7.1 Physico-chemical Principles of Air Stripping
The stripping process 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 from a liquid phase to a gas phase. The basic principle of this process of nitrogen removal is illustrated in Fig. 7.1.
Air + ammonia out
Influent, waste water with high pH.
Stripping unit for instance a packed tower
Effluent to pH adjustment or other treatment processes
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 ionic form, and ammonia, which is a highly water-soluble gas, is an acid-base reaction. The ammonia stripping is based on the following reaction:
The equilibrium constant for this process is 10-9 25 at 18°C, which means that: [NHa] [H+]
By separating H+ in this equation and converting to a logarithmic form, we get:
Knowing the ammonium concentration in an aquatic ecosystem, this relationship can be used to estimate the toxicity level of the water, see Section 1.4. From equation (7.3) we can see that at pH = 9.25, 50% of the total ammonia-nitrogen is in the form of ammonia and 50% in the form of ammonium. Correspondingly the ratio between ammonia and ammonium is 10 at pH 10.25 and 100 at pH 11.25. A graph showing the ratio ammonia to ammonium is given in Fig. 7.2. Consequently it is necessary to adjust the pH to 10 or more before the stripping process is used. The pKa value, which is the negative logarithm to the equilibrium constant, is dependent on the presence of other ions, or expressed in another way, of the ionic strength of the influent. The ionic strength is defined by the following expression:
where C = the molar concentration of the considered ions and Z = the charge.
On the basis of the ionic strength, it is possible to find the activity coefficient, f, from:
where I = ionic strength, Z = charge and f = activity coefficient. The activity coefficient, f, is defined as the activity a, divided by the concentration c. The activity is used in the mass equations to replace the concentrations, if the ionic strength is sufficient high to play a significant role, see also below.
Fig. 7.2. Distribution of ammonia and ammonium as function of pH and temperature.
If the ionic strength plays a role, the concentrations in equation (7.2) are replaced by activities. As pH is defined from the activity of hydrogen ions, (7.2) will be changed to the following expression in this case:
Equation (7.3) will be changed correspondingly:
or pH =9.25 +0.5 * Z2_M/L + log ([NHa] /[NH4+] ) (7.8)
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