How to Size a Sewage Basin

Liberty 1102/LE74M Sewage Package
Liberty 1102/LE74M Sewage Package

The sewage basin collects and contains wastewater before it is removed from a building’s system by way of a submersible sewage pump. It is a critical part of any wastewater (sewage, effluent, sump) system.

Commonly constructed with fiberglass or high grade plastic, basins are heavy duty containers. They come in a variety of system types (simplex or duplex), sizes and applications. Some basins operate with multiple types of pumps while others are designed expressly to be paired with a specific pump.

Basins sit below grade while the pump itself sits at the bottom of the basin. How it functions is simple: wastewater enters the basin through an inlet pipe or port. The wastewater accumulates in the basin until it reaches a certain level, at which point your sewage pump will activate (by way of a float switch or preset control) and pump the wastewater through a discharge pipe out of the building to a municipal sewer or septic tank.

A correctly sized basin is important to ensuring that the wastewater system and the pump itself runs smoothly and efficiently. For instance, a basin that is too small could fill up rapidly and contribute to improper pump cycling. If the pump turns on and off repeatedly, it will quickly burn out. Longer downtime between pump cycles keeps the pump cooler and results in longer life.

In this blog we will walk you through the process of sizing a basin to fit your needs and application by working through a specific example.

Sizing a Sewage Basin: An Example

Before ordering a basin, you need to understand what sewage pump you will be using, as well as that pump’s relevant performance and construction information. Specifically, make sure to know the pump’s height, its flow rate (GPM) and cycle time. This information can usually be found in a product description or installation manual.

Let’s start with a basic example: assume you need a simplex system with one submersible sewage pump operating at a flow rate of 100 GPM and a cycle time of one minute. The pump is 19 inches tall.

Once you are armed with the pump model information, it may be helpful to jot down all these specifications before you start researching basins. Now you can size a basin.

The two dimensions you are looking for are the basin diameter and the basin depth.

Luckily, determining the basin diameter is relatively easy. Simplex pump systems require a minimum of 24 inches, while duplex systems require a minimum of 36 inches. You have some leeway when it comes to choosing a basin diameter. Consider the size restrictions of the pit where you plan to install a basin. If you are digging an entirely new pit, consider how much space you want that pit to take up.

In our example, we are working with a simplex system, so we know that you need a basin of at least 24 inches (we’ll stick with the minimum for simplicity’s sake).

Basin Depth

Determining the basin depth is a little more complicated, but only slightly. You will need to know three variables: the height of the pump, the pump down or drawdown and the height of the area that extends from the basin top to the invert (bottom) of the inlet port. For convenience’s sake, we’ll refer to this area as the inlet height. The graphic below should give you an idea of how these sections relate to each other.

Both the pump height and inlet height are easy enough to obtain. You know the pump height in this example already: 19 inches. This is a typical height for a wastewater pump used in residential or light commercial applications.

Keep in mind that submersible sewage pumps should remain submerged while pumping so as to keep the pump cool and prevent overheating. The pumping action should not evacuate water to a point below the top of the pump.

Next, determine the inlet height. The height for this area in most sewage basins is 10 to 12 inches. Many sewage basins require the user to drill a hole for the inlet port, so you will be able to adjust this height to meet your own system needs. For the purposes of this exercise, let’s go with 12 inches.

Finally, you will need to calculate the pump drawdown.

Calculating Drawdown

The pump down or drawdown is the amount of wastewater that should be discharged from the basin in a given cycle. The drawdown is measured in inches from the invert of the basin’s inlet port to the top of the pump itself.

We know from the performance specifications that the pump will move 100 gallons during its 1-minute cycle time. Now you must find the height in inches of the drawdown area. If you paid attention during high school geometry, you might remember the formula to determine the volume of a cylinder in cubic inches: π × radius² × height. You can divide this value by 231 (number of cubic inches in 1 U.S. gallon) to find the volume in gallons.

Thus the total formula is [(πr²h)÷231]= V. In this case, we already know the value for V, so we need to solve for h. So we have:

[(3.14)(144)(h)÷231]=100

452.16h=23100

h=51 inches

So now we know the pump drawdown is 51 inches.

Here is where you will take the variables you have calculated and put them together. Add the inlet height to the pump drawdown to the height of the pump in order to find the basin depth.

19 inches (pump height) + 51 inches (drawdown) + 12 inches (inlet height) = 82 inches (total height)

82 inches is the minimum height you need, but since basin heights are typically constructed in multiples of 1 foot, you can round up to 84 inches. So based on the submersible sewage pump you are purchasing, you should look for a 24″ × 84″ basin.

Checking the Math

The above example is intended to give you a picture of what to look for in a basin when you only have the basic pump information at your disposal. Depending on how many variables you know, you might be able calculate the dimensions with the cylinder volume formula. Keep in mind that not all basins are perfectly cylindrical; many narrow towards the bottom in a truncated cone.

Assuming that the basin is a perfect cylinder, let’s try to find the total volume in gallons that the basin can hold. Just use the same formula, this time with the total height of the basin representing h, and solve for V:

[((3.14)×(12²)×(84)÷231]=164.5≈165

So your sewage basin will be able to hold 165 gallons. Now that you know both the total gallons and the height of the basin, we can easily find a value for gallons per inch. Simply divide 165 by 84 to get 1.96 gallons per inch. The gallons per inch value is useful for understanding the level of water at any point in the sewage basin.

We can also use that value as another way to check our work. For instance, if you want to check that your drawdown height is correct, take the 100 gallons of drawdown and divide by the gallons per inch:

100÷1.96=51 inches.

Having all this information can help you understand the relationship between the physical space of a basin and the amount of wastewater that will be pumped. Understanding correct sizing ensures the best possible usage efficiency and a longer lasting pump life.

Sewage Basin and Sewage System

Hopefully by now you can grasp the underpinnings and importance of sizing. The math may always not work out precisely. In dealing with real world applications, environmental constraints will always play a role in what you can and can’t do.  Installations don’t happen on paper.

Pump Products highly recommends working with qualified experts for sewage systems. Unlike with, say, sump pumps and sump pits, installing a sewage basin is not necessarily a straightforward do-it-yourself project. Sump pumps collect groundwater in a basement; sewage pumps need to transport all of a building’s wastewater which comes from any number of water fixtures. That means understanding a building’s piping system, complying with municipal codes, etc.

Another alternative is to buy a sewage system package which may include the pump, the basin, a control panel and other accessories.

Sizing a Sewage Basin Quiz

So now you know the steps to sizing a sewage basin. But can you calculate the dimensions on your own? We’ll give you some variables and formulas to work with and see if you can come up with the right answers. Complete the quiz and you’ll know you’re ready to select your sewage basin.

Goulds Pump Quiz

Take the Goulds pump quiz and get $5.00 off your next purchase of $100.00 or more.