Analytical Technology’s Entech FilterSmart Backwash Monitor is becoming a popular tool for managing backwashes in gravity filters. In two recent customer trials in Virginia and Colorado, FilterSmart revealed what may be a common error in the manual measurement of the media level (media expansion) during the backwash.

It's common to use a set of “pan pipes” or “organ pipes” — a long-handled device with graduated-length cylinders across the bottom — to measure the expansion of the media during a wash.

In typical use, the measurement tool is lowered to the elevation of the top of the static media and a mark or datum is made on the handle corresponding to the top guardrail or a concrete walkway. Then, during the high rate portion of the backwash, the device is lowered to this datum, and the successive cylinders will fill until the upper limit of the expansion (usually 20% or so of the media depth) is reached. So, if each cylinder is a multiple of 1-inch, the media for a bed 30 inches deep might fill up all the cylinders up to 6 inches in length, but no higher. The tool is then simply withdrawn, and the cylinders are visually inspected for media. The tool can then be emptied and the process repeated several times during the high rate portion of the wash.

Error discovery

What was recently discovered in these two locations, however, was that the tool was lowered into the filter at the beginning of the wash. Each facility has air scour, so when the backwash went into the air scour part of the process, the media was carried up and into the longest cylinders of the expansion tool, thereby overstating the degree of expansion in the filter during the wash. This of course was a potential problem in that the facilities’ high flow rates were adjusted accordingly, resulting in artificially low expansion rates, which can lead to mud-ball formation and poor overall filter health.

When the plant measurements were compared to the FilterSmart trends of each wash, it was clear there was disagreement. A simple review of each plant’s processes and procedures made it clear what was happening. Each plant subsequently lowered the expansion tool after the air scour had terminated, but during the high rate, and results were compared again. This time there was no disagreement (see image).

Real-time measurement

FilterSmart can enable you to see into your process like never before, with real-time measurement of media level and turbidity. If you would like a free trial of ATI’s FilterSmart Backwash Monitor, use the contact info below.

Analytical Technology Inc. designs, manufactures and distributes analytical instruments based on electrochemical and optical sensors. The company specializes in the areas of toxic gas detection and water quality measurements, continuing to develop reliable monitoring systems.

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