3.3. Nitrogen conversions and removals of the system
The variations of activated sludge and system effluent, TN removal efficiencies during the experimental period are shown in Fig. 1.
On the whole, the CW systems could slightly remove nitrogen pollutants from the nitrified wastewater, with overall removal efficiencies and loading rates of 46.6% and 940 mg/m2 d for TN. Effective nitrogen removals were achieved in an integrated vertical flow constructed wetland systems under inflow COD:N of 1.67, with mean removal efficiencies of 56.2% and 55.1% and mass removal rates of 762.1 mg/m2 d and 819.7 mg/m2 d for nitrate and TN, respectively (Chang et al., 2013).
At the beginning of operation AS reactor did not nitrify ammonium to nitrate, as possibly nitrification bacteria was not in enough population. After the 8th day of operation, NO3 appeared in the effluent of VSFCW reactor.
Treated wastewater from conventional activated sludge wastewater treatment plants with nitrification generally contains relatively high concentrations of nitrate but low levels of organic matter. Wetlands have recently been suggested as an alternative for treating nitrate contaminated aquifers, denitrification of nitrified sewage effluents and irrigation return flow. Denitrification efficiency in wetlands treating high-nitrate waters with low organic carbon has been shown to depend on C:N ratio, with peak efficiencies occurring at C:N ratios> 5:1 (Baker, 1998). By the last of operation C:N ratio reached to maximum levels due to higher C from decomposition of roots and NO3-N concentrations, which stimulate denitrification in CW particularly after the 15th day of operation (Fig. 2). Carbon source has been substantially proven to be a crucial limiting factor for nitrate removal especially when the COD:N was below 3:1 (Misiti et al., 2011).