Application of diatomite filter in water treatment of swimming pool

The turbidity limit in the water quality standard of swimming pools tends to decrease. The use of a diatomite filter in the water treatment system of the swimming pool can obtain lower turbidity of pool water and achieve a better water treatment effect.

Development trend of swimming pool water treatment system

The main body of swimming venues is water, and the pool water is closely related to the human body. The quality of water is the core of the quality of swimming venues, and also closely related to the comfort and health of swimmers.

The water quality standard of pool water is the basis to determine the water purification system of the swimming pools. All countries in the world and organizations such as fina and who have formulated corresponding water quality standards for swimming pools.

Turbidity is an index reflecting the physical properties of water and one of the important indexes to measure the good degree of water quality.

The colloidal substance in water is the main factor to form turbidity. Colloidal substances in raw water can be in a stable state for a long time, and it is difficult to be removed by gravity natural sedimentation.

In the swimming pool water treatment system, the colloid can not be oxidized by disinfectant and can only be removed by filtration due to the limitation of CT value (CT ≥ 1.6, C is disinfectant concentration, mg/ L; t is contact time, min) in swimming pool water treatment system.

Therefore, the use of filters with higher filtering precision in the water treatment system of swimming pools is an effective measure to obtain lower turbidity of pool water.

Comparison of quartz sand filter and diatomite filter technology

The filters used in swimming pool water treatment mainly include quartz sand filter, diatomite filter, gravity filter, wedge wire filter, and other forms.

All types of filters are suitable for certain places. In the water treatment process of most public swimming places in China, medium speed (filtration intensity is about 25 m3 / (H · m2)) single medium homogeneous quartz sand filter and diatomite filter are the main filtration equipment.

Quartz sand filter

According to the media, quartz sand filter can be divided into single media and multi-media filter; according to the particle size of quartz sand, it can be divided into homogeneous filter material and graded filter material; according to the filtration rate, it can be divided into low speed (filtration rate ≤ 10 m / h) and medium speed

(11 m / h ≤ filtration rate ≤ 30 m / h), high speed (30 m / h < filtration rate < 50 m / h) filter.

Although the low-speed quartz sand filter has a good filtering effect, its economic performance is poor; the high-speed quartz sand filter has a thin filter layer, large quartz sand particle size, short recoil cycle, high maintenance, and management requirements, and the filter layer is easy to be penetrated. Multi-media filter is less used in China.

The medium-speed single medium homogeneous quartz sand filter with a filtration rate of about 25 m / h is widely used in the water treatment of civil public swimming pool. The filtration accuracy of this kind of filter is about 10 μ m, and it can reach about 7 μ m with coagulant.

When the head loss of the filter layer reaches the allowable value or the effluent turbidity cannot meet the requirements, the filter layer must be backwashed with the combination of water or air water. Generally, the backwashing water intensity of homogeneous quartz sand filter is 12-15 L / (s·m2), and the backwashing time is 7-10 min. The backwashing process of quartz sand filter needs a large amount of water, and the backwashing water is the main water consumption in the operation of swimming pool. At the same time, due to the wear of filter material during backwashing, quartz sand needs to be replaced once every 5-7 years, which is complicated and complicated.

Diatomite filter

Diatomite is a porous biological siliceous rock formed by the accumulation of the remains of the ancient monocellular and low-level plants diatoms.

Diatomite has a large specific surface area (3.1-60 m2 / g). Under the microscope, it can be observed that diatomite has a unique microporous structure with multi-level, large number and orderly arrangement, with porosity of 90% – 92%. This microporous structure is the reason for diatomite’s special physical and chemical properties.

Diatomite is mainly composed of amorphous SiO2, and contains a small amount of Al2O3, Fe2O3, Cao and organic matter. The purified diatomite is fine and smooth, with uniform particles and clean surface. The particle size is 0.001-0.5 mm, and its SiO2 content is more than 90%.

Diatomite filter is a kind of filter which uses the purified diatomite as the filter medium and adheres to the surface of the filter element evenly to intercept impurities and dirt.

According to the data, diatomite filtration technology originated from the water filter developed in the Second World War to solve the clean and safe drinking water. In the 1950s, some countries in the United States and Western Europe used it for beer filtration, and then it was widely used in the treatment of alcohol, beverage and water purification. In 1950s, the United States has adopted a lot of diatomite filtration technology in the circulating water filtration of swimming pools.

The filtration accuracy of diatomite filter is related to the thickness of pre coated diatomite. The test results of Tongji University show that for swimming pool water, the appropriate amount of diatomite used for pre coating is 0.8 kg / m2 per unit filtration area (the coating thickness is 0.5-1 kg / M2 as stipulated by WHO).

Diatomite filter is mainly used in beer and beverage industry, diatomite filter equipment has been widely used in swimming pool water treatment abroad.

Since 2004, with the shortage of water resources and people’s increasing demand for the water quality of swimming pool, diatomite filter’s treatment effect and advantages of water saving and energy saving have been gradually recognized by professionals. More and more public swimming places choose diatomite filter.

Compared with the traditional sand filter equipment, diatomite filter has the following characteristics:

• High filtration precision. The filtration accuracy of quartz sand filter is generally 7-10 μ m; while diatomite filter can filter all kinds of impurities of 1-2 μ m and above, and the filtration accuracy is higher. The water quality after filtration is clear and transparent, and the turbidity is < 0.1NTU.
• The dosage in the process of treatment is less. In order to meet the requirements of filtration accuracy, the traditional sand filtration equipment needs to add coagulant, while diatomite filtration equipment itself has high filtration accuracy, so it does not need to add coagulant for flocculation treatment, which can reduce the dosing equipment and other supporting facilities, save the dosage of drugs, and make the comfort of water more suitable for human body. In addition, diatomite filter can remove most of the bacteria and microorganisms due to its high filtering precision, and reduce the use of disinfectant in disinfection process.
• The equipment needs less room area. Diatomite has a large specific surface area and unique structure. Under the premise of the same circulating water volume, the volume of diatomite filtration equipment is much smaller than that of sand tank, and the required room area is small, which reduces the infrastructure investment. If the treatment capacity is 250 m3 / h, the area of quartz sand filter is about 10 m2, while diatomite filter is only about 4 m2.
• Water consumption is low. The management of water resources is one of the most important links in the swimming pool, and diatomite equipment is the equipment with the lowest discharge (backwash discharge) among all kinds of equipment at present. The significance of the lowest discharge for energy saving is not only in the saving of water charges, but also in the saving of heat loss in the constant temperature swimming pool.。
• Lower operating costs. There is no need to add coagulant, and the dosage of medicine is saved. At the same time, the backwashing cycle is longer than that of quartz sand filter, and the single backwashing water volume is smaller.

Key points of diatomite filter design

The quality of the Diatomite Filter Wedge wire filter element is the key to the quality of the filter. If the filter cloth is not of good quality, the soil may run into the tank after the filter cloth is damaged.

• Taking the wedge wire candle diatomite filter as an example, each diatomite filter system has three main processes: membrane hanging, filtration and backwashing. The design points and precautions of each process are as follows:
• The process of coating diatomite on the surface of wedge wire filter element. The power of this process comes from the circulating pump of the system. The dosage of diatomite per unit filtration area should be 0.5-1.0 kg / m2, the thickness of membrane should not be less than 2 mm, and the thickness should be uniform. Membrane hanging can occur when the system is restarted or when diatomite is added after backwashing, so diatomite in membrane hanging process can come from the mixing tank inside the filter body or outside the filter body. In order to prevent some fine soil from entering into the pool body at the initial stage of membrane hanging, and to make diatomite uniformly adhere to the filter element, whether it is to restart or supplement diatomite, it is necessary to close the water outlet valve before the water is discharged from the filter system. The water is discharged from the tank to the suction pipe of the circulating water pump, and then sent to the tank body through the circulating water pump. After membrane hanging is completed, the valve is opened and the water is discharged to the swimming pool.
• Filtration is a process in which the water in the pool is lifted by the circulating pump, intercepted by Diatomite on the surface of the filter element, and then re entered into the pool. The filter strength of candle diatomite is designed as 3.5-4 m3 / (H · m2).
• Backwashing, after a period of sewage interception, when the filter element loss reaches a certain limit, the filter material is discharged through pressure and replaced again. The filter intensity of candle type diatomite backwashing is designed as 5 m3 / (H · m2). The composition of diatomite backwashing drainage is relatively complex. As the main composition of diatomite is SiO2, the abandoned diatomite in backwashing drainage is non viscous and non caking, keeps fluid state, and will not cause secondary pollution to the environment. However, the residual diatomite should be calculated according to the backwashing water quantity to confirm whether it meets the requirements of “water quality standard for sewage discharged into urban sewers”, Otherwise, the backwash water shall be treated as necessary.

Conclusion

The water distribution system, return water system and water treatment system constitute a complete swimming pool water circulation treatment system. This paper discusses the filtration link in the water treatment system, which focuses on the selection of filter form and relevant parameters. The water recycling system of the swimming pool is the core of the quality of swimming venues. Different water treatment equipment should be selected reasonably according to the actual engineering conditions and compared with technology and economy.