2.2 Materials and methods
2.2.1 Experimental apparatus
A constructed wetland(CW)was installed.As shown in Figure 2.1,the length×width×height was 59cm×45cm×40cm,respectively.Quartz sand with a grain size of 1-2cm was filled in the CW evenly.The packed height was 32cm.The active volume was 40L and the void ratio was 0.47.The synthetic water was entered from the upside of the CW used a peristaltic pump and flowed out of the bottom of the CW.An inlet and an outlet(also the sample connection)were set at the both sides of the CW.The biofilm-electrode reactor(BER)consisted of plexiglass cylinders,DC power supply,electrode,peristaltic pump,and influent tank.The active volume was 15L,and the diameter and the height were 19cm and 66.5cm,respectively.A bottom retaining plate in the BER was used to fix the electrode and homogenize the influent water.One carbon rod(diameter 1cm,length 31cm)was put in the center of the BER as anode.The cathode(diameter 14.2cm,length 45.8cm,width 25.2cm)was made of foamed nickel and made into cylinder-shaped that encircled the anode.The cathode and anode were all connected with the adjustable DC power supply(APS3003Si)and the supply current was adjusted through the supply voltage adjustment.The synthetic water was entered from the bottom of the BER and flowed out of the outlet situated on the upside of the BER.One sample connection was set at the nether of the outlet.The CW and BER were all located indoor to control the environment temperature at 20-25℃.
Figure 2.1 The schematic diagram of constructed wetland(a)biofilm-electrode reactor(b)
2.2.2 Microorganisms,CW plants and synthetic water
Bacteria which formed the biofilm on the surface of the foamed nickel cathode was collected from the Songjiang wastewater treatment plant(Shanghai,China).The CW plant chosen the ripe and similar cannas.Cannas planted every other 16 cm in CW and there are 3 pants in total.The synthetic water was prepared by dissolving 0.2429 NaNO3 g/L,0.0764 NH4Cl g/L,C6H12O6·12H2O according to different C/Ns and other mineral elements and microelements,such as CuSO4·2H2O,H3BO3,MnSO4·7H2O,ZnSO4·7H2O,and NH4Mo7O24·4H2O required by microorganisms into tap water.The original pH of the synthetic water was normally about 7.5 and needed no further adjustment.
2.2.3 Experiment procedure
About 10L and 5L of active sludge were added into the synthetic water according to C/N=3,NO3-N=30mg/L,NH3-N=15mg/L and the peristaltic pumps were used to put the active sludge with nutritive material into the CW and BER,respectively.Electric current did not apply to the BER in cultivating stage.The water in the two reactors was changed every day by the synthetic water mentioned before.After 10 days,the electric current was applied to the BER and gradually increased from 5mA to 20mA with an interval of 5mA.Meanwhile,the C/N ratio gradually declined from 3 to 0 with an interval of 1.The CW and BER started to run when more than 80% of the nitrate was eliminated and the removal efficiency of the nitrate was stable.The foamed nickel cathode of BER turned into a dark grey and the pH of the CW and BER effluent was automatically at 7.0-8.0.The whole biofilm immobilization and domestication process took 40 days.
After biofilm immobilization and acclimation,the CW and BER were operated under different experimental conditions of C/N ratio,electric current intensity,HRT and pH using synthetic water as described in Section 2.2.The C/N ratio was tested for 0.50,0.75,1.00,2.00,and I was tested for 5mA,10mA,15mA,20mA.The HRT was selected as 8h,12h,16h,20h(only for BER,the HRT of CW maintained of 24h),and the pH was chosen as 6.50,7.00,7.50,8.00,respectively.The operational conditions for each run were maintained until steady state in the nitrate removal for 5 days.At every 24h interval,100mL sample were taken from the CW and BER effluent for nitrate,nitrite,ammonia nitrogen and total nitrogen(TN)concentration analysis,and each sample was taken for at least two successive HRTs.
2.2.4 Analytical methods
NH3-N and TN both in influent and effluent were measured by ultraviolet spectrophotometer(HACH DR890),and NO3-N was measured by ultraviolet spectrophotometer according to Chinese standard method(UV2000).NO2-N was measured using Gas-phase molecular absorption spectrometry(GMA3202).The pH was measured by pH meter(HACH HQ40d dual input multiparameter digital analyzer).