Thesis - faculty.ait.ac.th - Asian Institute of Technology

Ammonia Stripping
Air stripping of ammonia involves passage of large quantities of air over the exposed
surface of the leachate, thus causing the partial pressure of the ammonia gas within the
water to drive the ammonia from the liquid to the gas phase. Ammonia stripping can also
be undertaken by water falling through a flow of air as in stripping towers or by diffusion
of air through water in the form of bubbles. Stripping towers are more efficient since there
is better contact between the gas and liquid phases when dispersion of liquid takes place in
the form of fine droplets. Since, ammonia stripping is mass transfer controlled, the surface
area of the liquid exposed must be maximised. This can be achieved by creating fine
droplets with the help of diffusers or sprayers. The process is further subject to careful pH
control and involves the mass transfer of volatile contaminants from water to air.
The formation of free ammonia is favoured when the pH is above 7. At pH greater
than 10, over 85 % of ammonia present may be liberated as gas through agitation in the
presence of air (Reeves, 1972). Ammonium hydroxide (NH4OH) is formed as an
intermediate at pH between 10 and 11 in the reaction. The bubbling of air through
ammonium hydroxide solutions results in the freeing of ammonia gas. This process is
subject to temperature and solubility interferences. Since ammonia is highly soluble in
water, solubility increases at low ambient temperatures.
To review the effectiveness of ammonia stripping as a pre-treatment option for
landfill leachate, Cheung, et al. (1997) investigated air flow rate and pH as critical
parameters for the optimisation of ammonia stripping in a stirred tank. In the study, to
evaluate the effective pH, air flow rate of 0, 1, 5 mL/min and lime dosage of 0-10,000
mg/L was varied. The study revealed an enhanced ammonia removal (86-93 %) could be
achieved at air flow rate of 5 mL/min and pH greater than 11. It was realized that
effectiveness of the process was also dependent on area (A): volume (V) ratio of the tank
and leachate quality. The efficiencies in previous studies by other researchers were 40 to
53 % for A: V = 23 m -1 and 19 % for A: V = 1.8 m -1 (Cheung, et al., 1997). This indicated
that the mass transfer governed the mechanism for ammonia stripping and it was further
revealed that ammonia desorption into the air bubbles was less significant than the airwater
interfacial area. The provision of air to the system promotes air bubble formation and
turbulence at the air-water interface, which aids in increasing the surface area for ammonia
removal. Thus, an indefinite increase in air flow rate could greatly enhance ammonia
stripping efficiency over a short detention time. The practicality of this approach depends
on the power mixer efficiency and mass transfer rate, which should be optimised to render
the process cost-effective. Further, ammonia stripping has the advantage of precipitating
organics and heavy metals present in the leachate.
There has been a plenty number of investigations performed on the physico-chemical
treatments to investigate their potential in treating leachate. A comparison of different
studies with physico-chemical treatments is presented in Table 2.15.
2.8.4 Natural Leachate Treatment Systems
Natural leachate treatment is distinguished from conventional systems based on the
source of energy that predominates in both the systems. In conventional systems, forced
aeration, mechanical mixing and chemical addition are input for the pollutant degradation.
Natural systems however, utilize renewable energy sources such as solar radiation or wind.
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