What is a multimedia filter

Author: Hongjie Water

Multi media filter is a process of using one or several filter media to pass water with high turbidity through granular or non granular materials with a certain thickness under a certain pressure, so as to effectively remove suspended impurities and clarify the water. Common filter media include quartz sand, anthracite, manganese sand, etc., which are mainly used for water treatment turbidity removal, softened water, pre treatment of pure water, etc., and the effluent turbidity can reach below 3 degrees.

The meaning of filtering

In the process of water treatment, filtration generally refers to the process of using quartz sand, anthracite and other filter material layers to intercept suspended impurities in water, so as to make water clear. The porous material used for filtration is called filter material, and quartz sand is a common filter material. There are various types of filter materials, including granular, powdered, and fibrous. Common filter materials include quartz sand, anthracite, activated carbon, magnetite, garnet, ceramics, plastic balls, etc.

Multi medium filter (filter bed) is a medium filter that uses two or more types of media as the filter layer. In industrial circulating water treatment systems, it is used to remove impurities, adsorbed oil, etc. from wastewater, so that the water quality meets the requirements of recycling. The function of filtration is mainly to remove suspended or colloidal impurities in water, especially small particles and bacteria that cannot be removed by sedimentation technology. BODs and COD also have a certain degree of removal effect.

Filter composition

A multi medium filter mainly consists of a filter body, supporting pipelines, and valves.

The filter body mainly includes the following components: simplified; Water distribution components; Support components; Backwashing the trachea; Filter material; Exhaust valve (external), etc.

Selection basis of filter material

(1) Must have sufficient mechanical strength to avoid rapid wear and breakage during backwashing;

(2) Chemical stability is better;

(3) Does not contain substances that are harmful or toxic to human health, and does not contain substances that are harmful or affect production;

(4) The selection of filter materials should try to use filter materials with high adsorption capacity, pollutant interception capacity, high water yield, and good effluent quality.

In the filter material, pebbles mainly serve as support. During the filtration process, due to their high strength, stable spacing and gaps between them, and large pores, it is easy for the filtered water to pass through smoothly during the washing process; Similarly, in the backwashing process, backwashing water and air can pass smoothly. In the conventional configuration, pebbles are divided into four specifications, and the bedding method is from bottom to top, first large and then small.

Multi media filter

The common multi-media filters used in water treatment are: anthracite quartz sand magnetite filter, activated carbon quartz sand magnetite filter, activated carbon quartz sand filter, quartz sand ceramic filter, etc.

The main factors to consider in the design of the filter layer of a multimedia filter are:

1. Different filter materials have significant density differences, ensuring that mixed layer phenomena do not occur after backwashing disturbance.

2. Select filter media based on the purpose of water production.

3. The particle size requirement is that the particle size of the lower layer filter material is smaller than that of the upper layer filter material to ensure the effectiveness and full utilization of the lower layer filter material.

In fact, take the three-layer filter bed as an example, the upper filter material has a large particle size and is composed of light filter materials with small density, such as anthracite and activated carbon; The middle layer filter material is in the middle of particle size and density, generally composed of quartz sand; The lower filter material is composed of heavy filter material with small particle size and high density, such as magnetite. Due to the limitation of density difference, the selection of filter material for three-layer media filters is basically fixed. The upper layer of filter material plays a coarse filtering role, while the lower layer of filter material plays a fine filtering role, fully utilizing the role of a multi medium filter bed. The effluent quality is significantly better than that of a single layer filter bed. For drinking water, filter materials such as anthracite and resin are generally prohibited.

Quartz sand filter

Quartz sand filter is a filter that uses quartz sand as filter material. It can effectively remove suspended solids in water and has a significant removal effect on pollutants such as colloids, iron, organic matter, pesticides, manganese, bacteria, viruses, etc.

It has the advantages of small filtration resistance, large specific surface area, strong acid and alkaline resistance, oxidation resistance, PH application range of 2-13, good pollution resistance, etc. The unique advantage of quartz sand filter is that it realizes the adaptive operation of the filter by optimizing the design of filter material and filter. The filter material has strong adaptability to raw water concentration, operating conditions, pretreatment process, etc., that is, the filter bed automatically forms a state of upper drainage and lower density when filtering, It is beneficial to ensure the effluent quality under various operating conditions, and the filter material is fully dispersed during backwashing, resulting in good cleaning effect.

Sand filters have the advantages of fast filtration speed, high filtration accuracy, and large interception capacity.Widely used in various processes such as power, electronics, beverages, tap water, petroleum, chemicals, metallurgy, textiles, papermaking, food, swimming pools, municipal engineering, etc., for pre-treatment of water, domestic water, recycled water, and wastewater.

The quartz sand filter is simple in structure, can realize automatic control in operation, has large processing flow, less backwashing times, high filtering efficiency, small resistance, convenient operation and maintenance, etc.

Activated carbon filter

The filter material is activated carbon, which is used to remove color, taste, residual chlorine, and organic matter. Its main function is adsorption, and activated carbon is a manually made adsorbent.

Activated carbon filters are widely used for pre-treatment of domestic water and water in industries such as food industry, chemical industry, and power. Due to its well-developed fine pore structure and huge specific surface area, activated carbon has strong adsorption capacity for dissolved organic compounds in water, such as benzene and phenolic compounds. It also has good removal effects for organic pollutants that are difficult to remove using biological and chemical methods, such as chromaticity, odor, surfactants, synthetic detergents, and dyes. Granular activated carbon has a plasma removal rate of over 85% for Ag ^+, Cd ^ 2+, and CrO4 ^ 2- in water. [3] After passing through the activated carbon filter bed, the suspended solids in the water are less than 0.1mg/L, the COD removal rate is generally 40%~50%, and the free chlorine is less than 0.1mg/L.

Backwash process

The backwashing of filters mainly refers to the interception and adsorption of a certain amount of impurities and stains by the filter material layer after a certain period of use, which leads to a decrease in the water quality of the filter's effluent. The main characteristics are: the normal filtered water quality of the filter deteriorates, the pressure difference between the inlet and outlet pipelines increases, and at the same time, the flow rate of a single filter decreases.

The principle of backwashing: The water flow reversely passes through the filter material layer, causing the filter layer to expand and suspend. With the help of the shear force of the water flow and the collision friction force of particles, the filter material layer is cleaned to separate the dirt inside the filter layer and discharge it with the backwashing water.

The necessity of backwashing

(1) During the filtration process, suspended solids in the raw water are intercepted and adsorbed by the filter material layer, continuously accumulating in the filter material layer. As a result, the pores of the filter layer are gradually blocked by dirt, forming filter cakes on the surface of the filter layer, and the filtration head loss continues to increase. When a certain limit is reached, the filter material needs to be cleaned to restore the working performance of the filter layer and continue working.

(2) Due to the increase in head loss during filtration, the shear force of the water flow on the dirt adsorbed on the surface of the filter material increases. Some of the particles move to the lower layer of the filter material under the impact of the water flow, eventually causing the suspended solids content in the water to continuously increase and the water quality to deteriorate. When impurities pass through the filter layer, the filter loses its filtering effect. Therefore, to a certain extent, it is necessary to clean the filter material in order to restore the pollutant holding capacity of the filter material layer.

(3) The suspended solids in sewage contain a large amount of organic matter, and long-term retention in the filter layer can lead to the enrichment and reproduction of bacteria and microorganisms in the filter layer, leading to anaerobic decay. Regular cleaning of the filter material is necessary.