Definition of Membrane Contamination

Membrane contamination usually refers to the process whereby substances in the mixture collect on the membrane surface (outside) and within the membrane pores (inside), causing the membrane pores to become blocked and contributing to a reduction in porosity, resulting in a reduction in membrane flux and an increase in filtration pressure.

In the operation of membrane filtration, water molecules and fine substances are constantly passing through the membrane, while some substances are retained by the membrane and block the membrane pores or are deposited on the membrane surface, thus causing membrane contamination. It can be said that membrane contamination is caused by membrane retention. The direct manifestation of membrane contamination is a decrease in membrane flux or an increase in operating pressure.

The presence of nutrient substrates, mycolloids, microbial cells, cell debris, microbial metabolites (EPS, SMP) and various organic and inorganic dissolved substances in the activated sludge mix system all contribute to membrane contamination.

The development of membrane contamination can usually be divided into 3 stages (also referred to as 2 stages)

(1) Initial contamination: Occurs at the initial stage when the membrane system is put into operation, with strong interactions between the membrane surface and colloids and organics in the mixture, with contamination in the form of adhesion, charge action and blockage of membrane pores. Under the conditions of staggered flow filtration, fine bioflocs or extracellular polymers can still adhere to the membrane surface, while substances smaller than the membrane pore size will adsorb in the membrane pores and cause membrane contamination through concentration, crystalline precipitation and growth and reproduction.

(2) Slow contamination: Initially, the membrane surface is smooth and large particles are not easily attached, mainly by EPS, SMP, biocolloids and other viscous substances adsorbed on the membrane surface through adsorption bridges, net trapping and other effects to form a gel layer, resulting in a slow rise in membrane filtration resistance and enhanced retention performance of contaminants in the mixture. Contamination of the gel layer is unavoidable and the effect is a slow rise in membrane resistance. This manifests itself as a slow rise in TMP in constant flow operation and a slow decay in flux in constant pressure mode.

(3) Rapid contamination: The gel layer formed in stage 2 is gradually dense with the deposition of contaminants under the action of the continuous filtration pressure differential and permeable water flow, resulting in a quantitative to qualitative change in membrane contamination, with flocs in the mixture rapidly collecting on the membrane surface and forming sludge filter cake and a rapid rise in trans-membrane pressure differential.

Contamination of the gel layer is inevitable and has the effect of a slow rise in membrane resistance. This manifests itself as a slow rise in TMP in constant flow operation and a slow decay in flux in constant pressure mode. The main considerations for the MBR O&M process are to retard gel layer contamination (maintaining good hydraulic conditions, in-situ cleaning, controlling the rate of membrane contamination development and extending the run time for slow contamination) and to control mud cake layer contamination (rapid contamination).