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,

J. R. Owen, Electrochem. Commun. 11, 2089 (2009). 7. J. W. Cahn, J. E. Hilliard, J. Chem. Phys. 28, 258 (1958).

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: m.c.m.vanLoosdrecht@tudelft.nl

Published by AAAS