Tertiary Treatment Of Domestic Sewage With A Constructed Wetland In North China

An improved 3-stage vertical flow constructed wetland was used for tertiary treatment of effluent from demestic sewage treatment plants in Dalian(one of the cities in north China), where the wastewater was treated through a secondary biological treatment process.During Nov.2006 to December 2007,the effect of seasons and operation mode on purification of low concentration municipal sewage by constructed wetland was investigated.In order to study the feasibility of constructed wetland as secondary treatment unit of domestic sewage,the treatment effect of high pollution load municipal sewage treated by constructed wetland was investigated during Jan.2008 and Feb.2008,with HRT of 8h and sequencial batch operational mode.Results showed that under different climate conditions(summer,winter and prespring), the removal efficiencies of all pollutants are affected more by influent pollution load than seasons.In different seasons,the influent loads of COD and TP are stable,and the removal rate are over 80%and 95%,respectively,which meet the needs of Environmental Quality Standard for Surface Water(GB 3838-2002)â… ,â…¢(15 mg·L^-1 and 0.2 mg·L^-1, respectively).Due to the high influent loads of NH3-N and TN in winter time,the effluent quality can only meet the needs of Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant(GB 18918-2002) class I B.In summer,while running in a sequencing batch mode,with different retention time(8h and 16h),TP disolves from fillers,which results in the decreaseing of TP removal rate when retention time prolongs.However,the removal efficiency of other pollutents increases slightly with longer retention time.Considering the performance difference,shorter retention of 8h is suggested as reasonable,while effluent COD,TP and NH₃-N meet the needs of Environmental Quality Standard for Surface Water(GB 3838-2002)â… ,â…¢andâ…¤.In winter,when running in a sequencing batch mode with 8h of retention time,effluents from primary sedimentation tank as well as secondary biological treatment pond were investigated.Results showed that the effluent quality is better than the needs of Discharge Standard of Municipal Wastewater Treatment Plant(GB 18918-2002 ) class I A,while treating secondary effluent.However,the effluent quality only meets the needs of Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant(GB 18918-2002) class I B,while treating the primary sedimentation tank effluent, Multi-stage vertical flow constructed wetland performs well for removing COD,TP,TN and NH₃-N from secondary effluent from ChunLiu sewage plant in Dalian.As a tertiary treatment technique,constructed wetland shows a great potential for preventing water eutrophication,reuse of wastewater,ecological restoration,landscape construction and water resource protection.However,more investigations have to be done with constrtucted wetland as secondary treatment process.