The influence of operational and design parameters on vertical redox profiles in sub-surface flow constructed wetlands: surveying the optimal scenario for microbial fuel cell implementation

Abstract

The objective of the present work was to determine the optimal redox gradient that can be obtained in sub-surface flow constructed wetlands (SSF CWs) to maximize the energy production with microbial fuel cells (MFCs). To this aim, a pilot plant based on SSF CW was evaluated for vertical redox profiles. Key operational and design parameters surveyed that influences redox conditions in SSF CW were the presence of plants (Phragmites australis) and the flow regime (continuous and discontinuous flow regime). Redox potential (EH) within the wetlands was analysed at a 5, 15 and 25 cm depth. Results obtained indicated that EH daily variation within the wetlands was small for depths of 5 cm and 25 cm, regardless operational or design conditions. On the contrary, the redox variation was of notable extent at 15 cm depth and ranging from +50 mV to – 300 mV, and especially pronounced for discontinuous flow planted wetlands. Overall, discontinuous flow and planted wetlands showed a higher redox potential at the bottom of the wetlands (between 50 and 100 mV higher) than those unplanted operated at a continuous flow. Furthermore, the maximum attainable redox gradient under the conditions here considered (redox difference between the surface of the wetland and the bottom or the middle part of the bed) is between 350 and 450 mV, being the continuous flow planted wetlands the configuration that offers the best balance between the maximum redox gradient and its stability along the day. Finally, the redox gradients obtained for planted wetlands operated under continuous flow regime would lead to a power production ranging of some 40 mW/m2 of wetland.