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Regeneration and Cleaning of Ceramic Membrane

Some measures can be taken to prevent and control the pollution process of ceramic membrane during the filtration process,but membrane fouling cannot be completely avoided. With the extension of the running time, the permeation flux of the ceramic membrane decreases rapidly, and it cannot meet the requirements of industrial design. Pollutants attached to the membrane surface and exist in the pores for a long time, which may cause chemical reaction with the membrane and reduce its service life. Therefore, in order to recover the performance parameters of the membrane as much as possible, the ceramic membrane must be cleaned regularly.

Types of pollutants

1. Inorganic fouling
In a pressure-driven membrane separation system, due to the retention of the membrane, the concentration of the components in the system will occur on the membrane surface, resulting in concentration polarization. For soluble components, when the content of ions exceeds its solubility, precipitation or fouling will form on the membrane surface and in the pores. The most important inorganic pollutants are the so-called scale layer formed by calcium and barium sulfate and carbonate, among which CaCO3 and CaSO4 are the most common. In most cases, there is a mutual promotion between inorganic and organic pollutants, aggravating membrane pollution.

2. Organic fouling
Organic pollutants are mainly bacterial extracellular polymers (EPS), proteins, peptides, fats and polysaccharides and other macromolecular substances. Macromolecules containing active groups may interact with metal ions such as Ca2+, Mg2+ and Ba2+ to form a gel layer on the surface of the membrane, thus reducing the flux of the membrane or increasing the filtration resistance of the membrane.

3. Microbial contamination
Microbial contamination is mainly a sticky substance composed of microorganisms and their metabolites. The surface of the membrane easily adsorbs macromolecular substances such as humus, glycan lipids, and the products of microorganisms' metabolic activities, which have the conditions for the survival of microorganisms, and it is easy to form a biofilm, which causes irreversible blockage of the membrane and decreases the water flux.

4. Colloidal fouling
Algae, bacteria, and organic matter may all be in colloidal size, and these colloidal substances may adsorb on the membrane surface and cause pollution. Colloidal substances have different origins, and the membrane fouling they produce is also very different. The colloidal substances from non-biological processes include inorganic substances such as silt and clay. The attenuation of water flux caused by them is often caused by the pollution of the filter cake layer. They generally do not adsorb thermodynamically and irreversibly on the membrane surface; these types of accumulated on the membrane surface The colloid is easily removed by hydraulic cleaning (such as backwashing and air scrubbing). The colloidal substances produced by microbial metabolism are often permanently adsorbed on the membrane surface, causing irreversible adsorption pollution. Colloid contamination from microbial processes is classified as microbial contamination.

Controlling measures of membrane fouling

1. Modification of membrane surface
The modification of membrane surface can be divided into physical modification and chemical modification.Physical modification includes blending and surface layer. Blending is to blend hydrophilic polymer with membrane forming material to improve the hydrophilicity of membrane; surface coating refers to the formation of a functional precoating layer on the surface of membrane to prevent the adsorption between membrane material and components in solution, so as to improve the anti pollution property of separation membrane.

2. Pretreatment of feed liquid
The pretreatment of feed liquid is to meet the water inflow standard of membrane module (for example, the feed pollution index SDI of reverse osmosis membrane is required to be less than 5). Pretreatment refers to adding one or more substances to the feed solution before filtration, which changes the properties of the feed solution or solute.Pretreatment includes chemical treatment and physical treatment.
Physical treatment usually includes pre-filtration and centrifugation to remove suspended particles that may block the pores of the membrane.Chemical treatment includes adjusting the pH of the feed solution to keep macromolecules or colloidal pollutants away from the isoelectric point to reduce the tendency to form a gel layer . Divalent ions, such as Ca2+ and Mg2+, can form precipitation by bridging the macromolecular chain, while monovalent ions can prevent precipitation and pollution on the contrary. Therefore, people often use ion exchange to remove multivalent ions. The chemical process also includes precipitation and flocculation, or the use of special chemicals to resist pollution or sterilization.
Membrane cleaning methods can generally be divided into physical methods and chemical methods. The physical method refers to the use of high-flow water washing, sponge ball mechanical cleaning, etc. to remove pollutants, and chemical method is to clean the membrane with chemical agents that do not damage the membrane material itself, but can dissolve or replace the pollutants.Due to the different membrane materials, separation systems, and operating conditions, membrane fouling mechanisms and fouling causes are also different, so far no suitable cleaning method for all systems has been obtained. Therefore, it is necessary to carry out experimental work for the specific system, and for the regeneration of the membrane after a system is polluted. First, the main pollution resistance and the main components of the pollutants should be clarified, and on this basis, the appropriate cleaning agent and corresponding cleaning conditions should be selected in a targeted manner.
As far as inorganic membranes are concerned, their excellent chemical stability and high mechanical strength enable them to be cleaned by a wider range of cleaning methods.At present, the general rules for chemical cleaning of polluted inorganic membranes are as follows: strong inorganic acid turns some insoluble substances in pollutants into soluble substances; organic acid mainly removes the deposition of inorganic salts; complexing agent can complex with inorganic ions in pollutants to produce substances with high solubility, which can reduce salt deposition on membrane surface and pore and adsorbed inorganic pollutants;surfactants are mainly used to remove organic pollutants; strong oxidants and strong bases are suitable for removing the pollution of oil, protein, algae and other biological substances; for the pollution system such as cell debris, enzyme cleaning agent is often used.  For membranes with very serious pollution, strong acid and strong alkali are usually used alternately to clean, and sodium hypochlorite and other oxidants and surfactants are added.
It is also very important to choose the appropriate cleaning conditions and operation mode. The same cleaning agent, under different operating conditions and operation modes, has great difference in cleaning effect. In the process of inorganic membrane cleaning, the operating conditions of high speed and low pressure are often used, and sometimes with backwash ,in order to play the role of physical methods and recover the membrane flux to the maximum extent.
In addition to chemical cleaning, high temperature resistance of inorganic membranes makes it possible to use the so-called thermal cleaning method to achieve the purpose of regeneration, especially in biological and food applications, which can be sterilized by steam or autoclave, etc. It is still necessary to perform chemical cleaning first to remove contaminants. The disinfection and regeneration of the membrane by steam, etc. can reach 120~130℃. In this process, it is necessary to pay attention to the heating and cooling speed not too fast to avoid damage to the membrane due to impact, especially Ceramic Membrane.
For inorganic membranes that can withstand 250℃ treatment, high temperature burning can also be used to remove some organic pollutants, but attention should be paid to the changes in the pore structure and surface morphology of the membrane after heating. In addition, this method is more suitable for the components with high temperature resistance of sealing materials, otherwise the treatment process is complex and not economical.
Table 1. Cleaning agents and purpose of ceramic membrane

Types

Main items

Purpose

Acid

Strong acid: HCI, HNO3

Weak acid: H3PO4, citric acid

pH regulation; dissolution of organic precipitates; hydrolysis of acidified macromolecules

Alkali

Strong alkali:NaOH、KOU
Weak alkali:Na2CO3

pH adjustment; surface charge change; alkaline protein hydrolysis

Oxidant

NaClO;H2O2

Oxidation of organic matter; disinfection and sterilization

Surfactant

Anion: SDS

Cation: CATB

Dispersed / suspended sediment

Chelating agent

EDTA

Control catalytic decomposition of metal ions

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