RO Membrane Care: Fouling Diagnosis, Cleaning and Correct Handling

Reverse osmosis, also called RO, is a membrane separation operation that uses a pressure difference as the driving force to separate solvent from solution. Pressure is applied to the feed on one side of the membrane; once it exceeds the osmotic pressure of that solution, solvent passes in the direction opposite to natural osmosis. Permeate — the solvent that has passed through — is collected on the low-pressure side, and a concentrated solution is left on the high-pressure side. Applied to seawater, reverse osmosis produces fresh water on the low-pressure side and brine on the high-pressure side.

What an RO Membrane Rejects

Reverse osmosis normally uses asymmetric or composite membranes, in hollow-fibre or spiral-wound modules. An RO membrane retains inorganic ions, colloidal matter and high-molecular-weight solutes, which is how it produces purified water; it can also be used to pre-concentrate solutions of macromolecular organics. Because the process is simple and its energy consumption low, it has developed rapidly over the past two decades. It is now applied at large scale to seawater and brackish water desalination, boiler feedwater softening and wastewater treatment, and combined with ion exchange it produces ultrapure water. Its range is still widening: concentration of dairy products and fruit juices, and the separation and concentration of biochemicals and biological preparations.

The Three Classic Mechanism Models

Several classic models describe how solvent passes through the membrane.

(1) Preferential adsorption capillary pore model. Proposed by Sourirajan. In dry-state electron micrographs no pores are visible; the wet membrane specimen is not a suitable electron-microscope sample, so the absence of visible pores proves nothing about the wet state.

(2) Solution-diffusion model. This model does not accept that pores exist at all: solvent dissolves into the membrane material, diffuses across it and desorbs on the other side.

(3) Dry-closed, wet-open model. Proposed by Deng Yu and co-workers in the 1980s and 1990s, this is the modern mechanism that reconciles models (1) and (2). When the membrane is dry it contracts and densifies, the pores close and nothing is seen under the electron microscope; when the membrane is wet the material swells, solvent swells the pores and they open. Combining the two states gives the dry-closed, wet-open desalination model.

Flux and Osmotic Pressure

The solvent permeation rate — the liquid flow capacity N — is:

N = Kh (Δp − Δπ)

where Kh is the hydraulic permeability coefficient, which rises slightly with temperature; Δp is the hydrostatic pressure difference across the membrane; and Δπ is the osmotic pressure difference between the solutions on the two sides.

For a dilute solution the osmotic pressure π is:

π = iCRT

where i is the number of ions produced by dissociation of the solute molecule, C is the molar concentration of the solute, R is the molar gas constant and T is the absolute temperature.

Diagnosing the Fouling Before You Clean

Cleaning only works if the foulant is identified first. Before cleaning, analyse the condition of the membrane, establish what kind of contaminant is present, and select a cleaning agent matched to the actual fouling and scaling.

(1) Analyse the operating data of the system.

(2) Analyse the potential fouling and scaling constituents in the feedwater.

(3) Analyse the contaminants collected on the membrane filter of the SDI instrument.

(4) Analyse the contaminants on the cartridge filter elements.

(5) Inspect the internal surface of the pipework and both ends of the membrane elements.

(6) Where necessary, cut open an element and analyse it to find the fouling and scaling constituents.

When fouling is severe, the cause and the location of the fault can be established by dye testing, microscopic analysis, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX). In practice the simpler methods below are usually enough.

Once fouling is confirmed and chemical cleaning is required, open the end plate of the pressure vessel first and look directly at how contaminants have accumulated in the gap between the end plate and the element. That direct observation normally identifies the type of foulant and therefore the cleaning programme.

Where pre-treatment media (quartz sand, activated carbon) have escaped, or where there is colloidal, organic or biological fouling, the front end is worst affected: particulate matter and slimy deposits can be seen at the inlet of the first element. Biological fouling has a fishy, unpleasant smell, and burning a scraped sample of the biological slime gives the burnt-hair odour of protein.

Chemical Cleaning Procedure

The general sequence for cleaning RO elements is as follows.

1. Pump clean, chlorine-free RO permeate from the cleaning tank (or a suitable water source) into the pressure vessel and discharge it for a few minutes.

2. Prepare the cleaning solution in the cleaning tank using clean permeate. Different membranes need different cleaning solutions.

3. Circulate the cleaning solution through the pressure vessel for one hour or another preset time.

4. After cleaning, drain and flush the cleaning tank, then fill it with clean permeate ready for rinsing.

5. Pump clean, chlorine-free permeate from the cleaning tank (or a suitable source) into the pressure vessel and discharge it for a few minutes.

6. After rinsing the system, run it with the permeate discharge valve open until the permeate is clean, free of foam and free of cleaning agent — normally 15 to 30 minutes.

Storage and Loading of Elements

Do not open the sealed packaging of an element before it goes into the pressure vessel. Store elements in a cool, dry place out of direct sunlight. Before loading elements into a pure-water machine, make sure the system has already been cleaned.

When an element is loaded, lubricate the O-rings and the brine seal properly with silicone grease or a 50% glycerine-water solution. Never use oil, grease, petroleum jelly or petroleum-based compounds.

As elements are loaded one by one, take up the clearance at the end plate with shims. This stops the element from shifting inside the pressure vessel during start-up and shutdown, and reduces the risk of leakage at the element interconnectors.

Start-up Rules

1. Flush a new system thoroughly after the elements are installed, so that all residual debris, solvent and preservative are removed. Where the permeate is for drinking water, flush for at least 24 hours.

2. Before the system starts, the concentrate valve must be fully open. Once running, close the concentrate valve slowly and gradually until the design recovery is reached. Never start the unit with the concentrate valve closed.

3. At no time during operation — including pre-start, normal running, flushing and chemical cleaning — may the valve on the permeate line be closed.

4. Before high-pressure operation, carry out a low-pressure flush through the soft-start arrangement or a variable-frequency drive to expel air from the system.

Operating Limits

1. Keep feedwater turbidity below 1.0 NTU, or SDI15 below 5, and feedwater temperature below 45 °C. The feed must contain no substance that could damage the membrane physically or chemically.

2. Free chlorine in the feed to the elements must never exceed 0.05 mg/L at any time. Above that level the membrane suffers irreversible oxidative damage.

When to Clean, and Short Shutdowns

After a period of normal operation the elements become fouled by suspended or sparingly soluble matter in the feed. Clean promptly when system performance has dropped by 10% under standard conditions, or when scaling or plugging is clearly visible. Periodic water flushing and chemical cleaning restore element performance and extend service life.

During short shutdowns, flush the system every five days and close the valves after flushing to prevent scaling and plugging.