science-2014-1452-van loosdrecht - anticipating the next century of wastewater treatment
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DOI: 10.1126/science.1255183, 1452 (2014);344 Science
Mark C. M. van Loosdrecht and Damir BrdjanovicAnticipating the next century of wastewater treatment
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INSIGHTS | PERSPECTIVES
1452 27 JUNE 2014 • VOL 344 ISSUE 6191 sciencemag.org SCIENCE
Anticipating the next century of wastewater treatmentAdvances in activated sludge sewage treatment can improve its energy use and resource recovery
WATER TREATMENTand the observed anisotropy could only be
modeled if that boundary extended to the
size of the crystallites.
Liu et al. therefore provide evidence
that disproportionation is not the domi-
nant mechanism during fast cycling of
nanosized LiFePO4. Instead, a metastable
nonstoichiometric phase exists through-
out the transition state accessed via the
applied overpotential. Hence, instead of a
local phase change, involving a progressive
structural reorganization near a moving
phase boundary, there exists a cooperative
structural rearrangement across a wide re-
gion of compositional variation as the struc-
ture changes continuously from one form to
the other with only a small degree of lattice
strain. The latter fact explains why it is fast
and reversible for thousands of cycles.
The above model seems reasonable; in-
stead of a phase boundary, there are con-
tinuous variations in both the lithium
concentration and the chemical potential
(energy) of lithium in a one-phase mate-
rial, as observed for normal electrodes. One
problem remains: The spinodal region near
the energy maximum still exists and can
cause havoc—the diffusion coefficient can
become negative, promoting rapid diffusion
up concentration gradients and thereby forc-
ing disproportionation. Maybe this effect is
too weak or too slow to make a difference.
However, there is another stabilizing influ-
ence, which is that the moving species is a
charged lithium ion; as a result, the electric
potential, which increases with the current
during discharge, adds another energy term
to the equation ( 4). If the current is high
enough, the miscibility gap, spinodal region,
and anomalous diffusion effect all vanish
and the behavior returns to normal as for a
non–phase-transforming electrode.
The value of the discovery by Liu et al.
lies not only in the already optimized
LiFePO4 but also in the prediction of how
it can be used to make better materials.
The hope is that the single-phase transfor-
mation pathway can be enabled in other
phase-transforming electrode materials
with high energy density, to reap the asso-
ciated benefits of higher power and longer
cycle life. ■
REFERENCES
1. A. K. Padhi, K. S. Nanjundaswamy, J. B. Goodenough, J. Electrochem. Soc. 144, 1188 (1997).
2. H. Liu et al., Science 344, 1252817 (2014); 10.1126/science.1252817.
3. R. Malik, F. Zhou, G. Ceder, Nat. Mater. 10, 587 (2011). 4. P. Bai, D. A. Cogswell, M. Z. Bazant, Nano Lett. 11, 4890
(2011). 5. N. Ravet et al., J. Power Sources 97-98, 503 (2001). 6. P. A. Johns, M. R. Roberts, Y. Wakizaka, J. H. Sanders,
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10.1126/science.1255819
Rapid urbanization and industrial-
ization in the 19th century led to
unhealthy environments and wide-
spread epidemic diseases. In re-
sponse, research was undertaken
that led to the development of sani-
tation technology. Exactly 100 years since
the activated sludge process was presented
( 1), it is still at the heart of current sewage
treatment technology. Activated sludge is a
mixture of inert solids from sewage com-
bined with a microbial population growing
on the biodegradable substrates present in
the sewage. The settling and recycling of
sludge inside treatment plants was the in-
vention of Ardern and Lockett. The current
demands from a rapidly growing human
population and the need for a more sus-
tainable society are pushing forward new
developments for sewage handling. These
developments have two main drivers: gen-
eral process improvements and the contri-
bution to the recycling of resources ( 2, 3).
The activated sludge process, combined
with a better drinking water supply, was
the main factor behind the increase in av-
erage life span in the previous century and
for minimizing the environmental impact of
human activities. Wastewater treatment is
in itself a relatively low-cost process (in the
Netherlands, 50 to 70 EUR per person per
year), consuming limited energy (<7 W per
person); its main limitations are the large
upfront investment costs (usually to be re-
covered from inhabitants within 20 years)
and land area requirements (mainly needed
for the gravity-based separation of flocculent
activated sludge and treated wastewater).
Attempts to intensify the separation process,
e.g., by membrane separation of the sludge,
have been technologically successful ( 4) but
not widely used because of the additional en-
ergy demand and capital costs.
The morphogenesis of the microbial com-
munities in activated sludge is a complex
process based on the interaction of micro-
biological, chemical, and physical processes
( 5). Only in recent years has it become pos-
sible to engineer these microbial structures
to allow bacteria to form a stable granular
sludge instead of flocculent sludge (see the
first figure) ( 6). This form of sludge makes
gravity-based separation a compact process
that can be integrated inside the treatment
reactor and greatly reduces area require-
ments and costs (by roughly 75 and 25%,
respectively) ( 7).
Activated sludge technology is based on
a complex microbial ecology process, in
100 years of activated sludge—quo vadis?
Two reactors at the wastewater treatment plant
Garmerwolde in the Netherlands (A) using aerobic
granular sludge technology (B) are treating the
wastewater of 235,000 persons.
By Mark C. M. van Loosdrecht 1 and
Damir Brdjanovic 1 ,2
A
Published by AAAS
SCIENCE sciencemag.org 27 JUNE 2014 • VOL 344 ISSUE 6191 1453
which sewage and microorganisms are re-
cycled to different tanks with different re-
dox conditions. Modern genomic tools ( 8)
are improving the understanding of inter-
actions among organic carbon-, nitrogen-,
and phosphate-converting bacteria and
have led to better process designs. More-
over, in granular sludge technology, these
activities have been integrated inside the
granules. With the different redox condi-
tions present inside the granule, the trans-
port of compounds occurs by diffusion,
replacing transport by the pumping of sew-
age and activated sludge between different
reactor compartments, thus minimizing en-
ergy needs.
Sewage treatment by activated sludge
is a technology that allows for closing cy-
cles and reuse of resources such as water
( 3), energy ( 9), and chemicals ( 2). With
an increasing global population demand-
ing more resources, this aspect is becom-
ing even more important. Effective sewage
treatment makes the recovery of water
by the use of membrane technology fea-
sible. Furthermore, water stress can be
minimized by the use of alternative water
sources, e.g., by using harvested rainwater
or seawater for toilet flushing ( 10). Energy
generation in the form of biogas produced
from the sludge has been practiced since
the early days of the activated sludge pro-
cess. In the 1970s, a high-rate, two-stage ac-
tivated sludge process was developed that
maximized energy recovery in the form of
biogas, but because of the need for organic
carbon, the interest in this process ceased.
With the recent advance of Anammox tech-
nology, a net energy-producing treatment
plant, including effective nutrient removal,
is becoming feasible ( 11).
Although the recuperation and produc-
tion of energy at sewage works is currently
getting most attention, the resource recov-
ery from wastewater and sludge should not
be overlooked. It is even more important
with respect to developing a more sustain-
able society. Phosphate recovery from sew-
age is increasingly being used, and other
options for the production of valuable ma-
terials from sludge are also emerging, e.g.,
the recovery of cellulose fibers ( 12) and the
production of bioplastics ( 2) and biopoly-
mers ( 13). The initial results show that valu-
able products can be produced in quantities
and at costs that match the current market
demand and prices (see the second figure).
In contrast to high-income industrial-
ized countries, where coverage by sewage
facilities is high and practically all waste-
water is treated at an advanced level (car-
bon, nitrogen, and phosphorus removal),
the sewerage coverage and sewage treat-
ment in developing countries and countries
in transition are overall very low (less than
10%). In these regions, centralized conven-
tional activated sludge (CAS) systems are
competing with decentralized technology.
Replicating CAS designs (mostly for sol-
ids and carbon removal) too often has not
taken into account the differences in local
conditions, such as sewage characteristics
and temperature. Also, the dumping of
large quantities of septic sludge in sew-
age works and lack of operator experience
with CAS systems has limited successful
applications.
The main constraint identified
as contributing to or causing the
numerous failures of CAS systems
in the developing world is poor
governance by the responsible
institutions ( 14). However, most
of these developing regions now
have an economic and technical
level well above those in Europe
and the United States a century
ago. Long-term investments in
sanitation are economically fa-
vorable because of improved
public health and thereby the in-
creased productivity of society. Cost recovery
structures and proper infrastructure asset
management (i.e., governance) are the main
prerequisites for the successful application
of activated sludge technology.
The solutions for these issues may be the
construction of smaller and simpler, decen-
tralized systems that are community-man-
aged, thus minimizing costs (e.g., anaerobic
treatment or aerobic granular sludge needs
much less mechanical equipment and fewer
imports), or enhancing resource recovery
(higher temperature-assisted biogas pro-
duction, nutrients, and water). An illustra-
tive example is Windhoek Goreangab in
Namibia ( 15, 16), where a 21,000 m3/day
water reclamation plant for the pioneering
production of potable water from treated
sewage (an activated sludge process fol-
lowed by maturation ponds and advanced
multibarrier treatment system) is used. Its
realization confirms that advanced treat-
ment technology combined with proper
governance can be successfully applied in
the developing world. ■
REFERENCES
1. E. Ardern, W. T. Lockett, J. Soc. Chem. Ind. 33, 523 (1914). 2. J. S. Guest et al., Environ. Sci. Technol. 43, 6126 (2009). 3. S. B. Grant et al., Science 337, 681 (2012). 4. S. Judd, Trends Biotechnol. 26, 109 (2008). 5. H. Daims, M. W. Taylor, M. Wagner, Trends Biotechnol. 24,
483 (2006). 6. M. K. de Kreuk, J. J. Heijnen, M. C. M. van Loosdrecht,
Biotechnol. Bioeng. 90, 761 (2005). 7. A. Giesen, R. Niermans, M. C. M. van Loosdrecht, Water 21,
28 (2012). 8. R. J. Siezen, M. Galardini, Microb. Biotechnol. 1, 3330
(2008). 9. G. Olsson, Water and Energy: Threats and Opportunities
(International Water Association, London, 2012). 10. M. C. M. van Loosdrecht, D. Brdjanovic, S. Chui, G. H. Chen,
Water 21, 17 (2012). 11. B. Kartal, J. G. Kuenen, M. C. M. van Loosdrecht, Science
328, 702 (2010). 12. C. J. Ruiken, G. Breuer, E. Klaversma, T. Santiago, M. C. M.
van Loosdrecht, Water Res. 47, 43 (2013). 13. Y. Lin, M. de Kreuk, M. C. M. van Loosdrecht, A. Adin, Water
Res. 44, 3355 (2010). 14. K. Vairavamoorthy, D, Brdjanovic, “Engineering infrastruc-
ture: Water supply and sanitation,” in UNESCO Report on Engineering: Issues, Challenges and Opportunities for Development, UNESCO, ISBN 978-92-3-104156-3 (2010).
15. I. B. Law, Water 30, 31 (2003). 16. P. L. du Pisani, Desalination 188, 79 (2006).
B CA
Struvite (A), polyhydroxyalkanonate bioplastic (B), and alginate biopolymers (C) are examples of recycled materials produced
by wastewater treatment.
B
10.1126/science.1255183
1Department of Biotechnology, Delft University of Technology, Julianalaan 67, 2628 BC Delft, Netherlands. 2UNESCO-IHE Institute for Water Education, Westvest 7, 2611 AX Delft, Netherlands. E-mail: [email protected]
Published by AAAS