Q: Why can't I use chlorine to disinfect RO membranes?

Polyamide thin-film composite (TFC) membranes — the dominant RO membrane type — are irreversibly degraded by free chlorine above 0.1 ppm. Chlorine breaks the amide bonds in the polyamide rejection layer, causing permanent salt rejection loss from >99% to <90%. The chlorine tolerance of polyamide TFC membranes is expressed as 'ppm × hours' (ppm·h) — most FILMTEC, Toray, and Hydranautics membranes have a chlorine tolerance of <200 ppm·h over membrane lifetime. Meaning: 1 ppm free Cl₂ for 200 hours, or 0.1 ppm for 2000 hours — very small exposures cause cumulative damage. The solution is: (1) de-chlorinate the feed water using sodium bisulfite (SMBS, 3–5 ppm active bisulfite per 1 ppm free chlorine) or sodium metabisulfite (SMBS) upstream of the RO; (2) for biofouling control in the RO system, use non-oxidising biocides compatible with polyamide — DBNPA is the standard (2,2-dibromo-3-nitrilopropionamide, dose 0.5–2 ppm for shock treatment); alternatives include isothiazolone blends, glutaraldehyde (compatible but slower), and THPS (tetrakis-hydroxymethyl phosphonium sulfate, for sulfate-reducing bacteria). Cellulose acetate (CA) membranes (rare, older systems) tolerate 0.5–1 ppm residual chlorine but are pH-sensitive and hydrolyze in caustic or strong acid CIP — know your membrane type before specifying chemicals.

Q: What is SDI and how does it relate to antiscalant dosing?

SDI (Silt Density Index) measures the fouling potential of RO feed water from particulate and colloidal matter — not scale-forming ions. SDI < 3 is required for RO; > t0). SDI > 5 means severe colloidal fouling — RO won't survive without UF pretreatment. SDI 3–5 requires excellent cartridge filtration and potentially coagulation. The relationship to antiscalant: high-SDI water means coagulant (PAC, alum) is often dosed upstream to remove colloids — this raises calcium concentration slightly in the treated water, increasing scaling risk and potentially requiring higher antiscalant dose. Always analyze the SDI of post-coagulation treated water before specifying antiscalant grade and dose. For the membrane flocculant (iron-based coagulant for RO pretreatment): it improves SDI directly by coagulating colloids ahead of the membrane; the coagulated solids must be removed in a subsequent sand filter or UF before the RO train — never dose membrane flocculant directly into RO feed without downstream filtration.

Question 1

How do I choose between general-purpose, high-hardness, and high-silica RO antiscalants?

Accepted Answer

Run a scaling potential calculation (Langelier Saturation Index for CaCO₃; solubility product check for CaSO₄, BaSO₄, SrSO₄; and silica saturation index at your target recovery) on the concentrate water chemistry. The dominant scale type dictates the antiscalant chemistry. The decision tree: (1) First calculate concentrate chemistry at your target recovery (typically 75% for brackish, 40–50% for seawater). At 75% recovery, all ions in the concentrate are 4× the feed level (concentration factor = 1/(1−0.75) = 4). (2) Calculate LSI for the concentrate: LSI = pH − pHs, where pHs = (9.3 + A + B) − (C + D), with A = f(TDS), B = f(temperature), C = f(calcium), D = f(alkalinity). LSI > +2 requires aggressive antiscalant or acid dosing; +1 to +2 is standard territory for general-purpose antiscalant; <+1 is low risk. (3) Check CaSO₄: if [Ca²⁺] × [SO₄²⁻] in concentrate > 2.1 × 10⁻⁴ (Ksp at 25°C) → high-hardness antiscalant required. (4) Check SiO₂: if SiO₂ in concentrate > 100 ppm → use high-silica antiscalant; above 150 ppm SiO₂ recovery is limited even with antiscalant unless pH is depressed below 7. AQUChem provides free scaling calculations via Langelier, Stiff-Davis (for seawater), and SDI projection — send us your feed water analysis.

Question 2

When should I perform RO membrane CIP and what cleaning sequence do I use?

Accepted Answer

Trigger CIP when normalized permeate flux drops >15% from baseline, or normalized differential pressure rises >15%. Standard sequence: low-pH acidic clean (dissolve inorganic scale) followed by high-pH alkaline clean (remove biofilm and organics) — or reverse the order if biofouling is the primary problem. CIP protocol for a brackish water RO system: (1) Flush the system with permeate or low-SDI feed water to remove loose particles. (2) Prepare acidic cleaning solution: pH 2.0–2.5 using citric acid (2% w/v) or hydrochloric acid — AQUChem acidic RO cleaner contains citric acid + low-foam surfactant + corrosion inhibitor. (3) Recirculate at low pressure (<60 psi) for 60–90 min at 25–35°C; let soak 30–60 min. (4) Flush with RO permeate to pH 6–7. (5) Prepare alkaline cleaning solution: pH 11–12 using AQUChem alkaline RO cleaner (NaOH + EDTA + non-ionic surfactant). (6) Recirculate at 25–35°C for 60–90 min; soak 30–60 min. (7) Flush to neutral pH. (8) Return to service at reduced pressure and check normalized flux recovery. For seawater RO (SWRO), temperature control is critical — clean below 40°C to avoid membrane compaction; SWRO membranes need more frequent CIP (every 3–6 months vs 6–12 months for brackish). Note: never use oxidizing chemicals (hypochlorite, hydrogen peroxide) on polyamide TFC membranes without supplier confirmation — irreversible salt rejection loss results.

Question 3

Why can't I use chlorine to disinfect RO membranes?

Accepted Answer

Polyamide thin-film composite (TFC) membranes — the dominant RO membrane type — are irreversibly degraded by free chlorine above 0.1 ppm. Chlorine breaks the amide bonds in the polyamide rejection layer, causing permanent salt rejection loss from >99% to <90%. The chlorine tolerance of polyamide TFC membranes is expressed as 'ppm × hours' (ppm·h) — most FILMTEC, Toray, and Hydranautics membranes have a chlorine tolerance of <200 ppm·h over membrane lifetime. Meaning: 1 ppm free Cl₂ for 200 hours, or 0.1 ppm for 2000 hours — very small exposures cause cumulative damage. The solution is: (1) de-chlorinate the feed water using sodium bisulfite (SMBS, 3–5 ppm active bisulfite per 1 ppm free chlorine) or sodium metabisulfite (SMBS) upstream of the RO; (2) for biofouling control in the RO system, use non-oxidising biocides compatible with polyamide — DBNPA is the standard (2,2-dibromo-3-nitrilopropionamide, dose 0.5–2 ppm for shock treatment); alternatives include isothiazolone blends, glutaraldehyde (compatible but slower), and THPS (tetrakis-hydroxymethyl phosphonium sulfate, for sulfate-reducing bacteria). Cellulose acetate (CA) membranes (rare, older systems) tolerate 0.5–1 ppm residual chlorine but are pH-sensitive and hydrolyze in caustic or strong acid CIP — know your membrane type before specifying chemicals.

Question 4

What is SDI and how does it relate to antiscalant dosing?

Accepted Answer

SDI (Silt Density Index) measures the fouling potential of RO feed water from particulate and colloidal matter — not scale-forming ions. SDI < 3 is required for RO; < 1 for SWRO. Antiscalants do not reduce SDI — SDI is controlled by pretreatment (multimedia filter, UF, cartridge filter). SDI is measured by the ASTM D4189 method: filter 500 mL through a 0.45 μm membrane at 30 psi; record times t0 and t15 (after 15 min); SDI = (1 − t0/t15) × 100/15. A fresh clean membrane filters instantly (t0 low); a high-SDI feed clogs the membrane rapidly (t15 >> t0). SDI > 5 means severe colloidal fouling — RO won't survive without UF pretreatment. SDI 3–5 requires excellent cartridge filtration and potentially coagulation. The relationship to antiscalant: high-SDI water means coagulant (PAC, alum) is often dosed upstream to remove colloids — this raises calcium concentration slightly in the treated water, increasing scaling risk and potentially requiring higher antiscalant dose. Always analyze the SDI of post-coagulation treated water before specifying antiscalant grade and dose. For the membrane flocculant (iron-based coagulant for RO pretreatment): it improves SDI directly by coagulating colloids ahead of the membrane; the coagulated solids must be removed in a subsequent sand filter or UF before the RO train — never dose membrane flocculant directly into RO feed without downstream filtration.

Question 5

What documentation do RO antiscalants need to meet NSF/ANSI 60 for drinking water?

Accepted Answer

NSF/ANSI 60 certification requires third-party toxicology testing and annual auditing — not just a self-declaration. For drinking water RO antiscalants, this means listing on the NSF Certified Products database and compliance with maximum use levels (typically 5–10 ppm for polyacrylate antiscalants). All AQUChem drinking-water grade antiscalants carry NSF/ANSI 60 documentation. NSF/ANSI 60 'Drinking Water Treatment Chemicals — Health Effects' is the North American standard; EU equivalent is EN 1913 / EN 12905 for polymers used in water intended for human consumption. For drinking water RO antiscalants, the key parameters are: (1) total organic carbon (TOC) contribution to permeate from antiscalant bleed-through at maximum use level; (2) heavy metal content of the antiscalant formulation (As, Cd, Cr, Hg, Pb, Ni); (3) specific polymer residuals (acrylamide monomer if polyacrylamide-containing). Chinese-produced antiscalants sold for drinking water export increasingly carry NSF/ANSI 60 certification through NSF International's China audit program. For municipal water utilities in North America and Australia, NSF/ANSI 60 is mandatory. Middle East desalination (SWRO) utilities generally accept either NSF 60 or WRC (UK Water Research Centre) certification. Industrial process RO producing non-potable water has no NSF 60 requirement.

Water Source / Fouling Risk	LSI / Scaling Index	Recommended Antiscalant Grade	Typical Dose (ppm)	Notes
Municipal tap water, low hardness (<200 mg/L CaCO₃)	LSI 0 to +1.5	General-purpose RO antiscalant	2–4	Standard choice for most brackish RO
Well water, high hardness (200–800 mg/L CaCO₃)	LSI +1.5 to +3	High-hardness RO antiscalant	4–8	High CaCO₃ + CaSO₄ inhibition required
Well water / geothermal, silica > 20 ppm at concentrate	Silica SI > 1.0	High-silica RO antiscalant	6–10	Silica polymerization inhibitor essential
Seawater desalination (SWRO)	BaSO₄ / SrSO₄ risk	High-hardness RO antiscalant or SWRO-specific	3–6	Confirm Ba²⁺ and Sr²⁺ in feed analysis
NF nanofiltration (softening)	CaCO₃ dominant	NF antiscalant (lower MW polymer)	2–5	NF operates at higher flux — higher risk
Biological fouling (SDI > 5, warm climate)	Biofouling	RO non-oxidising biocide (DBNPA / DBNPA-blend)	0.5–2 (shock)	Dose upstream of cartridge filter; check compatibility
Post-CIP or offline membrane	—	RO membrane preservative (sodium bisulfite 1%)	Per volume	Store membranes wet; change solution every 3 months

International Brand / Grade	China Equivalent	Major Chinese Producers
Dow FILMTEC Acid Cleaner C-104	Acidic RO cleaner (citric acid-based, pH 2)	上海宝翼, 广州西力, 深圳优泉
Dow FILMTEC Alkaline Cleaner C-111	Alkaline RO cleaner (NaOH + EDTA + surfactant, pH 12)	上海宝翼, 深圳优泉, 武汉天源
King Lee Technologies Hypersperse MDC700 (general antiscalant)	General-purpose RO antiscalant (polyacrylate blend)	杭州春来, 广州西力, 北京赛特
King Lee Technologies Hypersperse MSI300 (high-silica)	High-silica RO antiscalant	杭州春来, 上海宝翼, 郑州鑫恒
Avista Technologies RoQuest 3000 (high-hardness)	High-hardness RO antiscalant (phosphino-polyacrylate)	广州西力, 杭州春来, 深圳优泉
BWA Flocon 260 (general RO antiscalant)	General-purpose antiscalant (phosphonate-polymer blend)	杭州春来, 广州西力
Albemarle DBNPA 20% solution (RO biocide)	DBNPA 20% liquid biocide	山东赤诚, 广州赛特, 南京凯泰
Dow FILMTEC preservative solution (sodium bisulfite 1%)	RO membrane preservative (SMBS 1%)	国产通用 SMBS，客户自配

RO Antiscalants & Membrane Chemicals for Reverse Osmosis Systems

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