1、 Purification principle and filter material of sand filtration (quartz sand filtration)
（1） Purification principle: multi-layer physical interception and adsorption
Mechanical screening function: The pores between quartz sand particles form a “sieve” to intercept suspended solids in water (such as sediment, algae, colloids), and impurities with particle sizes larger than the pores are directly intercepted.
Deep interception effect: The thickness of the sand layer is usually 0.8-1.2 meters, and impurities are gradually captured by the sand particles in the middle and bottom layers during the water flow infiltration process, achieving “deep filtration” (such as water with turbidity>5NTU can be reduced to below 1NTU after sand filtration).
Adsorption and electrostatic neutralization: The surface of quartz sand carries a weak negative charge, which can adsorb some positively charged colloidal particles (such as iron and manganese oxides), or cause colloidal destabilization and precipitation through particle collision.
（2） Types and characteristics of filter media
Types of filter media, material characteristics, particle size range, application scenarios
Quartz sand has a silica (SiO ₂) content of over 99%, high hardness (Mohs hardness 7), acid and alkali resistance, and strong chemical stability. 0.5-1.2mm conventional surface water and groundwater pretreatment to remove suspended solids and colloids.
Anthracite has a high carbon content, a density lower than quartz sand (which can form a graded filter layer with light on top and heavy on bottom), a high porosity, and strong retention ability. A double-layer filter material composed of 0.8-1.8mm quartz sand is used for the pretreatment of high turbidity water or oily wastewater.
The main component of manganese sand is MnO ₂ · nH ₂ O, which has a catalytic oxidation effect on iron and manganese ions and can simultaneously remove Fe ² ⁺ and Mn ² ⁺ from water. 0.6-1.2mm groundwater iron and manganese removal (if iron content>0.3mg/L).
The main component of magnetite sand is Fe ∝ O ₄, with a high density (4.5-5.2g/cm ³), commonly used in the bottom layer of three-layer filter media to improve its ability to intercept pollutants. 0.25-0.5mm industrial wastewater deep treatment or high load filtration scenarios.
（3） Operating characteristics
Backwash frequency: determined by the turbidity of the incoming water, usually 1-2 times a day, using a combination of “water flushing+air flushing” to remove impurities trapped by the filter material.
Filtration rate: The conventional sand filter has a filtration rate of 8-12m/h, while the high-efficiency filter can reach 15-20m/h (requiring an increase in filter layer thickness or optimization of filter material grading).
2、 Purification principle and filter material of activated carbon filtration
（1） Purification principle: synergistic effect of physical adsorption and chemical adsorption
Physical adsorption (main function):
Activated carbon has well-developed internal pores (with a specific surface area of 500-1500m ²/g), and can adsorb organic matter (such as humic acid, pesticides, residual chlorine), heterochromatic and odorous substances (such as phenols, algal metabolites), and some heavy metal ions (such as Hg ² ⁺, Cd ² ⁺) in water through van der Waals forces.
The higher the proportion of micropores (pore size<2nm), the better the adsorption effect on small molecule organic compounds; Mesopores (2-50nm) can adsorb large organic molecules or serve as diffusion channels.
Chemical adsorption:
The surface functional groups of activated carbon (such as hydroxyl and carboxyl groups) undergo chemical reactions with pollutants, such as the oxidation and decomposition of residual chlorine in water (ClO ⁻+C → Cl ⁻+CO ₂), or the formation of complexes with heavy metals.
Biodegradation (applicable to activated carbon):
After running for a period of time, microbial films will grow on the surface of the activated carbon filter, which can degrade some biodegradable organic matter (such as low molecular weight fatty acids) and prolong the adsorption period of activated carbon.
（2） Types and characteristics of filter media
Types of filter media, materials, sources, structural characteristics, typical applications
Coal based activated carbon is made by high-temperature activation of bituminous coal and anthracite coal, with low cost and developed mesopores. Removal of residual chlorine from tap water and removal of large molecular organic compounds (such as dyes and surfactants) from industrial wastewater with a particle size of 1-2mm.
Activated carbon from fruit shells, coconut shells, apricot shells, and other materials are activated at high temperatures, with micropores accounting for over 80% and high adsorption capacity. Deep treatment of drinking water with a particle size of 0.5-1.0mm (such as removing odors and trace organic matter), and purification of water quality in the food industry.
Wood activated carbon is made from activated wood chips and bamboo chips, with uniform pores and fast adsorption speed. Liquid phase decolorization (such as syrup and pharmaceutical intermediate purification) with particle size of 0.3-0.8mm, and treatment of low concentration heavy metal wastewater.
Fiber like activated carbon is made from materials such as polyacrylonitrile, and is transformed into forms such as felt and cloth. Its specific surface area is greater than 1800m ²/g, and its adsorption rate is 10-100 times that of granular activated carbon. Fiber diameter 10-20 μ m for emergency water treatment (such as sudden organic pollution) and gas purification (VOCs adsorption).
（3） Operating characteristics
Adsorption saturation period: depends on the concentration of organic matter in the influent. For example, the use period of activated carbon in tap water treatment is usually 3-6 months, while in industrial wastewater treatment, it may be shortened to 1-2 months.
Backwash requirement: Use low-intensity water flushing (to avoid damaging the pore structure). If organic matter is mainly adsorbed, ozone regeneration (O3 oxidation decomposition of adsorbate) can be combined to extend the service life of the filter material.
3、 The synergistic application and differences between sand filtration and carbon filtration
Comparison Dimension Sand Filter Carbon Filter
Core removal of suspended solids, colloids, some inorganic particles, organic matter, residual chlorine, color and odor, heavy metals
Mechanism of action: Physical interception, mechanical screening, physical adsorption, chemical adsorption, biodegradation
Filter material replacement cycle: Quartz sand can be used for 5-8 years (with regular replenishment of losses), while activated carbon usually takes 3-12 months (depending on load)
The preprocessing position is often used as a primary filter (pre-processing) after sand filtration, as a deep processing step
4、 Examples of application scenarios
Waterworks: Raw water → coagulation and sedimentation → sand filtration → carbon filtration → disinfection, where sand filtration removes turbidity, carbon filtration adsorbs organic matter and residual chlorine, enhancing the taste of drinking water outlets.
Industrial wastewater treatment: During the pretreatment stage, sand filtration is used to remove suspended solids, followed by carbon filtration to adsorb recalcitrant organic compounds (such as benzene derivatives in chemical wastewater), ensuring that the effluent meets the standard.
Reverse osmosis pretreatment: The combination of sand filtration and carbon filtration can effectively remove particulate impurities and residual chlorine from water (preventing RO membrane oxidation), extending the service life of RO membrane.
By selecting appropriate filter media and process combinations, sand filtration and carbon filtration can complement each other in water treatment, creating favorable conditions for subsequent deep treatments such as RO membranes and ion exchange.
Post time: Jul-04-2025