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Ultra-filtration (UF) is the physical removal of particles and microbiological contaminants from an aqueous solution using a membrane filter with pore sizes less than 0.1 micron.  It does not remove dissolved ions and small molecules. Membranes may be made from several polymers including polyacrylonitrile (PAN), polyether sulfone (PES) and polyvinylidene fluoride (PVDF). The membrane material is chosen for characteristics such as strength, durability, ease of manufacturing, resistance to pH extremes and tolerance to a wide range of chemical cleaning agents.

Filtration Spectrum

Membranes typically have a pore size of 0.03 (microns), meaning they can be an effective barrier for bacteria, viruses and cysts.  The MWCO (molecular weight cut off) is approximately 100,000.  Large organic molecules can also be held back, but the membrane should not be applied for taste, odor or color removal without first running a pilot test on the actual water supply.  There are many variables to be considered when determining whether the system will successfully remove dissolved organic material from a particular water supply.

Membranes may be supplied as flat sheet, spiral wound or hollow fibre modules. Flat sheet membrane systems are contained in bulky structures and generally operated at low flux rates. Spiral wound modules are operated in continuous cross flow mode similar to reverse osmosis membranes. They do not perform a filtrate backwash cycle to lift foulants from the membrane surface. Hollow fibre membranes may be submerged in a membrane bioreactor or encased in pressurised modules.

Pressurised hollow fibre membranes may be designed to operate in either continuous cross flow or semi dead-end modes.
Cross flow mode is advantageous for higher suspended solids but usually requires more pumping energy. Both modes usually accommodate membrane backwashing using filtrate. Feedwater flow path may be outside to inside or inside to outside. The porous membrane structure is usually asymmetric and the incorrect flow direction should not be applied.  Both processes have their advantages.

While there are many variables in determining ultrafilter performance we believe the following generalized characteristics apply.

Outside-In                                                                                      Inside-Out
Higher tolerance of suspended solids                                       Lower tolerance of suspended solids
Minimal prefiltration requirement                                                Additional prefiltration requirement
Larger filtration surface area                                                       Smaller filtration surface area
Non-defined feedwater flow path                                                Defined flow path permits feed water flush
Usually requires air scouring                                                       Air scouring often not required
Higher volumes of filtrate used for backwash                            Lower volumes of filtrate used for backwash

DOW Ultrafilter

Outside-in flow systems usually tolerate higher levels of suspended solids in the feedwater to be treated. This reduces prefiltration requirements. Also for fibres of similar outside and inside diameter the outside filtration surface is greater resulting in lower flux rates. As the flow path on the outside of the fibre is non-defined, feedwater flush is of limited effect. Thus membranes rely mainly on backwash with filtrate plus air scouring. Scouring with significant volumes of air requires additional capital expenditure for compressors.

Inside-out flow systems may be less tolerant of suspended solids thus requiring additional prefiltration where the feedwater contains higher levels. For modules of similar dimensions and fibre sizes, filtration surface area is lower. However, a defined flow path on the inside of the fibre allows for efficient feedwater flushing. This may reduce the demand for backwashing with filtrate thus improving the total system recovery. i.e. the amount of feedwater recovered as filtrate.

System designers should account for these various characteristics when choosing an appropriate membrane system.

When the pressure drop across the membrane (transmembrane pressure) increases by approximately one bar (15psi), the accumulated material on the filtration surface of the membrane needs to be removed.  This is done using a Hydraulic Cleaning (HC) process. The process can be done in different ways, depending on the filtration process employed.

Inside-out membrane systems will use a feedwater flush through the inside of the fibre followed by a backwash with filtrate. Outside-in membrane systems will use a combination of air scouring, feedwater flushing and backwashing with filtrate.

Eventually, the transmembrane pressure does not return to its original performance due to a build up of particulate, inorganic, organic and biological material. When, the post HC transmembrane pressure impacts on the system performance it is necessary to perform a Chemically Enhanced Backwash (CEB). The CEB is part backwash and part chemical soak for approximately 10 minutes. The chemical used depends upon the material collected on the membrane surface but often includes sodium hypochlorite, sodium hydroxide and hydrochloric acid. CEB usually incorporates pre and post flush and backwash sequences.



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