Dredging effects on nutrient release of the sediment in the long-term operational free water surface constructed wetland

Abstract

Sediment is one of the key components of free water surface constructed wetland (FWS), and sediment dredging has become common practice in the long-term management of FWS. However, insufficient research has been carried out on the effects of sediment dredging in FWS, and limited information is available regarding appropriate dredging depth and methods in FWS sediment dredging. This study investigated the physicochemical characteristics of sediment in a five-year operational FWS. The effects of different dredging depths (0 cm, 1 cm, 10 cm, and 25 cm) and different dredging methods (mechanical excavation dredging and hydraulic purged flushing dredging) were compared based on the improvement of overlying water quality via indoor static simulating experiments. The results showed that the 0–10 cm depth of sediment was heavily contaminated by organic matter and nitrogen, and slightly contaminated by phosphorus. A boundary layer (0–1 cm) was formed with obvious different physicochemical characteristics which could suppress nutrient release into the overlying water. The 25 cm dredging depth was the appropriate dredging depth according to engineering practice and control performance of water quality. The pollutant diffusion fluxes of 25 cm dredged depth were 31.8%, 58.3%, 30.7%, 41.5% and 20.6% lower than those of undredged sediment for total phosphorus, soluble reactive phosphorus, total nitrogen, ammonia nitrogen, and permanganate index, respectively. Mechanical excavation dredging had a better control of water quality than hydraulic purged flushing dredging due to the low sediment water content and low residual sediment on the dredged sediment surface.