| TECH-TIP: Methods for managing filter ripening spikes - part 2
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TECH-TIP: Methods for managing filter ripening spikes - part 2

Methods for Managing Filter Ripening Spikes - Part 2 - Extended Terminal Subfluidized Wash

David HardyDavid K. Hardy, Utah Valley Water Treatment Plant Manager
Central Utah Water Conservancy District
Phase IV Excellence in Water Treatment Award Recipient - 2003

Since the early days of surface water treatment and granular filtration, water professionals have been challenged with turbidity spikes associated with filter startups. These events are called ripening spikes (see Figure 1). 



(Figure 1 - Filter Profile Highlighting Ripening Spike)
Figure 1







Over the years, methods and theories have been developed as to why this happens and what can be done to reduce or eliminate these spikes.  More than 90 percent of all particles that pass through a well operated filter do so during the ripening period.  Our goal then, as operators, is to eliminate these particles and events.

Why do Ripening Spikes Occur?
For this discussion, it is assumed that plant chemistry is optimal and is not part of the issue with filter performance.  However, if the optimization status of plant chemistry and chemical dose is unknown, it is suggested that utility staff further investigate this area during the process of examining filter ripening spikes.

Ripening spikes occur mainly due to remnant particles that remain in the filter box following the backwash cycle.  What is meant by “remnant particles”?  During a typical backwash cycle, the filter is subjected to some sort of surface wash or air scour cycle to break up the filter media and allow for a more complete detachment of trapped particles that have accumulated in filter beds and on the surface of the media itself.  Following the air scour, the backwash cycle is ramped up to a high backwash rate to achieve the fluidization and expansion of the filter media and to wash the accumulated solids from the filter bed.    During the high backwash rate period, wash water pushes out suspended particles and removes almost all of the solids from the filter bay.  But because the filter media is fluidized, media particles (sand/anthracite) continue to collide with each other, causing additional scouring action and the detachment of microscopic particles.  At the end of the backwash, if the high backwash rate is reduced to zero without a sub-fluidized (or low flow) step in the backwash cycle, remnant particles can remain within the filter box no matter how visibly clean the filter appears. 

In the previous issue of the Partnership for Safe Water newsletter, filter to waste was discussed as a viable means of managing ripening spikes, and guidelines were provided to optimize that process.  However, not all plants have the ability or infrastructure to adequately perform filter to waste.   In this article, “Extended Terminal Sub-fluidized Wash” (ETSW) will be discussed as an alternative and effective method for managing ripening spikes.

EXTENDED TERMINAL SUBFLUIDIZED WASH (ETSW)
ETSW is the process of removing remnant particles from the filter that, using typical backwash techniques, remain in the filter box and later show up as a ripening spike at filter startup.  Our utility calls ETSW “Rinse to Waste” because unlike Filter to Waste (which ripens a filter by passing water downward through the filter) ETSW  works by flowing wash water upward through the media and rinsing particles from the filter box during the final minutes of the backwash; and it is done at a sub-fluidized wash rate.

How it’s Done
Note that you will need to have manual control for the necessary filter components if ETSW is a new procedure for your plant.

1) The filter is washed clean following the treatment plant’s standard SOP for filter backwash.

2) Once the filter has concluded the high backwash portion of the wash, the wash rate is lowered to a sub-fluidized flow rate (about 6 gpm/ft²).

3) The filter flows at this sub-fluidized rate until one complete volume of water has been rinsed from the filter box.  [One volume equals the amount of water in the filter from the under drain to the lip of the trough or weir]

4) Once the filter has been “rinsed”, shut the backwash down following the plant’s standard SOP for this procedure.

5) Return the filter to service normally and watch the results! 

Why it Works
Particles that remain in the filter box following a standard backwash cycle are stable and do not filter well (see Figure 2 below).

(Figure 2)



As the filtration cycle begins, these remnant particles may pass through the filter and manifest themselves in the form of a ripening spike (see Figure 1).  By rinsing these particles out of the filter box using ETSW technique prior to filter startup, the ripening spike in most cases is significantly lower, as measured by both particle counters and turbidimeters (see Figures 3 & 4).

(Figure 3- Filter Profile Generated Without ETSW)


Figure 3






















(Figure 4 - Filter Profile Generated Utilizing ETSW)

Figure 4






















Pros and Cons to ETSW 

Pros

1) It works!

2) Minimizes filter start up spikes before they happen and improves filtrate quality at the all important beginning of the filter run.

3) Allows plants lacking proper Filter to Waste capabilities to manage ripening spikes.

4) Allows plants that DO HAVE Filter to Waste capabilities to dramatically shorten and, in some cases, eliminate the need for Filter to Waste.


Cons
1) ETSW requires between 5 -15% more backwash water to be used.

As indicated by the lists above, ETSW can be an effective technique for managing filter ripening spikes.  If ETSW is new to a facility but is something staff is interested in implementing, don’t hesitate to reach out to other utilities or resources for assistance.  As a trial, plants may wish to consider implementing ETSW on a limited number of filters, initially, to compare the results of ETSW with conventional filter backwash techniques.  As when making any plant operational modification, always remain focused on ensuring and maintaining water quality.  Finally, the Partnership for Safe Water reminds readers that every plant is unique and different.  The effectiveness of ideas and suggestions presented in this article may vary from one plant to another.

Acknowledgements:  Dr. James Amburgey (Georgia Tech);  “An Enhanced Backwash Technique” AWWA Journal, December 2003; “Optimizing ETSW Filter Backwashing”  Water Research 39 (2005); and my good friends and co-workers at the Utah Valley Water Treatment Plant for their undying pursuit for excellence.

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