Filter Backwash Methods

 

Cleaning gravity filters is necessary to remove the solids collected by the media during the filtration run. As solids are accumulated in the filter media, the headloss or force required to maintain the flow increases. A point is reached where either the flow cannot be maintained or solids are driven through the filter.

There are three common methods of cleaning filters in use today: hydraulic backwash only, hydraulic backwash plus surface wash, and hydraulic backwash plus air scour.

 

Hydraulic Backwash
The traditional method of cleaning a filter has been to reverse the flow and bring clean water up from beneath the bottom of the bed at a rate sufficient to fluidize the media and shear off the floc. (See References.)

Chemicals and trapped solids can adhere tightly to filter media grains. The cleaning of granular filters by the upward flow of backwash water alone to fluidize the filter bed is inherently a weak cleaning method because it is very sensitive to flow and the shearing action may not be adequate to remove chemical floc from the media. If the media is not completely cleaned each time, dirt can accumulate causing mudballs. If mudballs are allowed to accumulate, they can sink to the bottom of the media and plug the gravel. The bed will then become upset, resulting in poor performance and loss of media.

 

 

Hydraulic Backwash Plus Surface Wash
Another problem encountered in filters is the build-up of a crust of solids on the surface of the media. Chemicals and dirt can attach to the upper surface of the media and form a crust which may be too heavy to fluidize and backwash off. If it is not removed, this crust can form mudballs and ruin the filters.

The fact that a high rate backwash is not enough to eliminate mudballs has been recognized for a long time. Mudball formation is especially prevalent with highly turbid waters. Under these conditions some kind of auxiliary scour is considered necessary.

A common practice in the U.S. has been to install a rotary distributor to clean the media surface. This distributor, or "surface wash" mechanism, is suspended at the center, approximately two inches above the media. Nozzles at an angle of 15 degrees below the horizontal distribute high-pressure water to scour the media surface as well as provide motion to the distributor.

The surface washer is normally turned on for 1 or 2 minutes to scour the surface. Then the backwash flow is begun, expanding the media up past the surface washer and allowing it to continue its scouring action deeper in the bed.

Although surface wash is a substantial improvement in cleaning a filter, it does not completely eliminate mudballs. Nor does it clean the entire bed. The circular agitator has difficulty cleaning the corners where mudballs can form.

 

Hydraulic Backwash Plus Air Scour
The current practice in Europe and in the U.S. is to provide an air scour in place of the surface wash. Air scour in the range of 3 to 5 scfm/sf provides much more violent agitation of the media than surface wash. In addition, air scour combined with a low rate backwash (concurrent air and water) is the most effective way to remove solids from a filter. The media grains have a greater potential to collide, increasing the scrubbing action during the air scour. Side by side tests have shown that air scour is the most effective backwash method and uses the least water. (See References.)

 

 

Subfluidized Air/Water Backwash
Some manufacturers use strainers and a subfluidized backwash with air during the entire backwash cycle, claiming complete mixing of the media and good solids removal. However, the mixing may not be very uniform. Since the strainers are not very close, the entire floor is not swept and the bottom 8 to 10 inches of media is usually ineffective.

The subfluidized concurrent method requires baffles around each backwash water trough to keep the backwash from washing out media. In addition to deflecting the media, the baffles also deflect much of the solids. They only allow solids to be collected from directly below the trough. Most of the solids can be seen recirculating up and down between the troughs without being removed.

 

In addition to the solids removal problem, there are several other deficiencies with the baffled trough design: scum removal, media attrition, and fines removal.

Scum Removal
Since all water must go out through the bottom of the trough area, there is no way to remove scum from the water surface. Any chemical scum, leaves, or other debris must be removed manually.

Media Attrition
Most modern media designs use anthracite. Anthracite breaks down slowly with time. The shielded trough design is commonly used with a continuous air/water backwash. Most backwash systems use air for 2 to 3 minutes. A continuous backwash cycle may last 8 to 12 minutes. Air scour provides a high degree of agitation. Therefore, the extra time required by continuous operation can result in faster attrition of the anthracite.

Fines Removal
According to conventional wisdom low backwash rates can be used with shielded troughs. However, if anthracite media is used, a high-rate backwash is required to move the fines to the surface for skimming. A high-rate backwash is not required all of the time, but the capability must be available for periodic use.

If it is not possible to fluidize and skim off the fines, the filter will deteriorate rapidly and have to be rebedded more frequently than it should. If dual media is used, a fluidizing backwash is mandatory to classify the two media types.




Monomedia and Subfluidized Backwash
Even with the highest quality sand and anthracite, there is a typical percentage of fines present as a result of the manufacturing process. When it is not possible to fluidize and skim, fines occupy the voids between the larger grains and reduce the effective size of the media. Without a fluidization step, the media will be mixed up, which means there will be fines in the bottom of the bed. Many of the fines may, therefore, work their way through the filter.

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