When Do You Give Sewage a Lift?

Strategic pumping of effluent along the treatment train will improve downstream flows

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In recent columns, we focused on pumping into drop boxes and then utilizing flow by gravity into sewage treatment trenches. We wanted to follow up with a general discussion of pumping situations an installer or service provider may encounter and how and why pump requirements are different for each situation. In some applications, more than one pump is required.

There are some common elements to all pumping situations, involving four components: a pump tank or sump; a discharge assembly; pump controls; and the pump. The most trouble-free pumping situation is to pump sewage effluent after solids have been removed. Except for one notable exception, most pumps in our systems are used to pump effluent. 

The major exception is when there is a lower level in the house and raw sewage must be lifted to the septic tank at a higher elevation. Effluent from the septic tank at a higher level can then either flow by gravity into drop boxes and trenches if there is suitable soil, or flow into a pump tank and effluent pumped to trenches higher in elevation or to a pressure distribution system.

Pump to elevation

Raw sewage is collected in a sump in the basement or walk-out level, and a sewage ejector pump is used to deliver sewage from a sealed, vented sump to the septic tank. The sump is vented back through the plumbing stack. The amount of sewage delivered to the septic tank each time the pump runs should be less than 5% of the septic tank operating volume. This is to minimize disturbance in the tank interfering with solids settling. It is recommended a compartmented tank or two tanks in series be used to provide the septic tank volume.

An advantage of this scenario is that the septic tank is installed shallow, allowing the opportunity to deliver effluent from the tank by gravity to the rest of the system. In addition, having the pump in the basement means it is readily accessible for maintenance or replacement. Also, until the pump is replaced most of the plumbing system can continue to be used.

Another option to pump to a higher elevation is where raw sewage flows by gravity from the house to the septic tank, then by gravity to a pump tank, where it is delivered to a drop box. Flow from the drop box occurs by gravity into a sequence of trenches. This allows location of the soil treatment area in suitable soils at an elevation higher than the septic tank and pump tank. And the sequence allows easy access to septic and pump tanks near the house for maintenance.

We like to talk about a pump having two names that define and identify the size of the effluent pump needed for the situation. In this case, any pump selected to lift effluent to the trenches will have to have a pump curve that indicates it will deliver 10 to 45 gpm at a total dynamic head, reflecting friction loss in the piping and the elevation difference between the pump and the drop box.

The minimum rate is set so it keeps ahead of high discharge appliances such as automatic washers or dishwashers. The maximum rate is so sewage effluent will have time to flow out of a single 4-inch pipe outlet to a gravity sewage treatment trench. In addition, the discharge line from the pump must be directed against a wall of the drop box where there is no outlet or into a device installed to dissipate the force from the pump.

Another scenario is where a pump is used to deliver effluent to a pressure distribution network. The network can be in another pretreatment device such as a media filter, mound or at-grade system. In the case of the additional pretreatment device, flow out can be by gravity if appropriate, or will require an additional pump to the final dispersal and treatment area.

By the numbers

Each of these situations have their own unique sizing specifications depending on the network. The amount of effluent must fill up the piping and meet the discharge rate requirements based on the size and number of orifices. It also must operate with the necessary head based on elevation difference, friction loss in the pipes and the required head at any distribution manifold.

In these situations, the pump and the distribution network operate together, and when a pump needs to be replaced it is critical the pump meet the system specifications according to the initial design. It is important for the installer to keep a record or copy of the design specifications for your files as well as provide them to the homeowner. This gives future service providers the opportunity to look in two places for design specifications. We all know maintainers are met with a blank stare when asking homeowners for information about their systems. It’s also important to leave your contact information where it can be found when the system is accessed so you can be contacted. 


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