Reverse Osmosis Chemical Programme — Pretreatment, Antiscalant, CIP
Direct answer
An RO programme is four chemicals doing four jobs in sequence: dechlorination to protect the membrane, antiscalant to raise recovery, occasional pH adjustment, and clean-in-place to recover what fouled anyway. Polyamide membranes tolerate a lifetime chlorine exposure of only about 1,000 ppm-hours; a 0.5 mg/L residual reaching the membrane destroys it within months, so sodium metabisulphite is dosed at roughly 3 mg/L per 1 mg/L of free chlorine with a 1.5–3× excess and verified by ORP, not by pump setting. Antiscalant at 2–5 mg/L is what lets you run 75 % recovery instead of 50 %. Cleaning chemistry is chosen by what fouled: high pH for organics and biology, low pH for carbonate scale and metals.
Reading the symptoms before buying chemicals
Normalised data, not raw readings, tell you what to dose:
| Symptom | Likely cause | Action |
|---|---|---|
| Differential pressure up, salt passage flat | Biofouling or particulate on the lead element | High-pH CIP, review biocide and cartridge filters |
| Salt passage up, ΔP flat | Scale on the tail element, or membrane oxidation | Low-pH CIP; check ORP history for chlorine breakthrough |
| Both up gradually | Mixed organic and scale fouling | Two-stage CIP, high pH then low pH |
| Permeate flow down, pressure up | General fouling or low feed temperature | Normalise for temperature before acting |
Antiscalant and recovery
Calculate the limiting salt at the design recovery — usually calcium carbonate (LSI in the concentrate), calcium sulphate, barium sulphate or silica. Choose an antiscalant rated for that salt and confirm the manufacturer's saturation limit at your recovery. Never dose a cationic polymer or a cationic coagulant ahead of a polyamide membrane; it binds irreversibly to the anionic surface and the element cannot be cleaned. For potable systems, every dosed chemical must be NSF/ANSI 60 certified, and the certificate of analysis should state it lot by lot.
Dechlorination
Sodium metabisulphite is the standard. It also removes dissolved oxygen, which lowers ORP and can encourage biological growth downstream — this is why plants that switch from chlorine to bisulphite sometimes see biofouling rise. Options are a short chlorine contact upstream with GAC or bisulphite downstream, or periodic DBNPA shock. Interlock the RO high-pressure pump to the ORP reading so the unit stops rather than running on a failed bisulphite pump.
Clean-in-place
Clean when normalised permeate flow drops 10–15 %, salt passage rises 5–10 %, or ΔP rises 15 % from baseline — waiting longer usually means the foulant no longer comes off. High-pH clean: sodium hydroxide to pH 11–12 (with a surfactant or EDTA for organics), 30–35 °C, one hour recirculation plus soak. Low-pH clean: citric acid or hydrochloric acid to pH 2–3 for carbonate and metal oxides. Always run high pH first when both are needed — acid first can set organic foulant hard. Respect the membrane manufacturer's temperature and pH limits; exceeding them voids warranty and hydrolyses the polyamide layer.
What to send with your enquiry
Feedwater analysis with silt density index, design and current recovery, array configuration and membrane make, feed chlorine residual, normalised performance trend, existing chemicals and doses, permeate use (potable, boiler feed, process), and whether NSF/ANSI 60 or pharmaceutical documentation is required.