Reverse Osmosis Membrane Cleaning Guide

I. When Should RO Membranes Be Cleaned?

1. Significant decline in permeate flow: Under unchanged operating conditions and after temperature correction, the normalized permeate flow decreases by more than 10%–15% compared with the initial value or the value recorded after the previous cleaning.
2. Significant increase in operating pressure differential: The normalized pressure differential between the feedwater and concentrate increases by more than 15%, or the pressure differential across an individual stage exceeds 0.25 MPa or reaches more than twice its initial operating value.
3. Deterioration in salt rejection: Permeate water quality declines, the normalized salt passage increases by more than 5%, or the permeate conductivity rises significantly.
4. Scheduled maintenance: Routine cleaning must be performed before the RO system enters a long-term shutdown to prevent contaminants from hardening or becoming firmly deposited during the shutdown period.

II. Standard Cleaning Procedure

Chemical cleaning, also known as Clean-in-Place (CIP), is a controlled process that generally includes the following steps:

1. Pre-flushing: Flush the membrane system with clean RO permeate at low pressure and high flow rate to remove loose particles and deposits from the membrane surface.
2. Preparing the cleaning solution: Prepare the cleaning solution in the cleaning tank using RO permeate free of residual chlorine. Select an acidic cleaner for inorganic scale or an alkaline cleaner for organic matter and biofilm. Strictly control the pH and temperature of the solution.
3. Low-flow circulation: Start the cleaning pump and introduce the cleaning solution into the pressure vessels at a low flow rate to displace the raw water inside the system. Circulate for 30–60 minutes to ensure sufficient contact between the cleaning chemicals and the contaminants.
4. Soaking: Stop circulation and allow the cleaning solution to soak inside the membranes for 1–2 hours. For severe fouling, the soaking time may be appropriately extended to soften stubborn deposits.
5. High-flow circulation: After soaking, increase the cleaning solution flow rate. The shear force created by the higher flow helps remove loosened contaminants from the membrane system.
6. Thorough rinsing: Completely drain the cleaning tank and thoroughly flush the system with clean RO permeate. Normal operation may only be resumed when the discharge pH is neutral, between 6 and 8, and no foam or chemical residue remains.

III. Important Cleaning Precautions

1. Diagnose first, then select the chemicals: Never clean the membranes blindly. Identify the type of fouling through water-quality analysis and inspection of the membrane elements, and select the appropriate cleaning chemicals accordingly. For mixed fouling, a step-by-step process is generally recommended: first perform acid cleaning to remove inorganic scale, followed by alkaline cleaning to remove organic matter. The system must be thoroughly flushed with clean water between the two cleaning stages.
2. Strictly control cleaning parameters to prevent membrane damage: It is safer to use a lower cleaning-chemical concentration than an excessively high one. The cleaning temperature should be strictly controlled below 35°C and must never exceed 40°C, as excessive heat may cause irreversible damage to polyamide membranes.
3. Cleaning-water requirements: RO permeate must be used throughout the preparation and rinsing processes. Never use tap water, as it may cause secondary scaling. Acids and alkalis must never be mixed directly. Non-oxidizing biocides must not be mixed with oxidizing chemicals.
4. Safety first: Operators must wear acid- and alkali-resistant gloves, safety goggles, and chemical-protective clothing. The work area must be well ventilated. Used cleaning solution must be neutralized and treated to meet applicable discharge standards before disposal.