How to improve the filtration performance of a filter?

The main problems with single-layer filter media
The pollution capacity of the filter layer in a filter refers to the amount of impurities retained in the filter material per unit volume of the entire filter layer at the end of the filtration cycle, measured in kg/m3 or g/cm3. The high pollution capacity indicates that the entire filter layer plays a significant role, and the filtration efficiency, working cycle, and water production of the filter tank are high. The main disadvantage of a single-layer filter sand filter is that the impurities trapped in the filter layer are extremely uneven, with the highest amount on the surface (upper layer) and the lower the amount. There are basically no impurities trapped below 30cm. Therefore, during the filtration process, the head loss on the surface increases rapidly, and it can be said that the head loss is mainly concentrated on the surface, resulting in a significantly shortened filtration cycle. The main reason is that the particle size of the filter material in the filter layer is fine above and large below, which gradually increases along the direction of water flow. If the particle size of the filter material gradually decreases in the direction of the water flow (from large to small), a so-called "reverse particle size" filtration is carried out, which means that the water flow during filtration first passes through the coarse-grained filter material, and then sequentially passes through the small-sized filter material, that is, the particle size of the filter material gradually decreases in the direction of the water flow. The advantages of "anti particle size" filtration are: uniform distribution of impurities in the filter layer; increased ability of the filter layer to contain pollutants; and gradually reduced head loss in the filter layer; The filtering cycle has increased significantly.
Based on the concept of "anti granularity" filtration, in practice, upward flow filtration, bidirectional flow filtration, double-layer filter material filtration, and multi-layer filter material filtration have emerged. In practice, in addition to single-layer quartz sand filtration, the most commonly used double-layer filter media for anthracite and quartz sand filtration. The particle size of the filter material in a general filter is arranged in a fine top and coarse bottom, with water entering from the top and water exiting from the bottom. The particle size arrangement of the filter material in the current filter layer remains unchanged, and the inlet and outlet directions are opposite, that is, changed to lower inlet. The upper effluent forms a "reverse particle size" filtration method. This type of "anti particle size" filtration needs to control the filtration rate well, because when the filtration rate is high, the filter layer will expand, especially to prevent the loss of fine sand in the upper layer and affect the water quality of the effluent. Therefore, a grid or grid should be set up on the surface. Although this filtering method has the advantages of anti particle filtering, its disadvantage is that the expansion of the filter layer is limited by the grid during flushing, which increases the difficulty of flushing; The direction of flushing water flow is consistent with that of filtering water flow, which affects the flushing effect; During flushing, a large amount of sludge needs to pass through the entire filter layer to be discharged, which often leads to incomplete sludge discharge. These drawbacks are difficult to overcome, so upward flow filtering is rarely used in practice. The upward flow of the filter is affected by the installation of a grid, which affects the expansion of the filter layer during flushing, resulting in incomplete flushing and sludge discharge. To overcome these shortcomings and shortcomings, bidirectional flow filtration has emerged, where water flows into the bottom and top of the filter respectively, and exits from the middle of the filter. The upper filter layer adopts the downward flow method of a regular filter; The lower filter layer adopts reverse particle size filtration method. The upper filter layer can prevent the expansion of the lower filter layer during the upward flow filtration process. Although this filtering method has a good effect, the structure of the filter is complex and the operation is cumbersome, so it is rarely used in practice.
The use of double-layer or multi-layer filter media filtration is currently a widely concerned and valued filtration technology both domestically and internationally. The structure of the filter, the direction of water flow, backwashing, and filtration methods are basically the same as ordinary single-layer filter media filters, except that the composition of the filter media in the filter layer has been changed. The composition of the filter layer of the double-layer filter material is: the upper layer uses lightweight filter material with relatively low density and large particle size; The lower layer uses heavy filter media with high relative density and small particle size. Due to the difference in relative density between the two types of filter media, under a certain backwash intensity, the light filter media remains in the upper layer after flushing, while the heavy filter media remains in the lower layer, forming a double-layer filter media filter. Although each layer of filter material still increases from top to bottom, the average particle size of the upper layer is greater than that of the lower layer. Currently, the commonly used double-layer filter media is anthracite and quartz sand. The comparison of pollutant interception capacity between single-layer filter media and double-layer filter media has been proven in practice: the pollutant content of double-layer filter media is more than twice that of single-layer filter media, and at the same filtration rate, the filtration cycle is longer; Under the same filtration cycle, the filtration rate can be increased. The area enclosed by the cut-off curve and coordinate axis of the double-layer filter material is much larger than the corresponding area of the single-layer filter material, indicating that under the same thickness of the filter layer, the fouling capacity of the double-layer filter material is generally more than twice that of the single-layer filter material. If the water quality entering the filter is the same, the working cycle of the double-layer filter material filter is twice that of the single-layer filter material.
Multi layer filter media generally refers to three layers, with the upper layer being lightweight filter media with large particle size and small relative density, such as anthracite; The middle layer is a medium particle size and medium relative density filter material, such as quartz sand; The lower layer is a heavy filter material with small particle size and high relative density, such as garnet or granular magnetite. The average particle size of each layer of filter material decreases from top to bottom. The three-layer filter material not only has a high pollution capacity, but also the lower layer of heavy and fine filter material plays a significant role in ensuring the water quality after filtration. Therefore, the filtration rate can be slightly higher than that of the double-layer filter material. When the turbidity of the raw water is low and the water quality is good and stable, direct filtration of the raw water can be used to achieve the purpose of primary purification. Direct filtration generally uses anti granularity filtration method, otherwise the filter will quickly become clogged. Currently, double-layer and multi-layer filter media are commonly used for direct filtration. To ensure that the filtered water quality meets the standards for drinking water quality, polymer coagulants such as polyacrylamide or activated silica can also be added before the raw water enters the filter. Direct filtration of raw water eliminates the need for sedimentation or clarification processes, simplifies the treatment process, and saves investment. Using double-layer filter media and multi-layer filter media for direct filtration, the same as ordinary double-layer filter media or multi-layer filter media filters. But the composition and thickness of the filter layer should be reasonably selected according to the raw water quality, and the filtration rate should be low, generally around 5m/h.