Water Treatment-Sand Filters vs. Diatomaceous Earth Filters

As is widely known, swimming pool filters mainly consist of traditional pool sand filters and modern diatomaceous earth filters. Quartz sand and diatomaceous earth are important filtration media in the swimming pool water treatment process. Users often ask which type of filter, pool sand filter or diatomaceous earth filter, is better? Before answering this question, let’s do a detailed comparison of the two filters.

pool sand filter
A pool sand filter is filtering pool water by the pool

Comparison of operation management in pool water treatment process

Pool sand filter

The main operation contents of the quartz sand filtration system include: backwashing and coagulant dosing. Because the quartz sand filtration is deep filtration, the filtered particles gather in the whole filter bed, and the whole filter bed can be washed clean with greater washing intensity and longer washing duration. The backwashing strength of the pool sand filter is 12-15L/(S.2m), the backwashing duration is 8-10 minutes, and the interval between backflushing is less than 5 days. After each recoil, the system should be washed for 2-3 minutes. Sand filtration in the filtering condition and backwash condition required water flow difference of nearly twice, in the design and operation should pay attention to the circulation flow and backwash flow match, it is advisable to set up a dedicated backwash pump. The flocculation of coagulant is mainly affected by the variety of coagulant, dosage concentration, water temperature, PH and reaction time, so it is necessary to have rich management experience of the operator to ensure the effect of coagulation.

Diatomaceous earth filter

  • At present, there are two kinds of diatomite filter systems, reversible and renewable. Reversible diatomaceous earth filtration systems have filtration speeds of up to 6-10m/h. The main operation contents of renewable diatomite filter include: pre-coating, filter material regeneration and filter material replacement.
  • Pre-coating refers to the process of evenly distributing diatomite filter material to the surface of the filter element by hydraulic power. At the end of each “regeneration” or after replacing the filter material, a pre-coating is required to start the next round of filtration.
  • Filtration in diatomaceous earth filters mainly occurs on the surface of the filter membrane. The filtered particles and dirt are covered on the surface of the filter membrane or adsorbed by the diatomaceous earth particles on the surface of the filter membrane, which reduces the filtration effect and the passing capacity of the filter membrane. Therefore, it is necessary to restore the filtration capacity of the filter membrane through “regeneration”.
  • The so-called “regeneration” is the process of redistributing the diatomaceous earth originally distributed on the surface of the filter membrane and the diatomaceous earth in the deep layer of the filter membrane to achieve the filtration effect on the surface of the regenerative filter membrane. The utilization rate of diatomite can be improved by regenerating the filter membrane. The frequency of filter material regeneration should not exceed once per day.
  • When the filter resistance reaches 0.7mpa, the filter material should be replaced. The replacement filter material can be washed from the filter element thoroughly by the pool water recoil and then discharged with the water flow, or by the use of machinery to shake the filter material off the filter element, followed by gravity discharge. Then the new filter material is added again, and the next round of filtration cycle is started by pre-coating.
  • The filter replacement cycle is usually 15 to 60 days. Taking the swimming competition pool as an example, the running operation time is compared.

Brief summary

Through the above comparison, the operation of diatomite filtration is more complicated than that of quartz sand filtration, and its monthly operation time is about 1.18 times that of quartz sand filtration, so it is recommended to use an automatic control system to reduce the operation intensity.

Comparison of filtration mechanism in pool water treatment process

Action mechanism and filtration rate of pool sand filter

Quartz sand is a filter medium widely used in swimming pool water treatment. Quartz sand filtration is deep filtration, and filtration occurs in the entire depth of the filter bed. When the water needs to be filtered through the filter layer, the suspended substances contained in the water flow into the tiny gap of the upper filter material, subject to adsorption and mechanical obstruction, the suspended substances are intercepted by the surface of the filter material, at the same time, overlapping and bridging between the suspended substances that are intercepted, forming a filter membrane in the filter material layer, the filter membrane continues to filter, and the water quality is purified through such repeated filtration. The best filtration efficiency occurs during the last third of the filtration cycle. The usual sand filtration speed is 20-30m/h, with a filtration accuracy of 15-20umm, when the filtration speed is reduced to less than 25m/h and combined with the use of flocculant, its filtration accuracy can reach 7-10um.

Filtration mechanism and filtration rate of diatomite filter

  1. Diatomaceous earth is a filter medium widely used in beverage, brewing, medicine and other water treatment, and is also used in swimming pool water treatment. As a water treatment filtration medium, it uses the natural characteristics of porous diatomite filter material, with screening interception and adsorption and other functions, not only to remove turbidity in the water, but also to remove organic matter, ammonia nitrogen, cryptosporidium and bacteria in the water.
  2. In the diatomite filter, the diatomite is uniformly attached to the surface of the filter element to form a filter membrane. The filter membrane acts as a filter by intercepting and absorbing fine particles flowing through the water. Interception refers to the process of particles whose particle size is larger than the filter material gap are intercepted when passing through the filter membrane. The interception process mainly occurs on the surface of the filter membrane and is the main mechanism of diatomite filtration.
  3. The porous structure of the diatomaceous earth filter material makes it have good permeability. The fine particles in the pool water and the filter material friction form static electricity during the process of permeating the filter membrane. Under the combined action of static electricity and microporous mechanical obstruction, the finer particles are absorbed into the holes of the filter material to play an adsorption role and achieve a finer filtration effect.
  4. The diatomite filtration speed usually used in the swimming pool water treatment process is 3-5m/h, the filtration accuracy can reach 1-2um, the effluent turbidity can be extremely low, and the removal rate of cryptosporidium can reach 99.996%.

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