Modelling transportation and transformation of nitrogen compounds at different influent concentrations in sewer pipe

A mathematical model was established to describe the transportation and transformation of nitrogen compounds in the sewer pipe. In order to verify the consistency between the experimental data and model simulation data, four runs of experiments were carried out in a 21 m long, 0.15 m diameter model sewer. The results showed a good consistency between the experimental and simulation values (all correlation coefficients >0.81). According to the good consistency, it was proved that the attached heterotrophic biofilm on the sewer bottom played a dominant role on degradation of compounds in the system. Readily biodegradable substrate (SS) decreased with the test time due to aerobic and anoxic growth of heterotrophs. Ammonia and ammonium nitrogen (SNH) increased with the test time. But nitrate and nitrite nitrogen (SNO) and soluble organic nitrogen (SND) decreased. Dissolved oxygen (SO) declined due to the microbial consumption and subsequently increased due to reaeration, forming a sag curve. Furthermore, the transformation pathways of different compounds were identified in this study.