The stripping process (Chapter 7) is used to remove volatile gases such as hydrogen sulfide, hydrogen cyanide and ammonia by blowing air through the waste water. The process is, therefore, to be considered as a transfer of a compound from a liquid phase to a gas phase. The basic principle of this process of nitrogen removal is illustrated in Figure 7.1.
The rate at which ammonia can be removed by air stripping is highly dependent on pH, because the exchange between the two forms, ammonium which is the ion form, and ammonia, which is a highly water soluble gas, is an acid base reaction. High efficiency in ammonia removal requires adjustment of the pH to about 11.0 prior to the stripping process.
The principal problems associated with ammonia stripping are its inefficiency in cold weather, required shut down during freezing conditions, and the formation of calcium carbonate in the air stripping tower.
The best practical results are achieved by the use of countercurrent packed towers (0degaard 1988). As the amount of air needed is roughly independent of the ammonia concentration, the cost per kg of ammonia removed is much lower at high ammonia concentrations. Stripping is, therefore, most attractive for industrial waste water with a high concentration of ammonium.
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