Water Treatment 101: A Filtration Primer

Studying for an exam? Refreshing your knowledge? Whatever the case, go back to the basics with this filtration primer.

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Editor's Note: This article is part of our "Water Treatment 101" series, which covers basic treatment processes. To read other articles in this series, see "6 Tips for Improving Coagulation, Flocculation and Clarification" and "A Disinfection Primer."

Filtration.

You pass water through a media bed of sand or gravel. The particles are removed and clean water flows out the bottom.

Simple, no?

Not exactly.

“If it were that easy, anybody could do it,” says Brock McEwen, water technology director for CH2M HILL.

Filter operators need to comprehend the basic design principles of their systems, pay close attention to pretreatment, understand filter ripening and be smart about back washing, he says.

Filters come in all shapes and sizes, from shallow bed to deep bed, single media to multimedia, with sand, coal, activated carbon and combinations of media. Backwash mechanisms and underdrains differ in design. Filters follow different pretreatment schemes.

“While design and installation are really the province of engineers and contractors,” says McEwen, “it’s the operator who has to deal with day-to-day operation. Too often, operators rely only on empirical knowledge [what they see and experience] without understanding the underlying science of filtration.

“Know your filters,” he says. “Each one behaves differently. If you understand the basic mechanisms, you can do a better job.”

It’s the pretreatment, stupid
Of all the design variables, McEwen says pretreatment is the most critical.

That’s because the surface charge of the particles you are trying to remove is determined by coagulation.

“Look at chemical addition,” McEwen says. “Head loss, turbidity problems — if your filters aren’t working right, it’s almost always pretreatment.”

Granular media filters don’t remove particles by acting as a sieve or strainer, explains McEwen, and they are not membranes which rely on a barrier to reject particles.

Rather, they work by first getting a layer of particles to stick or attach to the media, and then by getting more and more particles to attach to the particles that are already stuck to the media.

This stickiness is critical, explains McEwen. “The particles must be properly chemically pretreated in order for them to attach to the media and also to other particles.”

The time is ripe
The process of particles building up and providing more and more filtering capability is called ripening, and it’s an important principle for operators to understand. After the filter bed has been cleaned, McEwen explains, the particle layer needs to built up again, restoring the filtering surface to its most effective removal rate.

In other words, your filter may be cleaned, but until it ripens, or adds that first layer of particles sticking to the media, it won’t start filtering at maximum effectiveness.

That leads us to backwashing. And media maintenance.

“Some operators tend to backwash too much or too long,” McEwen says. “You can get your filter bed too clean, and the ripening process has to start from square one. That wastes water.

“The backwash water doesn’t have to be drinking water clean,” he adds. “A turbidity level of 10 to 20 NTUs in the backwash water is a good starting point, but you’ll know only if you track initial head loss [and flow], ripening time [and flow] and see ultimately long filter run times between backwashes.

Apples to apples
He also points out that although backwashing is a vital maintenance procedure, filter effectiveness shouldn’t be judged solely on backwashing frequency.

“I’ve had some operators compare their filter’s performance against others by talking about how long they can run between backwashes,” he says. “They’re not comparing apples to apples. What’s the bed depth? What’s the loading rate? The media type?”

A better measure of filter performance is unit filter run volume. That’s calculated by recording the gallons per minute per square foot of filtering surface, and then multiplying it by the filter run time in minutes.

“That’s a better indication of filter performance,” McEwen says.

That doesn’t diminish the importance of backwashing. McEwen cites instances where filter beds have become so fouled that mud balls have formed and high-pressure water wands were required to break them up, or worse yet, the media had to be removed from the box and cleaned.

“Backwashing is critical,” McEwen says, “and with proper backwashing, a filter will last a long time.”



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