Backwashing process analysis and its importance

Multi-media filter

Multi-media filters used in water treatment commonly include anthracite coal -Quartz sand - Magnetite filter, activated carbon - Quartz sand - Magnetite filter, activated carbon - Quartz sand filter, quartz sand - Ceramic filter, etc.

The filter layer design of a multi-media filter mainly considers the following factors:

1 Different filter media have a large density difference to ensure that no mixing occurs after backwashing.

2 Select filter media according to the purpose of water production.

3 The particle size of the lower filter media should be smaller than that of the upper filter media to ensure the effectiveness and full utilization of the lower filter media.

In fact, taking a three-layer filter bed as an example, the upper layer filter media has the largest particle size and is composed of light filter media with low density, such as anthracite coal and activated carbon; the middle layer filter media has a medium particle size and density, generally composed of quartz sand; the lower layer filter media is composed of the smallest particle size and the highest density heavy filter media, such as magnetite. Due to density differences, the choice of filter media for three-layer media filters is basically fixed. The upper layer filter media performs coarse filtration, and the lower layer filter media performs fine filtration, thus fully utilizing the multi-media filter bed, and the effluent water quality is significantly better than that of a single-layer filter bed. For drinking water, anthracite coal, resin, and other filter media are generally prohibited.

Quartz sand filter

A quartz sand filter is a filter that uses quartz sand as the filter media. It can effectively remove suspended solids from water and has a significant effect on removing colloids, iron, organic matter, pesticides, manganese, bacteria, viruses, and other pollutants in water. It has the advantages of low filtration resistance, large specific surface area, strong acid and alkali resistance, oxidation resistance, The applicable PH range is 2-13 and good pollution resistance. The unique advantages of quartz sand filters also lie in the fact that through the optimization of filter media and filter design, the adaptive operation of the filter is realized. The filter media has strong adaptability to raw water concentration, operating conditions, and pretreatment processes, that is, during filtration, the filter bed automatically forms a loose-top and dense-bottom state, which is conducive to ensuring effluent water quality under various operating conditions. During backwashing, the filter media is fully dispersed, and the cleaning effect is good. Sand filters have the advantages of fast filtration speed, high filtration accuracy, and large dirt-holding capacity. They are widely used in various process water, domestic water, circulating water, and wastewater pretreatment fields, such as power, electronics, beverages, tap water, petroleum, chemical industry, metallurgy, textiles, papermaking, food, swimming pools, and municipal engineering.

Quartz sand filter equipment has the characteristics of simple structure, automatic control operation, large processing flow, few backwashing times, high filtration efficiency, low resistance, and convenient operation and maintenance.

Activated carbon filter

The filter media is activated carbon, used to remove color, odor, residual chlorine, and organic matter. Its main function is adsorption, and activated carbon is an artificially made adsorbent.

Activated carbon filters are widely used in the pretreatment of water in domestic water and the food, chemical, and power industries. Because activated carbon has a developed pore structure and a huge specific surface area, it has a strong adsorption capacity for soluble organic matter in water, such as benzene and phenolic compounds, and also has a good removal effect on organic pollutants that are difficult to remove by biological and chemical methods, such as chroma, odor, surfactants, synthetic detergents, and dyes. Granular activated carbon removes Ag^+, Cd^2+ ， CrO4^2-, etc., with a removal rate of 85% or more. After passing through the activated carbon filter bed, the suspended solids in the water are less than 0.1mg/L， COD removal rate is generally 40%~50% and free chlorine is less than 0.1mg/L。

      Filter backwashing mainly refers to the fact that after a certain period of use, the filter media layer retains and adsorbs a certain amount of debris and stains, which causes the effluent water quality of the filter to decrease. The main characteristics are: the normal filtered water quality of the filter deteriorates, the pressure difference between the inlet and outlet pipes increases, and the flow rate of a single filter decreases.

        Backwashing principle: Water flows backward through the filter media layer, causing the filter layer to expand and suspend. The shear force of the water flow and the collision and friction force of the particles clean the filter media layer, allowing the dirt inside the filter layer to detach and be discharged with the backwash water.

Necessity of backwashing

1 During the filtration process, suspended solids in the raw water are intercepted and adsorbed by the filter media layer and continuously accumulate in the filter media layer. As a result, the pores of the filter layer are gradually blocked by dirt, and a filter cake is formed on the surface of the filter layer, and the head loss of the filtration water continuously increases. When it reaches a certain limit, the filter media needs to be cleaned to restore the working performance of the filter layer and continue working.

2 During filtration, due to the increase in head loss, the shear force of the water flow on the dirt adsorbed on the surface of the filter media increases. Some particles move to the lower filter media under the impact of the water flow, which eventually causes the content of suspended solids in the water to continuously increase, and the water quality deteriorates. When impurities penetrate the filter layer, the filter loses its filtration effect. Therefore, to a certain extent, it is necessary to clean the filter media to restore the dirt-holding capacity of the filter layer.

3 Suspended solids in wastewater contain a large amount of organic matter, which will cause the accumulation and reproduction of bacteria and microorganisms in the filter layer for a long time, leading to anaerobic decay. Therefore, regular cleaning of the filter media is required.

Backwashing parameter control and determination

1 Expansion height: During backwashing, in order to ensure that the filter media particles have sufficient gaps for the dirt to quickly be discharged from the filter layer with the water, the expansion rate of the filter layer should be larger. However, when the expansion rate is too large, the number of particles in the unit volume decreases, and the chance of particle collision also decreases, which is unfavorable for cleaning. For double-layer filter media, the expansion rate is 40 % ----50 %.

Note: During production operation, randomly check the filling height and expansion height of the filter material, because some filter material will be lost or worn during normal backwashing, and needs to be supplemented. A relatively stable filter layer has the following advantages: ensuring the stability of the filtered water quality and ensuring the effectiveness of backwashing.

2 Backwash water volume and pressure: Generally, the design requirement is that the intensity of the backwash water is 40 m3 /( m2•h ), the pressure of the backwash water ≤0 ． 15 MPa。

3 Backwash air volume and pressure: The intensity of the backwash air is 15 m /( m •h ), the pressure of the backwash air ≤0 ． 15 MPa。

Note: During the backwashing process, the backwash air is collected at the top of the filter, and most of it should be discharged through the double-hole exhaust valve. In daily production. The patency of the exhaust valve needs to be checked frequently, mainly manifested in the degree of freedom of the valve ball rising and falling.

Air-water combined backwashing

1 First use air to rinse, then backwash with water: First, lower the water level of the filter pool to the surface of the filter layer 100 mm and introduce air for several minutes, then backwash with water. Suitable for filter pools with heavy surface pollution and light internal pollution.

Note: The corresponding valves must be closed in place; otherwise, when the water level drops below the surface of the filter layer, the upper part of the filter layer will not be wetted, and during the process of the particles being disturbed up and down, the pollutants cannot be effectively discharged, but will instead move to the depths of the filter layer.

2 Air and water combined backwashing: Air and backwash water are simultaneously sent from the bottom of the static filter layer. During the rising process, the air forms large bubbles in the sand layer, and when it encounters the filter material, it becomes small bubbles, and at the same time produces a scrubbing effect on the surface of the filter material; the backwash water loosens the filter layer, making the filter material in a suspended state, which is conducive to the air scrubbing the filter material. The expansion effects of the backwash water and backwash air are superimposed, which is stronger than when performed individually.

Note: The backwashing pressure of water and the backwashing pressure and intensity of air are different, and the order should be noted to avoid backwash water entering the air pipe.

3 After the air-water combined backwashing is completed, stop the air, keep the backwash water at the same flow rate, and continue to rinse 3 min ~5 min to remove the bubbles remaining in the filter bed.

Note: Pay attention to the status of the top double-hole exhaust valve.

Analysis of the causes of filter material caking

1 The pollutants retained on the surface of the filter layer, if they cannot be effectively removed within a certain period, in the subsequent backwashing process, if the distribution of the backwash air is uneven, it will lead to uneven expansion height. As the backwash air rubs, in places with less rubbing, the oil stains and other impurities on the surface of the filter material cannot be effectively removed. After entering the next normal water filtration cycle, the local load increases, and the impurities will sink from the surface to the inside, the agglomerates gradually increase, and at the same time extend to the filling depth of the filter, until the entire filter fails.

Note: In actual operation, the phenomenon of uneven backwash air often occurs, mainly due to perforation of the bottom air distribution pipe, blockage or damage of local filter caps, or deformation of the grid spacing.

2 The surface filter material particles of the filter layer are small, and the chance of collision during backwashing is small, and the momentum is small, so it is not easy to clean. The attached sand particles are easy to form small mud balls. When the backwashing is completed and the filter layer is re-graded, the mud balls enter the lower filter material, and as the mud balls grow, they continuously move deeper.

3 The oil contained in the raw water is retained in the filter, and the remaining part after backwashing accumulates over time, which is the main factor leading to the caking of the filter material in the filter. When to perform backwashing can be determined according to the characteristics of the raw water quality and the requirements of the effluent water quality, using standards such as limited head loss, effluent water quality, or filtration time.

Precautions for the processing and acceptance procedures of the filter

1 The parallel tolerance between the outlet water tank and the filter plate is required to be no more than 2 mm。

2 The levelness and unevenness of the filter plate are both less than ±1.5 mm . The structure of the filter plate is optimal for integral processing. When the diameter of the cylinder is large, or when it is restricted by raw materials, transportation, etc., it can also be formed by splicing two petals.

3 Reasonable treatment of the joints of the filter plate and the cylinder is particularly important for the air backwashing process.

（ 1) In order to eliminate the radial gap between the filter plate and the cylinder caused by errors in the processing of the filter plate and the rolling of the cylinder, arc ring plates are generally welded section by section. The contact parts must be fully welded.

（ 2) The radial gap between the central pipe and the filter plate is treated in the same way.

Note: The above measures ensure that filtration and backwashing can only be connected through the filter cap or the gap between the pipes. At the same time, it also ensures the uniformity of the distribution of the backwashing and filtration channels.

4 The radial error of the through holes processed on the filter plate is ±1.5 mm . The increase in the matching size between the filter cap guide rod and the filter plate through hole is not conducive to the installation or fixing of the filter cap. The processing of the through holes must use mechanical equipment.

5 The material of the filter cap is best nylon, ABS second. Because the filter material added to the upper part has a large compressive load on the filter cap, the strength is required to be high to avoid deformation. An elastic rubber pad should be added to the contact surface (upper and lower surfaces) of the filter cap and the filter plate.