Understanding Pumping Systems

Here are the fundamentals for selecting and installing pumps that deliver wastewater to a gravity-based onsite treatment system
Understanding  Pumping Systems
A pump tank with controls and alarm as well as a riser brought to the surface.

In keeping with our tour of onsite treatment systems, we now turn to pumping systems. Of course, one aspect of pumping systems is a tank, which we’ll discuss only from the perspective of proper sizing for the application, and not installation (since we covered that earlier). So now we’ll embark on a series on pumping system design and installation.

Pumps are used to move raw sewage or septic tank effluent to different parts of the onsite treatment system. A pumping system consists of four parts regardless of the application:

• Pump tank or sump.

• Discharge assembly.

• Controls.

• Pump.

How sewage moves through the system determines the placement of the pump. The pump then affects the sizing and appearance of all the system components. Some pumps are used to move raw sewage to a pretreatment device, such as a septic tank. Others move septic tank effluent to another pretreatment device, such as a media filter, or the final soil dispersal area. Some applications use more than one pump.


Pumping to gravity systems

There are two main gravity applications. The first involves pumping raw sewage from a basement sump up to the house sewer, where it flows by gravity into the septic tank. The second involves lifting septic tank effluent to the final soil treatment area.

When the pump is in the basement or lower level of the home, it is installed in a sump basket. This is not to be confused with a sump pump, which is used to pump clear water. Sump pumps should never be used for sewage applications.

If there is a toilet on the lower level, a sewage ejector or solids-handling pump is used. In this application, a two-compartment septic tank or two tanks in series should be used to prevent turbulence from pushing solids through the system. Effluent screens at the outlet of the septic tank will also help keep this from occurring.

In the event of pump failure, only the basement plumbing cannot be used, because the rest of the household sewage is delivered by gravity to the septic tank.

Sump baskets in the basement are usually made of plastic and hold 30 to 50 gallons. They can be smaller than other pump or dosing tanks, since the delivery of sewage to the pretreatment device should be continuous. When a pump problem occurs, it will be apparent immediately, since there is limited storage capacity. Water use in the basement will have to stop until the problem is corrected.

This sump needs to be vented; that requirement will likely be covered in the state plumbing code, which may mandate that a licensed plumber install and work on the sump. The vent must extend through the roof and must be large enough in diameter to maintain atmospheric pressure within the sump. The cover needs to be gastight and of a bolt-and-gasket type that allows access for maintenance and replacement. It must also be strong enough to support any anticipated loads in the area.


From pump to field

In the second application, the raw sewage flows from the house by gravity to the septic tank, and the tank effluent flows by gravity to a pump tank. From the pump tank the effluent is delivered to the soil treatment unit for final dispersal.

If the pump fails here, water use in the house needs to be restricted until repairs can be made. The reserve storage capacity for sewage from the house is determined by the pump tank capacity above the high-water alarm level.

A pump should never be installed directly in the septic tank, because in that event, solids are likely to cause plugging. Typically, two-compartment tanks are installed, where the first compartment is used as the septic tank and sized according to state code, and the second is used as the pump tank.

Often, a separate watertight tank is installed to act as the pump or dosing tank. Any of these tanks have to meet the same construction and installation requirements as the tanks we have discussed in previous articles. Some proprietary products use pump vaults to protect the pump in either a septic tank or pump tank. Make sure if you are using one of these that the vault and the pump capacity match.


Sizing the pump tank

The size of a pump tank is determined by the total daily flow. It needs to be large enough to supply the dose volume and provide storage capacity in case of pump failure. The tank should be large enough to hold the average daily sewage flow from the home. If a smaller tank is used, an alternating two-pump system should be installed.

Pump tanks can be round or rectangular. As with any tank but especially with pump tanks, a riser is needed to provide easy access to the pump for maintenance and replacement. We are always amazed at how many times we are called to systems that are having problems, only to find that access to the pump tank and pump are restricted due to poor installation.

The pump should be set up off the bottom of the tank at least four inches. This keeps the intake out of any solids that have carried over from the septic tank. The block or blocks used should be wide enough to accommodate the pump base so that when it is removed, the replacement or repaired pump can be easily set back onto the pedestal.

A pump basket may be added to the tank so that the pump draws effluent from a higher elevation in the tank. Again, make certain that the basket will work for the pump capacity.

When the pump delivers effluent to a series of trenches that are higher in elevation, the inlet into the drop box or distribution box should be equipped with a device to dissipate the force of the incoming effluent. The bottom of the discharge piping from the pump must be at least two inches higher than the supply line to the next box. This avoids potential drainback to the pump station other than the amount contained in the supply pipe.

In a drop box system this also ensures that the effluent gets delivered to the first trench in sequence. In the next article, we will discuss pumping to pressure systems, including mounds, at-grades, and low-pressure-pipe trenches.


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