Causes of Nanofiltration Membrane Fouling and Operational Analysis

I. Analysis of Fouling Encountered During Operation of Nanofiltration Membranes

Microbial contamination

Microorganisms include bacteria, algae, fungi, and viruses. Bacterial particles are extremely small, typically ranging from 1 to 3 μm, while viruses are even smaller, measuring approximately 0.2 to 0.01 μm. Microbial fouling imposes at least two adverse consequences on nanofiltration membrane systems: first, the prolific growth and metabolic activity of microorganisms generate substantial colloidal substances, leading to membrane fouling and a sharp decline in permeate flux; second, it results in an increase in the total bacterial count in the permeate. Microbial fouling of nanofiltration membranes is highly detrimental to the long-term operation of the entire system; therefore, it must be given utmost attention.

The common causes of biological contamination are:

(l) The influent contains a high concentration of microorganisms;

(2) The system’s shutdown, protection, flushing, and other procedures were not carried out in strict accordance with the technical manual.

(3) No disinfection of the influent, or the disinfectant dosage is too low.

(4) The influent water contains nutrients that readily promote microbial growth, leading to excessive microbial proliferation.

(5) Regular sterilization and disinfection of the piping system have not been performed. Membrane surfaces contaminated by microorganisms become extremely slippery and often emit an unpleasant odor; the smell produced when incinerating biofilm samples is similar to that of burning hair.

(For example, when the influent ammonia‑nitrogen concentration significantly exceeded the limit, it triggered extensive microbial growth within the piping and on the membrane elements. After chemical cleaning of the membrane system, failure to disinfect and sterilize the piping resulted in most of the residual microbial particles being carried by the flow into the membrane modules upon startup, causing a severe drop in permeate production and a sharp increase in pressure drop between membrane stages. Ultimately, the contamination was resolved through offline cleaning.)

Organic and mineral oil contamination

Membrane system failures caused by organic matter account for 60% to 80% of all system malfunctions. Organic compounds in the feed water adsorb onto the membrane element surface, leading to flux decline—particularly in the first stage. In many cases, the adsorbed layer on the membrane surface acts as an additional separation barrier, blocking the membrane pores and increasing the salt rejection rate. High‑molecular‑weight organic substances bearing hydrophobic functional groups often give rise to this phenomenon; for example, trace oil droplets and recalcitrant high‑molecular‑weight organics can result in organic fouling of the membrane system.

(For example, petrochemical wastewater is characterized by a complex composition, high concentrations of organic matter, and trace amounts of oil; consequently, organic fouling is one of the most common types of fouling in nanofiltration membrane systems used for advanced treatment of such effluents. Organic fouling of nanofiltration membranes can typically be assessed by analyzing the concentrations of oil and organic contaminants in the feed water, and conventional organic fouling can usually be mitigated through periodic chemical cleaning.)

Pollution caused by flocculants

During the system’s pretreatment stage, in the shallow flotation unit, a specific high-purity polyaluminum flocculant is added to precipitate colloids and large particulate impurities, as well as oil‑containing substances. Flocculants are broadly categorized into inorganic and organic types; inorganic varieties typically include polyferric and polyaluminum compounds. Due to their lower cost, inorganic flocculants are more widely used. To prevent iron ion contamination of the membrane system, high‑purity polyaluminum is generally preferred as the flocculant. Organic flocculants commonly comprise polyacrylamide and polyphosphate derivatives. In certain membrane systems, combining inorganic and organic flocculants can yield better results. However, in practice, the choice of flocculant type and concentration must be determined through empirical screening, taking into account differences in process design and feedwater quality. During operation, not all flocculant will form flocs; regardless of the type, some residual material will remain in the water. Under normal conditions, this residual flocculant is discharged with the concentrate stream. Yet, if the flocculant dosage is excessively high, leading to an excessive residual load in the feedwater, secondary flocculation and deposition may occur on the surface of the nanofiltration membrane, causing fouling. Moreover, fouling caused by over‑dosage is often difficult to remove during cleaning and may even necessitate premature membrane replacement.

Fouling caused by scaling

Scaling refers to the precipitation of sparingly soluble salts as solid deposits on the membrane surface. To prevent scaling, it is essential to ensure that the concentrations of these sparingly soluble salts do not exceed their saturation limits. In nanofiltration systems, the predominant scale-forming species are inorganic compounds, primarily calcium carbonate. In addition to carbonates, many other inorganic salts also exhibit low solubility at saturation, such as calcium sulfate, barium sulfate, strontium sulfate, and certain hydroxides. To inhibit fouling on the membrane surface, an appropriate amount of membrane‑compatible antiscalant is typically dosed upstream of the security filter, with dosing levels generally maintained within the range of 4–12 mg/L.

Sometimes, interactions between different chemicals dosed can lead to the precipitation of poorly soluble compounds, which in turn contaminate the membrane elements. For example, when polymeric organic scale inhibitors come into contact with polyvalent cations such as aluminum or residual polymeric cationic flocculants, gel‑like precipitates may form, severely fouling the upstream membrane elements; such fouling is often difficult to remove. Therefore, when dosing multiple chemicals, it is essential to consider their chemical compositions and, based on water quality data, the reverse osmosis design approach, and the selected membrane type, conduct laboratory tests to verify their compatibility and determine the appropriate scale inhibitor type and dosage.

Colloidal contamination

Colloids are fine particles with diameters ranging from 1 nanometer (nm) to 1 micrometer (μm), which, like clay, are difficult to degrade naturally and typically carry a negative charge in water. Common causes of colloidal contamination include organic colloidal substances in wastewater, excessive dosing of flocculants, and hydroxide colloids formed by the hydrolysis of metal ions in the effluent. Typical colloidal pollutants in wastewater include ferric hydroxide, aluminum hydroxide, and silica colloids.

(For example, colloidal fouling can be caused by excessive chemical dosing, pipeline corrosion, and the presence of high-molecular-weight organic compounds in the membrane system.)

Long-term operational experience with nanofiltration systems

Maintain the stability of the pretreatment effect.

During the pretreatment stage, most of the contaminants in the raw water are removed. Effective pretreatment significantly reduces the likelihood of fouling and other forms of contamination in the nanofiltration system.

(For example, regularly replace the security filter cartridges and inspect the security filter to prevent short‑circuiting and the growth of biofouling that could contaminate the membrane elements; strictly control feedwater turbidity and the fouling index (SDI), keeping turbidity below 0.5 NTU and the fouling index below 5; disinfect and sterilize both the pre‑membrane process streams and the membrane system itself, as disinfection is an essential step for controlling microbial contamination. System disinfection can be carried out using either shock disinfection or continuous disinfection, with the appropriate method selected based on the specific system requirements.)

II. Long-term Operational Experience with Nanofiltration Systems

Control of low operating pressure and recovery rate

Pressure serves as the driving force for nanofiltration desalination. As pressure increases, the permeate flux of the membrane module rises linearly, and the salt rejection initially improves; however, once the pressure reaches a certain threshold, the rejection rate levels off. Consequently, in practical operation, excessively high pressures should be avoided, as they accelerate membrane degradation and may even damage the membrane module. To extend the service life of the membrane module, it is common practice to operate at a slightly lower pressure—provided that both salt rejection and permeate flux meet process requirements—which significantly benefits long‑term system performance.

When a nanofiltration system operates at a higher recovery rate, the salinity of the concentrate increases accordingly. This not only promotes concentration polarization on the concentrate side but also raises the system’s osmotic pressure. To maintain the same permeate flow, the operating pressure must be increased, leading to higher specific energy consumption, deteriorating permeate quality, accelerated membrane fouling, and an increased risk of scaling and microbial contamination. Based on operational experience, it is advisable to keep the nanofiltration system’s recovery rate below 75%.

Perform physical cleaning of the membrane (product water flushing)

Backwashing involves flushing the membrane elements with low-pressure, high-flow feed water to remove fouling and deposits adhering to the membrane surface. Low-pressure backwashing helps reduce concentration polarization and prevents membrane dehydration. When conditions permit, it is recommended to perform frequent backwashes; increasing the frequency of backwashing is more effective than conducting a single chemical cleaning.

Standardize system start-up and shutdown procedures as well as outage protection measures.

During system startup and shutdown, flow rate and pressure may fluctuate. Excessive or rapid fluctuations in flow and pressure can lead to extreme pressure drops, causing water hammer effects that may result in membrane element rupture. Therefore, when initiating or terminating operation, it is essential to increase or decrease pressure and flow gradually.

Before startup and during shutdown, ensure that the pressure vessel is not under vacuum; otherwise, water hammer or hydraulic shock may occur at the moment the membrane elements are restarted. This phenomenon can also arise when a system that has already lost water is started up for the first time or during routine operation.

The system should maintain a low back pressure (on the permeate side), with the permeate-side pressure higher than the feed-side pressure. When the pressure differential exceeds 0.05 MPa, the membrane elements may suffer physical damage. Before starting up or shutting down the system, carefully verify the valve positions and monitor pressure changes to ensure that back pressure does not occur during operation. If the membrane system is to remain offline for an extended period, follow the technical manual’s recommendations by introducing a protective solution into the system or periodically flushing it with water to maintain the membrane elements in proper standby condition.

Perform online chemical cleaning of membrane elements on a regular basis.

Even with a well-designed pretreatment system and sound operational management, the degree of fouling on membrane elements can only be reduced; complete elimination of membrane fouling is impossible. Consequently, after a period of operation, nanofiltration systems are susceptible to contamination by various foulants—particularly when employed in advanced wastewater treatment applications, where fouling occurs frequently. Under normal operating conditions, once the normalized permeate flux declines by approximately 15%, the pressure drop across the system between the feed and concentrate streams increases to about 1.5 times its initial value, and the quality of the permeate deteriorates noticeably, necessitating chemical cleaning of the membrane elements.

During chemical cleaning, the first step is to identify the type of contaminants, followed by selecting an appropriate cleaning formulation and process based on the membrane’s characteristics. During cleaning, it is essential to carefully control the pH, temperature, and flow rate of the cleaning solution. To ensure effective rinsing, where conditions permit, a staged cleaning approach may be employed. Currently, both domestically and internationally, specialized membrane‑specific cleaning agents are available for use. The effectiveness of the cleaning can be verified by comparing key performance parameters—such as desalination rate, water production, and pressure drop—before and after treatment.

For membrane systems used in advanced treatment of petrochemical wastewater, chemical cleaning typically begins with biocidal treatment, followed by alkaline washing to remove microbial, organic, and oil‑related fouling. This is then succeeded by acid washing to eliminate scale and metal hydroxide deposits. The cleaning cycle is determined based on the actual operating conditions of the system.

Perform offline chemical cleaning of the membrane elements.

When a membrane system cannot regain its performance after multiple online chemical cleanings, or when it has suffered severe fouling, offline chemical cleaning of the membrane elements is required. Severe fouling of membrane elements is defined as a single‑stage pressure drop that exceeds twice the initial operating pressure drop of the system, a reduction in reverse osmosis permeate flow of 30% or more, or a mass increase of a single RO membrane element by 3 kg or more above normal values.

Based on the user’s complete raw water analysis report, performance test results, and available system information, the type of fouling is identified and an appropriate cleaning procedure is determined. When necessary, additional verification is conducted using specialized equipment and tools to pinpoint the specific foulant and select the required cleaning formulation. The removed membrane elements are then cleaned in a dedicated offline cleaning system; following successful testing, they are reinstalled and put back into service.