1. Core Advantages of Gravel in the Drainage and Filtration Field
Multi-Stage Filtration Capacity, Efficient Impurity Interception: By combining gravel of different particle sizes (e.g., 15-30mm coarse gravel for the bottom layer, 5-15mm medium gravel for the middle layer, and 2-5mm fine gravel for the surface layer), a gradient filtration hierarchy can be constructed. Coarse gravel first intercepts large-particle suspended solids (such as sediment and dead branches), while medium and fine gravel further filter tiny impurities (such as colloidal particles and organic debris), with a filtration efficiency of over 85%. This effectively reduces the load on subsequent treatment systems, especially suitable for the pretreatment of municipal sewage treatment.
Stable Chemical Properties, Ensuring Water Quality Safety: Natural gravel is mainly composed of inert substances such as silica, with stable chemical properties. During long-term contact with water, it will not release pollutants such as heavy metals and harmful ions into the water, nor will it react with chemical substances in the water to produce harmful substances. This characteristic allows it to continuously ensure effluent safety in scenarios with strict water quality requirements, such as drinking water pretreatment and landscape water circulation filtration.
Strong Corrosion Resistance, Adapting to Complex Environments: Gravel has excellent tolerance to acid-base environments. In water with a pH value of 3-11, its physical structure and filtration performance remain stable, without dissolution, weathering, or structural damage due to acid-base erosion. Whether in industrial acidic wastewater treatment or drainage systems in saline-alkali areas, gravel can work stably for a long time, with a service life of 15-20 years, significantly reducing the maintenance and replacement costs of water conservancy projects.
2. Potential Disadvantages of Gravel Application
Pores Prone to Blockage, Reducing Filtration Efficiency: After long-term use, suspended solids and colloidal particles in water will gradually adhere to the pores of gravel. If not handled in time, this will cause pore blockage, slow down the water flow rate, and reduce the filtration efficiency from over 85% initially to below 50%. In severe cases, it may cause problems such as system water accumulation and excessive pressure, affecting the normal operation of water conservancy projects.
Hidden Risks of Dissolvable Substances, Affecting Effluent Quality: Gravel from some geological areas (such as gravel containing carbonates or sulfates) may contain trace amounts of dissolvable substances. Under long-term immersion or water scouring, these substances may dissolve slowly, leading to changes in water pH value or increased ion concentration. If used in drinking water source filtration or sensitive industrial wastewater treatment, it may have an adverse impact on subsequent water quality.
Low Porosity of Smooth Particles, Limiting Filtration Effect: Smooth gravel formed by natural water scouring has high adhesion between particles, with a porosity of only 30%-35% (lower than the 40%-45% of angular gravel). The low porosity not only limits the water flow rate but also reduces the impurity adsorption space, resulting in weak filtration capacity, which is difficult to meet the high-flow and high-precision filtration requirements (such as industrial circulating water filtration).
3. Targeted Improvement Methods for Disadvantages of Gravel Application
Design Multi-Level Gradation, Optimize Filtration Structure: According to specific filtration needs, adopt a "coarse-medium-fine" multi-level gradation scheme. For example, lay 20-30mm coarse gravel on the bottom layer (large pores, fast flow rate, intercepting large impurities), 10-20mm medium gravel on the middle layer (transition filtration), and 3-10mm fine gravel on the surface layer (fine filtration). This structure not only ensures smooth water flow but also maximizes impurity interception, delaying the pore blockage rate.
Regular Backwashing, Restore Filtration Capacity: To solve the problem of pore blockage, a regular backwashing mechanism needs to be established. For municipal drainage or small-scale water treatment systems, high-pressure water can be used to backwash the gravel layer 1-2 times a week, using the impact force of water flow to flush out impurities in the pores; large-scale water conservancy projects (such as constructed wetlands and industrial water treatment plants) can be equipped with automatic backwashing equipment, using a combination of "water flushing + air flushing" to improve cleaning efficiency and quickly restore gravel filtration efficiency.
Preliminary Component Testing, Eliminate Harmful Substances: Before using gravel, its chemical composition needs to be tested in the laboratory, focusing on the content of heavy metals (such as lead and cadmium) and soluble salts (such as sulfates and carbonates), to ensure compliance with the "Quality Standard for Filter Media in Water Conservancy Projects". If used for drinking water-related filtration, additional testing of organic matter content is required to avoid using gravel with hidden risks of dissolvable substances, ensuring water quality safety from the source.
Combine with Artificial Filter Media, Improve Filtration Precision: To address the low porosity and insufficient filtration precision of smooth gravel, gravel can be used in combination with artificial filter media (such as quartz sand, activated carbon, and ceramsite). For example, lay a 5-10cm thick layer of quartz sand (to improve fine filtration effect) above the gravel filter layer, and a layer of activated carbon (to adsorb organic matter and odors) below, forming a "gravel + artificial filter media" composite filtration system. This not only retains the durability of gravel but also meets the high-precision filtration requirements.