Experimental Study on Purification of Reclaimed Water by River Sediment under Different pH Conditions

Reclaimed water is the primary source of urban landscape rivers in northern China, but its high nitrogen content can lead to river eutrophication and groundwater contamination. However, river sediment is the medium between river water and groundwater, which can help to purify water through physical, chemical and biological reactions, and the acid-base conditions of river water environment have a certain impact on the purification effect of sediment NO₃^--N. Using a river channel simulator, the aim of this experiment is to determine how well sediment in the trough system can remove NO₃^--N from inlet water with pH levels of 4, 7, and 10, respectively. The results indicated that sediment in the trough system had a significant impact on the removal of NO₃^--N under all the three pH conditions, with the overall removal efficiency being 51.3%, 57.5% and 69.9%, respectively. The impact is more pronounced at the depths of 10, 20 and 30 cm of trough sediment, where the removal efficiency of NO₃^--N was higher than 89.0%. The removal efficiency differed considerably across the three pH treatments at 50 and 70 cm, with the best removal efficiency occurring at pH=4. In the vertical variation, ρ(NO₃^--N) in the sediment was relatively lower at 10, 20 and 30 cm, and relatively higher at 50 and 70 cm under all three conditions. It was also observed that ρ(NH₄⁺-N) of bottom water at pH values of 4 and 10, was higher than that with pH values of 7, indicating that the removal ability of sediment to ammonia nitrogen was poor under acid-base conditions. NO₂^--N accumulated in the initial phase of pH=7 and 10 and the intermediate phase of pH=4, but it was finally eliminated. When pH=7 and 10, there was a significant negative correlation between bottom water pH and NO₂^--N concentration (P < 0.05). When the pH of the influent was 4, 7 and 10, the pH of the sediment remained between 7 and 9, indicating that the water environment provided a certain buffer against external acid-base changes, i. e. it has the ability to resist acid and alkali. The study demonstrates the purification effects of riverbed sediment on reclaimed water under different acid-base conditions, the sediment under alkaline conditions has a better purification effect on NO₃^--N, whereas the sediment under neutral conditions has a greater purification effect on NH₄⁺-N. This information can be used as a reference for the safe recharge of reclaimed water to rivers and lakes.