Effect Of External Carbon Sources On Nitrogen Removal In Constructed Wetlands Treating Tail Water From Wastewater Treatment Plant

Sewage treatment plant tail water is of large quantity, poor biodegradability and relatively high concentration of TN, which will result in eutrophication. It will be also waste of potential renewable water resources when the tail water is derectly discharged into surface water and city inland rivers. Therefore, further study on denitrification of tail water is of great significance. In this thesis, to deal with the problem of high concentration of TN of the tail water from ChangGe Wastewater Treatment Plant effluent, three sets of small scale horizontal subsurface flow constructed wetlands (HSFCWs) devices were set up on site. Field experiments were conducted to investigate the effect of external carbon and substrate on nitrogen removal efficiency. The operating parameters of HSFCWs were optimized according to the results of Response surface methodology (RSM). A polit scale process design of denitrifying treatment project dealing with the tail water was provided, which was based on the HSFCWs with corn-cob as external carbon sources. The main achievements accuqiured were as follows:Laboratory test results showed that water level and HRT had a significant impact on nitrogen removal in HSFCWs. With the influent of TN 18 mg/L, COD 60 mg/L, HRT 2.5 d and water level 10 cm below filter surface, the effluent concentrations of TN and COD was 8 mg/L and 50 mg/L, respectively. With the influent concentration of COD 70 mg/L, the TN concentration in effluent increased significantly when inflow load of TN exceeded 13 mg/L.To the HSFCWs with 1.5% of corn-cob as external carbon, gravel:volcanic rock:zeolite= 10:1:1 (mass ratio), water level 10 cm under filter surface and HRT 2.5 d, the result of field operation showed that the TN and COD concentration decreased from 25.37 to 9.23 mg/L and 55.43 to 31.34mg/L, respectively. To the HSFCWs with gravel as substrate, with and without 1.5% of corn-cob as external carbon, the effluent concentration of TN and COD were 11.07 and 36.93 mg/L,15.23 and 33.32 mg/L under the same experimental conditions.Optimized operating parameters derived from RSM were:middle water level (13 cm under filter surface), HRT 2.2 d. The effluent concentration of TN was 6.5 mg/L, which was 53.8% lower than that of the result before optimization. Meanwhile, the removal efficiency was increased 17.5%.A polit scale process design of denitrifying treatment project dealing with the tail water from ChangGe Wastewater Treatment Plant was provided, which was based on the HSFCWS with corn-cob as external carbon sources. The designed influent and effluent concentration of TN and COD were 30 to 3 mg/L and COD 60 to 20 mg/L. And the treatment scale, project investment and the cost of the treatment were 50 m3/d,249,600 Yand 0.32￥/t, respectively.