Under the Microscope
From wastewater treatment to biorefining
of the total electricity consumption in the US 1. At least as much
energy again is invested in handling the sludge generated during
the treatment.
Korneel Rabaey
Advanced Water Management
Centre (AWMC)
Gehrmann Building, 4th floor
The University of Queensland,
Brisbane QLD 4072
Tel (07) 3365 7519
Fax (07) 3365 4726
Email [email protected]
To address the energy issue, anaerobic digestion has emerged
in the past 2 decades. Anaerobic digestion creates methane gas
from waste streams that are preferentially quite concentrated 2. It
is a highly successful process, which is increasingly implemented
worldwide. Its drawbacks are the limited effluent quality,
often limited energy recovery and high cost for peripheral
infrastructure.
Wastewater, whether it is domestic or industrial,
represents a great opportunity to recover water, energy
or chemicals, and nutrients. Today, wastewater treatment
is energy-consuming, and does not recover the resources
from the wastewater. Bioelectrochemical systems (BESs),
which have recently been developed, allow for adequate
harvesting of the energy or for the production of high
quality chemicals. In this article, the basic principles
and opportunities of BESs in the context of wastewater
treatment are explained.
In the slipstream of research on extracellular electron transfer
(see further), microbial fuel cells (MFCs) emerged. MFCs use
whole microorganisms as catalysts to oxidise organics (as present
in wastewater) and transfer the electrons to an electrode,
the anode. From the anode, the electrons travel to a second
electrode, the cathode, where generally oxygen is reduced 3.
The current flowing from anode to cathode allows harvesting
electrical power. The domain of MFCs has exponentially grown
in the past few years, with power production going from a few
milliwatts to effective watts 4. In 2007, the first pilot scale MFC was
constructed at Foster’s brewery, in Australia, by a team from The
University of Queensland.
Take a closer look at what domestic wastewater actually is, and
you come to the conclusion that it is 99.5% pure water, and 0.5%
organics and nutrients. This 0.5% contains over 2kWh of energy
per cubic metre of wastewater. Thus, wastewater could be an
excellent source for water, energy and nutrients. Now take a
closer look at present-day wastewater treatment – it is an energyintensive activity. Wastewater aeration alone consumes about 1%
The whole process relies on the unique capability of
microorganisms to transfer electrons from their internal
metabolism towards an insoluble electron acceptor outside
the cell, the electrode. This mechanism is called extracellular
electron transfer. According to our present knowledge, bacteria
can achieve this by either producing soluble redox shuttles 5, or
directly transferring electrons via membrane bound complexes
Figure 1. Image of S. oneidensis MR-1 grown in electron
acceptor limited conditions. Possible nanowires are seen
extending from the bacterium 11.
Figure 2. The BESs. Microorganisms can catalyse oxidation
reactions of organics and potentially sulfide at an anodic
electrode. At the cathodic electrode, microorganisms can
catalyse reduction reactions such as denitrification, perchlorate
removal, oxygen reduction 11.
M I CROB I O L O G Y A U S T RALIA • MAY 2 0 0 9 87
Under the Microscope
or (potentially) via electrically conductive ‘nanowires’ (Figure
1). Key organisms known for this mechanism are Geobacter
sulfurreducens 6, Shewanella oneidensis 7 and Pseudomonas
aeruginosa 8.
One of the key bottlenecks for MFCs was the need for chemical
catalysts (often platinum) at the cathode. Quite recently it was
found that also the cathodic catalysis could be performed by
microorganisms. Microorganisms can receive electrons from an
electrode and transfer them not only to oxygen but also to other
electron acceptors, leading to nitrate 9 or perchlorate 10 removal,
CO2 reduction to methane and so on. In the latter cases, the
focus of the technology moved away from energy generation.
To account for this broadening of the scope from energy
generation to bioremediation and even bioproduction, the
technology was relabelled to BESs 11, in this context with whole
cell biocatalysts (Figure 2). This evolution also considerably
changes the value of the technology. Electricity generation is, due
to the low energy prices worldwide, still a marginally profitable
activity from wastewater. However, the generation of bioproducts
can deliver more than an order of magnitude in value, which will
be key to introduce the technology to the market.
BESs that treat wastewater and use the enclosed energy for
bioproduction can become a cornerstone of future biorefineries.
A biorefinery is a facility that integrates biomass conversion
processes and equipment to produce fuels, power and chemicals
from biomass. Together with other technology, BESs will be able
to produce chemicals/fuels from wastewater, using the energy
enclosed in the waste itself. Recovery of the nutrients (N and
P) would furthermore allow harvesting the third major product,
water itself.
Acknowledgement
Korneel Rabaey is supported through the Australian Research
Council (ARC DP0879245). For more information, please visit
www.microbialfuelcell.org
References
1.
Burton, F.L. (1996) Water and Wastewater Industries: Characteristics and
Energy Management Opportunities. Electric Power Research Institute Inc.
2.
Angenent, L.T. et al. (2004) Production of bioenergy and biochemicals from
industrial and agricultural wastewater. Trends Biotechnol. 22, 477-485.
3.
Rabaey, K., & Verstraete, W. (2005) Microbial fuel cells: novel biotechnology for
energy generation. Trends Biotechnol. 23, 291-298.
4.
Logan, B. et al. (2006) Microbial fuel cells: methodology and technology.
Environ. Sci. Technol. 40, 5181-5192.
5.
Newman, D.K., & Kolter, R. (2000) A role for excreted quinones in extracellular
electron transfer. Nature 405, 94-97.
6.
Bond, D.R., & Lovley, D.R. (2003) Electricity production by Geobacter
sulfurreducens attached to electrodes. Appl. Environ. Microbiol. 69, 15481555.
7.
Kim, B.H. et al. (1999) Electrochemical activity of an Fe(III)-reducing bacterium,
Shewanella putrefaciens IR-1, in the presence of alternative electron acceptors.
Biotechnol. Techniques 13, 475-478.
8.
Rabaey, K. et al. (2005) Microbial phenazine production enhances electron
transfer in biofuel cells. Environ. Sci. Technol. 39, 3401-3408.
9.
Clauwaert, P. et al. (2007) Biological denitrification driven by microbial fuel
cells. Environ. Sci. Technol. 41, 3354-3360.
10. Thrash, J.C. et al. (2007) Electrochemical stimulation of microbial perchlorate
reduction. Environ. Sci. Technol. 41, 1740-1746.
88
11. Rabaey, K. et al. (2007) Microbial ecology meets electrochemistry: electricitydriven and driving communities. ISME J. 1, 9-18.
Dr Korneel Rabaey is a senior research fellow in the Advanced Water
Management Centre at the University of Queensland QLD.
Proposal to develop a resource for teachers
The ASM, which became an incorporated professional society in
1976, is a not-for-profit organisation, formed in 1959 as a learned
society devoted to furthering the science of microbiology. In
this, our 50th anniversary year, we can all take pride in the fact
that much has been achieved with this purpose in mind.
The society has recognised that one of the most important
and effective ways of furthering our science is to reach out to
students, hence the prominence given to the Becton Dickinson
awards at our annual scientific meeting and the support and
rewards given to teachers by the David White teaching award
and the Teachers’ travel award.
Another strategy is to develop programs for teachers in
secondary schools to give them the confidence to teach
some microbiology and awaken their students’ interest in our
discipline. Many biology and general science curricula already
contain some basic microbiology but often teachers are not
confident about how to teach it. With this in mind, EDSIG ran
a workshop for teachers in the ACT a few years ago and on the
Gold Coast the following year. Elwyn Oldfield from UQ has
continued to run the workshop she designed for the Gold Coast
over the last 3 years and has found a strong supporter in Mary
Rowland, the past President of the science teachers association
of Queensland (STAQ). An attempt to run a workshop in South
Australia in 2007 was not met with enthusiasm by the local
teachers’ association and did not proceed. The absence of a
person able to drive the process meant that it did not happen
in Melbourne last year either. So the success of the venture
would appear to be largely determined by the partnership of a
committed microbiologist and a science teacher contact and a
program that teachers find useful and manageable.
Currently enrolments in science subjects are failing at secondary
schools and tertiary institutes alike. This is a major concern
when so many problems facing our continued existence on
this planet need at least an understanding of basic science
to underpin their solutions. Microbiology is a particularly
accessible and relevant science and ideal for exploring in a
simple, practical way.
I am sure many of you have been involved in talking to or
showing school students what microbiology is all about, what
microbiologists do, and how fascinating microorganisms are.
This is great, for a limited number of students. What we need
is a way to reach more students on a regular basis with a
consistent approach.
The proposal is to develop a resource in the form of a workshop
protocol or handbook. As Elwyn has shown, a ‘hands-on’
workshop, based on demonstrating and working through a
protocol, is the best way to support and train teachers who
want to show students some basic microbiology. The other
essential components are a local microbiologist, preferably with
access to a teaching laboratory, and a member of a teachers’
association with access to registered teachers. Put them all
together and we could have many more students aware of, and
interested in, our discipline. Do you agree?
Cheryl Power, Co-convener EDSIG
6/3/09
MICROBIOLOG Y A U STRA LIA • M AY 2009