Response Surface Analysis of Nitrite Oxidation with respect to Copper, Zinc, and Microbial Concentrations

Abstract

Nitrification is a two-step process by which ammonia is converted to nitrate via nitrite. This process plays a key role in transformation of fertilizer nitrogen in agricultural systems and is a key component of nitrogen removal in wastewater treatment. Production of soluble inorganic nitrogen by nitrification can lead to the contamination and eutrophication of terrestrial and aquatic ecosystems, while the gaseous products of nitrifier denitrification, nitric oxide (NO) and nitrous oxide (N2O), destroy stratospheric ozone and greatly contribute to global warming. In this study, simultaneous effects of three important factors including copper, zinc and NOB concentration on nitrite oxidation using a response surface analysis (RSA) were investigated. The range of copper, zinc and NOB concentration was from 0.01 to 2 mg/L, from 0.01 mg/L to 3.5 mg/L, and from 0.15 x 107 to 1.5 x 107 copies/mL, respectively. Design of experiment was followed a Center-Faced Central Composite Design, with a total number of 17 experimental runs. The regression equations obtained give the lag phase period (h), and ntrite oxidation rate (mg/L/h) Lag (h)= 66.06 + 0.67 x1 + 69.95 x2 - 261.96 x3 + 2.18 x1x1 + 0.63 x2x2 + 148.05 x3x3 - 4.58 x1x2 + 1.90 x1x3 - 49.5 x2x3 NO rate (mg/L/h)= 2.86 - 2.83 x1 - 0.1x2 + 7.18 x3 + 0.64 x1x1 + 0.003 x2x2 + 0.81 x3x3 - 0.12 x1x2 + 0.53 x1x3 +0.08 x2x3 where x1= copper (mg/L), x2= zinc (mg/L), x3 = NOB ( x 107 copies/mL)The analysis of variance (ANOVA) indicated that all regression models were significant at P value not higher 0.001. The coefficients of determination (R2) were from 0.9509 to 0.9886. The lag period was significantly affected by the NOB concentration and zinc, whereas copper concentration had no significant effect. The response surface of the NOR was nearly linear to the variation in the independent variables for the entire region of investigation.Two different species of Nitrobacter were identified- Nitrobacter winogradskyi and Nitrobacter hamburgensis by using denaturing gel gradient electrophoresis. The population change of NOB was different under the different combination of the three environmental variables. This study suggested the way for stable process operation when the wastewater treatment system having simultaneous effect of copper and zinc. The statistical method used in this study can be extended to estimate the quantitative effects of other environmental factors that can change simultaneously