Filter Run Time does NOT tell the entire story for conventional granular media filtration. Here’s why:

Operators aim for long filter runs to minimize backwashes. However, lightly loaded filters may not perform well. Instead, focus on throughput per area before backwash, or unit filter run volume (UFRV), calculated as filter rate times run length divided by filter area.

Identical filters can have similar throughput with different run times due to varying loading rates. UFRV also allows comparison between filters of different areas.

At a conference, Orthos presented a paper on Nashville’s Omohundro Water Treatment Plant. Its 1929 nozzle-based monolithic filters achieve UFRVs of at least 10,000 gallons per square foot, showcasing long-term performance.

The UFRV calculation is relatively easy. Each cell of Omohundro’s double filters is 12.5’ x 33’ and each filter produces at least 1.9 MGD before backwash with run times of approximately 105 hours.

The loading rate is 1,900,000 gallons per filter divided by 1440 minutes per day divided by the filter area of 12.5’ x 33’ x 2 cells, which equals 1.6 gpm per square foot.

Next calculate UFRV by multiplying 1.6 gpm per square foot by 105 hours and by 60 minutes per hour, resulting in 10,000 gallons per square foot.

In contrast, Omohundro’s sister plant, KR Harrington WTP, had filters averaging only 5,600 gallons per square foot UFRV before rehabilitation. After installing the Orthos-Centurion system, UFRV exceeded 22,000, indicating four times the throughput and improved performance.

If you operate conventional gravity filters, regularly calculate UFRV. If values are below 10,000 gallons per square foot, investigate potential improvements. Trending UFRV can also identify changes over time.

Source: AWWA ACE17 SUN01 Workshop, 06/11/17

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