Here’s How To Calculate Sewer Capacity

July 14, 2022

On average, over a billion gallons of waste may travel through the New York City sewer system daily. With such a colossal amount of material, it’s easy to believe that the system can continue adding to its carrying load indefinitely. But even a system as extensive as this has its limits. Fortunately, it’s easy to determine that limit using this simple guide for calculating sewer capacity.

Why Should You Calculate Sewer Capacity

Every sewer is designed to help manage a community’s waste. However, our communities change over time. Our population increases, we add new buildings, and with all this growth, the amount of material that goes into the system increases.

If we are unaware of what our sewer systems can handle, we risk putting too much strain on the system, increasing the risk of damaging pipes. This damage can cost home and business owners a great deal of money and create health concerns, such as the spread of waterborne illnesses.

Factors That Contribute to Sewer Capacity

There are numerous factors that come into play when you are trying to determine how to calculate a sewer system’s capacity. A few of these factors include:

  • Width and length of pipes
  • The slope of the pipe
  • Shape and setup of the pipe system
  • Material and texture of pipes
  • Speed of water going through the pipes

Calculating sewer capacity becomes a relatively simple process when you have this information.

The Equations for Sewer Capacity

Continuity Equation

The continuity equation is the simpler equation that will give you the peak carrying capacity for the section of pipe. All you need to do is multiply the velocity of the water measured in feet per second by the area of the section of pipe in square feet.

This equation is usually written in this way: Q = V x A, with Q being the peak flow measured in cubic feet per second.

Manning Equation

The manning equation is the more complex equation and can be used to find the velocity of the water, which you can then plug into the continuity equation. This equation is typically written out as V = (1.486R2/3S1/2)/n.

Here, the R stands for the hydraulic radius of the pipe, while S stands for the slope. The n represents a number known as the Manning coefficient. This number represents how rough the pipe material is. A copper pipe would have a Manning coefficient of about .011, while a vitrified clay sewer pipe may have one as high as .015.

If this seems too complex, don’t despair. At Harris Main and Sewer, we are well aware of the NYC dep standard sewer and water main specifications and know how to ensure your sewer systems meet them.

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