16 Sep Three Ways to Prevent Future Filter Problems
THREE EASY WAYS ENGINEERS CAN AVOID FUTURE PROBLEMS
FIRST: UPLIFT RESISTANCE VALUE
List an uplift resistance value that will provide substantial underdrain resilience. Uplift of an underdrain is a much more serious risk than a compression failure downward, in large part because upward backwash flow rates are a multiple of downward filtration rates. Uplift force requirements are project-specific and should include a safety factor. Typical values range upward from 1400 psf, or 10 psi, and depend on pressures from backwash pumps, elevated tanks, constant head boxes, and other structures. Recently, we’ve seen specifications for a stainless-steel underdrain requiring only 850 psf, or 5.9 psi, which is far too low.
UNDERDRAIN UPLIFT FAILURES
SECOND: ENSURE THE MEDIAS ARE COMPATIBLE
For dual media filters, ensure that the medias are compatible. An AWWA OpFlow article from March 1999 provides important guidance regarding relative sizes of sand and anthracite. To ensure that the entire bed fluidizes at the selected backwash rate, the sand must have approximately the same or lower required fluidization backwash rate as for the anthracite. The effective size of the sand should be selected to provide coarse-to-fine filtration, without causing excessive intermixing. An intermix ratio, calculated as the d90 of the anthracite divided by the effective size of the sand, of 4 results in undesirable and substantial media intermixing. A ratio of 2 – 2.5 causes a sharp interface, which may bind the filter. An intermix ratio of around 3 is recommended and provides effective transition between the two medias.
THIRD: ROBUST POST-INSTALLATION PERFORMANCE TESTING
Require robust post-installation performance testing to prove effective distribution. In a previous video, we highlighted the importance of stringent commissioning testing to make the filter underdrain manufacturer prove that what was installed is what the utility paid for. Orthos has created a free CSI-format testing spec section for owners and engineers to use to validate gravity filter performance. This Word doc download details 5 important tests for numerous types of filter underdrains—click here to download.
RECAP: To prevent future filter problems, in your specifications, (1) list a substantial uplift resistance value, (2) ensure the medias are compatible, and (3) require robust commissioning testing. These three easy spec features will go a long way to ensure excellent filter performance.
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