Study on the treatment of domestic sewage by stable surface flow - subsurface flow combination constructed wetland

The stable surface flow - subsurface flow combination constructed wetland system was designed and constructed, which was composed of collecting tank, anaerobic pool, stable surface flow constructed wetland(FSSFW) and horizontal subsurface flow constructed wetland(SFCW). The whole system was built based on local topography, and the sewage flowed from the former unit to the next unit only by gravity. After a month of domestication, the purification effect in pilot-scale of domestic sewage was studied. The removal effects of SS, COD, NH₃-N, TN and TP in each unit and the system of the stable surface flow - subsurface flow constructed wetland were monitored and analyzed when the design of hydraulic load and high hydraulic load were 0.108 and 0.180 m³/(m²·d), respectively. The results showed that the average removal rate of SS, COD, NH₃-N, TN and TP was 91.6%, 81.2%, 87.7%,77.3% and 86.3% respectively under the design hydraulic load. Moreover, the average removal rate of SS and COD by FSSFW was higher than that by SFCW and anaerobic pool, but the average removal rate of NH₃-N, TN and TP by SFCW was higher than that by anaerobic pool and FSSFW. In addition, the anaerobic pool had the lowest contribution to the removal rate of each pollution index. Under the high hydraulic load, the removal rates of SS, COD, NH₃-N, TN and TP was 91.2%, 73.1% ,84.2% ,69.0% and 82.7%, respectively. SFCW played a major role in the removal of SS, COD, NH₃-N and TN while FSSFW and SFCW had no significant difference in the removal contribution rate of TP, however, the anaerobic pool had the lowest contribution to the removal rate of each pollution index. The operating hydraulic load of the system could be set between 0.108 and 0.180 m³/(m²·d) in summer, while the operating hydraulic load should be set strictly according to the design in winter. During the pilot test of design hydraulic load rate, all the effluent indicators of the system could meet the Grade Ⅰ level B standard of Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant(GB 18918-2002).