Effect of hydrodynamics on the transformation of nitrogen in river water by regulating the mass transfer performance of dissolved oxygen in biofilm

Biofilms drive crucial ecosystem processes in rivers. This study provided the basis for overall quantitative cal-culations about the contribution of biofilms to the nitrogen cycle. At the early stage of biofilm formation, dis-solved oxygen (DO) could penetrate the biofilms. As the biofilm grew and the thickness increased, then the mass transfer of DO was restricted. The microaerobic layer firstly appeared in biofilm under the turbulent flow con-ditions, with the appearance of the microaerobic and anaerobic layer, the nitrification and denitrification re-action could proceed smoothly in biofilm. And the removal efficiency of total nitrogen (TN) increased as the biofilm matured. Under the turbulent flow conditions, mature biofilms had the smallest thickness, but the highest proportion the anaerobic layer to the biofilm thickness, the highest density, and the highest nitrogen removal efficiency. However, the nitrogen removal efficiency of biofilm was the lowest under laminar flow conditions. The difference of layered structure of biofilm and the DO flux in biofilm explained the difference of nitrogen migration and transformation in river water under different hydrodynamic conditions. This study would help control the growth of biofilm and improve the nitrogen removal capacity of biofilm by regulating hydrodynamic conditions.

Automated summary

Biofilms play a crucial role in driving ecosystem processes in rivers. This study provides a basis for quantifying the contribution of biofilms to the nitrogen cycle. The study shows that biofilms go through different stages of growth, and the removal efficiency of nitrogen increases as the biofilm matures. The layered structure of the biofilm and the dissolved oxygen flux explain the differences in nitrogen migration and transformation under different hydrodynamic conditions. This study suggests that controlling biofilm growth and optimizing hydrodynamic conditions can improve the nitrogen removal capacity of biofilms.