Evolution of pool filtration

by Sally Bouorm | August 1, 2010 8:11 am

By Bob Hawken

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Commercial swimming pool filtration has evolved significantly over the past six decades, encompassing many changes in methods, materials and technology. Many earlier filtration systems were similar to what was used for municipal water treatment, as that was the prevalent knowledge at the time. Gravity sand filters and open-top vacuum diatomaceous earth (DE) filters were among the methods used. As the commercial and municipal pool market grew, filtration products were developed to better satisfy these needs.

A discussion about filtration systems can not take place without realizing the filter is but one major component of the overall recirculation system. The proper sizing of commercial pool filter(s) is done in concert with the sizing of the other components in the system, such as piping, pumps, etc.

First, however, it should be determined what type of pool a filtration system is going to be used on. Over the years, many engineering changes have been made with regards to sizing a recirculation system. Most of these changes were made in conjunction with health department codes governing commercial swimming pool design.

Older commercial and municipal pools typically had one body of water, which was mainly used for recreational swimming. In most cases, these were large bodies of water that incorporated one filter/recirculation system, with the entire water volume being recirculated approximately every eight hours.

As the industry progressed, facilities were built with multiple pools to satisfy different needs for various water activities, e.g. competition and toddler pools, as well as whirlpools. Each new development required modifications to the size of the recirculation system, as well as turnover rates.

Challenging filtration needs

Today’s aquatic facilities are more challenging than ever, with the incorporation of lap, activity and slide pools, as well as lazy rivers and splash pads. A recirculation system for a competition/lap pool should be sized to provide a six-hour turnover rate, whereas the recirculation system for an activity pool or lazy river should provide a one to one-and-a-half hour turnover rate. Although the water volume in an activity pool may be less than a competition pool, filters, pumps and piping may be designed larger due to the faster turnover rate required.

The early years of the waterpark industry provide a great example of how recirculation system design has evolved. Most original waterparks were built with a common filter plant to handle the filtration needs for all bodies of water. However, operators soon realized this was not the best way to design a waterpark, as inconsistencies with water and bather volumes in the different pools gave rise to water clarity and sanitation issues. Also, if there was a fecal accident in one pool, the entire park was at risk. As such, waterpark design parameters were improved to provide a separate filter/recirculation/sanitation system for each body of water.

Sand filters: An early favourite

As design parameters in the industry improved, sand filters were the product of choice for most facilities. They progressed from earlier rapid-rate filters, which had a 7.33-m³/hr/m² (258.8-cf/hr/sf) maximum filtration rate, to current day high-rate sand filters, which offer a 48.89-m³/hr/m² (1,726.5-cf/hr/sf) maximum filtration rate. (The filter rate is the velocity at which water passes through the filter media.)

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High-rate sand filtration works on the premise of bed depth penetration; the faster the filter rate, the further the particulates will be driven into the sand bed. Vertical sand filters have a deeper sand bed than horizontal sand filters, so a higher filter rate can be utilized.

Most sand filters are National Sanitation Foundation (NSF) listed up to 48.89 m³/hr/m² (1,727 cf/hr/sf); however, this is the maximum. Good design practices (and most health department codes) limit this rate to 36.67 m³/hr/m² (1,295 cf/hr/sf). As horizontal sand filters have a shallow media bed depth, the target filtration rate is between 24.44 to 29.33 m³/hr/m² (863 to 1,036 cf/hr/sf).

As pool design and engineering requirements changed over the years, filter manufacturers responded to accommodate the new challenges by offering sand filters in different configurations, sizes and materials.

Larger bodies of water and faster turnover rates required larger filters, which also meant bigger equipment rooms. As construction costs increased, mechanical room space was at a premium; multi-cell sand filters were developed to overcome this challenge. These are single-tank units, which incorporate two to three filter cells within the tank. This doubled or tripled the basic filter area without increasing the footprint required in the equipment room.

Horizontal sand filters were also used to increase filter area in a smaller space, as a cylinder on its side provides a far greater filtration area than a vertical-oriented cylinder. Horizontal filters can also be stacked in pairs, which allows the filtration area to be doubled without increasing space requirements in the filter room.

Filter tank construction

The materials used in filter tank construction is another area that has improved over the years. Most earlier rapid- and high-rate sand filters were constructed of carbon steel, with different interior linings to protect the metal from corrosion. Many systems are still constructed in this fashion; however, current high-tech lining materials provide long-term security to the tank’s integrity.

Fibreglass materials were first used widely in the commercial pool industry a few decades ago and continue to be popular today. The material’s non-corrosive nature makes it a good choice for commercial pool applications. A lot of the technology for composite tank construction came from the aquarium industry, where constant exposure to salt water made fibreglass the material of choice for life support system designers. Most horizontal filters available on the market today are constructed of fibreglass; however, this type of construction is not always available due to tank design, structural requirements and cost factors to achieve the finished product.

Consider the pool operator

When selecting the best type of filter for a particular application, it is important to consider the swimming pool operator. An operator can face many challenges throughout the course of a day while ensuring a facility is functioning properly. Therefore, the facility’s filter system should be easy to operate and maintain.

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For example, the backwashing process on a sand filter can be automated, while also having manual override capabilities. These filters can be programmed to automatically backwash at a pre-determined time of day or when a pressure differential between the influent and effluent sides of the filter is achieved. Different face piping arrangements and component types are also available to allow manual movement of linkages, which connect all of the critical backwashing valves. These types of features make it simple for pool operators to maintain the system properly.

Although cartridge and smaller-pressure DE filters are mainly used in the residential pool market, there are some commercially sized cartridge systems available; however, they have not become popular in this industry segment. Also, while DE filters provide excellent water clarity due to the media’s small-micron particle removal capabilities, in the commercial market these filters (mainly the open-tank vacuum variety) raise many health and safety concerns. In addition, handling and disposal issues, along with the labour intensity involved in maintaining a DE filter, has dramatically reduced their popularity, thus making them undesirable for this market.

Regenerative media filtration

In the mid-2000s, it became apparent ‘green’ and sustainable design was the direction in which new projects were heading. Facility designers and owners were looking at ways to conserve energy and gain Leadership in Energy and Environmental Design (LEED) points for their designs, to participate in grant funding and/or to show concern for the environment.

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One way this can be achieved when developing an aquatic facility’s recirculation/filtration system or replacing an older filtration system, is by using a regenerative media filter (RMF) system. These systems use Perlite media, a volcanic rock material that is mined, crushed and processed into a fine white powder. It is considered a great filter aid and its use as a DE media alternative is growing in popularity. It is considered an amorphous silica product, which does not have any adverse health effects and can be easily disposed of directly into the sanitary sewer.

RMF technology came from industrial applications where high-purity liquids were desired. This technology was refined and re-designed for utilization in the commercial pool market and received a NSF listing for use in public pool facilities.

The technology works on the premise of surface area filtration. These filters contain hundreds to thousands (depending on the filter size) of flexible tube elements, which provide a tremendous amount of filter area, while taking up very little space in the equipment room. These tubes are coated in Perlite filter media, which has the ability to remove particles as small as one micron. This not only produces crystal clear water, but is important in the battle against waterborne illnesses, such as Cryptosporidium (Crypto). Oocysts from this parasitic disease measure between four-to-six microns and can be captured in the filter.

No backwashing required

What makes RMF systems unique is they do not require backwashing. Depending on the aquatic facility’s size, this can save hundreds of thousands, even up to millions, of litres of water annually. Operating expenditures can be reduced significantly when considering costs for make up water, sewer discharge fees, in addition to chemically retreating and/or reheating the new water.

RMF’s ‘regenerate’ the media within the filter by going through a daily ‘bump’ cycle—an automatic function that shuts down the pump and closes the necessary valves to isolate the filter. During normal filter operation, flex tubes, which are attached to a tube sheet at the top of the unit, are held in place via air pressure. During a bump cycle, air pressure is quickly released and the tube sheet drops slightly, agitating the tube elements, which release the media into suspension within the tank. The cycle will occur 10 times and take approximately 10 minutes to complete before the pump comes back on. When it does, water is recirculated around the filter through a pre-coat loop in the piping and the filter media is redistributed onto the tube elements to create a new outer layer.

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Due to the large amount of filter area and what is considered ‘void space’ in the media, it takes a long time before the filtration media becomes saturated and needs replacement. A typical filter cycle runs an average of 30 days.

When the influent vs. effluent pressure differential is achieved, this indicates the end of the filter cycle. Spent media, water and particulates are merely drained from the tank via gravity through the drain connection on the tank bottom. The tank is then refilled to rinse the flex tubes and drained again—it only takes two tank volumes of water to clean the filter. Once the filter is clean, the drain valve is closed and new filtration media is introduced via a vacuum transfer system, starting the whole process over again. As RMF systems use less water when being cleaned, these filters are being utilized on more facilities around the world, as water conservation is a huge issue in many countries.

The space needed for a RMF system in an equipment room is typically one-fourth to one-sixth of the space required for a similar-sized sand filter. With current construction costs, the ability to reduce the equipment room space requirements and the facility’s overall carbon footprint, can represent significant savings for a facility.

Filter media comparison
Perlite advantages	Diatomaceous earth (DE) disadvantages
·         Nuisance dust (same as kitchen flour);	·         Carcinogen (causes cancer);
·         Amorphous (uncrystallized) silicate (containing less than .001 per cent crystalline);	·         Crystalline silicate (can cut the throat and lungs if inhaled);
·         Can be discharged into sanitary sewers (holding tank not required);	·         Must be disposed of as a hazardous waste (holding tank required);
·         Lighter weight (4.03 kg/m3 [0.25 lbs/cf]);	·         Heavier weight (6.8 kg/m3 [0.42 lbs/cf]);
·         Use less (0.41 kg/m2 [0.08 lbs/sf]); and	·         Use more (0.75 kg/m2 [6.8 lbs/sf]); and
·         Lighter and less volume of media ensures even distribution.	·         Heavier weight and increased volume of media causes frequent clogging.

Bob Hawken is vice-president/sales manager for Neptune-Benson Inc., in Coventry, R.I. He has worked in the pool industry for 40 years, the last 26 of which in commercial pool construction and equipment sales. Hawken specializes in competition pools, waterparks, aquariums and other large aquatic facilities. Most recently he was involved in some of the world’s largest facilities, including Great Wolf Lodge Resorts, The Georgia Aquarium and the Atlantis Resorts in the Bahamas and Dubai.

Source URL: https://www.poolspamarketing.com/trade/evolution-of-pool-filtration/

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