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Bioelectrochemical systems for energy recovery from wastewater

Cheng, Ka Yu (2009) Bioelectrochemical systems for energy recovery from wastewater. PhD thesis, Murdoch University.

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The global concerns of climate change and energy crisis have provoked research efforts to develop energy-efficient alternatives to conventional activated sludge wastewater treatment processes. Recently, bio-electrochemical systems (BES) such as microbial fuel cells (MFCs) and microbial electrolysis cells (MECs) have emerged as a promising technology for simultaneous energy recovery and wastewater treatment. These systems harness the capacity of microorganisms for the catalysis of electrochemical reaction. In MFCs, chemical energy in the form of organic compounds in wastewater is directly converted into electricity. While in MECs, external electricity is provided to enable more valuable products (e.g. hydrogen) to form at the cathode.

Over the past decade, our knowledge on BES is gaining momentum. However, wastewater treatment using BES has not been successful on an industrial-scale. Many technological bottlenecks still remain unresolved and our understanding of microbe-electrode interactions in BES is incomplete. The overall aim of this thesis is to generate understanding that will be helpful in the development of an energy recovering wastewater treatment process using BESs. The scope of this thesis comprises two themes. The first theme is to study the fundamental aspect of BESs with an emphasis on microbe-anode interactions, the reaction that makes it possible to use organic waste substances as the electron donor for BES. The second theme is to quantify rate limiting steps and to develop practical solutions to overcome the established bottlenecks making room for the development of new technologies of BES for wastewater treatment and related purposes.

Most of the experiments were conducted using a sub-liter scale two-chamber BES equipped with a cation exchange membrane. An electrochemically active biofilm was established at the anode (graphite granules) from an activated sludge inoculum. A synthetic wastewater with acetate as the sole electron donor was used throughout the study. The cathodic electron acceptor was either potassium ferricyanide (K3Fe(CN)6) or dissolved oxygen. No chemical catalyst (e.g. platinum) was applied to the electrodes. In some experiments, the BES was coupled with a potentiostat to precisely control the biofilm-electrode potential or to perform voltammetric analysis.

The results showed that activated sludge bacteria could readily initiate a highly effective anodophilic biofilm (415 W•m-3 after five days with a Fe(CN)63+ cathode), provided that factors such as electrolyte pH, external resistance and cathodic oxidizing power were not limiting. From the Coulombic efficiency of over 80% the microbial activity could be recorded by online monitoring of the current. This allowed a detailed study of the affinity for the anode of biofilm. In analogy to the well known Michaelis-Menten kinetics, a half-saturation anodic potential (kAP) was established at which the microbial metabolic rate reached half its maximum rate. This kAP value was about -455 mV (vs. Ag/AgCl) for our acetate-driven biofilm. A critical anodic potential (APcrit.) of about -420 mV (vs. Ag/AgCl) was defined that characterizes both the bacterial saturation by the electron-accepting system and the maximal MFC power output. This information is useful for MFC modeling and optimization.

Although online process control is used for many bioprocesses it is not established for MFC. A new approach was developed that enabled voltammetric studies of the biofilm-anode, without using potentiostats but by feedback controlling a variable external resistance of the running MFC. This approach could perform the conventional cyclic voltammetry of a MFC without interrupting its operation.

In MFC the anodic reaction is proton liberating while the cathodic reaction is proton consuming. This leads to perhaps the major limitation in MFC operation known as a pH gradient. This limitation was addressed by using a novel operational regime: the intermittent polarity inversion. At electrode potential of -300 mV (vs. Ag/AgCl) the alternating supply of acetate and oxygen to the biofilm resulted in the generation of an anodic and cathodic current of +240 and -80 mA (+1500 and -500 A•m-3), respectively. Since the anodic reaction is proton-liberating while the cathodic reaction is proton-consuming, such operational regime prevented the detrimental build-up of a pH gradient enabling a prolonged operation of the MFC without using costly pH control methods (dosing of acid/base or chemical buffers).

The intermittent polarity inversion showed signs of the ― “anodophilic bacteria” being able to catalyze not only the anodic oxidation of acetate but also the cathodic oxygen reduction. The presence of ― “anodophilic bacteria” at the cathode could enable a 5-fold increase of power output (from 5.6 to 27 W•m-3). This is the first evidence that a BES biofilm can catalyze both the forward and backward electron flow with a single electrode. Based on this finding, a novel scalable, membrane-less BES configuration, termed rotatable bio-electrochemical contactor (RBEC) has been developed.

Similar to rotating biological contactors (RBC), the RBEC consists of a cylindrical water-holding vessel (ca. 3 L) which houses an array of carbon cloth coated discs (electrodes) mounted onto a central horizontal rotatable shaft. Each disc consists of a water-immersed anodic and an air-exposed cathodic half connected via a resistor. No ion-exchange membrane and wastewater recirculation were required as the air-water interface separated anode from cathode. A polarity inversion aiming at overcoming the pH gradient and enable the biofilm to catalyze both the anodic and cathodic reaction could be obtained by merely turning the disc array a half turn.

An electron flow from the submersed half disc to the air exposed half disc established with the moisture film on the air exposed cathode allowing the ionic charge transfer. As with other MFC any measured current could be documented to be linked to COD oxidation. The COD removal caused by the action of the intermittently turning discs was increased by about 30% by merely allowing an electron flow between the anodic and cathodic disc halves. This result suggests that the treatment performance of traditional RBC may be significantly increased by using suitable conductive material as the discs.

By raising the cathodic potential from about -500 to -1200 mV (vs. Ag/AgCl) using a potentiostat the cathodic limitation could be alleviated allowing an increase in electron flow and COD removal rate to 1.32 kg COD m-3 day-1 (hydraulic retention time 5h). While the COD removal rate was comparable to that of an activated sludge system, the potentiostatically supported RBEC removed COD more energy-efficiently than activated sludge systems (0.47 vs. 0.7-2.0 kWh kgCOD-1), even though it was not optimized. The RBEC could also enable electrochemically-driven hydrogen gas or methane gas production when operated as a MEC under fully anaerobic condition.

Overall, this thesis has extended our understanding on how electrochemically active microorganisms behave in BES. Especially, the discovery of the bidirectional microbial electron transfer property may shed light not only on BES development, but also on the context of fundamental microbiology. The new RBEC configuration may widen the functionality or suitability of BES for a large-scale wastewater treatment application. Nevertheless, further process optimization is needed. In particular, the cathodic reaction still remains as the key process bottleneck. Future efforts should thus be oriented towards improving and elucidating in details the mechanisms of the microbe-cathode electron transfers.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Environmental Science
Supervisor(s): Ho, Goen and Cord-Ruwisch, Ralf
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