Biofouling layer formation

Removes coarse materials that may cause cake/gel layer formation on downstream membranes Selectively removes contaminants or other particles that are sticky to filter media and downstream membranes Partially removes microorganisms that can cause biofouling Particulate and colloidal organic/inorganic substances, microbiota... 

The processes of interest are NFand RO where the membranes are either nanoporous or essentially nonporous. In these processes the fouling is a surface layer, the effects of which maybe exacerbated by the high retention of solutes by the membrane. Operation is with crossflow and in industry fixed flux is commonly used. This section considers particulate fouling, biofouling and scale formation and then discusses the implications of cake enhanced concentration polarization on fouling outcomes. 

Any surface subjected to water-containing micro-organisms will sooner or later become coated with a layer of bacteria. Deposits that contain a high amount of micro-organisms are referred to as (biological) slime or biofilm, because of their slimy or gel-like character. Slime formation is most obvious in areas where the water flow is insufficient to dislodge the accumulation. The process of biofilm formation is known as biofouling. Biofihns occur in most water systems, and the pulp and paper industry is just one of many industries where this occurs. 

While water permeates through the membrane, the rejected matter accumulates behind the membrane and forms a layer of high concentration of salts that increases the osmotic pressure and reduces the permeability. Also over time, organic and suspended matter adsorb on the membrane and reduce membrane permeability. These materials may also serve as food for bacteria that attach on the membrane surface. The excessive bacterial growth may cause membrane biofouling. Sparingly, soluble salts may precipitate on the membrane surface and impact membrane performance. The phenomenon of the formation of a concentrated layer close to the membrane is called concentration polarization. The ratio... 

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