Bio-electrochemical systems as a tool for the enhancement and monitoring overall heterotrophic bacterial activity in hydraulically saturated gravel-based wastewater treatment systems (constructed wetlands)
A cargo de: Jaume Puigagut (GEMMA - Group of Environmental Engineering and Microbiology, UPC)
Bioelectrochemical systems (Microbial fuel cells - MFCs) implemented in constructed wetlands (CW-MFCs), albeit a relatively new technology which is still under study, have shown to improve the treatment efficiency of domestic wastewater. So far the vast majority of CW-MFC systems investigated are designed as lab-scale systems working under rather unrealistic hydraulic conditions (up-flow, batch feeding) 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 domestic wastewater. The potential use of MFC electric signal as overall hetrotrophic bacterial activity indicator was also assessed. 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 (unpolarized – external circuit between anode and cathode is not connected) and closed- circuit CW-MFC (polarized – external circuit is connected).
For the purpose of this work, eight meso-scale horizontal subsurface flow CWs were operated for about four months. Each wetland consisted of a PVC reservoir of 0.193 m2 filled up with 4/8 mm granitic riverine gravel (wetted depth 25 cm). All wetlands had intermediate sampling points for gravel and interstitial liquid sampling. The CW- MFCs were designed as three MFC electrodes in series along the flow path of the wetland. The a anodes consisted of gravel with an incorporated current collector (stainless steel mesh) and the cathode consisted of a graphite felt layer. The eElectrodes of closed-circuit CW-MFC systems were connected externally over a 220 Ω resistance.
Results showed no significant differences between tested organic loading rates, hydraulic regimes or electrical connections regarding the treatment of domestic wastewater. However, on average, systems operated in closed-circuit CW-MFC mode under continuous flow outperformed the other experimental conditions, with open-circuit MFC and CW control being the worst performin experimental conditions. Closed-circuit CW-MFC compared to conventional CW control systems showed around 6% and 19% higher COD and ammonium removal, respectively. Correspondingly, overall bacteria activity, as measured by the fluorescein diacetate (FDA) technique, was higher (between 4% and to 30%) in closed- circuit systems when compared to CW control systems. Finally, in spite of hight results variability, MFC electric signal correlated well with overall heterotrophic bacterial activity (r2= 0.95), which evidences the potential use of MFC electric signal as a continuous indirect tool for bacterial activity monitoring.