Filtration: You're Doing It Wrong

Filter System 1017 Feat

To the unknowing eye, a crystal-clear pool looks perfectly clean and safe. There's no sign of turbidity or algae, nor the telltale "chlorine" smell we all recognize as unwanted chloramines due to improperly treated water.

Pool professionals and public health officials, however, know invisible dangers sometimes lurk beneath the sparkling surface, ready to harm unsuspecting bathers. What many do not know, however, is that traditional methods of removing these dangerous pathogens often fall short and leave the public at risk of infection, or, if left to fester in the pool, a wider outbreak. That's the view of Dr. James Amburgey, associated professor of civil and environmental engineering at the University of North Carolina at Charlotte. He began studying water treatment as an undergrad at UNC-Charlotte and went on to earn his master's and Ph.D. from Georgia Tech.

"In grad school I studied nothing but water treatment, which included some research on cryptosporidium, for four years," he says, "and to be honest I really had no interest in swimming pools."

That changed when Amburgey took a job at the Centers for Disease Control and realized the other scientists there really didn't know a lot about filtering water.

"While I was at CDC, we started working on a model pool code, and it was clear that I could really help with that, especially in the filtration area," he says. That assignment led Amburgey down a rabbit hole of research on pool filtration and coagulation, and he emerged with a grim assessment of the industry's practices: "Well, in drinking water treatment we know how to make filters work. But in swimming pools we kind of do everything in a way that makes them not work."

Amburgey, who has presented his ideas for improving pool water filtration to public health officials and pool operators at NSPF's World Aquatic Health Conference for years, will teach a course at the International Pool | Spa | Patio Expo, Oct. 29 to Nov. 3, in Orlando, Fla., as part of the Genesis Education track focusing on design, construction and drawing.

"The idea behind this class is to kind of teach the basics of filtration, in particular what works and what doesn't work," he explains. "Everybody wants cleaner water, but most people don't understand what it takes to produce cleaner water."

Removal Mechanism

The class will begin with a review of the way filters remove tiny particles — including those like crypto that have an exterior that's resistant to chlorine — from the water. An understanding of these principles, he says, forms the basis for all filtrations decisions.

"So we're looking at microscale microorganisms," he says. "Are we looking at making sure the particles can't fit through a certain size pore? A lot of times in a sand filter you've got pores that are on the order of 50 to 75 microns, but most of the particles we're trying to remove are 5 microns or less. So it creates a situation where the filter won't remove the particles just by being there. You have to enable the filter to work. That's what the drinking water industry does well, and the swimming pool industry can improve on."

One way to address the problem is to add a coagulant to take away the charge on the small particles, which allows the tiny particles to mass together into larger ones that can be then removed by contact with the filter. Then, according to conventional wisdom, efficiency improves as the filter catches more and more of these clumped particles and the "holes" in the filter decrease in size. It seems reasonable, but, according to Amburgey, it's a fallacy.

"Basically, what happens is the filter begins to clog. Then there's less area for the water to go through, so the water has to go faster, and as the water goes faster the sheer forces increase and you can actually pull particles back out of the filter and into the water.

"Typically pools are backwashed when the pressure differential gets to about 10 psi. And, depending on the coagulant used and the conditions in the pool, that number could be more like 7 or 8 psi. I don't think we need to go all the way to 10, much less push it to 12 like some people want to. It's not a case where the longer you go the better. It's just the opposite. I think if you go too far it just won't work at all."

Amburgey speaks from experience, having tried and failed to recreate the myth of dirty filter efficiency in a lab. "I've never once been able to show that that works," he says.

Media Matters

A lot of Amburgey's research, and a lot of what he presents at the conference, will focus on sand filtration, but he won't give the short shrift to cartridge or D.E. filters. He'll dedicate time to explaining ways to improve filtration efficiency on them all.

"All three are different," he says. "They all have different mechanisms and there are different ways of making them work. And by virtue of the way they work, they all have different limitations and different things you can do to address them."

For example, you can put some precoat media on a cartridge filter to improve its efficiency. A cartridge filter will already catch particles smaller than a sand filter, but adding Perlite or D.E. helps them capture particles as small as a few microns. "Nothing magical about it," Amburgey says.

"And some of the filter cartridge manuals even recommend this if a user is having a hard time getting the media to perform as expected."

Precoat media, on the other hand, already filters down to the 2- to 5-micron level needed to trap small pathogens like cryptosporidium. On a pore-size level, obviously, the things that do get through are vanishingly small. That efficiency means that it will clog a lot faster than a sand filter will.

"Say you have a D.E. filter with a 3-foot diameter. That's all the surface you get," he explains. "But with a sand filter, that same 3 feet in diameter is also a foot deep, so you get all that extra surface area in the foot of depth."

For D.E. filters, the amount of media you use, how evenly coated the support material is and whether that support material is structurally sound all play a role in the unit's effectiveness. Fortunately, Amburgey has some tips he'll share at the conference, including using the right type of media; using the right amount of media (not always the same thing as what's recommended in the manual, by the way); avoiding turning the flow on and off (also called "bumping" the filter), which can make media fall off the septum; and keeping the septum clean and in good repair.

"There's research that I've done and published on this topic, and I'll share what I know with the people at the presentation."

Errors and Omissions

In his years of research, and in conversations he's had with pool operators while working at the CDC and presenting at the World Aquatic Health Conference, Amburgey has seen and heard about nearly every mistake a person can make with a pool filter. Chief among these are mistakes made during backwashing.

"A lot of the conventional wisdom says that you should replace the sand once a year and try to figure out a filtration rate and a backwash rate that match," he says. "In drinking water treatment there may be a filtration rate of 2 to 4 gallons per minute per square foot, and a backwash rate of 20. But in a swimming pool we try to have just one rate for both, so we try to filter at 12 or 15 and we try to backwash at 12 or 15."

In that scenario, the filter rate is too high and the backwash rate is too low. This scheme reduces efficiency doubly. "If we're not getting the filter clean, that affects performance, and if we're filtering too fast, that also affects performance," he says.

RELATED: The Dirt on Filter Cleaning


Back in the 1920s, two researchers named Gage and Bidwell looked at pool recirculation efficiency and formulated what became known as the Gage and Bidwell Law of Dilution. According to Amburgey, understanding this law is crucial in determining how many "turnovers" are needed to filter all the water in a pool a single time.

"The concept of a turnover is that you're filtering the volume of water that is in a pool," he explains. "So if you've got a 1,000-gallon pool and you filter 1,000 gallons from the pool, theoretically you've filtered the pool one time. But in reality you've only turned over 63 percent of that volume. That's because you take some water out and put some back in for every cycle and it all mixes. You're just randomly taking whatever water is in there – some having been filtered and some not – and running it through again."

According to Amburgey, a second turnover gets you up near 85 percent, and a third takes it above 90. To get all the water through filtration takes about seven turnovers. That's where things get tricky.

"If you're looking at getting 99.9 percent particle removal, well, first you've got to get 99.9 percent of the water through the filter, and then the filter would have to be 100 percent efficient. But the reality is that sand and cartridge filters are typically only about 25 percent efficient at removing crypto-sized particles."

In short, you've got a recirculation system that's only getting 63 percent of the water to the filter and a filter that's only removing 25 percent of the small particles from that 63 percent. That's low treatment efficiency.

Faced with those facts, an operator may be tempted to increase the filtration rate. Makes sense, right? Instead of turning the water over every six hours, turn it over every four. Bad idea, Amburgey says.

"That will probably make the filter perform even worse because you'd be putting more water through the same filter," he explains. "And so even if you're treating more water, you are treating it less efficiently. So there are mathematical tradeoffs and calculations you can do to determine what actually makes sense instead of relying on what conventional wisdom or standard practices would dictate."

The objective is simply keeping the public safe from waterborne pathogens. No amount of filtration and chemical treatment will keep a pool constantly clear of contaminants, but a well-executed plan will prevent most serious outbreaks.

"Once people start jumping into a pool, the water is going to get some level of contamination in it," Amburgey says. "And if somebody has cryptosporidiosis and they have an accident in the pool, that pool is going to be contaminated. Now the question becomes how fast can we clean that up, and that becomes a function of the recirculation efficiency and the filtration efficiency.

"We can't keep crypto out of the water. The idea is to not let those contaminants just stay there and keep infecting people. The goal is to at least filter and clean things up on a daily basis so you get things down to an acceptable, non-infectious level."

Dr. Amburgey will address the above topics and more, with an emphasis on best practices for real-life aquatic venues. A list of learning objectives and details about registering for this and other Genesis University courses is available at 


SERVICE 321: Introduction to Advanced Pool Filtration

Presented by Dr. James Amburgey

Where: International Pool | Spa | Patio Expo
Orange County Convention Center (North Hall)
Orlando, Fla.

Room N320C

When: Wednesday, Nov. 1, 7:30 AM

This course will provide an overview of how sand and precoat (D.E.) filters work at the microscopic scale and extend these concepts into performance expectations in full-scale aquatic venues. Information will be provided on how to improve (and sustain) the performance of each type of filter. Guidelines will be provided on how to avoid common mistakes in designing new filtration systems and comply with current and future pool code requirements. Finally, discuss the role of the filter in the overall recirculation system regarding particle/pathogen removal.

Learning Objectives:

  • Review the mechanism by which different pool filters remove particles and pathogens from pool water.
  • Identify techniques to improve the performance of pool filters.
  • List common mistakes that lead to filtration failures.
  • Calculate the percent removal of particles/pathogens over time in a swimming pool system.
  • Analyze existing pool code requirements for filtration systems.

Credits: 1.30hrs / 0.2 IACET CEUs

Education Tracks: Genesis Education, Service


Buyer's Guide
Find manufacturers and suppliers in the most extensive searchable database in the industry.
Learn More
Buyer's Guide
Content Library
Dig through our best stories from the magazine, all sorted by category for easy surfing.
Read More
Content Library