Effects of wastewater flow direction on sulfur-based autotrophic denitrification processes: operational patterns, denitrification performance and response mechanisms

The sulfur autotrophic denitrification packed-bed (SPB) process is widely used in wastewater treatment due to its simplicity, cost-effectiveness, and sustainability. However, in practical SPB applications, the selection of wastewater flow direction was arbitrary, and its differences were completely overlooked, which may lead to insufficient filter bed efficiency and unnecessary economic losses. This study systematically evaluated the differences between upflow SPB (USPB) and downflow SPB (DSPB) in terms of clogging rate and denitrification performance, and explored the mechanisms underlying these differences. The results showed that the head loss growth in DSPB followed an exponential increase with an exponent of 4.5, while that in USPB was much lower, with an exponent of 0.6. Meanwhile, USPB also exhibited 1.28 times higher denitrification efficiency than DSPB, demonstrating significantly better performance. The performance difference between the two modes arose from the different accumulation of N₂ bubbles. In DSPB, the greater accumulation of N₂ bubbles led to the occupation of the voids between sulfur particles (which are used for water flow), resulting in an increase in head loss. The occupation of voids also led to a decrease in the actual hydraulic retention time (AHRT), which in turn resulted in lower denitrification efficiency in DSPB. Moreover, the mixed flow pattern in DSPB was another factor that resulted in worse clogging (higher flow collision) and lower denitrification efficiency (greater short-circuiting) compared to USPB (piston flow). Overall, this study provides fundamental insights for the selection and operation of the SPB process in practical wastewater treatment applications.