Stacked multi-electrode design of microbial electrolysis cells for rapid and low-sludge treatment of municipal wastewater

Hui Guo,Younggy Kim

doi:10.1186/s13068-019-1368-0

Abstract

BackgroundMicrobial electrolysis cells (MECs) can be used for energy recovery and sludge reduction in wastewater treatment. Electric current density, which represents the rate of wastewater treatment and H2 production, is not sufficiently high for practical applications of MECs with real wastewater. Here, a sandwiched electrode-stack design was proposed and examined in a continuous-flow MEC system for more than 100 days to demonstrate enhanced electric current generation with a large number of electrode pairs.ResultsThe current density was boosted up to 190 A/m3 or 1.4 A/m2 with 10 electrode pairs stacked in an MEC fed with primary clarifier effluent from a municipal wastewater treatment plant. High organic loading rate (OLR) resulted in high electric current density. The current density increased from 40 to 190 A/m3 when the OLR increased from 0.5–2 kg-COD/m3/day to 8–16 kg-COD/m3/day. In continuous-flow operation with two stacked MECs in series, the biochemical oxygen demand (BOD) removal was 90 ± 2% and the chemical oxygen demand (COD) removal was 75 ± 9%. In addition, the sludge production was 0.06 g-volatile suspended solids (VSS)/g-COD removed at a hydraulic retention time of only 0.63 h. The electric energy consumption was low at 0.40 kWh/kg-COD removed (0.058 kWh/m3-wastewater treated).ConclusionsThe MECs with the stacked electrode design successfully enhanced the electric current generation. The high OLR is important to maintain the high electric current. The organics were removed rapidly and the total suspended solids (TSS) and VSS were reduced substantially in the continuous-flow MEC system. Therefore, the MECs with the stacked electrode design can be used for the rapid and low-sludge treatment of domestic wastewater.

Highlights

Microbial electrolysis cells (MECs) can be used for energy recovery and sludge reduction in wastewater treatment
The maximum current density in MEC with 1 electrode pair (MEC-1) (MEC of 1 electrode pair) was only ~ 4 A/m3 or 0.2 A/m2, which was much smaller than that in MEC with 10 electrode pairs (MEC-10). This result indicates that a large number of electrode pairs in the sandwiched design can significantly enhance the rate of wastewater treatment using MECs without any precious metal catalysts on the cathode, such as platinum
Note that the primary clarifier effluent was fed to MEC10 and the effluent from MEC-10 was introduced to MEC with 5 electrode pairs (MEC-5) during the continuous MEC operation

Summary

Introduction

Microbial electrolysis cells (MECs) can be used for energy recovery and sludge reduction in wastewater treatment. Electric current density, which represents the rate of wastewater treatment and H 2 production, is not sufficiently high for practical applications of MECs with real wastewater. In conventional wastewater treatment using activated sludge, organic substrates are oxidized in bioreactors by aerobic microorganisms [1]. In an MEC, organic substrates are removed at the bioanode through an oxidation reaction driven by exoelectrogenic bacteria, while hydrogen gas is produced at the cathode by applying a small electric voltage (> 0.13 V) [8,9,10,11]. The performance of MECs in terms of removing organics in wastewater and producing hydrogen gas is directly dependent on the magnitude of electric current. The limited electric current generation, especially with municipal wastewater, indicates that breakthrough improvements are necessary to magnify electric current generation and such improvements should be scalable for pilot-scale and continuousflow operation for practical wastewater treatment and energy recovery using MECs

Methods

Results

Conclusion