Treating surface water with low nutrients concentration by mixed substrates constructed wetlands

Constructed wetland (CW) has been widely applied in nutrients reduction for eutrophication control, especially in the advanced treatment of effluent of municipal sewage plants or the in-lake river treatment with high hydraulic loads and low nutrient concentrations. But in real application, it shows lower nutrient removal efficiency. The main reason is that traditional substrates, such as soil and gravel have low capacity for nitrogen and phosphorus removal. This study aims to enhance nutrients removal in constructed wetland systems by using series of substrates including calcium silicate hydrate (CSH), vermiculite and ceramsite which are all investigated individually in static experiment or mixed in batch and continuous flow experiments. The result showed that the efficiency of phosphorus removal by CSH could reach 97%, much higher than the other substrates. However, when it was applied in CW, the removal efficiency decreased. Although vermiculite showed the highest ammonia nitrogen removal efficiency of 65.91%, the ammonia nitrogen removal efficiency may have depended on the action of microorganism. High total nitrogen removal efficiency was obtained in continuous-flow mixed substrate CW. Under a hydraulic retention time (HRT) of 18h and hydraulic loading rate (HLR) of 0.496 m3/m3.d, average total nitrogen removal efficiency of above 91% was achieved, but the average phosphorus removal efficiency was around 65% and this needs to be improved further.