Sewage Pump System Tutorial

The information provided here is for educational purposes only. A qualified and competent contractor should install all new systems and replace existing pumps and motors. Failure to install in compliance with local codes and ordinances, national codes and manufacturers recommendations may result in electrocution, fire hazard, unsatisfactory performance, and equipment failure.Sump, effluent and sewage pumps are designed for use in liquids which can present a health hazard as well as a threat to the environment. So you should be aware of health hazards; check explosion risk before welding or using hand tools; Guard against possible negative environmental impact and observe strict cleanliness.

NATIONAL, STATE AND LOCAL CODES

• Always follow national, state and local codes and ordinances
• States, counties, or municipalities may have pre-established standards for the design, sizing, selection, and/or installation of submersible wastewater pumps and wastewater systems.
• Wherever these standards (codes and ordinances) exist, they can differ from area to area and state to state, they TAKE PRECEDENCE OVER the information given, as well as any other method of pump selection or system design.

FUNDAMENTALS OF WASTEWATER SYSTEMS

• The function of the wastewater system is to receive and collect used water, store it temporarily, and move it to a collection system.
• Wastewater consists of fluids containing unscreened solids or wastes; typically sewage and effluent. Most wastewater systems are built around submersible wastewater pumps.
• These are heavy duty units designed to work submerged in the wastewater they are pumping, and capable of pumping unscreened solids.
• A wastewater system is often called a sewage system, a lift station, or a pumping station. They usually consists of a submersible wastewater pump and other equipment necessary to receive wastewater and move it to a collection station.

TYPICAL SEWAGE SYSTEMS

• Residential (Urban/Rural) ALL contaminated water is drained into the basin, which is then pumped to a gravity sewer system.
• Mound System (STEP - Septic Tank Effluent Pump) Effluent from the septic tank being pumped to a “mound,” or leachfield. Local codes may require this type of system. Or it can be used where the homeowner desires a better distribution of effluent across the drainage field.
• Sewage Lift Station: Sewage lift station, equipped with one sewage pump, lifting sewage to a gravity feed collection system.
• Municipal and Commercial Sewage Station: Sewage station, designed for heavy duty service.

TYPICAL RESIDENTIAL PUMPING STATION COMPONENTS

• Submersible Water Pump
• Basin
• Basin Cover
• Vent
• Electrical Controls
• Check Valve (Swing Check)
• Shut-off valve
• Inlet Hub
• Discharge connection
• Float Switch(s)
• Float Switch Bracket AND
• Interconnecting piping and wiring

TYPES OF SUBMERSIBLE WASTEWATER PUMPS

• When sizing any wastewater system, first determine the size of the solids the system will be handling. This may determine the type of pump you will need to use.
• Submersible wastewater pumps are generally classified by the nature of the wastewater and the size of solids they are capable of handling, as follows:
  • Sump Pumps: 3/8” solids.Generally dirty water, solids not typically present.
  • Effluent Pumps: Less than 1” solids. Partially or completely treated wastewater flowing out of a septic tank or treatment plant.
  • Sewage Pumps (Residential): 2” solids. Household wastewater which may contain human waste.
  • Sewage Pumps (Municipal andCommercial): 3” and larger solids. Municipal or commercial wastewater.
  • Grinder Pumps: Grinder Pumps are specialty pumps designed for applications where a gravity system is not practical. They are equipped with hardened stainless steel cutters which cut solids into small pieces, so the resulting residue can be pumped under pressure through smaller diameter piping. These can be part of a pressure sewer system.
• Solids-Handling requirements may be determined by local codes and/or by the type of application and types of solids.
• Unless otherwise specifically stated, a residential sewage pump should have the capacity of handling spherical solids of at least 2” diameter; commercial sewage pumps should have the capacity of handling spherical solids of at least 3” diameter
• “Solids” do not mean things like bolts and stones. “Solids” mean things that can be broken up by human hands and that normally can be flushed down a toilet. Most sewage pumps have a thermoplastic impeller that could be damaged by very hard items.

HOW A SUBMERSIBLE WASTEWATER PUMP WORKS.

• The submersible wastewater pump is a centrifugal pump consisting of a rotating impeller working inside a stationary casing, or volute.
• The pump motor turns the impeller, which transfers velocity to the liquid which surrounds it. This liquid is collected by the pump casing, and directed to the discharge of the pump.
• The submersible wastewater pump differs from normal centrifugal pumps in that solids must be able to pass through the pump.
• This requires special open or “non-clog” impeller designs, as well as sufficient clearance between the impeller and the wall of the casing.

SIMPLEX AND DUPLEX SYSTEMS

• Most residential wastewater systems are simplex or single-pump systems. However, some residential/systems and most municipal and commercial systems are duplex or two-pump systems. (Larger municipal and commercial systems may use three or four pumps.)
• Simplex System.
  • In a simplex system, wastewater flows into the receiver basin through an inlet pipe.
  • As the basin begins to fill, a switch floats up and turns on the pump.
  • As the pump evacuates the system the wastewater level in the receiver basin falls until the switch turns the system off.
• Duplex System.
  • A duplex system typically uses a three or four switch system to divide the pumping between the two pumps.
  • If a pump fails to turn on in the proper sequence, a switch will turn on the other pump and sound an alarm in a three switch system. In a four switch system the top switch sounds the alarm.
  • The duplex system divides the pumping load between the two pumps and provides backup in case of failure. For this reason each pump and discharge pipe in a duplex system is sized to handle the maximum anticipated flow. Both pumps in a duplex system are placed in the same basin.
  • When sizing a duplex system, make sure that each pump and discharge pipe is sized to handle the total load.

CYCLE TIME AND DRAWDOWN

• Cycle Time.
  • The period of time from when the pump switches on until it switches off is called a cycle, or cycle time.
  • If a pump turns on and off (cycles) too frequently, there is a risk of motor damage from overheating.
  • A properly sized pump should cycle within manufacturer’s recommendations for proper cooling.
• Drawdown.
• The amount of wastewater pumped per cycle is called drawdown.
• When this drawdown is being distributed throughout an absorption field, it is also called a “dose,” as in “dosing” the field, or applying the maximum “dosage.”
• In many areas, “dosage” amounts and maximums are dictated by Local Codes. Be sure to check with the appropriate agencies.

CALCULATING DRAWDOWN

• To calculate drawdown you must first determine switch ON and OFF locations.
• Recommended locations are:
  • ON (or uppermost): Minimum of 3” below basin inlet.
  • OFF: 6” below top of pump dome.
• Then multiply the distance between the ON and OFF locations times the basin capacity in gallons per inch to get Total drawdown or Usable Volue.
  RULE OF THUMB for Basin Capacity in Gallons/Inch
  • 24 " Basin Diameter = 2 Gallons per inch of depth
  • 30 " Basin Diameter = 3 Gallons per inch of depth
  • 36 " Basin Diameter = 4 Gallons per inch of depth
  • 42 " Basin Diameter = 6 Gallons per inch of depth
  • 48 " Basin Diameter = 8 Gallons per inch of depth
  • 60 " Basin Diameter = 12 Gallons per inch of depth
  • 72 " Basin Diameter = 18 Gallons per inch of depth
• Total Drawdown or Usable Volume = Distance between On and Off Switch(in inches) x Capacity in Gallons/Inch

DETERMINING FLOW AND SYSTEM CAPACITY

• In order to size and select a wastewater pump, you must first determine the flow, or discharge capacity required.
• A number of factors are involved; including number of people, bathrooms, and plumbing fixtures, as well as total water consumption.
• Flow can vary widely, the average per capita flow varies from 50 to 500 gallons per day.
• There are also peak periods of use, usually in the morning and the evening when the family is at home.
• System capacity must be sufficient to handle maximum anticipated peak flow.
• One method of calculating system capacity (called the “Fixture Method”) is to calculate the number of plumbing fixtures, assign a value to each, and then divide by a pre-determined factor to arrive at the required system capacity.
• An easy-to use rule of thumb, which is practically foolproof:
  • 1 BATHROOM - 20 GPM
  • 2 BATHROOMS - 30 GPM
  • 3 BATHROOMS - 40 GPM
  • 4 BATHROOMS - 50 GPM
  • 5 BATHROOMS - 60 GPM
  • 6 BATHROOMS - 70 GPM

SCOURING VELOCITY

• A wastewater pump must o maintain sufficient velocity to scour or clean the walls of the pipe so the pipe doesn’t get clogged from buildup.
• The capacity or flow of the pump produces the velocity which scours the line.
• Minimum velocity needed to scour the line is 2 ft. per second.
• The required flow to maintain minimum velocity of 2 feet per second is:
  • 1.25” PIPE - 9 GPM
  • 1.5” PIPE - 13 GPM
  • 2” PIPE - 21 GPM
  • 2.5” PIPE - 30 GPM
  • 3” PIPE - 46 GPM
  • 4” Pipe – 78 GPM
• There may be State or Local Codes requiring a minimum sanitary or plumbing system capacity based on usage and/or occupancy.
• Required system capacity can vary widely depending upon family ages and lifestyle.
• Be sure to check with the appropriate agencies.

OVER-SIZING THE PUMP

• More horsepower or flow is not always better – especially in smaller basins.
• Short cycling may reduce the life of the pump.
• A longer pumping cycle will be better for pump longevity.
• The most efficient part of the curve is usually in the middle of the curve, away from maximum head or flow

TOTAL DYNAMIC HEAD

• In order to do its job, the pump must raise the liquid to the highest point in the system, as well as push it through all the pipe and fittings in the system.
• Dynamic Head can be expressed in feet of head or in pressure (pounds per square inch or PSI).
• To convert PSI to feet of head, multiply by 2.31; To convert feet of head to pressure, divide by 2.31.
• Total Dynamic Head (or Total Head) is the pressure the pump must provide to do the job.
• Total Dynamic Head (TDH) = Vertical Elevation (Static Head) + Friction Loss + System Pressure Requirements.
  • Vertical Elevation is the distance from the pump inlet to the highest point in the discharge system.
  • Friction Loss As the wastewater is pumped from the receiver basin to the collection point the inside of the pipe and fittings provide a resistance called friction.
    • Friction loss works against pump performance, so you want to keep it as low as possible.
    • Friction loss INCREASES as pipe length increases. The longer the pipe, the more the friction loss.
    • Friction loss INCREASES as flow rate increases. The greater the flow rate the greater the friction loss.
    • Friction loss DECREASES as inside pipe diameter increases. The larger the inside diameter of the pipe, the less the friction loss. Therefore, you can reduce friction loss by using a larger diameter pipe.
    • There are published pre-calculated Friction Loss Tables available for different flow rates and different types of fittings.
    • When calculating Friction Loss, always round UP to the nearest whole foot.
  • System Pressure Requirement
    • This is the pressure required or available at the end of the pipe as the wastewater enters the tank or collection site.
    • These pressure requirements are often dictated by Local Codes

SUBMERSIBLE WASTEWATER PUMP SIZING AND SELECTION CHECKLIST

• State and Local Codes. They always TAKE PRECEDENCE OVER other methods of sizing and selection, including the information found in this tutorial. State and Local Codes often contain system or size requirements based on local soil conditions and/or lot layout including Dosage Requirements.
• Size of Solids. Size of Solids. This will determine whether the application requires a sump, effluent, or sewage pump.
  • Sump - 3/8” solids (solids typically not present)
  • Effluent - Less than 1" solids
  • Sewage (residential) - 2" solids
  • Sewage (municipal and commercial) -3"+ solids
  • Grinder - Specialty pumps used in pressurized systems.
• Capacity Required: This is the flow or discharge capacity required by the installation. While flow can vary from installation to installation, these RULE OF THUMB: will handle most situations:
  • 1 BATHROOM - 20 GPM
  • 2 BATHROOMS - 30 GPM
  • 3 BATHROOMS - 40 GPM
  • 4 BATHROOMS - 50 GPM
  • 5 BATHROOMS - 60 GPM
  • 6 BATHROOMS - 70 GPM
• When in doubt, always select a pump with capacity greater than anticipated peak flow.