Hydraulic Short Circuiting

Hydraulic short circuiting has come up in a couple of my recent classes, so I decided to discuss it today. This is an issue that impacts treatment process performance, so it’s important for us to have a handle on what it is and what we can do about it.

If we were to be standing by the stream shown in today’s photo, we would notice that much of the water flows very swiftly through portions of the stream, but some of the water kind of gets lost in the quiet pools near the banks. This is a great way to visualize hydraulic short circuiting. In an “ideal” stream — from a hydraulic rather than a scenic standpoint — each molecule of water would line up in order. The first molecule in would also be the first one to leave any given reach of our stream. That clearly is not the case with our natural stream. So, as scenic as our stream might be, it is far from ideal in terms of its hydraulic performance.

In treatment plants, hydraulic structures are intended to be ideal; however, even then we rarely see completely ideal performance. Some of the water that enters a flocculation or sedimentation basin or a clearwell will zip through the structure quickly, just like the water in the middle of our stream. Some of the water will linger in the corners of our structures, just like the quiet pools of our stream.

The issue here is time. We calculate the detention time in a hydraulic structure by dividing its volume by the flow rate of the water passing through it. But this is an ideal — or “theoretical” — detention time. In reality, our fast-moving water gets through our structure in a much shorter time than this theoretical detention time.

That’s a problem, because flocculation, sedimentation, and disinfection are time-dependent processes. If this were not so, we would not see such huge structures constructed for these processes. We could save a whole lot of money if our sedimentation basins only needed to hold the water for a couple seconds, instead of a couple of hours!

So for today, we’ll leave this subject at this: hydraulic short circuiting is a common occurrence in treatment structures. And the result is that some of the water we are treating gets far less treatment time than what we might expect. And time is critical to the performance of many treatment processes.

Later, we’ll get a look at how we contend with hydraulic short circuiting.

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