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

Table 4.12 Variation in Ammonia Removal Efficiency
Velocity Gradient
(s
Ammonia Removal (%)
Contact Time (h)
-1 )
2 4 6
0 28-31 37-46 47-51
1,530 61-66 84-86 88-93
2,850 69-74 88-95 96-98
4,330 71-76 89-95 96-98
While Diamadopoulos, 1994 did the ammonia removal experiments using air
stripping at pH 11.5 with air flow rate 2-3.5 L air/L in leachate, he could achieve 95%
ammonia removal after a time period of 24 h. The removal efficiency of air stripping was
similar with the present study, with the advantage that the present study required a lower
time period. The main role of agitation was to create turbulence sufficient enough in the
free leachate surface, to increase the surface area for ammonia removal (Smith and Arab,
1988). In this case, the ammonia desorption would be less important than the surface area
similar to studies done by Cheung, et al. (1997). This could be the probable reason for the
efficient removal at a lower contact time. Another added advantage is that the present
process can withstand changes in the volume and leachate concentration in comparison
with the nitrification and denitrification processes for ammonia removal. It could also be
said that ammonia stripping is an appropriate option for pre-treatment of leachate even in
terms of cost-effectiveness (Cheung, et al., 1997).
In the second stage of ammonia stripping studies as mentioned above, the pilot-scale
studies were done to confirm the results obtained from laboratory-scale studies. The pilot
scale study was conducted with leachate volume of 40 L at pH 11-12 and velocity gradient
of 2,850 s -1 . The summary of the pilot scale studies are given in Table F-5 of Appendix F.
The contact time was varied from 1 to 5 h. At each hour, the removal efficiency was
measured. The average removal efficiency was found to be 89% at 5 h contact time. From
the pilot scale studies, similar removal efficiency was expected at 4 h. Pilot scale results
could be taken as a representative results as the standard error decreased with increasing
volume. When Yangin, et al. (2002) worked on ammonia stripping of domestic wastewater
mixed with leachate, it was found that 89% ammonia could be removed from the UASBR
effluent containing an ammonium concentration of 1,000-2,000 mg/L. So, with the pilot
scale study, we can be assured that the optimum condition persists at 5 h contact time. To
verify this result again, with varying leachate ammonia concentration, the experiment was
conducted and the average ammonia removal efficiency of 86% could be obtained with
standard deviation of 3 mg/L. The results of this experiment are given in Table F-6 of
Appendix F.
The mechanism in ammonia stripping could be due to the ammonia desorption from
the surface of the liquid leachate into the gaseous phase. It has also been said that the mass
transfer of ammonia from liquid to air is proportional to the concentration of ammoniacal
nitrogen in the solution and is a first order reaction (Srinath and Loehr, 1974). However,
this could not be proved significantly in the present study, as only a range of ammoniacal
nitrogen in the leachate was used in the experiment. Another aspect to be discussed in the
study would be biological removal of ammonia in the aeration process through agitation. It
was clear that the ammonia was removed through stripping rather than biological activity
as there wasn’t a significant increase in the concentration of the oxidized nitrogen
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