The corroercially available membrane processes for removal of metals from industrial wastewaters are microfiltration, ultrafiltration, reverse osmosis, and electrodialysis. Microfiltration (HF) and ultrafiltration (UF) are used in combination with chemical treatment for the physical separation of metal sludges. Reverse osmosis (RO) and electrodialysis (ED) are used to recover plating compounds from rinse water and to enable reuse of rinse waters. (Electrodialysis was discussed earlier in Section 3),
Membranes can be classified in several ways. Figure 3 provides a classification of various separation processes based on particle size. Because the distinction between ultrafiltration and reverse osmosis is not precise, reverse osmosis is sometimes called 'hyperfiltration". 'Nanofiltra-tion* is a term recently introduced for processes treating particle sizes in the range of 8 to 90 Angstroms. Table 29 shows some typical design characteristics for the two membrane processes. The flux of a process is defined as the volume flow rate of permeate per unit area and pressure. The flux obtainable with reverse osmosis and ultrafiltration is also shown in Table 29.
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