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Silica Removal Water Treatment Companies
Silica removal, lime softening, IX, RO, and specialty resins for boiler feed, power, and semiconductor water.
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Silica Removal from Water: Coagulation, Ion Exchange, and High-pH Precipitation Methods
Silica (SiO2) in water exists in two main forms: reactive (monomeric, H4SiO4, Si(OH)4) and colloidal (polymeric, non-reactive). Reactive silica is the dominant form in groundwater (5 to 100 mg/L), surface water (1 to 30 mg/L), and geothermal sources (200 to 1,000+ mg/L). Silica causes severe fouling in: RO/NF membranes (amorphous silica precipitation on membrane surface when concentrate SiO2 exceeds 100 to 120 mg/L solubility at pH 7 to 8); boiler steam generators (silica scales at greater than 150 degrees C forming anhydrite or cristobalite, causing severe heat transfer reduction and tube failures); semiconductor wafer manufacturing (ultrapure water requires SiO2 less than 10 ppb); and cooling water systems (silica scaling at Langelier-equivalent saturation for SiO2). Analytical methods: reactive silica by molybdate colorimetric method (ISO 16264, detection limit 0.01 mg/L); total silica after HF digestion or alkali fusion to convert colloidal to reactive form.
Silica removal methods depend on silica form and target effluent concentration: (1) Coagulation with iron or aluminium salts: ferric chloride (20 to 50 mg/L Fe) or alum (20 to 60 mg/L Al) co-precipitates reactive and colloidal silica by adsorption onto metal hydroxide floc; achieves 60 to 90 percent removal at pH 6.5 to 7.5; effective for surface water with silica 5 to 30 mg/L; (2) Lime softening at high pH: at pH greater than 10.5, Ca(OH)2 (lime) addition precipitates calcium silicate hydrate (CSH, similar to cement chemistry); achieves greater than 90 percent reactive silica removal; requires acid re-acidification after treatment; used in industrial boiler makeup water (achieving SiO2 less than 1 mg/L); (3) Strong base anion exchange (SBA resin, Type II): reactive silica (H3SiO4- at pH greater than 8) is removed by anion exchange; but requires high-pH feed or SBA resin operating in OH- form; limited to low-silica feeds and ultrapure water applications due to capacity and regeneration requirements.
For ultrapure water (semiconductor, pharmaceutical, power generation), silica removal requires a combination of ion exchange and membrane processes: mixed bed deionisation (strong cation + strong anion exchange resin) achieves SiO2 less than 10 ppb from feed water of 1 to 5 mg/L; electrodeionisation (EDI, continuous regeneration) achieves SiO2 less than 1 ppb; polishing with reverse osmosis (RO) upstream reduces load on ion exchange. Silica monitoring in UPW: online silicamolybdate colorimetric analyser (detection limit 0.1 ppb, Mettler Toledo, Hach) or ICP-OES for periodic validation. For RO systems with high-silica feed: silica antiscalants (polyacrylate-based, specialised silica inhibitors from BWA Water Additives, King Lee Technologies) extend maximum allowable concentrate SiO2 to 150 to 200 mg/L; maintain pH at 6.5 to 7.0 (lowest silica precipitation tendency); recovery limited to 60 to 75 percent for high-silica groundwater. Geothermal brine: silica scaling control requires rapid cooling to below 60 degrees C (reduces polymerisation rate), pH adjustment, or silica inhibitor injection within seconds of production well discharge.
Frequently Asked Questions
What silica concentration causes problems in RO membranes?
Amorphous silica scaling on RO membranes occurs when the concentrate-side silica concentration exceeds the solubility of amorphous SiO2. Solubility of amorphous SiO2: approximately 120 mg/L at pH 7.0 and 25 degrees C; decreases slightly with temperature; increases significantly at pH greater than 9 (ionisation of H4SiO4 to H3SiO4- increases apparent solubility). Rule of thumb: if feed silica times concentration factor (1 / (1 - recovery)) greater than 100 to 120 mg/L, silica scaling risk is high. Example: feed SiO2 40 mg/L at 75 percent recovery: concentrate SiO2 = 40 / (1-0.75) = 160 mg/L - exceeds solubility, scaling risk. Mitigation: (1) Reduce recovery to below 65 to 70 percent; (2) Dose silica-specific antiscalant (2 to 6 mg/L) to raise tolerable concentrate SiO2 to 150 to 200 mg/L; (3) Operate at elevated temperature (reduced viscosity but lower silica solubility - contradictory); (4) Raise feed pH to greater than 9.5 to 10 (increases silica solubility but may cause carbonate and hydroxide scaling). Silica fouling is very difficult to reverse; regular acid cleaning (pH 2 to 3) has limited effectiveness against silica scale; NaOH cleaning (pH 12 to 13) is more effective but risks membrane damage.
How does lime softening remove silica?
Lime softening removes reactive silica via co-precipitation with calcium silicate at high pH. Process: lime (Ca(OH)2) added to raise pH above 10.0 to 11.5; at these pH values, calcium ions react with silicate ions (formed by ionisation of H4SiO4 at high pH) to precipitate calcium silicate hydrate (xCaO.ySiO2.nH2O); precipitation is also assisted by magnesium hydroxide (Mg(OH)2) floc adsorption of silica. Silica removal achieved: 85 to 95 percent at optimum pH 10.5 to 11.5 with excess lime; achieves effluent SiO2 less than 1 to 5 mg/L from feeds of 20 to 60 mg/L. Soda ash (Na2CO3) is added simultaneously to remove calcium hardness as CaCO3. After lime softening: acid (CO2 recarbonation or H2SO4) is dosed to reduce pH to 7.5 to 8.5 before distribution or further treatment. Lime softening is used in power plant boiler makeup water treatment and large municipal softening plants. Temperature effect: warm lime softening (60 to 70 degrees C) achieves faster reaction rates and better silica removal than cold lime; used in steel and mining process water where waste heat is available. Magnesium silicate removal: if Mg2+ is present, coprecipitation of magnesium silicate improves silica removal efficiency.
What ion exchange process removes silica from water?
Strong base anion (SBA) exchange resin removes silicate ions (H3SiO4- and H2SiO4-2) which form at elevated pH. Key limitations: (1) Silicic acid (H4SiO4) is a very weak acid (pKa1 = 9.9); at neutral or acidic pH, silica exists predominantly as the uncharged H4SiO4 species which is NOT removed by anion exchange; (2) For SBA resin to remove silica, feed pH should exceed 8.5 to 9.0, or the resin must be operated in OH- cycle where the strongly basic resin can exchange OH- for even weakly ionised silicate. In demineralisation trains: SBA resin (Type II - N,N-dimethyl-2-hydroxyethyl quaternary amine) in OH- form removes both CO2 (as carbonate) and silica; achieves effluent SiO2 less than 10 ppb in ultrapure water systems. Mixed bed deionisers (cation + anion resin mixed) provide polishing to SiO2 less than 1 ppb. Regeneration: SBA resin regenerated with 4 to 8 percent NaOH solution; silica can be difficult to elute fully (silica 'hangover' or 'silica creep') from strongly basic resins - requires slow regenerant flow and extended rinse cycle; hot caustic regeneration (50 to 60 degrees C) improves silica elution efficiency.
How is silica controlled in cooling water systems?
Silica scaling in cooling towers forms when evaporation concentrates silica in recirculating water above amorphous silica solubility (120 mg/L at pH 7.0). Control strategies: (1) Cycles of concentration (CoC) control: limit CoC to keep silica below scaling threshold; if makeup water has 20 mg/L SiO2 and limit is 120 mg/L: maximum CoC = 120/20 = 6.0; (2) Silica-specific antiscalant: polyacrylate and co-polymer antiscalants (BWA Bellasil, Nalco, Solenis) allow operation at SiO2 up to 180 to 250 mg/L with threshold inhibition; dose 5 to 15 mg/L; (3) pH control: maintain pH 7.0 to 7.5 (lower pH increases silica solubility; higher pH increases SiO2 ionisation to H3SiO4- and colloidal SiO2 which may deposit differently); (4) Sidestream softening: treat a fraction of circulating water (5 to 10 percent of recirculation flow) through lime softening or ion exchange to remove silica continuously, reducing bulk concentration; (5) Blowdown: increase blowdown rate to reduce CoC when makeup water silica is high. Silica scale cleaning: NaOH solution (5 to 10 percent, 50 to 60 degrees C) dissolves amorphous silica deposits; hydrofloric acid (HF, controlled hazardous conditions) for crystalline silica (quartz) which resists alkali treatment.
A combined cycle gas turbine (CCGT) power station in the North East of England was experiencing repeated boiler tube failures and reduced heat transfer efficiency due to amorphous silica scaling in its high-pressure steam generators (operating at 165 bar). The raw water supply from a local borehole had reactive silica of 28 mg/L; at the design recovery rate of 75 percent, the BWRO concentrate was carrying 112 mg/L silica, which was then entering the mixed bed deionisation polishing system and precipitating in the boiler at elevated temperatures.
A process review identified that the BWRO system was being operated without a silica antiscalant and that the mixed bed deioniser resin was exhausted, allowing silica breakthrough of 0.8 mg/L (target below 0.02 mg/L). The corrective programme introduced BWA Bellasil 303 antiscalant at 4 mg/L to the BWRO feed (allowing safe operation at 120 mg/L concentrate silica), replaced the depleted SBA resin in the mixed bed units (hot NaOH regeneration at 60 degrees C to ensure complete silica elution), and installed an online silicamolybdate analyser at the deioniser outlet with an alarm at 10 ppb triggering automatic diversion of off-spec water to drain.
Boiler SiO2 below 0.02 mg/L restored within 48 hours of mixed bed resin replacement. No further boiler tube failures in 36 months of post-project operation. Annual maintenance cost for boiler tube replacement reduced from GBP 180,000 to GBP 12,000 per year. BWRO recovery maintained at 75 percent with no silica scaling events on membrane elements. The online silica analyser detected two subsequent mixed bed resin exhaustion events before they caused product quality failures, enabling planned resin replacement rather than emergency shutdowns.
Questions to Ask Shortlisted Providers
- 1
Has a full feed water analysis been completed including reactive silica, total silica, and colloidal silica fractionation, and has a silica scaling index been calculated for the proposed recovery rate?
Reactive and colloidal silica behave differently in treatment processes: colloidal silica is not removed by ion exchange and passes through RO membranes more readily than reactive silica; a treatment system designed only for reactive silica removal will fail if the feed contains significant colloidal silica that was not characterised during initial water analysis.
- 2
At the proposed system recovery rate, what is the calculated concentrate-side silica concentration, and has an antiscalant been selected and dosed specifically for silica inhibition (not generic scale inhibitors)?
Generic carbonate-targeted antiscalants (polyacrylate, phosphonate blends) are largely ineffective against amorphous silica; silica-specific antiscalants (specialised polysiloxane co-polymers) must be selected; an undersized or wrong antiscalant at 75 percent recovery on a 30 mg/L silica feed results in membrane silica fouling within weeks of commissioning, requiring acid and alkali CIP sequences that shorten membrane life.
- 3
For ion exchange demineralisation systems handling silica, what regenerant temperature, concentration, and flow rate have been specified for the strong base anion resin, and has silica creep been demonstrated to be manageable within the planned regeneration frequency?
Silica elution from SBA resin requires slow flow (half the normal regenerant service flow), elevated temperature (50 to 60 degrees C for hot caustic regeneration versus 20 to 25 degrees C for standard), and 4 to 6 percent NaOH concentration; systems designed for carbonate removal that now must handle silica often find their regeneration cycle is too fast, resulting in accumulating silica residual that progressively reduces bed capacity until breakthrough occurs at ever-shorter service intervals.
- 4
What target outlet silica concentration is required for the downstream application, and is continuous online monitoring of silica installed at the point of compliance?
Different applications have very different silica targets: cooling water treatment accepts 120 to 200 mg/L with antiscalant; RO permeate for industrial use typically targets below 1 mg/L; boiler make-up for high-pressure steam generators (above 100 bar) requires below 0.02 mg/L; without online silica monitoring at the point of use, off-spec water will reach the application and cause damage before the laboratory batch sample reveals the problem.
- 5
For cooling water systems, what is the proposed cycles of concentration limit and how has the blowdown rate been sized to maintain silica below the antiscalant-extended solubility limit under peak summer evaporation conditions?
Summer cooling load increases evaporation rate by 30 to 60 percent versus winter; if the blowdown rate has been sized for average conditions rather than peak summer, the cycles of concentration will exceed design in summer, driving silica above the antiscalant inhibition threshold and triggering scaling in the hottest heat exchanger surfaces where the cooling water is most concentrated.
What Drives Cost in This Category
Silica-specific antiscalant products (BWA Bellasil, King Lee Pretreat Plus 100, Nalco 73750) cost GBP 3,000 to 8,000 per tonne; a 10,000 m3/day BWRO system on high-silica groundwater (30 mg/L) at 4 mg/L antiscalant dose consumes approximately 15 tonnes per year (GBP 45,000 to 120,000 per year); underestimating antiscalant cost leads to budget shortfalls and operational decisions to reduce dose below the effective inhibition threshold, which is directly correlated with membrane fouling events costing GBP 20,000 to 60,000 per cleaning episode.
SBA resin (Purolite A400, Dow Amberlyst A26) in mixed bed service for ultrapure water production costs GBP 4,000 to 8,000 per cubic metre; a 2 m3 mixed bed vessel uses GBP 8,000 to 16,000 of resin; with silica creep under-managed, resin exhaustion frequency increases from planned 18-month to 6-month cycles, tripling resin cost from GBP 5,000 to 15,000 per year per vessel; hot NaOH regeneration requires heat exchanger installation (GBP 8,000 to 25,000) but extends resin service life by 20 to 40 percent compared to cold regeneration.
A silicamolybdate continuous online silica analyser (Mettler Toledo Thornton Si analyser, Hach SiVer) costs GBP 12,000 to 30,000 per unit; calibration reagent consumables cost GBP 2,000 to 5,000 per year; for a boiler make-up system requiring sub-ppb silica monitoring, two in-series analysers (one on deioniser outlet, one on boiler feed) are required; the capital cost is routinely justified by the cost of a single boiler tube failure event (GBP 50,000 to 200,000 including lost generation and tube replacement).
Sidestream lime softening (treating 5 to 10 percent of circulation flow through a lime reactor at pH 11) removes 85 to 95 percent of silica from the sidestream, allowing higher cycles of concentration in the bulk system without antiscalant; capital cost of a sidestream lime softening unit (reactor, dosing, clarifier, pH control) for a 10,000 m3/h cooling water system is GBP 200,000 to 600,000; operating cost (lime consumption, pH adjustment chemicals, sludge disposal) GBP 30,000 to 80,000 per year; the break-even versus antiscalant dosing depends on the antiscalant dose required and whether scaling events are occurring at current CoC.
Key Regulations & Standards
The VDI/VDMA/VGB guidelines (German industrial standard widely adopted in UK power industry) specify maximum silica in boiler feed water by operating pressure: below 40 bar, SiO2 less than 1.0 mg/L; 40 to 100 bar, less than 0.2 mg/L; above 100 bar, less than 0.02 mg/L; above 150 bar, less than 0.01 mg/L; BS EN 10523 specifies analytical methods for water quality monitoring in steam generators; both are referenced in UK power station operating procedures and insurance requirements.
Hydrofluoric acid (HF) used for cleaning crystalline silica (quartz) deposits from process equipment is a Schedule 1 (highly toxic) substance under COSHH 2002; a COSHH assessment must be completed before any HF use; HSE EH40 WEL for HF is 0.5 ppm (8-hour TWA) and 1 ppm (STEL); emergency procedures (neutralisation with calcium gluconate gel, PPE, emergency shower and eyewash) must be in place; most UK sites now use NaOH-based cleaning (effective for amorphous silica) to avoid HF risk.
Cooling tower blowdown containing silica antiscalant chemicals may require an EA Environmental Permit or consent if discharged to surface water or groundwater; the EA's Position Statement on Antiscalant and Antifouling Chemicals requires assessment of ecotoxicity and persistence before approving discharge; biodegradable antiscalants (polyaspartate, glutamate diacetate) are preferred by EA over polyacrylate-based products for cooling water discharges to sensitive watercourses.
Silica antiscalants and ion exchange resins used in drinking water treatment must hold WRAS approval (BS 6920 compliance for materials in contact with drinking water; NSF/ANSI 60 for treatment chemicals); the WRAS Approved Products List covers SBA resins, mixed bed resins, and antiscalant chemicals; non-WRAS-approved antiscalants may not be used in membrane systems treating drinking water in England and Wales without DWI agreement.