Tertiary treatment of municipal wastewater by a novel flow constructed wetland integrated with biochar and zero-valent iron

Abstract

To achieve high and stable nutrient removal efficiency for stricter discharge standards, three bench-scale integrated flow constructed wetlands (IFCWs) with different filler medium arrangements were built for tertiary treatment of municipal wastewater. We hypothesized the incorporation of biochar and zero-valent iron (ZVI) into an IFCW could enhance nutrient removal significantly. Results showed that W2 with biochar (89.1 ± 1.5%, 88.1 ± 1.1%, 75.9 ± 4.5%) and W3 with ZVI-biochar (86.4 ± 1.5%, 88.3 ± 0.7%, 91.6 ± 3.8%) had significantly higher removal efficiencies of NH4+-N, TN, and TP than that of the control group (68.8 ± 7.1%, 62.4 ± 4.6%, 68.1 ± 8.0%). Analysis of the microbial community revealed that nitrifiers such as Nitrosomonadales and Nitrospirales were enriched in W2, while denitrifiers like Rhodocyclales, Burkholderiales, Rhizobiales and Pseudomonadales were enriched in W3. Analysis of functional genes demonstrated that the biochar increased the abundance of AOB-amoA, while ZVI-biochar promoted the abundance of nosZ, nirS, nirK and hzsA. Principal component analysis (PCA) indicated that higher NH4+-N removal efficiency mainly resulted from the enhancement of microbial nitrification by biochar, and the improvement of NO3−-N removal efficiency was attributed to the enhanced denitrification and anammox by ZVI.