removable and replaceable without dewatering the wet well and with continuity of operation of the other unit or units.
2. Operation - Submersible pumps shall be capable of unsubmerged operation without damage or reduction of service capability or positive provision shall be made to assure submergence, e.g., backup controls.
3. Minimum Draw Down - See Table 1 in this Chapter.
4. Controls - The control panel and alarm system shall be located outside the wet well and suitably protected from weather, humidity and vandalism.
5. Valves - All control valves on the discharge line for each pump shall be placed in a convenient location outside the wet well and be suitably protected from weather and vandalism.
6. Electrical Equipment - See Section D-5 of this Chapter.
Alarm Systems...
Alarm systems should be provided for all pumping stations. The alarms shall be activated in cases of power failure, pump failure, or any cause of pump station malfunction. Pumping station alarms shall be telemetered to a facility or office
that is manned 24 hours a day. Where no such facility exists, an audio-visual device shall be installed at the station
for external observation. It is recommended that an alarm and shutoff control equipment be provided in dry well to protect motors if sump pump should fail or sudden break occurs that exceeds the capacity of the sump pump.
Emergency Operation...
1. Objective - The objective of emergency operation is to prevent the discharge of raw or partially treated sewage to any waters and to protect public health by preventing back-up of sewage and subsequent discharge to basements, streets, and other public and private property. There shall be no by-passing of sewage to the groundwater, surface of the ground or any watercourse.
2. Emergency Power Supply - Provision of an emergency power supply for all pumping stations should be made, and may be accomplished by connection of the station to at least two independent public utility sources, or by provision of portable or in-place internal combustion engine equipment which will generate electrical or mechanical energy, or by provision of portable pumping equipment.
3. Internal Combustion Equipment - Where in-place internal combustion equipment is utilized, the following guidelines are suggested for use :
a. The unit shall be bolted in place. Facilities shall be provided for unit removal for purposes of major repair or routine maintenance.
b. Provisions shall be made for automatic and manual start-up and cut-in.
c. Unit size shall be adequate to provide power for lighting and ventilation systems and such further systems affecting capability and safety.
d. The unit internal combustion engine shall be located above grade with suitable and adequate ventilation of exhaust gases.
4. Portable Equipment - Where portable equipment is utilized the following guidelines are suggested for use:
Pumping units shall have the capability to operate between the wet well and the discharge side of the station. The station shall be provided with permanent fixtures which will facilitate rapid and easy connection of lines. Electrical energy generating units shall be protected against burn-out when normal utility services are restored, and shall have sufficient capacity to provide power for lighting and ventilation systems and such further station systems affecting capability and safety.
5. Emergency Power Generation - All emergency power generation equipment should be provided with instructions indicating the essentiality of regularly starting and operating such units at full load.
Overflows...
A high-level wet well overflow to supplement alarm systems and emergency power generation may be provided. Where a high
level overflow is utilized, complete retention of all overflow in storage-detention tanks or basins shall be provided. Provision shall be made to drain or pump the tanks or basins to the station wet well. The basins or tanks shall not
discharge to the groundwater, surface of the ground or any watercourse.
Instructions and Equipment...
Sewage pumping stations and their operators should be supplied with a complete set of operational instructions, including emergency procedures, maintenance schedules, tools and such spare parts as may be necessary.
Force Mains...
1. Size - Force Mains shall be no smaller than 4-inch diameter except where grinder pumps are provided.
2. Velocity - At design average flow, a minimum cleansing velocity of at least two feet per second shall be maintained. A velocity of eight feet per second should not be exceeded.
3. Valves - Automatic air release valves shall be placed at high points in the force main to prevent air locking. A blow-off shall be placed at the low points where gritty material could accumulate and restrict flow through the force main. Access to air release facilities shall not be located in traffic-ways.
4. Termination - Force mains should enter the gravity sewer system at a point not more than two feet above the flow
line of the receiving manhole. The design shall be such as to prevent turbulence and deterioration at this point.
5. Leakage (Ex-filtration) - Force mains shall be tested at a minimum pressure of at least 50 percent above the design operating pressure, for at least 30 minutes. Leakage shall not exceed the amount given by the following formula :
L = NDP 1 / 2 / 1,850
Where :
L = The allowable leakage in gallons per hour
N = The number of pipe joints
D = The nominal pipe diameter in inches
P = The test pressure in psi
6. Maintenance - If the force main is taken out of service for repair or cleaning, the sewage shall be discharged to a storage-detention tank or basin and returned to the sewer system with no discharge to the groundwater, surface of the ground or any watercourse.
7. Restraints - Force mains shall be restrained at bends to prevent movement occurring from maximum operating pressures or surges.
Pressure Sewer System...
1. General :
a. The pressure sewer system shall be considered as a supplemental tool for wastewater collection system and not as a replacement for the conventional gravity collection system. It is expected that a pressure sewer system would generally be used in small sub-divisions, sanitary districts or communities. This system may be approved for use when justified by
unusual formations, low population density, difficult construction due to groundwater conditions, or other circumstances where it would offer an advantage over a gravity system.
b. A responsible management structure (whether private company, municipal or governmental body) for inspections, repair and maintenance of septic tanks, pressure sewers and grinder pump units shall be clearly defined and established prior to
the approval of any installation. The pressure sewer system including the grinder pump should be regarded as integral components of the system and not as a part of the individual home plumbing. Any industrial hook-up, such as a restaurant or service station, should have adequate pretreatment for grease and grit collection. The responsible entity must have full repair service capability available on short notice which should include a stand-by power generator, adequate spare parts, spare units and service tools.
2. Design Criteria - The following considerations shall be used for the design of a pressure sewer system including the grinder pump units :
a. If a septic tank is not provided before the pump, a grinder pump shall be required. Pressure sewer collection
systems shall be preceded by grinder pumps or at least 1,000 gallon capacity septic tanks. Pumping units should not be installed in the settling chamber of a septic tank if the septic tank is to be used for solids reduction.
b. No pressure sewer less than 1 1/4 inches inside diameter shall be provided. The required size shall be determined to maintain low frictional losses in the system and a minimum scouring velocity of 2 feet per second at all points in the system.
c. Special care shall be exercised in the hydraulic design of a pressure sewer system which is proposed to serve ultimately more houses than those expected to be served initially.
d. The determination of flow in the pressure sewer system shall be made on the basis of the maximum probable number of grinder pump units that would be expected to run simultaneously or some other accepted method of computing the peak sewage flow rate in the system.
e. The pressure sewer system shall be laid out in a branched or tree configuration to avoid flow-splitting at branches which cannot be accurately predicted.
f. The pressure sewer piping shall be installed in a depth sufficient to protect against freezing and damage from vehicular traffic.
g. Although any suitable pipe material can be used, plastic pipe such as PVC S.D.R.-26 or equivalent are considered suitable. A "C" factor of 130 to 150 is recommended to be used for plastic pipes in the Hazen- Williams formula. It is recommended that ball type check valves have a plastic or bronze body. Since valves are important components of the system, they should be reliable.
h. Clean-out connections shall be provided at distances not to exceed capability of available cleaning equipment (approximately 400-600 feet). Flushing clean-outs should be provided at the upstream end of every major branch.
i. Pressure and vacuum release valves shall be employed at appropriate locations. Pressure sewers should be constructed on a gradually ascending slope to minimize air binding. Valve boxes are recommended about every 1,000 feet or major change
in direction.
j. The pressure sewer should have appropriate identification to distinguish between sewer and potable water lines.
k. Current criteria as outlined in Chapter II and the State Plumbing Code shall be maintained in separating pressure sewer and water systems.
l. Pressure sewer system operating pressures in general shall not exceed a range of 40 to 50 psi for any appreciable period of time.
m. Thorough pressure testing of all lines, fittings, valves, etc. shall be made prior to start-up.
n. Details of construction shall be clearly stated in the drawings and/or specifications.
o. The minimum net storage capacity of the grinder pump unit shall not be less than 50 gallons. The grinder unit tank should be able to accommodate normal peak flows and emergency storage for a period of 8 to 12 hours during short power outages or equipment malfunctions.
p. When a holding tank is provided for emergency purpose, during an extensive power outage, the tank should be sized
for at least 3 days storage if continuously supplied by an outside water supply not dependent on same power source. Adequate provisions should be made to empty the holding tank when necessary.
q. The grinder pump shall have the characteristics which will continue to produce flows of at least 8 gpm even when conditions in the pressure system cause heads to rise temporarily to values as high as 50 psig. Generally 2 Hp grinder pumps are recommended for individual home applications.
r. Check and shut-off valves shall be employed to isolate the grinder pump unit from the house service line and the pressure laterals.
s. Appropriate high water and overflow detection devices such as visual and/or audio alarm shall be provided.
t. Provisions shall be made to insure that grinder pump operates even under temporary loads above normal and contains integral protection against back siphonage and over pressure.
u. The grinder pump unit shall be capable of reducing any material in the wastewater which enters the grinder unit to such size that the material will pass through the pump unit and pressure sewer without plugging or clogging. No screens or other devices requiring regular maintenance shall be used to prevent trashy material from entering the grinder pump. The pumps should be non-clog type with non-corrosive materials for inner parts.
v. Pumping units and grinder pumps used in pressure sewer systems should be reliable, easily maintained, and should
have compatible characteristics. At least one stand-by grinder pump unit for each 50 units or fraction thereof shall be provided for emergency replacement. Whenever any pumping unit handles waste from two or more residential units or from a public establishment, dual grinder pump units with twice the holding capacity shall be provided to assure continued service in the event of an emergency.
w. The grinder pump unit must be capable of being removed without dewatering the pumping compartment.
x. Provisions must be made for access as well as protection from weather and vandalism.
Deviations from Design Criteria...
The Department may consider and allow deviations where adequate documentation is provided to prove the need for such deviation.
