Purification Effect of Sequential Constructed Wetland for the Polluted Water in Urban River

Abstract

Constructed wetlands can play an active role in improving the water quality of urban rivers. In this study, a sequential series system of the floating-bed constructed wetland (FBCW), horizontal subsurface flow constructed wetland (HSFCW), and surface flow constructed wetland (SFCW) were constructed for the urban river treatment in the cold regions of North China, which gave full play to the combined advantages. In the Yitong River, the designed capacity and the hydraulic loading of the system was 100 m3/d and 0.10 m3/m2d, respectively. The hydraulic retention time was approximately 72 h. The monitoring results, from April to October in 2016, showed the multiple wetland ecosystem could effectively remove chemical oxygen demand (COD), ammonia nitrogen (NH4+-N), total nitrogen (TN), total phosphate (TP), and suspended solids (SS) at average removal rates of 74.79%, 80.90%, 71.12%, 78.44%, and 91.90%, respectively. The removal rate of SS in floating-bed wetland was the largest among all the indicators (80.24%), which could prevent the block of sub-surface flow wetland effectively. The sub-surface flow wetland could remove the NH4-N, TN, and TP effectively, and the contribution rates were 79.20%, 64.64%, and 81.71%, respectively. The surface flow wetland could further purify the TN and the removal rate of TN could reach 23%. The total investment of this ecological engineering was $12,000. The construction cost and the operation cost were $120 and $0.02 per ton of polluted water, which was about 1/3 to 1/5 and 1/6 to 1/3 of the conventional sewage treatment, respectively. The results of this study provide a technical demonstration of the restoration of polluted water in urban rivers in northern China.

Remediation of Polluted River Water by Biological, Chemical, Ecological and Engineering Processes

“…Biological methods are more environment-friendly, self-sustaining and less expensive than the physical and chemical processes. However, the processes need an extended time, ranging from several months to years for microbial growth and, sometimes, different environmental factors such as temperature and rainfall affect their performance [9]. Some of these technologies require high cost, labour and maintenance as well.…”

“…Potential increase of pollution, cost intensive mechanical process [9,19] Building hydraulic structures…”

“…Flow through riverbed and groundwater aquifer to the pumping wells Remove organic and inorganic contaminants through natural filtration process Slow process [22][23][24][25] Ecological engineering-based techniques, such as plant purification treatment, ecological floating beds, artificial floating islands and constructed wetlands, have attracted the greatest attention due to their overall economic, environmental and ecological benefits, but these methods demonstrate variable performances to remediate polluted river water [9,10,26,27]. The remediation of river water is a critical process which needs the combination of engineering and ecological technologies for successful treatment of river water.…”

“…These methods have some advantages over other traditional water treatment processes, which include low running cost, easy maintenance, no secondary pollution, economic and environmental benefits and high efficiency in the restoration of river ecosystems and river water quality [67]. However, they require a large area, have low hydraulic load and exhibit intolerance to heavy pollutant loading rate [9], including the observation of seasonal death and plant diseases [68]. Therefore, living plants are recommended to be replaced by aqua mats and artificial seaweed, which can enhance the growth of microorganisms and overcome the above drawbacks [69].…”

“…Therefore, living plants are recommended to be replaced by aqua mats and artificial seaweed, which can enhance the growth of microorganisms and overcome the above drawbacks [69]. Given the low removal rate of single constructed wetland, Bai et al [9] sequentially constructed floating bed wetland, horizontal subsurface flow constructed wetland and surface flow constructed wetland for the treatment of urban river water. The combination of three wetland systems effectively removed COD, NH 4 + -N, TN (total nitrogen), TP (total phosphorus) and SS from river water.…”