Silica in water predominantly exists in the form of inorganic silicic acid compounds, and the industry uniformly measures it as silicon dioxide (SiO₂). According to GB/T 12149-2017, Determination of Silicon in Industrial Recirculating Cooling Water and Boiler Water, silica in water exists in four forms:
1. Dissolved Silica (Reactive Silica) Composed mainly of monomeric orthosilicic acid (H₄SiO₄) and metasilicic acid (H₂SiO₃); detectable via the molybdate colorimetric method. Particle size < 1 nm, so it cannot be removed by ordinary filtration; under conditions of pH > 10.5, a small fraction dissociates into silicate ions.
2. Colloidal Silica (Non-Reactive Silica) Formed when silicic acid molecules polymerize into polysilicic acid colloids, with a particle size of 1 nm to 1 μm. Conventional coagulation removes only about 60%, and it can easily pass through ultrafiltration and reverse osmosis membranes.
3. Adsorbed Silica Silica adsorbed onto the surface of sediment, iron hydroxide, or calcium carbonate flocs; most of it can be removed through conventional sedimentation and filtration.
4. Particulate Silica Solid particles such as sand, gravel, and silicate minerals, which can be fully intercepted by bar screens and quartz sand filtration.
For zero-liquid-discharge (ZLD) and MVR evaporation systems, dissolved silica and colloidal silica are the most hazardous. As they pass through the membrane system, they are continuously concentrated and their concentrations multiply, making them the core cause of silica scale formation.
Core reasons why water treatment must remove silica, and the hazards of silica:
1. Damage to Reverse Osmosis / Nanofiltration Membrane Systems Colloidal silica forms a dense gel layer on the membrane surface, while dissolved silica precipitates as silica scale after continuous concentration. Per HJ 579-2010, Technical Specification for Membrane Separation Wastewater Treatment, the RO feed-water SDI should be ≤ 5. Excessive silica directly keeps the SDI elevated, manifesting as a month-by-month rise in transmembrane pressure, a production capacity decline of over 30%, and premature membrane element failure within 1-2 years. Silica scale cannot be dissolved by conventional acid or alkali cleaning; only hydrofluoric acid can remove it, significantly increasing chemical consumption and on-site safety risks.
2. Fatal Impact on MVR Evaporation-Crystallization Systems Industry standards clearly specify that the silica dioxide in MVR evaporator feed water should not exceed 50 mg/L, while salt-resource-recovery projects must control it within 10 mg/L. Under high-temperature concentration, silicic acid polymerizes to form dense, hard magnesium silicate and calcium silicate scale that firmly adheres to heat-exchanger tube walls. With a thermal conductivity only 1/100 of that of stainless steel, silica scale directly raises evaporation energy consumption by 40%. Being extremely hard, it is difficult to remove by ordinary acid washing and requires shutdown for high-pressure water flushing; each such maintenance stop can take 3-7 days. Once pipelines are blocked by silica scale, system treatment capacity drops sharply, and there is also an environmental risk of non-compliant effluent discharge.
3. Secondary Cascading Economic Losses Silica entering the crystallized salt product turns the salt black and fails quality standards, making it unsellable as industrial salt and forcing disposal as hazardous waste — at up to 10 times the cost. Meanwhile, frequent start-stop cycles and repeated chemical cleaning generate large volumes of hazardous sludge, and combined with chemical consumables, labor hours, and production downtime, overall operation and maintenance costs multiply.
Silica scale has three major characteristics — difficulty of cleaning, irreversible damage, and heavy equipment wear — and is a key hidden threat to the stable operation of membrane concentration and evaporation-crystallization systems.
All industry design codes make it clear that desilication cannot be remedied or repaired by downstream equipment; it must be a core part of pretreatment, with strict control over key process parameters such as pH, chemical dosage, and reaction retention time. Based on the actual water quality of the project and the resource-recovery objectives, the appropriate processes — such as magnesium-salt precipitation, tubular membrane, and resin polishing for silica removal — should be selected to control feed-water silica dioxide within the standard limits. Only in this way can the comprehensive costs arising from equipment failures, shutdown maintenance, and hazardous-waste disposal be effectively reduced, ensuring long-term stable and compliant operation of the entire water treatment system.
Your message must be between 20-3,000 characters!
