Filter Backwashing

by Jennifer Peters Dodd & James D. Fettig

Why Backwash?

During the filtration process, particles suspended in the filtered water are removed largely through entrapment within the filter media. As more and more fluid is passed through the filter, the suspended particles accumulate within the media, reaching levels that lead to one of two detrimental events.

Headloss monitor and recorder for sand filter. Photo by Dan Gallagher

They can either cause the head loss within the filter to reach excessively high levels (6 to 8 feet of hydraulic head), or they can become pushed through the filter, resulting in product water turbidities that reach unacceptable levels (greater than 1 NTU). Therefore, in order to maximize the use of a given filter, it becomes necessary to remove these entrapped particles from the media itself. Filter backwashing is the process by which this is accomplished. From an operational standpoint, backwashing is initiated when either one of the two aforementioned conditions occur, or more commonly, after a preset run-time interval has been reached.

Backwash Basics

The backwashing process begins by pumping product water from the clearwell to the filter bottom through an underdrain system designed to distribute this "washwater" evenly across the bottom of the filter (see Figure). Approximately 1-5% of the product water a filter produces during a run is ultimately used for backwash. Even distribution of washwater flow is necessary to prevent short-circuiting and channeling that might otherwise hinder the effectiveness of the backwash process and disrupt the filter structure. Wheeler Bottoms and Leopold Blocks are two examples of such underdrain systems specifically designed to distribute backwash flow. The backwash fluid velocity starts slow and is gradually increased to avoid disrupting the bed structure (see Figure 1). Typical backwash flow velocities are between 15 and 20 gpm/ft^2. As the washwater flows upward through the filter, it lifts the media, causing the bed to expand and assume a fluidized state. Usually the degree of bed expansion is in the range of 20 to 50% of the static bed depth. Bed expansion and fluidization permits entrapped particles to become released and flushed upward and out of the media. The dirty washwater that has passed through the filter is then collected in washwater troughs located about 3 feet above the static filter surface, and is typically sent back to the head of the treatment plant, or to drying ponds. In some locations, the washwater is sent to wastewater treatment facilities.

A series of movies showing a backwash in progress are available below.

Video clip of backwash startup (670 Kb)

Video clip of backwashing in progress (256 Kb)

Video clip of the end of backwash (359 Kb)

Supplemental Techniques

In many cases, bed expansion and fluidization is not sufficient to completely clean the bed. In these cases, surface wash and/or air scour techniques are employed in addition to backwash to increase the efficacy of the cleansing process. A surface wash system uses jets of water set at 1 to 2 inches above the surface of the static bed. These jets are turned on 1 to 2 minutes prior to backwash and continue to run until approximately 2 to 3 minutes before the backwash flow is shut down. The function of the jet wash is to increase the scouring effect necessary to dislodge stubborn particles from the media. Although such systems can be very effective for the upper regions of the filter bed, they tend to have little effect on particles in the lower reaches of the filter. Air scour systems, on the otherhand, function by pumping air through the bottom of the filter prior to backwash until just before the washwater begins to overflow into the washwater troughs. Because the air bubbles are less dense than the washwater, they travel at a greater velocity relative to the fluid, increasing the turbulence, and hence the shearing forces, within the media. Although air scour can be effective, it can be potentially damaging to the filter, as it increases the likelihood of the filter media itself being swept into the washwater trough. If too much media is lost, the filter would lose its functionality and expensive replacement would be required prematurely. Therefore, it is important to guard against this loss of media if air scour is to be used. This can be accomplished by either decreasing the backwash velocity while scouring is occuring or by shutting down the scour before the washwater overflows into the collection troughs.

REFERENCES

AWWA. 1990. Water Quality and Treatment. pp. 516-527.

Reynolds. 1982. Unit Operations and Processes in Environmental Engineering. pp. 131-169.

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