Selection Of Conmstructed Wetland Plant Species And Their Treatment Efficiencies

Constructed wetland is a clean and low-carbon sewage ecological treatment technology, which has low capital costs and energy consumption, good effluent quality, convenient operation and maintenance, flexible system configuration, good resistance to shock loadings, and effective pollutant removal ability. Currently, researches and applications of wetland-based wastewater treatment technology mostly focused on low-strength wastewaters. But for the high strength wastewater treatment, there was still little understanding. Therefore, in this study, the wetland treatment system for high strength wastewater was conducted using different plants, under sand and water culture conditions. The pollutant removal efficiency of different plant wetland systems was studied, the pollutant removal mechanism were analyzed, so as to provide the basis for the treatment of high strength wastewater using constructed wetland process and the selection of wetland plants. The results are as follows:In sand culture conditions, the wetland was constructed using Calamus, Loosestrife, Reed, Typha and Spathiphyllum, respectively. The pollutant removal efficiency of different wetland systems was studied, with the influent concentration of total phosphorus (TP) of 7.27mg/L,8.47 mg/L,6.39 mg/L, ammonia nitrogen (NH4+-N) of 134.35 mg/L,167.19 mg/L,157.63 mg/L and chemical oxygen demand (COD) of 580.71 mg/L,610.98 mg/L,615.80 mg/L, and the hydraulic retention time of 1 day,3 days and 5 days, respectively. The results showed that:the presence of plants had a significant role in promoting the phosphorus (P) removal in constructed wetland, especially in the Calamus system. For the removal of NH4+-N, the Calamus systems showed a little higher removal rate than the control, while other systems showed little difference from the control, the removal rates in all planted systems were between 30.0% and 50.0%. For the removal of COD, when the hydraulic retention time increased form 1 day to 5 days, the removal rates of COD enhanced form 60.0% to 90.0%, and there was little difference among all systems. In this experiment, the optimal hydraulic retention time was 5 d.In sand culture conditions, by using Calamus, Loosestrife, Reed, Typha and Spathiphyllum, the heavy metal removal efficiency of different wetland systems was studied. The results showed that, when the influent Cd and Zn was 7.32mg/L,7.87mg/L, and the retention time was 3 days, in an operation period of 36 d, all systems showed good removal of Cd and Zn. The effluent Cd and Zn was not detected. The removal of Cd and Zn was mainly through the substrate adsorption and plant uptake, and the contribution of substrate adsorption in heavy metal removal was higher than that plant uptake. Reed and Typha were the best on the enrichment of Cd and Zn. Cd and Zn were mainly enriched in the plant's roots. The enrichment of Cd was better than Zn for all five plants.In water culture conditions, Arrowhead, Alisma orientalis, Monochoria morsakowii, Thalia dealbata and Cress were used for wetland systems, and the pollutant removal efficiency of different wetland systems was studied, the results showed that:when the hydraulic retention time was 3 d, and the influent NH4+-N, TP, COD was 37.3 mg/L,2.1 mg/L and 71.6 mg/L, respectively. The presence of plants can promote the removal of organic matter and nutrient. For P removal, the Arrowhead system was the highest, up to 79.01%, following by Alisma orientalis system 25.09%, Cress system 20.76%, Monochoria morsakowii system 17.02%, Thalia system7.04%, and the blank system was 22.14%. For N removal, the average removal rate of Arrowhead system was the highest, up to 63.67%, followed by Alisma orientalis system 51.08%, Thalia dealbata system 50.68%, Monochoria morsakowii system 47.84%, Cress system 41.78%, the blank system was 30.24%. For COD removal, the highest removal rate was still observed in Arrowhead system, which reached to 63.27%, while little difference in the remaining system. In water culture conditions, the optimal plant was arrowhead, followed by Alisma.The pollutant removal mechanism of constructed wetland was analyzed in sand culture conditions. The relationship between the pollutant removal and the substrate microbe was investigated. And the contribution of substrate adsorption and plant uptake in pollutant removal was calculated. The results showed that:under sand culture conditions, the removal of organic matter and nutrients were mainly through microbial degradation, substrate adsorption and plant uptake. For N, the good relationship was shown between the removal rate of NH4+-N and the abundance of substrate nitrite bacteria, the removal was mainly realized through microbial nitrification and denitrification, and the contribution of plant uptake and substrate adsorption was 10.0%-20.3% and 4.8%-7.1%, respectively. For P, plant uptake accounted for a percentage of 35.9%-57.9% in total P removal, while substrate adsorption accounted for 23.5%-30.2%, plants uptake made larger contribution to the total P removal, since the P content in the aboveground part was 75%-87% of the whole plant, the maintenance of P removal performance in wetland would be achieved by harvesting the aboveground part of the plant.