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Abstract
Statistical methodology was applied to the optimization of the ammonium oxidation by Nitrosomonas europaea for biomass concentration (CB), nitrite yield (YN) and ammonium removal (RA). Initial screening by Plackett-Burman design was performed to select major variables out of nineteen factors, among which NH4Cl concentration (CN), trace element solution (TES), agitation speed (AS), and fermentation time (T) were found to have significant effects. Path of steepest ascent and response surface methodology was applied to optimize the levels of the selected factors. Finally, multi-objective optimization was used to obtain optimal condition by compromise of the three desirable objectives through a combination of weighted coefficient method coupled with entropy measurement methodology. These models enabled us to identify the optimum operation conditions (CN = 84.1 mM; TES = 0.74 ml; AS = 100 rpm and T = 78 h), under which CB = 3.386×108 cells/ml; YN = 1.98 mg/mg and RA = 97.76% were simultaneously obtained. The optimized conditions were shown to be feasible through verification tests.
Highlights
Ammonium is the main pollutant in waste water
Probability (P) values were used to check the significance of the coefficients, which are necessary to understand the pattern of the mutual interactions of the test factors
The significance of the regression coefficient was tested with the confidence of 95%, so p#0.0001 meant very significant; p#0.05 was considered to denote a statistically significant difference and p#0.01 shown the power of significance
Summary
Ammonium is the main pollutant in waste water. Ammonium removed from waste water is mainly based on biological technology, for it is more effective and relatively inexpensive [2]. The biological removal of ammonium is undertaken in biological nitrification/denitrificantion processes, which represents a key process in global nitrogen cycle [3]. Among all the microorganisms listed as good degrader of ammonium, Nitrosomonas europaea, the model chemolithoautotrophic ammonium oxidizing bacterium (AOB), obtains energy from the aerobic oxidation of ammonia (NH3) to nitrite (NO22) [4]. A significant amount of the energy obtained from oxidation of hydroxylamine has to be invested in ammonia oxidation to hydroxylamine, reverse electron transport to generate Nicotinamide adenine dinucleotide (NADH) and CO2 fixation. The growth rate and yield of N.europaea are relatively low [6]
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