Hydraulic systems filtration requirements

Two process modes, namely, dead-end and cross-flow modes, are widely used for microfiltration (14). For the dead-end mode, the entire solution is forced through the membrane. The substances to be separated are deposited on the membrane, which increases the hydraulic resistance of the deposit. The membrane needs to be renewed as soon as the filtrate flux no longer reaches the required minimum values at the maximum operation pressure. This mode is mostly used for slightly contaminated solutions, e.g., production of ultra-pure water. For the cross-flow mode, the solution flows across the membrane surface at a rate between 0.5 and 5.0 m/s, which prevents the formation of a cover layer on the membrane surface. A circulation pump produces the cross-flow velocity or the shear force needed to control the thickness of the cover layer. The system is most widely used for periodic back flushing, where part of the filtrate is forced in the opposite direction at certain intervals, and breaks up the cover layer. The normal operating pressure for this mode is 1-2 bars. 

Over long periods of operation, membrane fouling is generally not totally reversible by the hydraulic backwash procedure. As the number of filtration cycles increases, the irreversible fraction of membrane fouling also increases (see Fig. 6.13). In order to obtain the desired production flow rates (or flux), an increase in TMP is required. When this pressure reaches a maximum allowable mechanical resistance of the membrane, chemical cleaning is required for the membrane to regain most of its permeability. Regardless of the membrane system... 

The basic requirements of a filtration system for hydraulic services are as follows ... 

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