EVALUATION OF BACKWASH TIME EFFECT ON MESH SELF-WASHING FILTER PARAMETERS

Purpose: The study aims to determine the effect of the backwash time on the parameters of the mesh self-washing filter. Theoretical framework: The duration of the backwash process can significantly affect the operation parameters of the mesh self-washing filter Method: During the study, experiments were carried out using a laboratory stand on which a mesh self-cleaning filter was installed. In the experiments, the filter operation parameters were measured at different values of the backwash time. Mathematical modelling methods were also used to determine the filtration efficiency. Results and conclusion: The results of the study showed that the backwash time significantly impacts the operation of the mesh self-washing filter. The optimal backwash time is from 20 to 30 seconds, while the percentage of filter clogging due to removing impurities during backwash is reduced by 20-25% compared to a shorter or longer backwash time. Research implications: These filters work on the principle of direct filtration when water passes through a sieve to remove suspended solids. The filter material is periodically cleaned by backwashing, which involves changing the direction of water flow through a sieve to remove accumulated solid particles. Originality/value: Water purification by suspended solids with a size of more than 100 microns is carried out by an amount of more than 99.5%. The filter provides reproducible cleaning quality, and the cleaning efficiency does not decrease from cycle to cycle.


INTRODUCTION
Water is of exceptional importance and is widely used in technological processes at most industrial enterprises, especially in thermal power engineering, metallurgy, mechanical engineering and municipal facilities (Khah M.V,2023) Water sources are widespread. Recently, the demand for water in such industries as energy and heat engineering has increased significantly, associated with its use as a heat carrier and the need to compensate for water losses in the circuits of boiler houses and heating stations (Wang Q,2022). It makes it possible to significantly increase the performance of systems of heat power facilities and extend the service life of heat-generating equipment (Ilyina M.E., 2022). At the same time, for heat-generating plants, water is needed of a strictly defined quality, i.e. preliminary special water treatment, is necessary (Trivedi D.P 2013). Usually, such water is a chemical solution containing a diverse number of salts and various impurities, many of which change their properties when heated, fall out of the aqueous solution as suspensions, and settle in the form of insoluble scale.
This water cannot be used directly in thermal power plants (Kolosova,A,2021) Moreover, depending on the primary source (surface reservoirs, underground wells, etc.), the composition of impurities can be different and varies in a wide range (Uvarova A., 2022). In rare cases, the source waters used have characteristics corresponding to the regulatory documents for thermal power facilities: low hardness and alkalinity, low concentration of suspended solids, petroleum products, etc., but in most cases, mandatory preliminary water treatment is required (Chukhlanov V,2018).
One of the important and primary methods of water treatment in the thermal power industry is mechanical filtration, which performs the function of primary purification of the source natural water from undissolved contaminants, (Vujanovic M., 2023) which further makes it possible to remove many problems when solving subsequent tasks, largely reducing the load on the next stages of water treatment (Peng H.,2023).
The most optimal for preliminary mechanical purification of natural waters are regenerated mesh filters that effectively remove sedimentary particles, sand, and other suspended insoluble impurities. Unlike cartridge filters, in which the filter element is replaced when saturated with impurities, mesh filters are equipped with a system for effective cleaning of the filter surface of the filter. It significantly reduces operating costs and can allow the filters to be fully automated (Li J.,2023). At the same time, the effective operation of such filters largely depends on the correct organization of the flushing of the mesh filter element. (An B.-H., 2022). Self-cleaning strainers are widely used in water treatment systems to remove suspended solids from water.Mesh self-cleaning filters are widely used in many fields, including irrigation, industrial processes and municipal water treatment. These filters are designed to remove particles from liquids using a mesh element that captures impurities and then removes them through reverse-flow flushing. The process of reverse-flow flushing is critical for the operation of the filter, as it helps to remove accumulated dirt and restore its effectiveness (Shankara A.H.,2022).
These filters work on the principle of direct filtration when water passes through a sieve to remove suspended solids. The filter material is periodically cleaned by backwashing, which involves changing the direction of water flow through a sieve to remove accumulated solid particles .
Backwash is an important process in self-cleaning strainers because it maintains the filter's operability and prevents it from clogging. However, the backwash time can significantly affect the performance of the filter (Mamuad R.Y.,2022).
This work aims to evaluate the effect of the backwash time on the parameters of the mesh self-washing filter (Islam A.,2022).

Background
The duration of the backwash process can significantly affect the operation parameters of the mesh self-washing filter (Song S., 2023). If the backwash time is too short, the filter may not be clean enough, leading to decreased efficiency and an increased pressure drop. On the other hand, if the backwash time is too long, it can lead to excessive wear of the filter and increased water consumption (Kabiri B.,2022).

Review
Mechanical water purification filters are only the preparation of water for deeper purification by other methodschemical or biological (Duan D., 2022). Passing water through nets of various materials and structures extracts coarse impurities from river sediments from sand, silt and other suspended substances. The main elements used for these purposes in water intake units are flat and rotating grids. Drum nets and microfilters are mounted in the entrance structures of water treatment plants. In the practice of water purification, mesh filters are usually used (Huang M.-Y.,2020).
The separation of suspensions into filtrate and sediment occurs due to the pressure difference created. The filtrate passes through the mesh cells, and coarse, suspended, and floating particles in water linger on it. Modern equipment makes it possible to solve the layout of the pre-mechanical filtration stage at water treatment facilities in a new way. It should be taken into account that several parameters determine the performance and efficiency of the filter: the initial contamination of water and a given degree of purification, the size of the mesh cells, the technology of operation (scanner or brush cleaning of the mesh), the type of drive (electric or hydraulic), the material of manufacture and the conditions of equipment placement (Kim H.J., 2023).
The concept of scanner cleaning of the mesh provides that the filter can be in two states filtration, filtration and flushing. The main advantage is that the filtering process is ongoing without reducing productivity (Kadim, 2022). The criteria for choosing a scanner cleaning are: filtration of impurities of a minimum fractional size the absence of a significant amount of heavy petroleum products in the filtered liquid the absence of conditions for the occurrence of active fouling of the mesh, not only chemical and physical but also biological nature In general, scanner cleaning gives a better and more reliable result. At the same time, there is no mechanical wear on the mesh and the cleaning mechanism. The cost of scanner filters is slightly higher than other designs, but at the same time, they have lower operating costs. When using filters in circulating systems and at water intakes from open sources, a pre-cleaning grid ("sump") is used in the filter design to delay large particles and parallel arrangement of the supply and withdrawal ports of the source and purified water. The main drawback of these designs is that the filter stops working during flushing or sharply and noticeably reduces productivity. And this circumstance makes its application ineffective in many areas. Attempts to improve the design only led to the complication of the filter, increase in cost and decrease in reliability (Ke X., Shi Z.,2014).
At least two grids are used to solve the problem in filters cleaned by counterflowone is washed with filtrate, and the second is involved in the filtration process. Another solution is one mesh, but it is divided into two partsone half is washed, and the other half purifies the water for washing (Selivanov O.G.,2020).
Another solution to the problem is to use multiple grids in construction with a complex distribution of flows. While one mesh is being washed, the other prepares the water for washing, and the others filter the water. But still, the performance of the filter is not high enough (Santos,2023). The market offers various self-washing filters, differing in organizing filtration and flushing. Regeneration can be performed both simultaneously over the entire surface ("Honeywell", "Valtec", "RBM") and by point movement of the suction over the entire surface ("Arkal"), movement of brushes along the surface ("Hydac International"), increasing the gap at the time of backwashing ("Tekleen", "Arkal", Krapukhin filter). It is also possible to combine several methods, in particular, backwash, with the movement of brushes ("Aqua").
To create high-quality, reliable, simple and cheap filters, YAMIT has developed a completely new technology-focused flushing technology, according to which the filter is always in filtration mode, and cleaning of the mesh and removal of impurities always occurs from the "dirty side". In this case, the grid is cleaned sequentially site by the site over the entire area. As other promising devices for preliminary mechanical filtration in energy-and resourcesaving water treatment and water treatment plants, mesh self-washing filters with scraper brushes are considered. Filters of this design can be used to filtrate natural, recycled and wastewater in the water treatment of heat power facilities, industrial enterprises and public utilities (Yu C.,2021).

METHODOLOGY
The mesh self-washing filter developed by OOO BMT (Vladimir, Russia) is a housing with a stainless-steel mesh cartridge (see Figure 1). The filter is mounted on a metal frame. Inside housing 1, a mesh cartridge 4 is mounted on filter rod 5 and secured with a lock nut 8 (Maciel P.M.F., 2021). The mesh filter cartridge combines a perforated stainless-steel sheet (frame) and a steel mesh wound on the frame and is bonded by spot welding, with steel covers welded at the ends. Depending on the cleaning requirements, a fine-mesh steel mesh of different filtration ratings (5 to 500 microns) can be used. The pressure drop on the filter is not more than 0.15 MPa. The working area of filtration is 0.12 m2 (Judd S.J.,2021).
Regeneration of the self-cleaning filter involves flushing the mesh cartridge with a reverse flow of purified water with simultaneous rotation of the cartridge relative to the fixed brushes 6 built into the housing, which is carried out by an electric drive 9.
During washing, water enters the mesh cartridge, and passes out, knocking out the impurities trapped on the mesh surface from the inside (Yuan H., 2015). During the rotation of the filter cartridge, additional mechanical cleaning of the outer surface of the mesh occurs with fixed brushes mounted on the inner walls of the housing (Pan Y., 2023). Filtered mechanical impurities and sediment from the bottom of the housing and the washing water are removed through the drainage pipe 10. The general view of the self-cleaning filter is shown in Figure 2. Using mechanical filters that ensure the regeneration of the filter surface by the reverse flow of water with the simultaneous use of special brushes will allow removing dirt from the filter surface with a high degree of efficiency. This principle significantly reduces the amount of washing water with almost 100% restoration of the original characteristics of the filter. At the same time, the volume of washing water is minimized and is about 1% of the volume of water passed in the operating model (Liu H.-L.,2023).
To confirm these parameters, a series of tests of the developed design of a mesh selfwashing filter was carried out on model solutions containing 50 and 500 mg/l of suspended solids. The model solutions were prepared as follows: dry clay and peat were dried at a temperature of 105℃ and crushed. Next, the powder was fractionated on a sieve with a cell size of 100 microns (Wu Q., 2023). A particle fraction of more than 100 microns was isolated. Model solutions were prepared based on distilled water according to GOST R 58144-2018.

Research Technique
The turbidity of the kaolin model solution before and after filtration was determined according to GOST 3351-74, and the content of suspended substances in it was determined according to the guidance document RD 52.24.468-2019. The technological scheme of the installation for experimenting is shown in Figure 3. The model solution from tank T1 is fed by pump P1 to the self-washing filter F with a flow rate of 200 litres/hour. Water enters the filter, flows through the filter cartridge mesh, and dirt is trapped on the mesh surface, partially settling on the bottom of the housing. In filtration mode, the solenoid valves VE1 and VE2 are open, and VE3, VE4 are closed. The filtration process continues until the differential pressure value of 1.5 kgf/cm2 is reached, controlled by the differential pressure relay R.
When the set pressure drop is reached, the self-washing filter switches to the flushing mode. At the same time, the electromagnetic valves VE1-VE4 change the direction of fluid flow: the valves VE1 and VE2 are closed, and the valves VE3 and VE4 are opened. The water for the filter's backwash is supplied from tank T2 by pump P2 with a flow rate of 2.8 m3/hour for a specified time. The filter is flushed with a stream of purified filtrate.
In the flushing mode, the water flows back through the holes in the filter rod, knocking out the sediment trapped on the grid from the inside. The sediment washed off the grid and accumulated on the bottom of the housing is captured by the water flow and removed through the drainage pipe into the sewer. The counterflow effect is enhanced by simultaneous mechanical cleaning of the filter mesh with rigid brushes installed in the filter housing. The electric drive provides rotation of the filter cartridge in the housing. At the end of flushing, the filter automatically returns to the filtration mode.

RESULTS
During the tests, filtrate samples were taken for analytical control of turbidity and concentration of suspended substances, and a pressure drop was recorded depending on the volume passed. The experimental results for model solutions with a suspended particle content of 50 mg/l are presented in Table 1. As can be seen from the experiment results, a self-washing filter with a filtration rating of 100 microns provides consistently high cleaning efficiency for suspended particles larger than 100 microns (>99.5%) during the entire filtration cycle. The dependence of the increase in the pressure drop on the filter on the passed volume is shown in Figure 4. It is established that the resource before washing on the water of this composition is 17900 litres, and the continuous operation time is 89 hours. Then three more filtration cycles were carried out on a model solution of the same composition with washing times of 10, 20 and 30 seconds. The results of the experiment are presented in Table 2 and Figure 5.  As can be seen from the experimental data, the optimal flushing time, at which the filter life is restored to 100%, can be considered 30 seconds. Accordingly, with a flushing time of 10 seconds, the filter resource is reduced by 4800 litres, and with a time of 20 secondsby 2400 litres.
During further experiments, ten filter cycles were carried out on a model solution with back washings for 30 seconds. The results are presented in Table 3. As can be seen from the data in the table, a self-washing filter with backwash on the water of this composition for ten filter cycles provides a fairly stable cleaning efficiency for suspended solids of a given size and a stable value for the passed volume of water. Thus, as a result of the conducted studies on model solutions with a suspended solids content of 50 mg/l, the technical characteristics of the self-washing filter presented in Table 4 were obtained. At the next stage of the experiments, tests were carried out on a model solution with a suspended solids content of about 500 mg/l. The results of the experiment are presented in Table  5. As can be seen from the experiment results, after countercurrent flushing for 30 seconds, the pressure drops on the filter returned to its original value (0.15 kgf / cm2) and the resource of the second filtration cycle is almost equal to the first. It can be concluded that when the content of suspended solids in the source water is up to 500 mg / l, the washing time of 30 seconds is also sufficient.
Thus, the following technical characteristics of the self-washing filter were obtained from the conducted studies on a model solution with a suspended solids content of about 500 mg/l (Table 6). Table 6. Technical characteristics of a mesh self-washing filter obtained after resource tests on a model solution containing 500 mg/l of suspended solids.

Parameter Indicator
Water flow supplied to the filter, l/hour 200 Volume of water passed before washing, l 3200 Duration of washing, sec. 30 Volume of water for washing, l 24 Restoration of the filter performance to the initial after the first washing, % 100 Retention capacity of the filter for suspended solids larger than 100 microns after the first filter cycle, % >99,5 Source: Prepared by the Authors(2023).
Thus, the volume of water passed before washing is 3200 litres, while the water required for washing for 30 seconds is 24 litres. Restoring the filter performance to its original value after the first flush reaches 100%. The retention capacity of the filter for suspended solids larger than 100 microns after the first filter cycle is 99.5%.

CONCLUSION AND SUGGESTION
In the course of conducting experiments to assess the effect of the time of reverse-flow flushing with filtrate on the parameters of the mesh self-washing filter, it was found that the optimal flushing time is 30 seconds.
Filter performance was restored 100% to its original value after the first flush. The filter capacity of suspended solids larger than 100 microns after the first filter cycle is 99.5%.
Water purification by suspended solids with a size of more than 100 microns is carried out by an amount of more than 99.5%. The filter provides reproducible cleaning quality, and the cleaning efficiency does not decrease from cycle to cycle. After repeated flushes, the filter performance is restored almost to its original value. The volume of water required for flushing the filter is no more than 1% of the total volume of water passed through the filter in one filter cycle. This design of the self-washing filter will effectively replace the ones used today.

FINANCIAL SUPPORT
The work was carried out with the financial support of the Ministry of Science and Higher Education of the Russian Federation (Grant Agreement dated June 23, 2021, No. 075-11-2021-031 IGC 000000S407521QKN0002) as part of the implementation of complex projects for the creation of high-tech industries approved by the Decree of the Government of the Russian Federation dated April 9, 2010 No. 218.