Nitrogen compunds wastewaters must be removed because they cause environmental problems including dissolved oxygen depletion, eutrophication, odor, ammonia toxicity, and ground water contamination. Biological nitrogen removal involving nitrification and denitrification has been still adopted in wastewater treatment because it is inexpensive  and causes little envoromental damage, in contrast to physico-chemical treatments. Nitrification, the first step in biological nitrogen removal, involves two processes. Ammonia oxidation to nitrite by ammonia oxidazing bacteria and nitrite  oxidation to nitrate bay nitrite oxidizing bacteria. Ammonia oxidation though to be the rate-limiting step in nitrification, bacause ammonia oxidizing bacteria have lower growth rate than nitrite oxidizing bacteria, and are more sensitive to inhibition by environtmental factors. Therefore, to establish the stable nitrification reaction in waste-water treatment process the dynamics of ammonia oxidizing  bacteria in response to the operating condition must be understood.

The substrate consistency is one of the important factors to ensure process stability of biological system. However, substrate concentration often fluctuate in full-scale wastewater treatment plants. Substrate overload reduces  microbial activity, which results in the poor removal efficiency. Similarly, ammonia overload resulting from variations in substrate content can cause failure of ammonia oxidation in nitrification system. Ammonia oxidizing bacteria are much more sensitive to substrate concentration rather than are heterotrophic bbacteria grown in wastewater treatment plans. For example, ammonia oxidizing bacteria can be inhibited only 1.0mM of free ammonia concentration, although some heterotrophic bacteria can effieciently grow at 1.0% of glucose (55,5mM).

The steel manufacturing industry produces large amounts of steel wastewater that contains high concentration of ammonia (>100ppm) and inorganic salts as a by product of a process. Steel wastewater has 6,4<pH<8,9 This high concentration of ammonia can often cause ammonia overloading shock to biological nitrogen removal  system, thus its reducing nitrification efficiency. In some steel wastewater systemtreatment plans, the high strength of ammonia wastewater used introduced into nitrification system can change suddenly, thereby causing a drastic increase of ammonia concentration. Therefore, for nitrification of steel wastewaterto be succesful,  the effect of ammonia overloading shock on the nitrification resilience of the ammonia oxidizing bacteria must be determined.

Certain ammonia oxidizing bacteria species appear to adapt to biological nitrogen removal systems that are subject to ammonia overloading shock, and become the dominant species in the ammonia oxidizing bacteria community. Hence, microbial community resilience following ammonia overloading can contributing in accelerating nitrification step in full-sacle steel wastewater treatment facilities. However only a few  studies have been conducted regarding the process resilience under ammonia overloading shock. For example, actvated sludgepre-exposed to high ammonia level had higher resistance to ammonia than did un-acclimatedactivated sludge, and the dominant ammonia oxidazing bacteriacommunity was changed after the ammonia overload period in a sequential batch reactor system. However, the information of the effects of ammonia overloading shock on process resilience and ammonia oxidizing bacteria communities have been still limited.


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