RO Membrane Fouling: Types, Prevention and Cleaning

Four Types of Fouling

Type	Cause	Symptoms	Prevention	Cleaning
Scaling	CaCO₃, CaSO₄, BaSO₄, silica	ΔP increase, salt rejection increase	Antiscalant, pH adjustment	Acid cleaner (pH 2)
Colloidal	Silt, clay, iron oxide	SDI > 5, ΔP increase	Pretreatment (UF/MF, coagulation)	Alkaline cleaner (pH 12)
Organic	NOM, humic acids, oil	Normalized flux decline	Pretreatment (GAC, coagulation)	Alkaline cleaner + surfactant
Biological	Bacteria, biofilm	Increased ΔP, bad odor	DBNPA online dosing, pretreatment	Alkaline cleaner + biocide soak

Prevention Strategy

Step 1: Pretreatment

• SDI < 3 (or < 5 for spiral wound)
• Turbidity < 0.5 NTU
• Free chlorine = 0 (dechlorinate with sodium bisulfite)
• Iron < 0.05 ppm

Step 2: Antiscalant

• Select grade based on feedwater analysis (general/high-silica/high-hardness)
• Inject before HP pump at 2–8 ppm
• Verify dosing with online flow meter

Step 3: Biocide

• DBNPA 30–50 ppm intermittent (3x/week, 30 min)
• Or continuous low-dose biocide in pretreatment

When to Clean

Clean when ANY of these thresholds are crossed:

• Normalized permeate flow drops 10–15% from startup
• Normalized salt passage increases 50% from startup
• Normalized feed-concentrate ΔP increases 15% from startup

Do NOT wait until performance drops 25%+ — fouling becomes irreversible at that point.

Cleaning Protocol Summary

Alkaline CIP (organic/bio fouling):

1. Flush with permeate water
2. Circulate alkaline cleaner (pH 11–12) at 35–40°C for 2–4 hours
3. Soak 2 hours
4. Flush and return to service

Acid CIP (scale):

1. Flush with permeate water
2. Circulate acid cleaner (pH 1.5–2.5) at 25–35°C for 1–3 hours
3. Flush and return to service

Always do alkaline FIRST, then acid if both are needed.