How to Control and Clean MBR Membrane Fouling (Scaling) in 2023?
Table of Contents
MBR technology has been widely and maturely used in wastewater treatment, but the problem of membrane fouling has been plaguing the development and operation of MBR. Why MBR membrane fouling? What can MBR operators do to quickly find the source of membrane fouling and treat it accurately to reduce cleaning frequency?
What is MBR membrane fouling?
MBR membrane fouling usually refers to the process of adsorption and accumulation of substances in the mixture on the membrane surface (outside) and inside the membrane pores (inside), causing the membrane pores to be clogged and contributing to the reduction of porosity, resulting in the decay of membrane flux and the increase of filtration pressure.
In membrane filtration, water molecules, and fine substances continuously pass through the membrane. In contrast, some substances are retained by the membrane and block the membrane pores or deposited on the membrane surface, thus causing membrane fouling. Membrane fouling is caused by membrane interception. The direct manifestation of membrane fouling is the decreased membrane flux or increased operating pressure.
The nutrient substrates, zoogloeal, microbial cells, cell debris, microbial metabolites (EPS, SMP), and various organic and inorganic dissolved substances in the activated sludge mixture system all contribute to membrane fouling.
The development of membrane fouling can usually be divided into three stages.
- Initial membrane fouling. It occurs at the initial stage of membrane system operation when the membrane surface interacts strongly with the colloids and organic matter in the mixture. The fouling is in the form of adhesion, charge effect, and membrane pore blockage. Under the conditions of cross-flow filtration, fine biological flocs or extracellular polymers can still adhere to the membrane surface. In contrast, substances more minor than the membrane pore size will adsorb in the membrane pores and cause membrane fouling through concentration, crystalline precipitation, growth, and reproduction.
- Slow membrane fouling. Initially, the membrane surface is smooth, and large particles are not easily attached, mainly by EPS, SMP, bio-colloids, and other viscous substances adsorbed on the membrane surface through adsorption bridges, mesh capture, and other effects to form a gel layer, resulting in a slow rise in membrane filtration resistance. It will enhance the retention performance of pollutants in the mixture. Contamination of the gel layer is inevitable and brings about the effect of a slow rise in membrane resistance. It is manifested as a slow rise in TMP in constant flow operation and a slow decay in flux under continuous pressure mode.
- Rapid membrane fouling. Under continuous filtration pressure difference and permeable water flow, the gel layer formed in stage 2 is gradually dense with the deposition of pollutants. It leads to the membrane fouling from quantitative to qualitative changes. The flocs in the mixture rapidly accumulate on the membrane surface and form a sludge filter cake, and the trans-membrane pressure difference rises quickly.
What are the sources of MBR membrane fouling?
There are three main ways MBR membrane fouling is formed, which are as follows.
- Filter cake layer. Mainly water passes through the membrane. However, some of the retained activated sludge and colloidal substances are accumulated on the membrane surface under filtration pressure difference-permeable water flows, forming membrane fouling.
- Dissolved organic matter. The source of organic matter is mainly the metabolites of microorganisms, which can form a gel layer on the surface of the membrane or be adsorbed on the surface of the micro-pores inside the membrane to block the pores and the membrane flux.
- Microbial contamination. Microorganisms require nutrients on the membrane surface and in the micropores inside the membrane, so there will inevitably be many microorganisms breeding.
What are the types of MBR membrane fouling?
There are three main types of MBR membrane fouling as follows.
1. Classification by composition of pollutants
- Organic contamination. It mainly comes from the mixture’s macromolecular organic matter (polysaccharides, proteins, etc.), humic acids, microbial flocs, cell debris, etc. Among them are dissolved organic matter SMP and EPS; although the proportion is very low for MLSS, the membrane fouling caused by them accounts for 26%-52%. In addition, microbial growth and adsorption in membrane pores and on the membrane surface are also important factors of membrane fouling.
- Inorganic contamination. It is formed by the bridging action of metal salts and inorganic salt ions. Common inorganic pollution of membranes is mainly carbonate, sulfate, and silicate scaling substances generated by calcium, magnesium, iron, silicon, etc. Typically, such as calcium carbonate, calcium sulfate, and magnesium hydroxide.
2. Classification by nature of pollutants
- Reversible pollution (temporary pollution). Membrane fouling can be removed by specific hydraulic measures such as clear water backwashing and aeration shaking.
- Irreversible pollution (long-term pollution). Hydraulic cleaning measures cannot remove membrane fouling but can be removed by oxidizing agents, acids, alkalis, reducing agents, and other chemical cleanings.
In particular, users can clear out both reversible and irreversible membrane fouling. Those that any cleaning methods cannot clean out are called irrecoverable membrane fouling.
3. Classification by location of pollutants
The substances in the mixture are called internal pollution, formed by adsorption, concentration, crystallization, and aggregation inside the membrane pores. In contrast, those created by accumulation and deposition on the membrane surface are called external pollution.
What factors affect MBR membrane fouling?
The main factors affecting membrane fouling are membrane properties and module structure, sludge mixture characteristics, and membrane operating conditions. The specific performance is as follows.
- MBR membrane characteristics include membrane pore size and distribution, membrane material, membrane structure, membrane hydrophobicity, membrane-solute-solvent interactions, etc.
- The quality of the treated wastewater, especially the type and concentration of organic matter in the water.
- MBR technology’s operating conditions include sludge age, dissolved oxygen concentration, membrane surface flow rate, temperature, etc.
- MBR module structure, such as size, height, aeration system arrangement, etc.
- Other factors include the interaction between microbial populations, the effect of the membrane itself on microbial growth, the composition and concentration of bacterial extracellular polymeric substances (EPS), etc.
How can we control MBR membrane fouling?
Generally speaking, we can control and solve membrane fouling from the following aspects.
1. Selection and rational optimization of membrane modules
- Membrane material selection. When selecting membrane materials, technical analysis and economic evaluation should be made regarding strength, thermal stability, chemical stability, fouling resistance, water production performance, service life, and membrane cost. In addition, whether the organic membrane materials are hydrophilic materials such as PAN, hydrophobic materials such as PVDF, PET, PS, etc. (hydrophilic modification), or inorganic membrane materials such as alumina, silicon carbide, titanium oxide, zirconium oxide, and other ceramic membrane materials, users should carefully consider all.
- Membrane pore size selection. Theoretically, selecting a membrane with a large pore size can increase the water flux while ensuring that contaminants are retained. However, it was found that larger membrane pore size accelerated membrane fouling, and water flux decreased rapidly. In general, the membrane’s cut particle size (molecular weight cut-off) should be one order of magnitude smaller than the size of the contaminants to be separated.
- Membrane surface modification. Considering that the pollutants and perhaps sludge in wastewater are organic substances, to prevent membrane fouling, the manufacturer should reduce the interfacial energy between the membrane and raw water, and users should use excellent hydrophilic membranes with anti-pollution properties. In addition, membranes with the same charge of membrane material and solute are also more resistant to pollution.
- Membrane module installation. To prevent membrane fouling, the distance between the membrane module and the wall of the aeration tank, the distance between the membrane module and the air diffuser tube, the distance between the membrane module and the reactor liquid level, and the distance between the air diffuser and the bottom of the aeration tank should be reasonably determined during the installation of the membrane module.
2. Improvement of sludge mixture characteristics
To improve the sludge concentration in the bioreactor, ensure the treatment efficiency, maintain high membrane flux, and delay membrane fouling, adding fillers to the MBR reactor to achieve better results is recommended. The fillers used widely are powdered activated carbon (PAC), zeolite, foam filler, porous flexible suspension filler, polyamide elastic filler, chitosan, etc.
3. Optimization of separation operating conditions
The choice of membrane flux is an essential factor affecting membrane fouling. It is generally believed that there is a critical membrane flux for a specific membrane; when the actual membrane flux used is lower than the critical membrane flux, the membrane filtration pressure remains stable, and the membrane fouling is reversible; on the contrary, the membrane filtration pressure rises rapidly, and the reversibility of membrane fouling decreases significantly. It is believed that the operation mode of constant membrane flux can avoid excessive pollution of the membrane surface at the initial stage of operation, which is more conducive to the long-term stable operation of the membrane.
4. Strengthen pretreatment
Pretreatment is significant for MBR technology. Wastewater contains many impurities, fibers, and suspended solids, which will block the membrane surface and then reduce the porous area of the membrane and cause membrane fouling. The reasonable use of coarse and fine grids in the water reservoir and membrane pool can effectively alleviate such situations.
How can we control MBR membrane fouling?
Cleaning methods of membrane fouling mainly include physical cleaning and chemical cleaning.
1. Physical cleaning
It refers to removing pollutants on the membrane by manual or mechanical cleaning, mainly including intermittent operation and clean water backwashing. Backwashing is an effective way to remove reversible fouling and delay membrane fouling during MBR operation. At the same flux, low-frequency high-intensity backwashing is more effective in reducing membrane fouling than high-frequency cleaning.
It is worth mentioning that during intermittent operation, pollutants on the membrane surface are loosened and shed due to gravity, and the effect is more evident under aeration. Therefore, combining intermittent operation and backwashing can more effectively alleviate membrane fouling.
2. Chemical cleaning
Physical cleaning cannot prevent irreversible pollution, and the membrane flux must be restored by chemical cleaning. Chemical cleaning includes maintenance cleaning and robust (recovery) cleaning. The commonly used cleaning agents are NaCIO solution for removing organic pollutants, citric acid for removing inorganic contaminants, etc.
As a professional MBR product manufacturer, EcoLanTM also has efficient cleaning methods and unique formulation agents and welcomes customers to communicate with us.
There are various cleaning methods for MBR membranes, and it depends on the practical situation to decide which flushing method to choose. MBR membranes are commonly used as filtration membranes in wastewater treatment. Cleaning MBR membranes on time and on demand will restore the membrane flux and extend its service life to a certain extent, so cleaning MBR membranes on time is necessary. Please refer to Do You Know the Secret of MBR Membrane Cleaning and Maintenance for more details.
Finally, there is no doubt that MBR is an up-and-coming technology for sewage & wastewater treatment. By taking measures to control membrane fouling in MBR from various aspects such as process flow design, pretreatment, membrane material property, filler application, equipment operation, and cleaning after contamination, the concentration polarization and membrane fouling can be minimized. Therefore, MBR technology will eventually overcome the problem of membrane fouling and become one of the leading technologies for water pollution control
