A Study on Nutrient Removal from Municipal Wastewater by Struvite Formation Using Taguchi's Design of Experiments

Abstract

The aim of this study is to investigate the removal of NH4+-N and PO43−-P from the municipal wastewater by struvite (MgNH4PO4·6H2O) formation. The optimization of the NH4+-N and PO43−-P removals is made by changing the total concentrations of struvite forming ions (Mg2+, NH4+, PO43−) and pH of the wastewater. The experimental design was made using Taguchi's L9(34) orthogonal array technique in the first stage of the study. The conditions yielding maximum removals of NH4+-N and PO43−-P are found as molar concentration ratio of Mg:N:P of 2:2:2 and 3:3:2, respectively, using the signal-to-noise (S/N) ratio of experimental results. The removal of NH4+-N followed the stoichiometry of struvite formation, while PO43−-P removal required the excess amount of Mg2+. The pH of wastewater in which the maximum NH4+-N and PO43−-P removals are observed was 9.0. All of the factors tested influenced the removal of NH4+-N opposite to the removal of PO43−-P, in which the concentration of NH4+-N was found insignificant statistically. Therefore, the addition of NH4+ ion is eliminated and Taguchi's L4(23) orthogonal array was used in the second stage of experimental study. The molar concentration ratios of struvite forming ions (Mg:N:P) were found to be as 1:1:1 and 2:1:1 for maximum NH4+-N and PO43−-P removals at the levels of 52.4 and 67.7%, respectively, at pH 9.0 using the S/N ratios of experimental results. The performance of proposed method in removing NH4+-N and PO43−-P and foreseen problems related with effluent quality is discussed.