Purified Water Systems: Reverse Osmosis Contamination Types and Cleaning Options

01

Determine the type of pollution

1.1 Accurate determination of the type of contamination is the basis for choosing an effective cleaning method. The following data and information need to be analyzed.

1.1.1 Trend analysis of operating parameters (key indicators)

1.1.1.1 Standardized water production

A consistent decrease in water production at the same operating pressure, temperature, and recovery compared to the initial or clean state is the most common sign of contamination.  A drop of more than 10-15%  is usually a sign that you need to wash it.

Schematic diagram of the working principle of reverse osmosis membrane

1.1.1.2 Standardized salt permeability/desalination rate

A decrease in desalination rate (increased salt permeability) usually indicates fouling (such as salt deposits penetrating the boundary layer) or damage to the membrane surface/seal (less commonly). Abnormally elevated desalination rates can sometimes also be associated with dense contaminant cover.

1.1.1.3 Standardize the pressure drop/pressure difference between sections

A significant increase in pressure drop between feedwater/concentrate (often more than 10-15%) strongly suggests physical blockages within the membrane channels, such as colloids, particulate matter, microbial biofilms, or severe scaling.

1.1.1.4 Standardized operating pressure

In order to maintain the same water yield or recovery rate, the required operating pressure continues to increase, often accompanied by a decrease in water yield or an increase in differential pressure, indicating contamination.

1.1.2 Physical inspection and sampling analysis

1.1.2.1 Membrane element inspection (usually after disassembly)

Observe the appearance of the inlet, concentrated and producing ends.

1.1.2.2 Scaling (Inorganic Salt Scale)

White, off-white, or colored crystalline deposits (e.g., calcium carbonate scale, calcium sulfate scale, white/brown, silica scale/glassy, ironscale reddish-brown) are visible.

1.1.2.3 Organic matter pollution

May appear as a viscous, gelatinous substance that may be brown, yellow, or colorless.

1.1.2.4 Microbes/Biofilms

Sticky slippery, peculiar odor (such as musty smell, rancid smell), viscous biofilm or slime can be seen. It is common in the grid at the inlet end.

1.1.2.5 Colloidal pollution

It is usually a silt-like sediment with a variety of colors (e.g., silica gray-black, iron colloid reddish-brown).

1.1.2.6 Metal oxide pollution (iron, manganese)

Reddish-brown or black deposits.

1.1.2.7 Pollutant sampling and analysis

Scrape or rinse contaminant samples from membrane surfaces, inlet grids, and concentrated water ends.

1.1.2.8 Chemical analysis

Anions, cationic content (to determine inorganic scale type), total organic carbon (TOC - indicative organic pollution), burn reduction (LOI - indicative organic/biomass content)

1.1.2.9 Microbiological analysis

Microbial limits, endotoxins, and even culture and identification of scrapings or rinses (to determine the severity and type of biocontamination).

1.1.2.10 Instrumental analysis

Infrared spectroscopy (FTIR - identification of organic types), X-ray diffraction (XRD - identification of inorganic crystal structures), scanning electron microscopy (SEM - observation of microscopic morphology and biofilms).

1.1.3 Evaluation of influent water quality and pretreatment system

Review the changes in raw water quality (seasonal changes, water source switching).

Evaluate the operational efficiency and replacement/regeneration frequency of the pretreatment unit (multi-media filtration, activated carbon adsorption, softening, security filtration) as appropriate. Pretreatment failure is the main cause of downstream RO pollution.

Check whether the chemical agents (scale inhibitors, reducing agents, non-oxidizing fungicides) are appropriate and effective.

1.2 Summary of common pollution types and characteristics

1.3 Regulatory guidelines and industry practices

1.3.1 Data recording and trend analysis

GMP emphasizes continuous monitoring and documentation of all key operating parameters, and standardized calculations (eliminating fluctuations in temperature, pressure, recovery rates), and establishing a historical database for trend analysis and early warning (Chinese Guidelines for GMP for Pharmaceuticals, ISPE).

1.3.2 Preventive monitoring

Regularly (such as monthly and quarterly) check the influent SDI value, TOC, hardness, alkalinity, residual chlorine/ORP, microbial indicators, etc. to predict potential pollution risk (ISPE).

1.3.3 Root cause analysis

When contamination is suspected or confirmed, the source of contamination (especially microbial contamination) must be investigated, the effectiveness of the pretreatment system must be evaluated, and the recurrence of contamination must be prevented (Chinese Drug GMP Guidelines).

02

Choose the right cleaning method

The choice of cleaning method should be based on an accurate judgment of the type of contamination and follow the principle of "safe, effective, and minimal damage to the membrane".

2.1 Selection basis for cleaning method

2.1.1 Main types of pollution

It is the primary determinant in choosing a cleaning agent.

2.1.2 Severity of pollution

Light contamination may require only a single cleaning agent; Moderate-to-heavy or mixed contamination usually requires step-by-step cleaning (acid before alkali, or alkali before acid).

2.1.3 Membrane material compatibility

Cleaning guidelines provided by the membrane manufacturer must be strictly adhered to, confirming that the chosen cleaning agent type, concentration, temperature, and pH range will not damage the specific membrane material (polyamide composite membranes are the most common and sensitive to oxidizing agents and extreme pH).

2.1.4 Cleaning equipment capacity

The flow rate, pressure, and heating capacity of the cleaning pump need to meet the requirements (usually high flow and low pressure, the specific values refer to the membrane manufacturer and ISPE Vol 4 recommendations).

2.1.5 Security and Compliance

Consider the safety of chemicals (corrosiveness, toxicity), operational protection, and discharge treatment requirements (Chinese environmental regulations).

2.2 Types of commonly used cleaning agents and applicable pollution

2.3 Standard cleaning procedure steps

2.3.1 Low-pressure flushing

Rinse the membrane system with RO produced or pretreated water to remove loose contaminants.  (Required step).

2.3.2 Configure the cleaning fluid

In the cleaning box, use RO produced water or deionized water (it is strictly forbidden to use raw water or pretreated unqualified water) to prepare the cleaning solution according to the recommended concentration. Make sure it dissolves evenly. Control the temperature (usually < 45°C, as required by the membrane manufacturer).

2.3.3 Low flow circulation

Start the cleaning pump and pump the cleaning fluid into the RO pressure vessel at a low flow rate (membrane manufacturer's recommended value, usually 1/3 - 1/2 of the rated flow rate of a single element).  Avoid excessive flow rate and damage to membrane elements caused by differential pressure. During the circulation process, the initial part of the cleaning liquid that may be seriously contaminated is discharged back into the cleaning box to prevent secondary pollution. Continuous cycling (typically 30-60 minutes) closely monitors pH and temperature changes (adjustable if needed) and observes changes in wash solution color and turbidity.

2.3.4 Soaking

Stop the cycle, close the valve, and allow the cleaning solution to soak the membrane element (usually 30 minutes to 2 hours, depending on the level of contamination). Soaking is very important, especially for biofilm and stubborn dirt.

2.3.5 High flow cycle

Start the cleaning pump again and circulate the cleaning solution at a higher flow rate (membrane manufacturer's recommended value, usually a single element rated flow) (usually 30-60 minutes). This step uses shear forces to wash away loose contaminants.

2.3.6 Rinsing

Rinse the system thoroughly with RO produced water or pretreated water (water quality must be qualified),  and flush at low pressure until the flushing water is close to the inlet water pH, conductivity, no foam, and no cleaning agent residue (usually takes more than 15-60 minutes). This is a critical step in preventing cleaning agent residues and secondary contamination

2.3.7 Repeat Cleaning (if necessary)

For severe or mixed contamination, it may be necessary to change a different type of cleaning solution (e.g., pickling first and then alkali washing, or alkaline washing first and then pickling), repeating steps 2-6.  It must be thoroughly rinsed before each cleaning agent change. Alkaline washing is usually more effective for organic matter and biofilms, often as the first step; Pickling is more effective against scaling. The order can be adjusted according to judgment.

2.3.8 Final flushing and pre-operation preparation

Once the system is thoroughly flushed, the system can be returned to normal operating or the next steps (e.g., disinfection, performance testing) can be performed.

2.4 Regulatory guidelines and industry practices

2.4.1 Cleaning Procedures (SOPs)

Detailed and written cleaning procedures (SOPs)  must be formulated, clarifying cleaning conditions (trigger points), contamination judgment methods, cleaning agent selection logic, specific formula concentration, temperature range, flow pressure parameters, cycle soaking time, rinse end point standards, safety protection measures, discharge requirements, etc. (Chinese Drug GMP Guidelines).

2.4.2 Verification/Validation

The cleaning procedure and its effectiveness should be verified or confirmed. After cleaning, the performance recovery (water yield, desalination rate, differential pressure return to close to the cleaning level) should be evaluated, and the cleaning effect and residue should be proved to meet the standards (Chinese Drug GMP Guidelines) through sampling and testing (e.g., concentration of contaminants in the cleaning solution, conductivity/TOC/microorganisms of the flushing water).

2.4.3 Cleaning frequency

Based on monitoring data trends, not just fixed time intervals. Follow the "wash on demand" principle, but with maximum allowable intervals (e.g. based on time or water production accumulation).

2.4.4 Records

Complete record of each cleaning operation, including date, contamination performance, judgment basis, selected cleaning agent and concentration, temperature, flow rate, pressure, time, pH change, observed phenomenon (color, foam), rinse end point data, post-cleaning performance test results, operator, etc. (Chinese Drug GMP Guidelines).

2.4.5 Personnel training

Operators must be fully trained and familiar with SOPs, safe handling of chemicals, emergency handling, and GMP requirements.

2.4.6 Preventive maintenance

Developing an effective pretreatment system maintenance schedule (filter replacement, activated carbon regeneration/replacement, softener regeneration, scale inhibitor dosing monitoring) is fundamental to reducing the frequency of RO cleaning (ISPE, industry practice).

2.4.7 Cleaning system design

Cleaning devices (pumps, heaters, pipes, meters, cleaning boxes) should be reasonably designed, compatible with materials, easy to operate and clean, and avoid becoming a source of contamination (ISPE).

Summary

Determining the type of RO membrane contamination depends on continuous monitoring and trend analysis of system operating parameters, combined with physical inspection and laboratory analysis of pollutants. Cleaning methods must be selected for major types of contamination, strictly following the membrane manufacturer's restrictions on chemical compatibility, concentration, temperature, pH, and operating parameters, and performing proven standard cleaning procedures (including rinsing). The entire process must comply with GMP specifications, have detailed SOPs, be fully documented, and emphasize preventive maintenance and root cause analysis to prevent recurrence of contamination. Through this systematic method, the performance of RO membrane can be effectively restored, and the stable operation of the purified water system and the quality of the water produced can be ensured in accordance with the requirements of the Pharmacopoeia.

Attached: Pictures of various typical types of reverse osmosis pollution

Inorganic Scale Pollution (1)

Inorganic Scale Pollution (2)

Organic Pollution (1)

Organic Pollution (2)

Organic Pollution (3)

Microbial/biofilm contamination

Colloidal contamination