Effects of dissolved oxygen on water quality and biofilms in the raw water distribution pipeline for a reserved water source

Abstract

The raw water distribution pipeline for a reserved water source is relatively closed off with poor oxygen enrichment of the water. The long hydraulic retention time of raw water in the pipes will cause a decrease in dissolved oxygen (DO) concentrations, which may lead to the deterioration of the raw water quality. This study explored the effects of different influent conditions on the attenuation trend of DO in the reserved raw water pipeline with laboratory reactors; 2 mg L⁻¹ was set as the end of the reserved period. Under different initial DO concentrations of 4 mg L⁻¹, 6 mg L⁻¹ and 8 mg L⁻¹, key parameters of water quality were investigated, and it was observed that higher initial DO concentrations could enhance the nitrification reaction and purification process of the raw water. The results showed that after the reserved period, the pH value, total alkalinity, and calcium hardness in the effluent increased significantly. The microbial community structure of pipeline biofilms under three different DO concentrations showed great similarity, whereas the relative abundance of Nitrospirae in the sample corresponding to an initial DO concentration of 8 mg L⁻¹ was the highest (6.3%), which enhanced nitrification. The microbial community diversity and species richness increased with increasing DO concentration, making the ecosystem inside the pipe more stable. To manage the changes in the reserved pipeline and reduce water waste, this study provides first insights into the water quality and microbial communities of the raw water distribution pipelines for reserved water sources during the reserved period, providing a theoretical basis for water plants.