In the field of water treatment, multi-media filters are widely used devices. Their core function is to remove suspended solids, impurities, and pollutants from water through a combination of multiple filter media, thereby improving water quality. However, to achieve optimal filtration performance, the design of the filtration rate is critically important. This article will explore the key points in the design of filtration rates for multi-media filters and provide some optimization strategies to help you better understand and apply this technology.
First, it is necessary to understand the basic principles of filtration rate design for multi-media filters. These principles are the foundation for ensuring efficient operation of the filter and achieving the desired filtration effect. The following are some key basic principles of filtration rate design:
The filtration rate design of a multi-media filter must first consider the conditions of the inlet water. Factors such as water quality, viscosity, pollutant concentration, liquid viscosity, and liquid temperature all significantly affect the filtration rate. Generally, the better the inlet water quality, the higher the designed filtration rate can be. For example, when the inlet turbidity is less than 20 NTU, the outlet turbidity can reach below 3 NTU. Therefore, maintaining the stability of inlet water quality is the foundation for ensuring efficient operation of the filter.
Multi-media filters vary greatly in size, ranging from small household units to large industrial filters, with significant differences in tank height and overall height. Large multi-media filter tanks can reach over 2 meters in height, with an overall height exceeding 3 meters. The larger the size, the wider the design range for water flow speed, and the higher the filtration rate. Additionally, the diameter of the inlet and outlet ports also affects flow speed. For instance, with DN100 inlet and outlet ports, the flow can reach 34–42 tons per hour, which represents a considerable water production capacity.
Multi-media filters use a wide variety of filter media, including activated carbon, quartz sand, anthracite, and softening resin. Different media affect water flow velocity, particle size capture, and the ability to block impurities differently. Media with larger pore sizes allow faster filtration, while media with smaller pore sizes filter more slowly. Therefore, when designing the filtration rate, it is necessary to select the appropriate media according to specific filtration requirements.
Next, we will explore how optimization strategies can further improve the performance and efficiency of the filter. These strategies can not only increase filtration speed but also extend the service life of the equipment while ensuring that the water quality meets the expected standards.
To ensure efficient operation of multi-media filters, it is crucial to maintain the stability of inlet water quality and temperature. Multi-media filters can handle liquids up to 50°C, but in actual operation, it is best to control the liquid temperature between -5°C and 40°C. Stable inlet conditions can reduce fluctuations in filtration rate and improve filtration performance.
The horizontal level and flatness error of filter plates should be controlled within 1.5 mm to ensure even water flow distribution. The structure of the filter plate is preferably processed as a whole using automated equipment to improve precision. The radial error of through-holes on the filter plate should be controlled within ±1.5 mm. Additionally, the material of the filter cap can be nylon or ABS, and elastic rubber pads should be added to the upper and lower surfaces of the contact area between the filter cap and plate to prevent deformation.
Filter media is the core component of multi-media filters, and its material, gradation, particle size, and filling method directly affect filtration rate. To increase the filtration rate, the following optimization strategies can be applied:
Application of High-Density Media: Conventional media such as quartz sand have low density and are easily disturbed by high water flow, causing the filter bed to become disordered. High-density media, such as magnetite (density 4.5–5.0 g/cm³) or ilmenite (density 4.2–4.8 g/cm³), can be used instead. High-density media can withstand greater water flow impact, increasing filtration rate by 20%–30%.
Multi-Layer Media Combination: Using a layered filling method of "light coarse media on top + heavy fine media below" leverages the "reverse particle size effect" of different density media. The upper coarse media intercepts large particles, while the lower fine media intercepts small particles. This combination expands impurity retention capacity, reduces overall filter bed resistance, and stabilizes the filtration rate at 15–20 m/h.
Adjustment of Media Gradation: Improper media gradation can cause pore clogging too quickly. It is recommended to adjust particle size according to raw water turbidity, controlling the "uniformity coefficient" (K80) of the media between 1.8 and 2.2 to increase filter bed porosity.
Regular Replacement and Regeneration of Media: Over time, media can become compacted and aged, resulting in increased resistance and reduced flow. Quartz sand and anthracite are typically replaced every 1–2 years, while high-density media like magnetite can last 3–5 years. If media are not severely worn, regeneration through "acid washing + backwashing" can restore adsorption capacity and porosity.
Structural deficiencies in equipment can affect water flow distribution, causing local flow velocities to be too high or too low. Optimization strategies include:
Improvement of Water Distributors: Replacing traditional "central pipe + branch pipe" distributors with perforated plate distributors or filter cap distributors can significantly improve distribution uniformity by over 30%.
Installation of Curved Baffles: Installing curved baffles at the inlet prevents raw water from directly impacting the filter bed surface, ensuring horizontal water flow diffusion.
Adjustment of Filter Bed Height: Adjust filter bed height according to raw water turbidity. For low turbidity (<30 NTU), the bed can be reduced to 0.8–1.0 meters; for higher turbidity, the bed should be increased to 1.8–2.0 meters.
Expansion of Tank Diameter: If a single filter needs to handle higher water volume, the tank diameter can be increased. For example, increasing from 1.0 m to 1.5 m raises the filtration area by 125%, allowing greater water treatment without changing flow speed.
The filtration rate is closely related to inlet pressure, product size specifications, and media thickness. Higher inlet pressure increases filtration rate; larger equipment handles higher water volume, requiring higher filtration rates; thicker media layers require more pressure for water to pass through. Therefore, filtration rate design must consider these factors and be dynamically adjusted based on actual operation.
All multi-media filters have some head loss during operation. Excessive head loss can lead to operational issues. Head loss should be maintained within a reasonable range. Regularly monitor inlet and outlet pressure differences; if the difference exceeds 0.1 MPa, check and take measures such as cleaning or media replacement.
The theoretical filtration rate is often designed slightly higher than the actual rate to extend filter lifespan. The increased rate must ensure outlet water quality meets standards. This can be achieved by optimizing media characteristics, equipment structure, operating parameters, and pre-treatment processes. The key is to reduce filter bed resistance, improve media impurity retention, and maintain uniform water distribution.
To better understand optimization strategies for filtration rate design, consider a case from an industrial water treatment plant using a large multi-media filter with a tank height of 2.5 m, overall height of 3.2 m, and DN150 inlet/outlet ports. The plant faced poor inlet water quality, high turbidity, and significant temperature fluctuations. By optimizing the filtration rate, the plant implemented the following measures:
High-Density Media Application: Replaced quartz sand with magnetite, increasing filtration rate from 12 m/h to 15 m/h.
Multi-Layer Media Combination: Used "anthracite + quartz sand + magnetite" layered filling, stabilizing filtration rate at 18 m/h.
Water Distributor Improvement: Replaced traditional distributor with a perforated plate, improving uniformity by 35%.
Curved Baffle Installation: Added curved baffles at the inlet, preventing direct impact on the filter bed and ensuring even water diffusion.
Filter Bed Height Adjustment: Adjusted bed height to 1.8 m according to turbidity, ensuring impurity retention and reducing frequent backwashing.
Tank Diameter Expansion: Increased diameter from 1.2 m to 1.5 m, raising filtration area by 125% and proportionally increasing treated water volume.
Through these measures, the plant's multi-media filter achieved significantly improved efficiency, outlet water quality met standards, and equipment lifespan increased by more than 30%.
Filtration rate design in multi-media filters is a complex and important process that requires comprehensive consideration of inlet water conditions, equipment size, media characteristics, and structural design. By optimizing media selection, adjusting equipment structure, and maintaining stable inlet water quality and temperature, filtration efficiency and service life can be significantly improved. In practical applications, filtration rate design should be flexibly adjusted according to specific operating conditions to ensure efficient operation of the filter. This article aims to provide valuable guidance for the design and optimization of multi-media filter filtration rates.
