Are you performing the maintenance and cleaning of your ultrafiltration membrane correctly?

Are you performing the maintenance and cleaning of your ultrafiltration membrane correctly?

Ultrafiltration membranes are widely used and have proven highly effective in sectors such as the construction industry, pipe manufacturing, wastewater treatment, sealing materials, fibers, and packaging films. Their performance is particularly noteworthy, with significant social value and comprehensive benefits when applied in industrial settings like thermal power plants and chemical factories. However, as operating time increases, ultrafiltration membranes require regular maintenance and cleaning to maintain optimal performance and achieve their intended objectives.

Maintenance of Ultrafiltration Membranes

The maintenance of ultrafiltration membranes requires selecting appropriate methods based on specific operating conditions and stages to ensure maintenance quality. Specifically, maintenance measures can be analyzed from several perspectives, including initial startup or restart, long-term operation, and shutdown.

1. Maintenance during initial startup or after a restart

Ultrafiltration membranes and their components must be cleaned upon initial startup or after prolonged periods of inactivity, typically by rinsing with clean water for several hours. During the first use of ultrafiltration membranes and assemblies, the initial permeate flow rate should be strictly controlled—generally set at 60% of the design capacity—and can be increased to the design value after 24 hours of operation. The feed water temperature is usually maintained around 20°C. In the maintenance of ultrafiltration systems, prioritizing proper care during the initial operating phase helps ensure optimal performance and extend service life, thereby safeguarding both the system’s effectiveness and the user’s economic interests while reducing repair and replacement costs.

2. Long-term operation and maintenance

For long-term operation, ultrafiltration membranes and their modules used in industrial applications must meet specific influent requirements: the influent water quality must comply with standards, and the water source may be tap water, groundwater, surface water, seawater, or wastewater that meets the secondary effluent discharge standards of municipal secondary treatment plants as specified in GB 8978‑1996. The water temperature should not exceed 40°C, and the pH should be between 2 and 10. Backwashing frequency must adhere to the operating specifications of the membrane and module—typically 24 to 48 cycles per day—with a backwash duration of 30 to 60 minutes. Chemical cleaning should be performed promptly based on the membrane’s usage conditions.

3. Stop operation and maintenance

Maintenance of ultrafiltration membranes and their components during shutdown should be determined based on the duration of the shutdown, ensuring that maintenance procedures are both effective and appropriate, thereby preserving the performance and operational efficiency of the membranes and their assemblies.

(1) If the equipment is shut down, and the components are temporarily out of service (2–3 days), they may be operated for 30–60 minutes daily to prevent bacterial contamination.

(2) If a component is out of service for an extended period (7 days or longer), perform a manual air‑water backwash on the ultrafiltration unit prior to shutdown, and then inject a protective solution (1% NaHSO₄) into the unit. ₃ : solution), and close all inlet and outlet valves of the ultrafiltration units. Check the pH of the preservation solution once a month, and replace it promptly if the pH drops to ≤3.

(3) Upon resuming operation after a prolonged shutdown, the ultrafiltration unit shall be subjected to continuous flushing until the effluent is free of foam.

(4) During the shutdown period, the ultrafiltration membrane must remain wet at all times; once it dehydrates and dries, the membrane module will suffer irreversible damage.

Note: During preparation for a prolonged shutdown of the equipment, the control cabinet’s output power must be switched off, and the input power supply should also be de-energized.

Ultrafiltration Membrane Cleaning

1. Cleaning method

The cleaning methods for ultrafiltration membranes mainly fall into two categories: physical cleaning and chemical cleaning. Physical cleaning makes full use of mechanical forces to remove contaminants adhering to the membrane surface, without involving any chemical reactions. Common physical cleaning techniques include air‑backwashing, air‑forward flushing, backwashing, and positive‑pressure flushing. Chemical cleaning, on the other hand, employs chemical agents to induce chemical reactions with surface contaminants, thereby achieving the desired cleaning effect.

2. Physical cleaning

(1) Backwashing: This operation does not involve filtration; its purpose is to remove residual contaminants from the membrane elements using clean water. The concentrate valve is fully open, while the permeate valve is fully closed, and the backwash duration typically ranges from 3 to 10 minutes.

(2) Backwashing: This operation is the reverse of the filtration process. Under a specified pressure, clean water (membrane‑filtered water or higher‑quality water) is forced to permeate from the permeate side into the feed side, thereby flushing out contaminants on the feed side as well as any foulants that have entered the micropores.

(3) Soaking: Membrane modules may be soaked in clean water or a chemical solution. Soaking can loosen fouling substances, and prolonged soaking is often an effective method for removing fouling.

(4) Air–water backwashing: During backwashing of the membrane module, a controlled amount of compressed air is introduced through the feed inlet. The rising air flow causes the hollow‑fiber membranes to oscillate, inducing friction and collisions that dislodge and remove fouling particles adhering to the membrane surface, which are then discharged through the effluent outlet.

3. Chemical cleaning

(1) In continuous membrane filtration systems, chemical cleaning is typically performed every 3 to 5 weeks, using commonly available water treatment chemicals with excellent biocidal properties, such as NaClO and ClO⁻. ₂ Perform systematic disinfection; use NaClO/NaOH solutions to remove organic fouling; and employ acid or sodium EDTA solutions to eliminate inorganic salt scaling and other contaminants, thereby restoring membrane flux.

(2) Alkaline cleaning:

Use a 0.1%–0.2% NaOH solution to circulate within the membrane system at low pressure (＜0.05 MPa) for 20 minutes, followed by a soak of 20–40 minutes, then rinse with water until neutral. Solid caustic soda must be fully dissolved before preparing the solution.

(3)NaCIO Cleaning:

Clean with a 300–3,000 ppm NaClO aqueous solution for 40–60 minutes; this cleaning agent is suitable for microbial contamination. Commercial‑grade NaClO typically contains 10% available chlorine.

4. Maintenance cleaning

The basic procedure is the same as for restorative cleaning, except that the cleaning duration is shorter and the chemical concentration is lower. Maintenance cleaning is typically performed once every day or once every two days.

Conclusion

Practice has demonstrated that employing appropriate cleaning methods for ultrafiltration membranes and their components can effectively enhance cleaning performance, restore optimal filtration efficiency, achieve desirable results, and meet the intended treatment objectives. During operation, cleaning is an indispensable step and a critically important task for ultrafiltration membranes and their components.