  Microbial fuel cells implemented in constructed wetlands (CW-MFCs), albeit a
relatively new technology still under study, have shown to improve treatment
efficiency of urban wastewater. So far the vast majority of CW-MFC systems
investigated were designed as lab-scale systems working under rather
unrealistic hydraulic conditions using synthetic wastewater. The main objective
of this work was to quantify CW-MFCs performance operated under different
conditions in a more realistic setup using meso-scale systems with horizontal
flow fed with real urban wastewater. Operational conditions tested were organic
loading rate (4.9+-1.6, 6.7+-1.4 and 13.6+-3.2 g COD/m2.day) and hydraulic
regime (continuous vs intermittent feeding) as well as different electrical
connections: CW control (conventional CW without electrodes), open-circuit
CW-MFC (external circuit between anode and cathode not connected) and
closed-circuit CW-MFC (external circuit connected). Eight horizontal subsurface
flow CWs were operated for about four months. Each wetland consisted of a PVC
reservoir of 0.193 m2 filled with 4/8 mm granitic riverine gravel. All wetlands
had intermediate sampling points for gravel and interstitial liquid sampling.
The CW-MFCs were designed as three MFCs incorporated one after the other along
the flow path of the CWs. Results showed no significant differences between
tested organic loading rates, hydraulic regimes or electrical connections,
however, on average, systems operated in closed-circuit CW-MFC mode under
continuous flow outperformed the other experimental conditions. Closed-circuit
CW-MFC compared to conventional CW control systems showed around 5% and 22%
higher COD and ammonium removal, respectively. Correspondingly, overall
bacteria activity, as measured by the fluorescein diacetate technique, was
higher (4% to 34%) in closed-circuit systems when compared to CW control
systems.
