2.1. SAMFC construction and microorganism acclimation
Cube-shaped plexiglass with a volume of 84 mL was used as the setup of
SAMFCs. The anodes were graphite brushes with 2 cm in diameter and 2.5
cm in length. The air-cathodes were fabricated by rolling method as
previously described (Liu et al., 2014) using activated carbon as oxygen
reduction reaction catalyst. Nine SAMFCs were built and divided into
three groups with three in each group. The three groups of SAMFCs run at
conditions of open-circuit (OC), 1000Ω load and 20Ω load (denoted as
SAMFC-OC, SAMFC-1000Ω and SAMFC-20Ω), respectively. These SAMFCs were
inoculated from SAMFC that inoculated with domestic wastewater from
treatment plant (Qingshan, Nanchang) and run for more than one year. The
SAMFCs were firstly operated by feeding 50 mM phosphate buffer solution
(PBS) containing 1.64 g·L-1 sodium acetate (equal to
1280 mg·L-1 COD) for 30 days to acclimate
exoelectrogens onto the anodes, then run by feeding the 50 mM PBS
containing 1.64 g·L-1 and extra 1.21
g·L-1 sodium acetate (200 mg·L-1NO3--N) to acclimate denitrifers. The
detailed acclimation and characterization process of the SAMFCs is shown
in Figure S1.
All the SAMFCs were operated in fed-batch mode at a constant temperature
of 35±1°C. During the acclimation process, the anode potentials and cell
voltages were automatically recorded every 5 min using a data
acquisition system (digital multimeter, Keithley 2700, USA) connected to
a personal computer. Medium was refreshed when the anode potential rose
to over 0 V. All the electrode potentials were measured versus the
Ag/AgCl reference electrode (+0.198 V versus SHE, saturated KCl). The
concentrations of NO3--N in the SAMFCs
were measured at the 20th (t20),
40th (t40), 60th(t60), 80th (t80), and
100th (t100) day, respectively. The
microbial electrocatalytic activity and community analyses of the
microorganisms at the anodes were conducted at the initial
(t0), 20th (t20) and
100th (t100) day, respectively.
Microbial community analysis can damage the anode biofilm, therefore one
of the three SAMFCs in each group was used for
NO3--N concentration, CV
characterization and final microbial community analysis, while the other
two SAMFCs were only used for microbial community analysis.