Nitrogen and phosphorus transformations and balance in a pond-ditch circulation system for rural polluted water treatment

A pond-ditch circulation system (PDCS), which located along the bank of Donghu Lake (Wuchang District, Wuhan, China) and can effectively remove nitrogen and phosphorus, was proved to be a promising solution for the restoration of a degraded rural water environment. However, little is known about the removal process of nitrogen and phosphorus in the PDCS and the contribution of various pathways to nitrogen and phosphorus removal. This study aims to investigate the mechanisms of nitrogen and phosphorus removal for the PDCS. The results showed that (1) for nitrogen removal in the PDCS, it effectively reduced the total nitrogen level within the first 20 days, where dissolved oxygen (DO) played a pivotal role in the process. Based on the mass balance approach, the plant uptake and sediment storage removal contributed to 20.5% of the total nitrogen input removal, whereas other pathways such as N2 emission accounted for 79.5% of the nitrogen removal. These results indicated that nitrogen removal mainly depended on nitrification and denitrification. Meanwhile, in addition to microbial mechanisms, plant uptake and sediment storage were the major N transformation and removal pathways; in the static system, the pathways for nitrogen removal were similar with those observed in the PDCS system. (2) For phosphorus removal, microorganism processes coupled with plant uptake might be the major methods for phosphorus removal in the PDCS. In the static system, however, the phosphorus removal was mainly attributed to plant uptake.