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Comparison of Four Common Filtration Processes in Swimming Pools

Swimming pool water treatment is divided into physical and chemical processes, both of which are indispensable in the treatment process. The physical process purifies pool water through the filtration of circulating water treatment equipment. The chemical process involves adding chemical agents to the circulating pool water for disinfection, flocculation, and algae removal, followed by physical processes to clean and sanitize the water.

Turbidity in swimming pool water not only affects water quality but can also lead to safety accidents and reduce disinfection effectiveness, directly impacting swimmers’ comfort. The core of the pool water treatment process is to purify water quality. Currently, the filtration systems commonly used in the swimming pool water treatment industry are mainly based on quartz sand filtration, diatomaceous earth filtration, as well as relatively smaller wall-mounted integrated filters and gravity filters.

This article focuses on discussing four commonly used methods of physical filtration: quartz sand filters, diatomaceous earth filters, wall-mounted integrated filters, and gravity filters.

Introduction to Four Filtration Methods

1.1 Brief Introduction to Quartz Sand Filtration Process

Quartz sand filtration materials generally use high-quality carbon steel lined with rubber for corrosion resistance, stainless steel, and reinforced fiberglass materials, designed to resist double corrosion from ozone and chlorine ions, with a design pressure of 0.6MPa. An automatic air release valve is installed at the top of the equipment to promptly discharge air inhaled due to careless operation. The tank has openings in the middle or top for easy access to replace or refill the filtration material. The quartz sand filter material should use 0.45-0.8mm refined quartz natural sea sand filtration material, with a pebble (2mm-32mm in diameter) support layer at the bottom of the cylinder. To achieve deep layer filtration, the effective filtration layer thickness should not be less than 700mm, and it must be equipped with a 4-5 meter water head. Considering the increase in scale formation in the filter material during the filtration process and the increased pressure in the sand tank, the cylinder’s pressure capacity should exceed 0.6MPa. The tank is equipped with an observation mirror, and if necessary, a low-pressure waterproof light can be installed to observe the dirt level in the sand tank.

1.2 Brief Introduction to Diatomaceous Earth Filtration Process

Pressure diatomaceous earth filters can be divided into plate and frame diatomaceous earth filters and candle-type diatomaceous earth filters. Plate and frame diatomaceous earth filters consist of multiple filtration units, each composed of a filter plate, filter frame, and filter cloth, with the cloth sandwiched between the plate and frame as the adsorption filtration medium.

The candle-type diatomaceous earth filter is similar in shape to a vertical quartz sand pressure filter, with its interior consisting of candle-shaped filter elements, hence the name. During filtration, a diatomaceous earth pre-coating layer is first formed on the filter candles. As the material passes through the diatomaceous earth filter layer attached to the surface of the filter candles, it intercepts suspended particles and colloids for filtration.

Diatomaceous earth filter media is a siliceous biological sedimentary rock mainly composed of opal, primarily formed from the deposition of single-celled aquatic plants’ diatoms’ remains. It is characterized by being porous, having a large surface area, and chemical stability, making it a natural filter aid. Diatomaceous earth also has unique ion selectivity and the ability to kill chlorine-resistant pathogens. Over time, the pre-coating layer on diatomaceous earth filters accumulates more filtered impurities, blocking the filtration channels. Therefore, a certain amount of diatomaceous earth must be added, allowing it and the suspended solids in the filtrate to be simultaneously retained and adsorbed by the filter mesh, forming a new filter layer without completely blocking the micro-filter pores, maintaining its filtration performance, thereby prolonging the filter cycle and increasing the total amount of filtration per cycle.

1.3 Brief Introduction to Integrated Filtration Process

Integrated filters generally use high-quality paper core filter cartridges or PP filter cartridges as the filtration medium, with built-in circulating water pumps, automatic dosing devices, underwater lights, etc. Their circulation mode is suction-type circulation, drawing water from the equipment’s own inlet, filtering and purifying it, and then discharging it from the equipment’s own outlet. Their service range is limited to the immediate area around the equipment, and there are issues with dead zones and eddies during use. They are not mainstream filtration technology, have a low market usage rate, and are only suitable for pools without a machine room, small water volume, and fewer swimmers, such as high-end villa outdoor pools and private club pools.

1.4 Brief Introduction to Gravity Filter Process

Gravity filters are best suited for improving the water quality environment of natural or man-made landscape water bodies. Their aeration and oxygenation activation of the water body and sand bed filtration are beneficial for removing organic pollutants and reducing turbidity in such water bodies, belonging to the category of environmental microbiology water treatment processes. The water treatment for swimming and leisure uses belongs to the medical disinfection process of medical microbiology disinfection and sterilization, and there are essential differences between the two. Gravity filtration technology was originally used in the treatment of natural or man-made landscape water bodies. In the past decade, gravity filtration systems have also appeared in China in the treatment of recreational water such as swimming pools and water parks, but their overall market share is low.

The working process of a gravity filter involves sending raw water into a distribution tank for even distribution, then passing it through an air isolation device for aeration and oxygenation. Aeration and oxygenation involve releasing harmful gases such as nitrogen, phosphorus, and potassium from the water into the air and then absorbing oxygen from the air. This system is also known as a (respiratory system). After completion, the water enters the fine filter (composed of multiple layers of composite filter material) and filters from top to bottom, as shown in the following filtration process diagram. As the filter layer continuously traps suspended solids in the water body, the resistance of the filter layer gradually increases, causing the water level in the siphon pipe to rise. When the water level rises to the set position, it enters the auxiliary pipe of the siphon with a vacuum device. Due to hydraulic action, the air in the siphon pipe is removed, creating a negative pressure. When the negative pressure reaches the design value, siphoning occurs. At this time, the water in the tank forms a backflow, continuously washing the filter layer from the bottom up, regenerating the filter layer, as shown in the reverse washing process diagram below. Due to continuous backwashing of the filter layer, the backwash wastewater is discharged to the sewer, and when the water level in the tank drops to the specified value, the siphoning effect is destroyed, the backwash ends, and the filter starts working again.

Analysis of the Characteristics of the Four Filtration Methods

2.1 Characteristics of Quartz Sand Filtration Process

Advantages:

  • It is an international/domestic mainstream swimming pool water treatment filtration process, used in many high-level international and domestic swimming competitions, mature and reliable, suitable for national conditions (domestic swimming venues are basically operating at overcapacity during peak seasons), with a market share of over 95%.
  • The filtration medium is mainly quartz sand, which is readily available.
  • The equipment is stable in performance, cost-effective, and has a long service life.
  • The filtration speed is medium, and the filtration precision can meet international/domestic swimming pool water quality standards. It is also capable of handling high summer peak flows and can operate continuously.
  • The equipment is simple to operate, and the operating and maintenance costs are low, making equipment management in the machine room convenient.

Disadvantages:

  • Compared to other filtration processes, it requires more space and ceiling height in the equipment room.
  • Its filtration precision is slightly lower than that of diatomaceous earth filters.

2.2 Characteristics of Diatomaceous Earth Filtration Process

Advantages:

  • The filtration precision can reach 2-3┬Ám, higher than other processes, and belongs to low-speed precision filtration.
  • The filtration area is large, and the equipment volume is small, occupying less space in the machine room.
  • High degree of automation, automatic dosing of filter media, partially recyclable.

Disadvantages:

  • Under the same water treatment conditions, the investment cost of a single piece of equipment is higher than that of quartz sand filters.
  • Large amount of backwash water, high consumption of filter media, need to refill diatomaceous earth filter media after each backwash, requiring space to store enough diatomaceous earth for more than 15 days of operation, resulting in higher operating costs.
  • High automation of equipment requires high-quality operators, time-consuming and labor-intensive to completely disassemble and inspect in case of failure.
  • Weak capability to handle high summer peak flows, unable to operate continuously, and unable to guarantee water quality.
  • Diatomaceous earth filter media is lightweight, and backwashing easily carries away some of the diatomaceous earth filter media. A dedicated middle water treatment system is required; otherwise, it may cause secondary pollution and even clog the drainage pipes in the machine room.

2.3 Characteristics of Integrated Filtration Process

Advantages:

  • Low operating and maintenance costs, saving a significant amount of water, electricity, labor, etc.
  • Simple equipment structure, convenient operation, no need for special supervision.
  • No need for a dedicated equipment room, simple installation, and less investment.

Disadvantages:

  • It needs to be installed near the poolside.
  • Limited service area, lower filtration precision, and average water treatment effect.
  • Not suitable for high-grade competition venues, with a low market share.

2.4 Characteristics of Gravity Filter Process

Advantages:

  • Non-pressurized gravity design, power-free filtration, energy-saving;
  • The primary filtration medium is quartz sand, which is convenient to source;
  • No valves, no parts replacement, no maintenance, labor-saving;
  • Siphon principle for automatic backwash technology.

Disadvantages:

  • Not mentioned in the standard specifications and collections for swimming pools; the technology is still under verification;
  • Requires strict room area and especially ceiling height;
  • Shallow filter layer, low filtration efficiency, average water treatment effect;
  • Advantages like no maintenance, no parts replacement are yet to be verified.
  • As a niche technology, its effectiveness in swimming pools and water parks and similar recreational water treatment is yet to be validated.

Conclusion

This article lists four common swimming pool water treatment filtration technologies available in the market, each with its advantages and disadvantages. The choice of filtration method is not static. Even the best products cannot achieve optimal results without the correct method of use. Typically, the cycle time for swimming pools is 4-6 hours, and for hot spring baths, it does not exceed 1 hour. The cycle time should be designed according to local customs and foot traffic to meet the required standards and maintain clean and healthy water quality. The design of the cycle period plays a crucial role. Additionally, the reasonable configuration of various equipment, the operation method of filtration equipment, water quality disinfection and balancing, the nature of pool water use, pre- and post-swimming cleaning facilities, and many other factors collectively impact the outcome. One should comprehensively consider the project’s construction conditions, financial situation, pool characteristics, foot traffic, and the capabilities of the maintenance team post-construction to choose the most suitable filtration technology to achieve the desired results.

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