Do You Know the Secret of MBR Membrane Cleaning and Maintenance

During the operation of the MBR membrane, cleaning and maintenance are very important. Today, we discuss this issue in detail and show the secrets of MBR membrane cleaning and maintenance one by one to facilitate better use of the MBR technology and make a more outstanding contribution to the wastewater treatment industry.

What is MBR membrane cleaning?

MBR membrane cleaning usually refers to the removal of membrane surface contaminants. The cleaning strategies also differ depending on the membrane contamination, such as fouling and clogging. In the process of MBR application, the membrane treats more effluent, which will generate a certain amount of contamination, and membrane cleaning becomes especially important when observing a significant increase in trans-membrane pressure difference or a significant decrease in membrane flux.

At the same time, prevention is better than cure, and regular cleaning of MBR membranes is also essential to ensure the MBR process’s stable and efficient operation.

Why is MBR membrane cleaning required?

MBR is one of the main processes in wastewater treatment and is the preferred process for many wastewater treatment projects and wastewater treatment plant upgrading. However, MBR membrane modules operate in high sludge, high organic load environments for a long time and are prone to fouling. Hence, it is necessary to clean MBR membranes regularly to maximize the quality and quantity of product water.

In addition, once the biochemical operation is abnormal, humic acid, inorganic minerals, anaerobic sludge, and other pollutants will also erode the membrane surface, quickly decreasing system water production and shortening the life of MBR membrane modules. Therefore, timely and effective cleaning of MBR membrane modules is a prerequisite for maintaining membrane performance and ensuring water production.

How to determine MBR membrane needs to be cleaned?

During regular operation, MBR membrane filaments can become contaminated with inorganic salt scale, microorganisms, colloidal particles, and insoluble organic matter resulting in a decrease or deterioration of standardized water production and system desalination rates. Therefore, cleaning of MBR membranes is required when the following conditions occur.

A decrease of 10% or more in MBR module water production.
A 20% to 30% increase in trans-membrane pressure difference.
An increase of 5% or more in the standardized salt permeability.

In addition, periodic maintenance cleaning is required under regular operation to maintain stable water production from the membrane system. In other words, users should carry MBR membrane cleaning periodically instead of cleaning only when the membrane filaments are heavily fouled and produce abnormal data. Periodic cleaning ensures the membrane filaments are in good condition to improve water production and provide a suitable response resistance when the membrane module is impacted.

What are the cleaning methods of MBR membranes?

As explained below, MBR membrane fouling methods mainly include physical and chemical cleaning.

1. Physical cleaning. It refers to removing contaminants on the membrane by manual or mechanical cleaning, mainly including aeration, online water backwashing, and ultrasonic cleaning.

Aeration cleaning. Air aeration cleaning is a physical cleaning method to strengthen water circulation. When stopping the inlet and outlet water, significant intensity continuous aeration for 2~3 hours, using water circulation and shear force to flush off the sludge layer deposited on the membrane surface.When the sludge layer attached to the membrane surface greatly impacts the membrane flux, air aeration can effectively solve the membrane pollution problem. Still, air aeration can only remove part of the external pollutants of the membrane. The internal and external retention brought by the adsorption of large molecules needs chemical cleaning and other methods to remove.
Online water backwashing. Water backwashing is the process of flushing by applying a backwashing pressure to the membrane outlet so that the treated water or a mixture of treated water and air passes back through the membrane. Water backwashing requires high membrane performance and should be operated at low pressure to avoid damaging the membrane and causing deterioration of the effluent. In addition, periodic water backwashing can effectively delay membrane fouling.
Ultrasonic cleaning. Ultrasonic cleaning consists of an ultrasonic generator, a transducer, and a cleaning water tank. The ultrasonic generator converts the AC power into ultrasonic frequency power, then the power supply to the transducer. Several sound heads often combine the transducer in parallel. Electrical energy is converted into the same frequency mechanical kinetic energy and transferred to the cleaning water tank so that the cleaning fluid forms a back-and-forth vibration of the ultrasonic sound field.

Ultrasound in the liquid propagation, the action of ultrasound and the liquid will produce intense and rapid mechanical movement and cavitation phenomenon; the water shot to the surface of the membrane, the resulting high-energy impact liquid on the dirt layer directly and repeatedly impact, destroy the adsorption of pollutants and membrane surface, causing fatigue damage and detachment of the pollutant layer, coupled with the synergistic effect of ultrasonic vibration, bubbles can also enter the cracks to do shaking, so that pollutants off.

At the same time, the ultrasonic wave propagation in the cleaning solution will produce high-speed acoustic flow; on the one hand, the acoustic flow will bring the solution to the membrane surface and dissolve the particles on the membrane surface, while cavitation to remove the insoluble contaminants, reduce membrane surface fouling. On the other hand, the acoustic flow can directly destroy the pollutants, remove or weaken the boundary contamination layer, and increase the role of agitation and diffusion. Users should note that ultrasonic cleaning needs to control the ultrasonic time. Otherwise, it will damage the membrane structure.

2.Chemical cleaning. Chemical cleaning includes online maintenance chemical cleaning and offline recovery chemical cleaning.

Online maintenance cleaning. Specific steps are as follows.
- Online cleaning using a mixture of concentrations 200 ~ 500mg / L of sodium hypochlorite solution and a mix of concentrations 1000 ~ 2000mg / L of citric acid solution, sodium hypochlorite solution cleaning for once every 3 ~ 7 days, citric acid solution cleaning for once every 15 ~ 30 days.
- The process of chemical washing firstly stops water production and aeration. Then, within 5~10 minutes, sodium hypochlorite with a concentration of 10% or citric acid with a concentration of 50% is pumped into the main water production line by a metering pump. The water flow of the backwash pump mixes the agent in the pipeline mixer, beats it into the membrane interior, and soaks for 30~60 minutes.
- Turn on the aeration again after the chemical injection and resting, continue aeration for 2~5 minutes, and then restart the water production process.

Note: The aeration process needs to be stopped during the online cleaning process; the purpose is to ensure the contact reaction time between the chemical solution and the contaminants on the membrane surface. Continuous aeration during the cleaning and dosing process will dilute the chemical solution into the whole reactor, making the membrane cleaning effect unsatisfactory.

Offline recovery chemical cleaning. The specific steps are as follows.
- Slowly lift the membrane module out of the reactor during offline cleaning and rinse the membrane module with a water gun to flush out the sludge mixed in the membrane module.
- Put the membrane module into the cleaning tank and fill the cleaning tank with fresh water and aeration to further clean the adhering materials in the membrane filament.
- Add the corresponding cleaning solution, submerged the module entirely, and let it stand for 5 to 12 hours, with intermittent aeration during the standing process, aeration every hour for about ten minutes or whole stirring, with the temperature of the liquid being 30°.
- Remove the membrane module from the cleaning tank and wash the liquid residual with water after soaking. Cleaning generally uses multiple agents. When one agent cleaning is finished, remove the module, rinse the residual solution with water, put the membrane module into the cleaning tank of other corresponding chemicals, and repeat the operation. Finally, the membrane module is lifted from the cleaning tank and placed into the neutralization tank for one hour of continuous air aeration.
- Return the membrane module to the reactor, connect the aeration and water collection pipes, and start the water production.

How to choose the MBR membrane cleaning methods?

There are various MBR membrane cleaning methods, and the careful selection of which flushing method depends on the practical situation. Regardless of the cleaning method, it is crucial to choose a cleaning method suitable for the site’s operating conditions to make the membrane element operate stably for a long time with minimum operation and maintenance costs. To save energy and investment, combining physical and chemical cleaning methods can ensure the cleaning effect and achieve the lowest maintenance investment.

How to improve the cleaning efficiency of the MBR membrane?

Solve the problem from the source, pretreatment in the front section, increase the membrane screen to less than 1mm, and add PAM reasonably.
Do periodic cleaning, don’t wait until the water production decreases or the transmembrane pressure difference increases before cleaning.
Analyze what the chemical pollutants are and target dosing cleaning. For example, if the contaminant is organic, choose sodium hypochlorite, hydrogen peroxide, and other oxidant cleanings; if the hardness, alkalinity, and other inorganic ions are high to form inorganic salt scaling, choose citric acid or oxalic acid and additional acid cleanings.

Conclusion

Compared with the traditional process, the MBR process has higher requirements for pretreatment. Therefore, strictly controlling the amount of hair, fiber, and oil content in the feed water is beneficial to alleviate membrane fouling and prolong the service life of the membrane.

The maintenance of MBR membranes must be in place, and proper and compliant operation should be paid attention to in regular operation to reduce the probability of damage to the membrane modules so that users can efficiently exploit the advantages of the MBR process.

EcoLan^TM‘s whole series of MBR membranes are made of high-quality modified hydrophilic PVDF or PTFE materials with excellent wastewater treatment performance. Operate according to the rules; timely cleaning and maintenance will bring you the maximum benefit per unit of time. Welcome to discuss with us further.