Metal filters, microfiltration

This technology removes dissolved metals from liquid wastes at a lower cost then other treatment options, such as precipitation followed by clarification and conventional filtration, ion exchange, reverse osmosis, and electrolysis. An advantage of the DuPont/Oberlin microfiltration technology is that it produces a dry, stabilized cake that can be landfiUed when used in conjunction with a filter aid/cake stabilizing agent. 

The EPOC microfiltration process is based on the ability of a proprietary woven polyester filter to retain particles and allow water to permeate through the filter. The technology uses a three-step process in which (1) reagents are added to the wastewater to precipitate metals and/or sorb other contaminants (2) the microfiltration unit (known as EXXFLOW) removes and concentrates the precipitates, while allowing water (permeate) to pass and (3) the concentrated precipitate is dewatered (by a module called EXXPRESS) to produce a semidry filter cake containing the metal precipitates and other filtered solids. 

Membrane microfiltration at the Palmerton Zinc Superfund site in Pennsylvania VOCs (liquid), metals, and oily, inorganic, organic wastes E.I. DuPont de Nemours and Oberlin Filter Company... 

In cross-flow flltration, the wastewater flows under pressure at a fairly high velocity tangentially or across the filter medium. A thin layer of solids form on the surface of the medium, but the high liquid velocity keeps the layer from building up. At the same time, the liquid permeates the membrane producing a clear filtrate. Filter media may be ceramic, metal (e.g., sintered stainless steel or porous alumina), or a polymer membrane (cellulose acetate, polyamide, and polyacrylonitrile) with pores small enough to exclude most suspended particles. Examples of cross filtration are microfiltration with pore sizes ranging from 0.1 to 5 pm and ultrafiltration with pore sizes from 1 pm down to about 0,001 pm. 

In the last example crossflow microfiltration has been chosen because a slurry of a specified concentration can be achieved, cement is added when this has been obtained [Empsall and Hebditch, 1994]. The ability of microfiltration to provide particle-free permeate, above a certain threshold, has lead to the adoption of skid-mounted crossflow filters for heavy metal effluent treatment, see Figure 10.25 [Metal Finishers Association, 1992]. 

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