Are anti-fouling membranes inherently durable? How to clean anti-fouling membranes for minor contamination

Are anti-fouling membranes inherently durable? How to clean anti-fouling membranes for minor contamination

Anti-fouling membrane elements are widely used in power generation, petrochemicals, steel, electronics, pharmaceuticals, food and beverage, municipal utilities, and environmental protection sectors. They play a crucial role globally in seawater and brackish water desalination, boiler feedwater treatment, industrial pure water and electronic-grade ultrapure water production, drinking water purification, wastewater treatment, and specialized separation processes.

As the concept of reclaimed water gains increasing prominence, wastewater reuse and zero-liquid discharge systems are entering the public consciousness. So how should one select the core component of reclaimed water systems—membrane systems, specifically reverse osmosis membrane systems?

In tandem with the water treatment market's expansion over recent years, the reverse osmosis membrane market has also experienced exponential growth. With increasing membrane models tailored to diverse applications and environments, selecting the appropriate reverse osmosis membrane has become essential knowledge for water purification professionals.

When discussing reclaimed water reuse and zero liquid discharge systems, the immediate assumption is often that anti-fouling membranes are mandatory. However, this isn't necessarily the case. Why? Let's first examine the characteristics of anti-fouling membranes:

Anti-fouling RO membranes fall into two categories: 1. Membranes with wider flow channels that resist clogging—these are widely applicable. 2. Membranes modified with surface treatments to exhibit negative, positive, or neutral charge. Leveraging the principle that like charges repel, this prevents specific contaminants from depositing on the membrane surface. The effectiveness of this approach heavily depends on the water quality characteristics.

Got it? When selecting anti-fouling membranes, you must first understand the water's characteristics. Then choose anti-fouling membranes suitable for your specific water quality. This is especially critical in water containing high concentrations of chemical agents, where the surface properties of reverse osmosis membranes play a vital role. A moment's carelessness could render the entire batch unusable, as surface charge adsorption fouling is extremely difficult to clean.

What if contamination occurs? Next, let's discuss how anti-fouling membranes handle mild contamination cleaning.

After normal system operation, anti-fouling membrane elements may become contaminated by suspended solids or insoluble salts present in the feedwater. The most common contaminants include calcium carbonate precipitation, calcium sulfate, barium sulfate, strontium sulfate precipitation, metal oxide precipitation (iron, manganese, copper, nickel, aluminum, etc.), silica deposits, mixtures of inorganic or organic deposits, natural organic matter (NOM), synthetic organic compounds (e.g., scale inhibitors/dispersants, cationic polymer electrolytes), and microorganisms (algae, mold, fungi).　　

The nature and rate of fouling depend on various factors, such as feedwater quality and system recovery rate. Fouling typically develops progressively; if not controlled early, it can damage membrane elements within a relatively short timeframe. When anti-fouling membrane elements are confirmed to be fouled, or prior to extended shutdowns, or as part of routine maintenance, cleaning the membrane elements is recommended.

Chemical cleaning or physical flushing is required when anti-fouling membranes exhibit the following issues:　　

Under normal feedwater pressure conditions, temperature-corrected permeate flow decreases by 10-15% from normal values; To maintain normal permeate flow, temperature-corrected feedwater pressure increases by 10-15%;　　

Product water quality decreases by 10-15%, and salt rejection increases by 10-15%;

Feed pressure increases by 10-15%;

Significant increase in pressure differential across system stages.

These cleaning principles remain applicable even when operational data is not standardized, provided key parameters remain unchanged. Maintaining stable operating parameters primarily involves product water backpressure, recovery rate, temperature, and TDS. If these parameters fluctuate significantly, it is strongly recommended to standardize data to determine if fouling has occurred or to confirm the actual operational status of the reverse osmosis system before changes in critical parameters. Regular monitoring of overall system performance is the fundamental method to confirm whether membrane elements have become fouled. Fouling affects membrane elements progressively, and the extent of impact depends on the nature of the contamination.

When anti-fouling membrane elements exhibit only mild fouling, prompt cleaning is essential. Severe fouling impedes chemical penetration into the fouling layer, compromising cleaning efficacy. If membrane performance degrades to 30-50% of normal levels, full restoration to factory specifications becomes unfeasible.　　

In reverse osmosis system design, flushing contaminants with reverse osmosis product water can reduce cleaning frequency. Soaking membrane elements in product water aids in dissolving and dislodging fouling, thereby decreasing the need for chemical cleaning.

The type of contaminants to be cleaned and the cleaning method should be determined based on the specific on-site contamination conditions. For complex situations involving multiple simultaneous contaminants, the cleaning method involves alternating between low-pH and high-pH cleaning solutions.