Whether in industrial production or daily life, water quality issues are receiving increasing attention. As one of the important indicators for evaluating water quality, water turbidity is directly related to water transparency and purity. As a widely used water treatment device, the multi-media filter plays a crucial role in controlling effluent turbidity. This article will explore in depth how the multi-media filter, through its unique filtration mechanism and refined operational management, ensures that effluent turbidity reaches the ideal standard, thereby meeting water quality requirements under different application scenarios.
Water turbidity refers to the degree to which particulate matter in water scatters light, causing a decrease in transparency. It is usually expressed in NTU (nephelometric turbidity unit), and the lower the value, the clearer the water quality. Suspended solids and colloidal substances such as soil, silt, fine organic matter, anthracite, and manganese sand in water will cause turbidity. For example, 1 liter of water containing 1 mg of SiO₂ corresponds to a standard turbidity unit, referred to as 1 degree. Generally, the higher the turbidity, the more cloudy the solution.
A multi-media filter is a device that filters water under certain pressure using various filter media (such as quartz sand, anthracite, and manganese sand). It allows turbid water to pass through a certain thickness of granular or non-granular materials to effectively remove suspended impurities, resulting in clear water. The effluent turbidity of this filter can reach below 3 degrees or even lower, depending on the application scenario and the quality of the influent water.
The filtration performance of a multi-media filter largely depends on the filter media used. Common combinations of filter materials include anthracite, quartz sand, and garnet or magnetite. These filter materials are arranged in layers from top to bottom according to "large particle size → small particle size, light specific gravity → heavy specific gravity." This layered design effectively intercepts suspended particles of different sizes, thus achieving efficient filtration.
For example, in municipal water pretreatment, the multi-media filter is usually used as a pretreatment unit before the reverse osmosis (RO) system. Its purpose is to control the influent turbidity at ≤5 NTU and the effluent turbidity at ≤0.5 NTU to prevent RO membrane blockage caused by high turbidity and to extend the membrane's service life. Since the RO membrane feedwater requirement is generally ≤1 NTU, the pretreatment role of the multi-media filter is extremely important.
The effluent turbidity of a multi-media filter is not fixed but varies according to different application scenarios and influent water quality. The following are some common application scenarios and their corresponding effluent turbidity requirements:
Municipal Water Pretreatment (such as RO Pretreatment):
The influent turbidity is usually ≤5 NTU, and the effluent turbidity should be controlled at ≤0.5 NTU. This is because RO membranes have strict requirements for feedwater turbidity; excessive turbidity can cause membrane blockage, affecting system operation efficiency and service life.
Industrial Circulating Water Makeup:
The influent turbidity is between 5–20 NTU, and the effluent turbidity should be controlled at ≤1–2 NTU. This can reduce scaling and corrosion in the circulating system pipelines, prevent heat exchanger blockage, and ensure normal operation of the industrial circulating system.
Wastewater Treatment (Secondary Effluent Advanced Treatment):
The influent turbidity is between 10–50 NTU, and the effluent turbidity should be controlled at ≤5 NTU. In some reuse scenarios, such as landscaping or toilet flushing, the effluent turbidity requirement is even lower, ≤2 NTU, to meet the water quality requirements for subsequent disinfection and reuse.
Groundwater Purification (Low Iron and Manganese Content):
The influent turbidity is between 2–10 NTU, and the effluent turbidity should be controlled at ≤0.3–0.5 NTU. This is because the goal of groundwater purification is to meet the sanitary standards for drinking water (GB 5749-2022), which require drinking water turbidity ≤1 NTU and preferably ≤0.5 NTU.
Although the multi-media filter performs excellently in reducing water turbidity, its effluent turbidity can still be influenced by multiple factors. To ensure that the filter can consistently meet the turbidity standards, these factors must be carefully analyzed and effectively managed.
The filter media are the core components of a multi-media filter, and their condition directly affects the filtration performance. During long-term operation, water flow scouring and backwashing may cause wear, particle size reduction, and decreased filtration efficiency. For example, if the particle size of the upper anthracite layer is too small (e.g., <0.8 mm), it easily clogs quickly, causing flow short-circuiting. Conversely, if the lower quartz sand layer particles are too large (e.g., >2 mm), they cannot intercept fine suspended particles (e.g., colloids <10 μm), leading to increased effluent turbidity.
In addition, media fouling and compaction cannot be ignored. Organic matter, microorganisms, and colloids in water can adhere to the media surface. Over time, this accumulation reduces pore space or causes compaction, increases flow resistance, lowers impurity removal capacity, and affects effluent turbidity. For example, after long-term operation, the gaps between media can trap large amounts of oil, organic matter, or microorganisms (forming biofilms), leading to "media compaction," where water cannot flow through normally.
Operating parameters also significantly affect the effluent turbidity of multi-media filters. If the filtration velocity is too high, the water's residence time in the filter bed is too short for impurities to be fully captured; if too low, excessive deposition may occur, making backwashing difficult. For example, a typical filtration velocity design is 8–12 m/h. If the actual velocity exceeds 15 m/h, the residence time in the media bed is less than 2 minutes, preventing suspended particles from being trapped and resulting in turbidity exceeding the limit. If the velocity is too low (<5 m/h), excessive accumulation of retained solids occurs, making backwashing insufficient.
Backwashing is key to removing trapped impurities. If the backwash intensity is insufficient (typically 15–20 L/(m²·s)), the duration too short (typically 5–8 minutes), or air scouring is not used (for oily or organic-rich water), the retained particles cannot be completely detached, and during subsequent filtration, they may be "re-released," causing increased effluent turbidity.
Equipment failures are also important factors causing excessive effluent turbidity in multi-media filters. For instance, if the upper water distributor (such as perforated plates or spray nozzles) becomes clogged or damaged, the influent will locally scour the media bed, forming “channeling” (water bypassing filtration directly). If the bottom collector (such as the quartz sand support layer or filter screen) is damaged, media loss may occur, and unfiltered water can flow directly through. Moreover, if the vessel leaks (e.g., cracked welds, loose flange seals), external water such as rainwater or seepage may enter the filter, increasing turbidity. Detached internal coatings and corrosion products ("rust particles") can also contaminate the effluent.
To ensure that the multi-media filter consistently achieves the required effluent turbidity, a series of effective management strategies must be implemented, including regular media maintenance, optimization of operating parameters, and timely repair of equipment failures.
Regular inspection and replacement of filter media are key to maintaining filtration efficiency. It is recommended to replace the media every five years to prevent degradation caused by wear or breakage that reduces porosity and filtration performance. Periodic backwashing should be carried out to remove trapped impurities and prevent fouling or compaction. For oily or organic-laden water, "air scouring" should be added during backwashing to enhance cleaning effectiveness.
Adjust filtration and backwashing parameters appropriately based on influent quality and application. The standard filtration velocity is 8–12 m/h, backwash intensity 15–20 L/(m²·s), and duration 5–8 minutes. In actual operation, these should be flexibly adjusted to ensure adequate residence time for impurity capture and effective removal during backwashing.
Regularly check equipment conditions and promptly repair any detected faults. Inspect the integrity of water distributors and collectors to prevent clogging or damage; check vessel sealing and internal coating conditions to prevent leaks and corrosion. In cases of sudden turbidity increase in influent, adjust parameters or implement emergency measures promptly to ensure effective filtration of highly turbid water.
As an efficient water treatment device, the multi-media filter plays an important role in controlling effluent turbidity. Through proper selection and combination of filter media, optimization of operational parameters, and regular equipment maintenance, it can effectively reduce turbidity to meet various application requirements. Whether for municipal water pretreatment, industrial circulating water makeup, wastewater treatment, or groundwater purification, multi-media filters provide reliable water quality assurance. However, to ensure long-term stable operation, it is necessary to deeply analyze and manage the factors affecting effluent turbidity and adopt targeted maintenance strategies, thereby achieving continuous water quality improvement and efficient equipment performance.
