Treatment of small town mixed sewage by the combined process of biological methods and constructed wetlands

Key words: 
• biological methods /
• combined constructed wetlands /
• mixed sewage /
• removal rate constant

Abstract: In order to meet the requirements of high efficiency, low energy consumption and easy management for small town sewage treatment, a field study on processes of biological methods(anaerobic/biological contact oxidation) combined with constructed wetlands (A/BCO-CW) to treat mixed sewage from town areas was conducted. Present results showed that the A/BCO system was highly effecttive in removing suspended solids and organic matter under the condition of HRT 3～6 h. Removal rates of COD, BOD5 and TSS reached 63.1%, 67.7% and 66.2%, respectively, which helped avoid wetland jams and improve treatment efficiency, reducing land occupation by 44%. Results from 12 months running showed that average removal of COD, BOD5 and TSS loading reached 9.4、5.1 and 7.2 g/(m2·d) or were cut by 42%、71.2% and 85.2%, respectively, in the combined constructed wetland system of VF-SF-SSF at hydraulic loading rates of 0.3～0.59 m/d. The removal rate constants for COD and BOD5 were 74.6±12.1 m/y and 166±30.5 m/y， respectively, all ranging high among the k-values recorded in the literature. The A/BCO-CW system had high removal effects on COD, BOD5 and TSS. The effluents could stably meet the type Ⅳ standard specified in the Environmental Quality Standards for Surface Water (GB 3838-2002). The two methods could be combined to develop low-cost, efficient and sustainable systems for treating town sewage, which will be highly effective in practical applications, especially in treatment of high organic load sewage.