Swimming Pool Filtration

When beginning any course on filtration, my first question to the class is, “Have you ever walked up to a pool and it looked as if diamonds were jumping off the water?” This imagery of sparkling water is the result of a conscious effort to maintain water clarity through filtration.

Water clarity is dependent upon proper filtration, circulation and chemical treatment. These articles have discussed circulation (hydraulics) and pool chemical treatment, but to ensure optimum clarity, the filtration system must be addressed. The pool water is cleansed, and particulate matter is removed when it passes through the filter media.

The opposite of clarity is turbidity–the lower the turbidity, the better clarity. Water clarity can be determined by several mechanical methods. A nephelometer, or a turbidimeter, is a device that measures the turbidity of water and industry standards set the Nephelometer Turbidity Units, not to exceed .5 NTUs. Most pool managers do not have one of these meters, and a visual inspection is mandated. The pool manager must be able to see the pool drain clearly from the pool deck. It is standard practice to close a pool if turbidity is too high, which can be determined by viewing the main drain. Some pool managers have a small 2-inch disk with black and red quadrants they place on the floor of the pool. The ability to differentiate the colors is necessary for water clarity.

Understanding Filter Media

The filter media traps particulate matter as it flows through the filter vessel. The type of media used will determine the desired size of the suspended, trapped particle. The particle size is measured in microns1-millionth of a meter. Silica sand, zeolite, crushed glass, diatomaceous earth, cellulose powder, perlite, spun-bonded polyester material and antimicrobial cartridge fabrics are just a few types of filter media used today. Each type will filter down to a different micron, and is placed in a different type of vessel, called a filter system.

Past, Present And Future Of Filtration

In the earliest known filter, boiled water was poured through sand or charcoal. During the exploration period, seashells were used as media for clarifying water on ships. From 1740 to 1800, the Scottish Enlightenment created an outpouring of scientific thought. One of the founders of swimming pool filtration was Robert Thom, a Scottish scientist and engineer. He built the first municipal water treatment plant in Paisley, using slow sand filtration. Subsequently, the rapid sand filter evolved. This filter is still being used in various swimming pool applications where the vessel contains gradients of gravel, and then a layer of sand.

High Rate Sand Filtration

The advent of High Rate Sand Filtration came into existence during the Space Age. Newer materials were developed to withstand higher pressures within smaller vessels. The filter media is usually silica sand, but materials such as zeolite products have been introduced as a sand replacement.

The mechanics of high rate sand filtration is as follows: the water from the pool is forced down into the filter sand. The sand media traps the particulate matter, and the cleansed water is returned to the pool through small laterals in the bottom of the tank. As the sand traps dirt and debris, the pressure within the tank rises. Since the pool operator cannot tell how dirty the sand is, the only way to determine when to clean the sand filter is to look at the pressure gauge. The industry standard is to backwash the filter when the pressure gauge rises 8 to 10 pounds per square inch. The filter is not completely full of sand; there is an area above the sand bed called freeboard.

During the backwashing phase, the water is redirected from the bottom of the sand bed, and churns the water upwards into the freeboard area. The dirty water flows out of the top of the sand filter into the sanitary sewer. Depending on the size of the filter and the amount of dirt accumulated, the backwashing process will usually take 5 to 10 minutes. Pool operators can tell when the filter sand is clean by looking into the backwash site glass, located on the backwash line. When the water runs clear, the process can be reversed and filter valves can be changed back to filter mode. Larger pool facilities now incorporate automation to change the valves.

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