2.2 Measurements and calculations
The concentrations of NO3--N in the SAMFCs were measured regularly with the time interval of 20 days. For example, the first and second denitrification tests were performed at t20 and t40 respectively. During the whole period for denitrifers biofilm acclimation, the concentrations of sodium nitrate keep at 1.21 g L-1, which is corresponding to the NO3--N concentration of 200 mg L-1. In order to obtain optimal C/N ratio, three different concentrations of sodium acetate of 0.82 g L-1 (640 mg L-1 COD), 1.64 g L-1 (1280 mg L-1 COD) and 2.05 g L-1 (1600 mg L-1 COD) were used to control the C/N weight ratio of 1.2, 2.4 and 3, respectively. The denitrification tests under different C/N ratios were performed at t20. Three different SSAMFCs were sampled at 1h, 2h, 3h, 4h and 5h and immediately filtered through disposable Millipore filter with the pore size of 0.22μm for the analyses of nitrate, nitrite at the same condition simultaneously. NO3--N concentration measurements were conducted by applying a spectrophotometer (SpectroDirect, Lovibond, Gremany) using standard methods.
Denitrification rate (rDN) was fitting by pseudo-first order reaction with respect to concentration and calculated as follows according to Jia et al(Jiang et al., 2018),\(\text{Ln}\left(\frac{C_{t}}{C_{0}}\right)=-\text{Kt\ }\), where C0 is the initial concentration of nitrogen in the form of nitrate (NO3--N) (mg/L), t is the time, Ct is the NO3--N econcentration(mg/L) at t h. K is the observed reaction rate constant.