Log size pm

Figure 10.1. Membrane processes and particle size.

The relation between the process and the removable particle size; see Figure 10.1, indicates the possibilities of using membrane processes for nitrogen removal. Proteins can accordingly be removed from waste water and waste products such as whey by application of ultrafiltration. This has found a wide use particularly in the dairy industry. Ammonium and nitrate can be removed at least to a certain extent by use of reverse osmosis. This application has, however, some shortcomings:

1) The osmotic pressure increases to very high levels due to high concentrations of inorganic ions in the reject. This implies that the permeation rate decreases and the required size of the equipment increases. This means high installation costs. The alternative is to accept smaller concentrations of the reject, which, however, increases the problem of reject discharge.

2) It is difficult to avoid a certain clogging of the membranes, although removal of most organics and all suspended matter reduces the problem.

3) The high pressure needed for the process implies high energy costs and therefore high operation costs.

Membrane technology has developed rapidly during the last decades, and it cannot be excluded that reverse osmosis will find a much wider application in the neart future for nitrogen removal, too.

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