PERFORMANCE OF CONSTRUCTED WETLAND FOR LOW CONCENTRATION EFFLUENT PURIFICATION FROM SEWAGE TREATMENT PLANT

Constructed wetlands (CWs) have many advantages including high impact resistance, outstanding ecological value and low cost. Consequently, it is gradually becoming more widely used technique for wastewater treatment plant (WWTP) upgrading via effluent polishing. Hydraulic load, water depth, and hydraulic residence are three important factors that influence the operational stability of CWs. When the hydraulic load increased suitably, the area of the system can be effectively reduced. Therefore, given the complexity of construction and land deprivation, the application of high-load CWs needs further research and exploration. In addition, most of the studies on CWs are based on small pilot tests. Analysis and investigation of the purification performance and operation mechanisms of full-scale systems are lacking. Furthermore, the lack of carbon in the WWTP effluents leads to poor denitrification in CWs, whereas the accumulation of biofilm in large wetland systems over a long period can easily cause the system clogging. Thus, the use of CWs on a large scale in practical engineering is limited. In response to these problems, a CW system was designed with main feature of a bi-directional cross-flow filtration unit, while a WWTP effluent was used as feed water. A study of the purification performance of the CW system for low concentration of the WWTP effluent showed a highly efficient performance. Analysis of the bi-directional cross-flow filtration unit planted with different plants (Canna indica, Thalia dealbata and Arundo donax var versicolor) showed that: (1) All three different types of planted wetland units had great water quality enhancement when feeding with WWTP effluent. Nevertheless, the different types of aquatic plants showed significant variability in the purification performance for COD, TN and NH₄⁺-N. The average removal efficiencies of COD, TN, \rm NH_4^+ -N and TP in the planted unit with Arundo donax var versicolor were the highest, at 20.11%, 17.17%, 28.08% and 18.12%, respectively. (2) The bi-directional cross-flow filtration wetland allowed for the effective retention of carbon in the substrate, which was utilized to enhance denitrification by switching the inlet and outlet water flow directions. Consequently, the TN removal rate was increased by approximately 50%. (3) From the phylum classification level, the main microbial communities of the wetland system did not show significant changes between the forward influent-feeding stage and the reverse influent-feeding stage. Nonetheless, the wetland system had a high proportion of Proteobacteria and Bacteroidetes (around 65.00%) at the influent end of the system. This phenomenon also explained the rapid transition of the wetland system after switching the intake direction, restoring its efficacy in treating pollutants. The findings can provide theoretical guidance and technical reference for the operation of large-scale CWs.