climate

REVIEWS

Science and technology for water

purification in the coming decades

Mark A. Shannon 1,4, Paul W. Bohn 1,2, Menachem Elimelech 1,3, John G. Georgiadis 1,4, Benito J. Marin ˜as 1,5

& Anne M. Mayes 1,6

One of the most pervasive problems afflicting people throughout the world is inadequate access to clean water and

sanitation. Problems with water are expected to grow worse in the coming decades, with water scarcity occurring globally,

even in regions currently considered water-rich. Addressing these problems calls out for a tremendous amount of

research to be conducted to identify robust new methods of purifying water at lower cost and with less energy, while at the

same time minimizing the use of chemicals and impact on the environment. Here we highlight some of the science and

technology being developed to improve the disinfection and decontamination of water, as well as efforts to increase water

supplies through the safe re-use of wastewater and efficient desalination of sea and brackish water.

he many problems worldwide associated with the lack of

clean, fresh water are well known: 1.2 billion people lack

access to safe drinking water, 2.6 billion have little or no

sanitation, millions of people die annually—3,900 children

a day—from diseases transmitted through unsafe water or human

excreta 1. Countless more are sickened from disease and contamina-

tion. Intestinal parasitic infections and diarrheal diseases caused by

waterborne bacteria and enteric viruses have become a leading cause

of malnutrition owing to poor digestion of the food eaten by people

sickened by water 2,3. In both developing and industrialized nations, a

growing number of contaminants are entering water supplies from

human activity: from traditional compounds such as heavy metals

and distillates to emerging micropollutants such as endocrine dis-

rupters and nitrosoamines. Increasingly, public health and environ-

mental concerns drive efforts to decontaminate waters previously

considered clean. More effective, lower-cost, robust methods to dis-

infect and decontaminate waters from source to point-of-use are

needed, without further stressing the environment or endangering

human health by the treatment itself.

Water also strongly affects energy and food production, industrial

output, and the quality of our environment, affecting the economies

of both developing and industrialized nations. Many freshwater aqui-

fers are being contaminated and overdrawn in populous regions—

some irreversibly—or suffer saltwater intrusion along coastal regions.

With agriculture, livestock and energy consuming more than 80% of

all water for human use, demand for fresh water is expected to increase

with population growth, further stressing traditional sources. The

shift to biofuels for energy may add further demands for irrigation

and refining. Alarmingly, within 30 years receding glaciers may cause

major rivers such as the Brahmaputra, Ganges, Yellow (which already

at times no longer runs to the sea) and Mekong rivers, which serve

China, India and Southeast Asia, to become intermittent, imperilling

over 1.5 billion people during the dry months 4,5. Even industrialized

nations in North America and Europe, and those in Andean countries

in South America, could see major disruptions to agriculture, hydro-

electric and thermoelectric generation, and municipal water supplies

from reductions in snowmelt and/or loss of glaciers 6,7. In the coming

decades, water scarcity may be a watchword that prompts action

ranging from wholesale population migration to war, unless new ways

to supply clean water are found.

Fortunately, a recent flurry of activity in water treatment research

offers hope in mitigating the impact of impaired waters around the

world. Conventional methods of water disinfection, decontamination

and desalination can address many of these problems with quality and

supply. However, these treatment methods are often chemically, ener-

getically and operationally intensive, focused on large systems, and

thus require considerable infusion of capital, engineering expertise

and infrastructure, all of which precludes their use in much of the

world. Even in highly industrialized countries, the costs and time

needed to develop state-of-the-art conventional water and wastewater

treatment facilities make it arduous to address all the problems.

Furthermore, intensive chemical treatments (such as those involving

ammonia, chlorine compounds, hydrochloric acid, sodium hydro-

xide, ozone, permanganate, alum and ferric salts, coagulation and

filtration aids, anti-scalants, corrosion control chemicals, and ion

exchange resins and regenerants) and residuals resulting from treat-

ment (sludge, brines, toxic waste) can add to the problems of con-

tamination and salting of freshwater sources. Moreover, chemically

intensive treatment methods in many regions of the world cannot be

used because of the lack of appropriate infrastructure.

However, even within central Europe there has been a movement

towards reducing chemical treatment via engineered ‘natural’ sys-

tems for drinking-water production in order to reduce residual che-

micals in the distribution systems 8. Fortunately there is much more

that science and technology can do to mitigate environmental impact

and increase efficiency because current treatment methods are still

far from natural-law limits in their ability to separate compounds,

deactivate or remove deleterious pathogens and chemical agents,

transport water molecules, and move ions against concentration

gradients. Our expectation is that by focusing on the science of the

aqueous interface between constituents in water and the materials

used for treatment, new, sustainable, affordable, safe and robust

1NSF STC WaterCAMPWS, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA. 2Department of Chemical and Biomolecular Engineering and Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556, USA. 3Department of Environmental and Chemical Engineering, Yale University, New Haven, Connecticut 06520, USA. 4Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA. 5Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA. 6Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.

Vol 452 j20 March 2008 jdoi:10.1038/nature06599

implemented to serve people throughout the world.

Here, we highlight some of the science and next-generation systems

being pursued: to disinfect water, removing current and emerging

pathogens without intensive use of chemicals or production of toxic

byproducts; to sense, transform, and remove low-concentration

contaminants in high backgrounds of potable constituents at lower

cost; and to re-use wastewater and desalinate water from sea and

inland saline aquifers, all of which hold great promise for effectively

increasing water supplies. To realize these challenging goals, many

open research questions need to be addressed. Our thesis is that

research will enable improved disinfection, decontamination, re-use

and desalination methods to work in concert to improve health, safe-

guard the environment, and reduce water scarcity, not just in

the industrialized world, but in the developing world, where less

chemical- and energy-intensive technologies are greatly needed.

Disinfection

An overarching goal for providing safe water is affordably and

robustly to disinfect water from traditional and emerging pathogens,

without creating more problems due to the disinfection process itself.

Waterborne pathogens have a devastating effect on public health,

especially in the developing countries of sub-Saharan Africa and

southeast Asia 9. Waterborne infectious agents responsible for these

diseases include a variety of helminthes, protozoa, fungi, bacteria,

rickettsiae, viruses and prions 10. While some infectious agents have

been eradicated or diminished, new ones continue to emerge and so

disinfecting water has become increasingly more challenging. Viruses

are of particular concern, accounting, together with prions, for nearly

half of all emerging pathogens in the last two to three decades 9.

Enteric viruses received less attention in the past compared with

bacterial pathogens (for example, Vibrio cholerae ) and protozoan

parasites (for example, Cryptosporidium parvum ), partly because they

were difficult to detect, and partly because free chlorine (the main

disinfectant used worldwide because of its potency and low cost) was

very effective in inactivating them. However, free chlorine is ineffec-

tive in controlling waterborne pathogens such as C. parvum and

Mycobacterium avium. M. avium in particular is ubiquitous in

biofilms within water distribution systems around the world, with

remarkable resistance to chlorine at the high pH and low temperature

of natural water. Indeed, ageing and deterioration of drinking

water distribution systems and the associated growing of biofilms

within them has emerged as a key infrastructure rehabilitation chal-

lenge: significant resources are needed to maintain and upgrade

distribution systems. In the USA, where large numbers of such old

systems exist, disinfectants are required to suppress pathogens

within the system. Halogenated disinfection strategies for treatment

and distribution systems produce toxic disinfection by-products

(DBPs) such as trihalomethanes and haloacetic acids. Recent US

disinfection regulations 11,12 require the control of C. parvum oocysts

while minimizing the formation of certain DBPs, which might force

some drinking-water utilities to discard free chlorine disinfection

and implement alternative technologies.

Therefore, the effective control of waterborne pathogens in drink-

ing water calls for the development of new disinfection strategies,

including multiple-barrier approaches that provide reliable physico-

chemical removal (for example, coagulation, flocculation, sedimen-

tation, and media or membrane filtration) along with effective

photon-based and/or chemical inactivation. The 1993 outbreak of

cryptosporidiosis in Milwaukee, Wisconsin, USA, in which approxi-

mately 400,000 people were infected and more than 100 died, was a

wake-up call for the US drinking-water industry. They were

reminded that relying exclusively on physicochemical removal,

which can suffer from malfunctions arising from defects in manufac-

turing or operation, can have a devastating effect on public health.

The use of light from visible to ultraviolet (UV) to photochemi-

cally inactivate pathogens has recently seen a resurgence in interest,

notwithstanding the historical use of sunlight to disinfect water.

Sequential disinfection schemes such as UV/combined chlorine

and ozone/combined chlorine are being considered by many drink-

ing-water utilities as the inactivation component of their multiple-

barrier treatment plants because, compared with free chlorine, both

UV and ozone are very effective in controlling C. parvum oocysts. In

addition, combined chlorine can provide a residual in distribution

systems without forming high levels of regulated DBPs. However,

changing disinfection technologies has raised new concerns because

viruses, although effectively controlled by ozone, are resistant to both

UV and combined chlorine disinfection. Moreover, ozone can form

the DBP carcinogen bromate ion in water containing bromide ions,

and combined chlorine can form other unregulated DBPs, for

example, haloacetonitriles and iodoacetic acid 13,14 , that may be more

toxic and carcinogenic than those associated with free chlorine.

The situation in developing countries is similar. International

agencies and non-governmental organizations have introduced the

use of sunlight irradiation of water within PET (polyethylene ter-

ephthalate) bottles to kill pathogens, and are promoting the use of

sodium hypochlorite for point-of-use disinfection of drinking water

in rural areas (for example, the CDC SafeWater System) 15. Although

these initiatives have lowered the incidence of gastrointestinal dis-

ease, owing to the lack of adequate sanitation, the source waters in

these areas contain ammonia and organic nitrogen that react with the

sodium hypochlorite to form combined chlorine species that are

ineffective in inactivating viruses. Furthermore, relatively high levels

of toxic DBPs can form in the presence of high concentrations of

organic matter associated with inadequate sanitation.

To develop alternatives to chlorine (free and combined) and UV

disinfection for the control of waterborne viruses requires significant

advances in understanding how viruses are inactivated by the bench-

mark (chlorine and UV) methods and by any new technologies. The

goal is to match or improve on the positive aspects of chlorine and

UV disinfection while avoiding the negative effects. To do so requires

several questions to be answered. It is well established that both UV

light and the free chlorine species hypochlorous acid (HOCl) react

with various amino acids in the virus capsid proteins as well as with

the nucleic acid protected by the capsid 16,17 . However, the actual

limiting step (that is, the molecular target and its level of damage)

responsible for inactivation is not yet known. Developing a process

that targets that inactivation mechanism may create a new, safe, and

robust disinfection method.

For example, many species of adenovirus—the waterborne

pathogens with highest resistance to UV inactivation—use their fibre

head (Fig. 1a) to attach to the amino-terminal D1 domain of the

coxsackievirus-and-adenovirus receptor (CAR) of host cells 18.

Amino acid sequence alignments have shown that the hydrophobic

side group of tyrosine and ionizable basic side groups of histidine and

lysine in the fibre head associate with CAR amino acids (Fig. 1b) and

thus play a role in the attachment of adenovirus to the host cell 19.

Consequently, oxidization of the phenolic group of tyrosine, and

formation of reactive chloramines with the amino groups of histidine

and lysine 20–23 could contribute to changing the conformation of the

adenovirus head protein and inhibiting binding to receptors, thus

effectively inactivating the virion. HOCl also reacts with nucleic acid

and amino acid residues involved in many steps of the infection cycle

of viruses, such as cell entry (endocytosis and endosomal lysis), intra-

cellular trafficking and nuclear delivery, in the case of adenovirus 24.

Thus, even if the virus penetrates the cell, the infection cycle could be

inhibited at some subsequent step. A potential problem with this

strategy is that once inside the cell, the virion might manipulate

the host cell to repair the damage and subsequently complete the

infection cycle 25. Consequently, disinfection processes that target

the proteins responsible for attachment and penetration would avoid

the unwanted possibility of genome repair.

A new generation of disinfection processes to control viruses

should be capable of selective reactions with the key residues in

REVIEWS NATURE jVol 452 j20 March 2008

Heterogeneous processes are envisioned that would use comple-

mentary nanostructured and functionalized surfaces that mimic

the structure and functionality of the receptors of target protein

residues. These structures should have both high affinity and specifi-

city and be relatively inert to the large amounts of organic matter

ubiquitous in natural water. The surfaces of new materials could be

designed with arrays of sites that serve to trap all waterborne viral

pathogens via binding to host receptors.

A futuristic disinfection method involves the combined use of

photons and engineered nanostructures. Although UV is effective

for inactivating waterborne bacteria and protozoa cysts and oocysts,

it is not very effective for viral pathogens. However, UV light is

capable of activating photocatalytic materials such as titania

(TiO 2), which are capable of inactivating viruses. Furthermore,

new photocatalysts such as TiO 2doped with nitrogen (TiON), or

co-doped with nitrogen and a metal such as palladium, can be acti-

vated with visible light 26(which could potentially inactivate viruses

and other waterborne pathogens with much lower energy use than

UV), or even with sunlight (for deployment anywhere with bright

sunlight). Of particular interest are materials and systems that use

low-cost visible lamp light and sunlight to achieve sufficiently high

throughput. Low throughput rates have thus far limited adoption of

photoinactivation. Throughput rates depend on factors such as

incident light flux and wavelength, absorption length through water,

geometry, reactor hydrodynamics, contact efficiency of species in

water on the photocatalysts and, critically, the inactivation kinetics.

Moreover, we need to improve our understanding of the mechanisms

for the interactions of pathogens, in particular virions, with excited

photocatalyst surfaces and adherent active moieties, such as hydroxyl

radicals and superoxides. The physicochemical structure of such

surfaces would need to be optimized for maximum selective affinity

of target viral capsid molecular motifs.

Once these new materials are developed, they can be engineered

into flow-through reactors for high-throughput systems. The config-

uration and associated cost of such systems could make them eco-

nomically viable for applications ranging from large water-treatment

plants supplying potable and non-potable water to point-of-use sys-

tems with segregated lines dedicated to human consumption and

hygiene. Antiviral photocatalysts could be immobilized on fibres

and foams of various materials 27–29 , or incorporated into mem-

branes 30. Optical fibres could be used to bring photons into compact

configurations such as monolithic reactors 31. Reactors incorporating

visible-light photocatalysts could be designed using sunlight as the

source of photons 32,33 , a configuration that would be particularly

beneficial in developing countries. The resulting systems would pro-

vide a barrier against all pathogens by inactivating viruses and trap-

ping any larger bacteria and protozoa cysts and oocysts with relatively

high resistance to light and photocatalytic inactivation, all without

producing DBPs or extensive use of chemicals.

Decontamination

The overarching goal for the future of decontamination is to detect

and remove toxic substances from water affordably and robustly.

Widely distributed substances, such as arsenic, heavy metals, halo-

genated aromatics, nitrosoamines, nitrates, phosphates, and so on

are known to cause harm to humans and the environment. Two key

problems are that the amount of suspected harmful agents is growing

rapidly, and that many of these compounds are toxic in trace quan-

tities. To detect their presence and remove them in the presence of

safe and natural constituents that are 3 to 9 orders of magnitude more

concentrated is challenging, expensive, and unreliable at present.

Chemically treating the total volume of water to transform or remove

a specific trace compound is also expensive and potentially itself

harmful. Moreover, the treatment does not necessarily remove other

harmful compounds, and safe constituents may interfere with the

remediation. Thus, new methods to detect toxic compounds and

decontaminate water are urgently needed.

The problems of detecting and accurately measuring toxic com-

pounds in water and of selectively removing only these compounds

are tightly linked. Both are affected by the particular combination of

micropollutant classes (heavy metals, As(III/V), BTEX, pharmaceu-

tical derivatives, agricultural chemicals, endocrine disrupters, and

so on) 34relevant to a specific water source. Furthermore, viable ave-

nues for both detection and treatment are tied to the resource base

available. Approaches to speciation of As(III/V) or elemental pro-

filing relevant to western Europe are simply not an option for

Bangladesh 35,36 or Benin 37. Powerful methods of monitoring low

concentrations of contaminants are invariably built around sophis-

ticated laboratory instrumentation. It is extremely challenging to

develop robust, low-cost, effective means of chemical sensing rel-

evant to the water contamination problems of developing nations.

Similarly, affordably treating toxic compounds in water, such as by

AD2 fibrehead protein

AD37 fibrehead protein

Histidine

Lysine

Tyrosine

H-bondreceptors

CAR D1domain

Figure 1 |Waterborne virus attachment head and receptor on host cell. a, Schematic of adenovirus-2 attachment fibre head showing amino acidswith basic (magenta, orange, purple), acidic (yellow), and hydrophobic (red)side groups. b, Schematic of adenovirus-37 fibre head protein attaching to the D1 domain of the coxsackievirus-and-adenovirus receptor (CAR) on thehost cell. Basic ionizable (histidine, lysine) and hydrophobic (tyrosine) sidegroups of AD37 amino acids and those in CAR D1 domain involved inhydrogen bonding are highlighted. The figure was developed with ProteinExplorer 98based on a structure described in ref. 99 (courtesy of Martin A. Page, University of Illinois).

NATURE jVol 452 j20 March 2008 REVIEWS

safe ( ,10 parts per billion), without producing toxic waste disposal

issues has proved to be a major challenge. But although these goals

are beset by severe technical difficulties, they also present exciting

opportunities for the research community.

Speciation remains a challenging detection problem. For example,

As(III) is estimated to be ,50 times more toxic than As(V), so both

As(III) and total As must be measured. Anodic stripping voltamme-

try 38has sufficiently low limits of detection (LOD 51.2 mgl21)tobe

practical and is capable of measuring As(III) in the presence of large

excesses of As(V). Alternatively, ion exchange separations may be

combined with hydride generation atomic spectroscopy to measure

As(III) and As(V) separately 39. But neither method is suitable for

untrained workers. These methods also demonstrate the related

generic problem of the LOD dynamic reserve. The temptation, given

that detailed dose–response data frequently do not exist (especially at

low concentrations of toxic species), is to regulate to the existing

analytical capabilities, which can create new problems. For example,

if the total As concentration is regulated at a maximum contaminant

level of 10 mgl21, then the 1.2 mgl21LOD of As(V) represents only an

eightfold dynamic reserve. It might not be possible to achieve a

tenfold or greater dynamic reserve between the LOD and the maxi-

mum contaminant level using detection methods suitable for use by

untrained workers to enhance human health.

Beyond these quantitative issues lies the dichotomy between the

capabilities for detecting target compounds and for identifying

potentially troublesome non-target species. Even powerful multidi-

mensional analytical methods, such as liquid chromatography-mass

spectrometry (LC-MS), struggle to characterize waters containing

significant amounts of non-target species. These compounds must

often be pre-concentrated by factors of 10 2to 10 3and can only be

assayed accurately in the presence of a small number of potential

compounds whose liquid chromatography retention behaviour is

known 40. Such problems point to the critical need to develop mole-

cular recognition motifs (sensor reagents) that can be combined with

micro-nanofluidic manipulation 41and data telemetry to accomplish

single-platform chemical sensing having the requisite figures of merit

to be competitive with bench-scale instrumentation. In this regard

the recent combination of catalytic DNA (DNAzyme) in a micro-

nanofluidic platform is of considerable interest. Functional DNA,

obtained through in vitro selection, can be used to bind metal ions

with high affinity (yielding parts-per-trillion LODs) and specificity

(.106-fold over other cations) 42. When synthetically elaborated with

proximal fluorophore and quencher, the resulting molecular beacon

construct (see Fig. 2) may be placed in microfluidic formats to

achieve the double selection of a chemical separation followed by a

highly specific molecular recognition event 43. Significant opportu-

nities exist to exploit the in vitro selection process to achieve similar

performance characteristics for a wide range of micropollutants.

Biosensing strategies are also beginning to be applied to water-

borne pathogens. For example, capillary waveguide integrating

biosensors have been applied to detect waterborne Escherichia coli

O157:H7, an enterohaemorrhagic bacterium 44. However, given the

large fraction of the contaminated-water death toll that is due

to waterborne pathogens, there is enormous potential for future

development of bio-based measurement schemes.

Detection and remediation of toxic compounds are inextricably

linked, as treatment of anionic micropollutants demonstrates. Deter-

mination of the anionic constituents of aqueous systems remains

among the most challenging analytical problems. Typically, anions

are determined by ion chromatography coupled with conductance

detection, which is universal but does not have the sensitivity

required in all instances. Sensitivity can be grafted on through the

use of LC-MS 45at considerable added expense, although lowering

the limits of detection from the ,5mgl21level to ,0.05 mgl21may

well justify the cost. On the remediation side, compelling research

opportunities surround is the development of high-specificity

synthetic anion transporters although these have thus far been

focused on biomedical applications 46—for particularly refractory

micropollutant species, such as ClO 4–and NO 3–. Anions also illus-

trate the complexity of designing effective treatment/remediation

strategies. For example, disinfection of water sources with ozone

(O 3) is highly effective, but if the water contains significant amounts

of Br –, oxidation to the problematic BrO 3–takes place 47, effectively

substituting one water contamination problem for another.

Similar strategic considerations affect the treatment versus

removal decision. Whether to treat water via a chemical or bioche-

mical conversion of a micropollutant to an innocuous form, or to

remove the toxic contaminant via adsorption, chelation and filtra-

tion, or another method is a decision that rests largely on matching

the problem to the sophistication of the available technology and the

resource base to support the use of the technology, as well as how far

the target concentration is below the maximum contaminant level.

The use of Sono filter technology at local wells in Bangladesh and

reverse osmosis (RO) systems in central plants in the USA may both

represent optimized solutions for As removal within the context of

the local problem 48. However, here, too, opportunities exist for

research to make an impact.

The treatment protocols used widely and envisioned for the future

all encompass a complex interplay of elementary steps such as trans-

port, partitioning, reaction and conversion, and release. To this end,

fundamental advances in understanding these processes will neces-

sarily involve sophisticated modelling to assess the way in which the

basic steps are coupled most effectively 49, and modelling-based pre-

dictions of potential removal activity 50. Modelling is essential to

optimize multi-step strategies—for example, the capture of As by

monodisperse Fe 3O4nanocrystals followed by magnetic separation

of the waste stream 51—which are often the most effective, or perhaps

the only, possible approaches.

Another critical problem involves unintended transformations of

non-targeted pollutants. For example, treatment of wastewater with

No Pb 2+

NCAM

Receiving channel

Control Initial injection After 40 s

Pb2+

Sourcechannel

d c

Figure 2 |Lead DNA sensor with a micro-nanofluidic device. Immobilized DNAzyme sensor and micro-nanofluidic devices for detection of Pb 21by fluorescently labelled 17E DNAzyme. a, Schematic of immobilized DNAzyme showing catalytic beacon signalling of reaction on the surface,releasing fluorophore into solution for detection. b, Schematic of DNAzyme immobilized within the pores of a nanocapillary array membrane (NCAM)with inset showing the mode of ratiometric fluorescence signalling in theabsence or presence of Pb(II). c, Schematic representation of orthogonal microfluidic channels separated by a NCAM flow gate. d, Fluorescence micrographs of the receiving channel before injection of Pb 21sample into a receiving channel containing ,1mM 17E DNAzyme, after initial Pb 21

injection and after 40 s of total injection time for a DNAzyme–NCAMmicrofluidic device.

REVIEWS NATURE jVol 452 j20 March 2008

suspected carcinogen N-nitrosodimethylamine (NDMA). These

unintended secondary effects would seem to argue for separation

over transformation strategies, but the relative cost and effectiveness

of each approach needs to be considered on a case-by-case basis.

A promising workaround focuses on exploiting biology to effect

either transformation, such as the biodegradation of NDMA by

monooxygenase-expressing microorganisms 52, or removal, as exem-

plified by the Fe-specific siderophile desferrioxamine-B produced by

Streptomyces pilosus . Desferrioxamine-B exhibits stability constants

in excess of 10 26for Th(IV) and Pu(IV), and so may be useful in

actinide remediation strategies 53. Of course, organism-oriented strat-

egies must also be vetted to ensure that they do not introduce other

undesirable secondary effects.

Finally, an ubiquitous problem in remediation strategies is the cost

or use of critical components that are consumed in stoichiometric

reaction, which spurs interest in catalytic treatment approaches to

convert organic compounds to innocuous N 2,CO 2and H 2O. Major

anion pollutants such as nitrates and perchlorates are now removed

via ion exchange resins or RO, leaving a deleterious brine to be dis-

posed of. Next-generation remediation may use bi-metallic active

catalysts to mineralize the brine, such as Pd-Cu/ c-alumina catalysed

reduction of NO 3–(ref. 54). Future efforts may include incorporating

active nanocatalysts in a membrane barrier to transform anions at low

concentrations in a hybrid process. The combination of modelling

and experiments can reveal the mechanisms of these reduction reac-

tions, helping to identify potentially transformative catalytic reme-

diation strategies.

Re-use and reclamation

The overarching goal for the future of reclamation and re-use of

water is to capture water directly from non-traditional sources such

as industrial or municipal wastewaters and restore it to potable qua-

lity. Of all the water withdrawn from rivers, lakes and aquifers, the

majority is returned to the environment. Agricultural and livestock

users return the least at ,30–40%, whereas industrial users return

,80–90%, power generation returns considerably more at ,95–

98%, and public and municipal users return ,75–85%. The rest is

lost to the atmosphere or is consumed in biological or chemical

processes. A large part of the cost of water for human use is pumping,

transport and storage (particularly in developing countries whose

citizens often spend substantial time acquiring water). Thus recover-

ing water at or close to the point of use should be very efficient.

However, unlike the decontamination of trace compounds just dis-

cussed, wastewater contains a wide variety of contaminants and

pathogens, and has a very high loading of organic matter, all of which

must be removed or transformed to harmless compounds.

Municipal wastewaters are commonly treated by activated sludge

systems that use suspended microbes to remove organics and nutri-

ents, and large sedimentation tanks to separate the solid and liquid

fractions. This level of treatment produces wastewater effluent suit-

able for discharge to surface waters or for restricted irrigation and

some industrial applications. Similarly, biological treatment via

traditional trickling filters and aquacultures have been used exten-

sively to reduce solids and remove ammonia and nitrites from water.

Typically, these biological treatment systems are large with long water

residence times. A technology now actively being pursued is mem-

brane bioreactors (MBRs) 55–57 . This technology combines suspended

biomass, similar to the conventional activated sludge process, with

immersed microfiltration or ultrafiltration membranes that replace

gravity sedimentation and clarify the wastewater effluent. MBRs can

produce high-quality effluent that is suitable for unrestricted irriga-

tion and other industrial applications.

MBRs have also the potential for use in developing countries to

address the pressing need for improved sanitation 55. Possible appli-

cations in developing countries include the direct treatment of raw

sewage, particularly in rapidly growing megacities, and the extraction

of valuable resources from sewage, namely clean water, nutrients

(mostly N and P), and energy. The small footprint, flexible design,

and automated operation of MBRs make them ideal for localized,

decentralized sewage treatment in the developing world.

One of the growing applications of MBRs is as pretreatment

for RO, which, when followed by UV disinfection (or, potentially,

visible-light-activated photocatalysts), can produce water for direct

or indirect potable use (Fig. 3). Current wastewater re-use systems

use a conventional activated sludge process, followed by a microfil-

tration MBR pretreatment of the secondary effluent, which has high

quantities of suspended and dissolved solids. The effluent water from

the MBR still partially contains dissolved species and colloidial

substances that act to foul the membranes of the subsequent RO

system used as a final barrier to contaminants in the product water.

Employing a ‘tight’ ultrafiltration membrane in the MBRs lets

through fewer dissolved solids than does microfiltration, allowing

the RO system to operate with significantly less fouling. Futuristic

direct re-use systems envisioned involve only two steps: a single-stage

MBR with an immersed nanofiltration membrane (obviating

the need for an RO stage), followed by a photocatalytic reactor

to provide an absolute barrier to pathogens and to destroy low-

molecular-weight organic contaminants that may pass the nanofil-

tration barrier.

A major obstacle to the efficient application of MBRs in current or

next-generation re-use systems is membrane fouling, particularly

when it leads to flux losses that cleaning cannot restore 56,58 . Fouling

in MBRs is primarily caused by microbe-generated extracellular

polymeric substances, most notably polysaccharides, proteins and

natural organic matter. The development of economical, high-flux,

non-fouling membranes is therefore needed before viable MBR pro-

cesses, as well as other membrane-based approaches for wastewater

reclamation, can be achieved.

Fouling of polymer membranes is influenced by membrane che-

mistry and morphology. Polymers used in porous membrane manu-

facture have chemical and mechanical stability, but are generally

hydrophobic in nature, and as a consequence are highly susceptible

to adsorption of organic foulants. Commercial methods to reduce

fouling largely involve graft polymerization of hydrophilic mono-

mers on the membrane surface 59. The resulting ‘brush’ of hydrated

Potablewater Wastewater

MBR Using microfiltration or ultrafiltration (or nanofiltration?) RO Disinfection(UV or visible?)

Figure 3 |Membrane bioreactor treatment system for direct conversion to potable water. Depiction of a next generation MBR-based treatment method that can potentially take wastewater from municipal, agricultural,livestock or other high-organic-content sources and convert it to potable

water. Future methods may be able to omit the RO step with a nanofiltrationmembrane, and follow with a visible light disinfection step to ensure that allpathogens, including viruses, are inactivated.

NATURE jVol 452 j20 March 2008 REVIEWS

reduces intrinsic permeability owing to partial blocking of surface

pores, while internal pores may go unmodified and remain prone to

fouling 60. The extra manufacturing steps also add to membrane cost.

Alternative in situ approaches to membrane surface modification

under development may generate more efficacious brush layers

without the drawbacks of surface graft polymerization 61–64 . Comb

copolymers, having hydrophobic backbones and hydrophilic side

chains, function as macromolecular surfactants when added to mem-

brane casting solutions 62, lining membrane surfaces and internal

pores during the conventional immersion precipitation process

used in membrane manufacture. Order-of-magnitude flux enhance-

ments 63,65 and complete resistance to irreversible fouling by the three

classes of extracellular polymeric substance foulants 65,66 , recently

demonstrated for such ultrafiltration membranes (see Fig. 4), offer

substantial promise for decreasing operational costs of wastewater

treatment through reduced membrane cleaning and replacement and

increased process efficiency.

Next-generation membranes offer further opportunities for

improved contaminant retention or recovery of valuable constitu-

ents from wastewaters, without intensive chemical treatment and

while reducing the need for subsequent decontamination. These

advanced filtration processes require membranes with much nar-

rower pore size distributions than those derived from immersion

precipitation, in addition to fouling-resistant surface/pore chemis-

tries 61,67 . Approaches under investigation include block copolymers,

graft/comb copolymers, or lyotropic liquid crystals that self-

assemble to form nanodomains that are highly permeable to

water 68–71 , or can be selectively removed to create nanopores for

water passage 72,73 . Such nanostructured materials may be implemen-

ted as thin-film coatings on conventional ultrafiltration or micro-

filtration membrane supports 70,72,74 , or on novel high-flux base

membrane structures, such as electrospun nanofibres 75. Recently,

for example, rigid star amphiphiles with 1–2 nm hydrophobic cores

and hydrophilic side chains were coated onto polyethersulphone

ultrafiltration membranes to obtain nanofiltration membranes with

comparable or better rejection of As(III) and water permeability

several times greater than commercial nanofiltration membranes 76.

The commercial viability of this new class of thin-film composite

membranes for water re-use hinges on the development of inexpen-

sive coatings, chemistries and scalable processing methods that can

reproducibly achieve the desired membrane structure and yield

fluxes comparable to today’s ultrafiltration membranes.

Desalination

The overarching goal for the future of desalination is to increase the

fresh water supply via desalination of seawater and saline aquifers.

These sources account for 97.5% of all water on the Earth, so cap-

turing even a tiny fraction could have a huge impact on water scar-

city. Through continual improvements, particularly in the last

decade, desalination technologies can be used reliably to desalinate

sea water as well as brackish waters from saline aquifers and rivers.

Desalination of all types, though, is often considered a capital- 77

and energy-intensive 78process, and typically requires the conveyance

of the water to the desalination plant, pretreatment of the intake

water, disposal of the concentrate (brine), and process maintenance.

Estimated costs of pumping from sea intake to the desalination plant

vary widely with geographical location, height and distance from the

source water 77. But so far, the total cost and increased environmental

concerns have limited the widespread adoption of desalination tech-

nologies. Nevertheless, for a state-of-the-art RO system that uses as

little as ,2.2 kW h of electrical energy to produce a thousand litres of

drinking water inside the desalination plant, the total energy usage to

desalinate water will be ,0.005 kW h l 21, which includes the elec-

trical plus modest conveyance energy needs. Putting this estimated

energy use into perspective reveals that supplying even 50 litres a

day per capita of drinking water at 0.25 kW h can be a small fraction

of the daily energy required per capita (ranging from 3.2 kW h in

China to 30 kW h in the USA) for living in a world with strained

environmental resources (see http://telstar.ote.cmu.edu/environ/

m3/s3/02needs.shtml).

The major desalination technologies currently in use are based on

membrane separation via RO and thermal distillation (multistage

flash and effect distillation), with RO accounting for over 50% of

the installed capacity 77,78 . Conventional thermal desalination pro-

cesses are inefficient in their use of energy and suffer particularly

from corrosion, as well as scaling that also affects RO. Even where

fuel is readily available and low-cost, high capital and operational

costs limit adoption. Therefore, the market share of large conven-

tional thermal desalination plants will probably decline. However,

for family and very small community systems in remote locations,

especially in the developing world, solar thermal distillation and

humid air desalination technologies may find an increasing role,

particularly in inland semi-arid areas with access to saline lakes

and aquifers 79,80 . These thermal technologies may also find small-

scale applications in locations without ready sources of energy, other

than solar.

Although RO systems have a relatively low rate of energy con-

sumption, they use high-cost electrical energy. RO desalination,

however, can take advantage of low-grade heat energy to increase

flux through the membrane for a given pressure drop. This thermally

enhanced RO finds applications in locations where such heat energy

is available, typically using waste heat from a co-located electrical

power generation and RO desalination plant. We also envision

hybrid desalination plants that would combine thermally enhanced

RO and thermal desalination to lower electrical energy consumption

per unit of product water further, while achieving higher water recov-

eries than can RO alone. Desalinating inland saline waters, which are

present on most continents in quantities similar to fresh water, can

also be used to increase water supplies, but disposal of the residual

concentrate is a major problem. Hybrid desalination technologies

that concentrate precipitates and salts while extracting the water

with membranes can potentially process the brine. Two alternative

desalination technologies currently under investigation, forward

osmosis 81and membrane distillation 82, can also use low-grade heat

0 0

300

600

900

1,200

1,500

1,800 b

5Comb content (%)

Pure water permeability

(l m

–2 h–1 MPa

–1)

10 20

Figure 4 |Comb copolymer amphiphiles for fouling-resistant membranes. a, Schematic illustration of in situ approach using comb copolymer amphiphiles to modify ultrafiltration membrane surfaces and internal poresduring membrane casting. b, Pure water permeability of polyacrylonitrile ultrafiltration membranes incorporating 0–20% comb copolymer additivehaving a polyacrylonitrile backbone and polyethylene oxide side chains.White bars show the initial pure water permeability, and grey bars show thepure water permeability after 24 h of dead-end filtration of 1,000 mg per litreof bovine serum albumin in phosphate buffered saline, followed by adeionized water rinse. Initial flux and flux recovery increase with combadditive content. Membranes exhibit complete resistance to irreversiblefouling at 20% comb content (data from ref. 65).

REVIEWS NATURE jVol 452 j20 March 2008

achieve high water recoveries.

For large-scale desalination, RO has advanced significantly in the

past decade, particularly owing to the development of more robust

membranes and very efficient energy recovery systems. As a result,

the reduction in energy consumption of RO desalination has been

remarkable 77,78,83,84 . The specific (per unit of produced potable water)

energy of desalination has been reduced from over 10 kW h m 23in

the 1980s to below 4 kW h m 23(refs 78, 83). Any desalination system

will be most energy efficient if it involves a reversible thermodynamic

process, which is independent of the system and mechanisms used.

From the free-energy change on removing a small amount of pure

water from a mixture of water and salt, the theoretical lower bound of

the energy needed for desalination can be estimated 85. For zero per

cent recovery, that is, the removal of a relatively small amount of

water from a very large amount of sea water, the calculated theoretical

minimum energy for desalination is 0.70 kW h m 23of fresh water

produced. This theoretical minimum increases to 0.81, 0.97 and

1.29 kW h m 23for recoveries of 25, 50 and 75%, respectively, sug-

gesting that further improvements in the energy efficiency of RO

desalination are still possible.

Although RO is currently the state-of-the-art desalination techno-

logy, there are several challenges and opportunities that could result

in additional reductions in the total cost per unit of product water.

Among the major challenges of RO desalination are membrane foul-

ing, relatively low recovery for sea water desalination (less than

,55%), which results in large volumes of concentrated brine, and

relatively low removal of low-molecular-weight contaminants, most

notably boron in sea water. Future RO desalination membranes

will ideally have high water flux per unit of pressure applied, near-

complete rejection of dissolved species, low fouling propensity, and

tolerance to oxidants used in pretreatment for biofouling control.

The total cost for RO involves three strongly interrelated components

that depend upon region, source water, and energy sources: capital

(infrastructure, equipment, membrane replacement), energy (ther-

mal and electrical), and operation (pretreatment, post-treatment,

concentrate disposal and cleaning). Lowering the flux to save energy

may be offset by the consequent increase in capital costs. Increasing

the permeability of the RO membranes decreases both capital and

energy costs, but may increase the cost for pretreatment and cleaning.

Improvements must be made in all three components to lower the

total cost of the product water.

Recent work by the Affordable Desalination Coalition 78,84 has

demonstrated a remarkably low specific energy of seawater desalina-

tion, at 1.58 kW h m 23, under ideal conditions (that is, new mem-

branes, no fouling, and low water flux) at 42% recovery. This value

is relatively close to the theoretical minimum energy for seawater

desalination at that recovery, suggesting that next-generation

fouling-resistant RO membranes will be able to desalinate sea water

with lower energy consumption. Asymmetric membranes depicted

in Fig. 5 (a) currently used for RO have relatively large random pore

size distributions. Therefore, the separation layer is thicker than ideal

to ensure adequate salt rejection, reducing the flux rate. Desalination

typically involves ions with small hydration diameter dH, which

require pores with a hydraulic diameter of dRO ,dHto exclude them,

increasing mainly enthalpic energy requirements. The energy of

desalination depends critically on pore diameters, and the chemical

affinity of water and ions with the pore wall.

Approaching the theoretical minimum energy is impractical for

desalination plants, because it would require huge facilities with high

capital costs. Moreover, in real desalination processes, energy is lost

because of inherent thermodynamic irreversibilities that arise from

diffusion, viscous dissipation and flux-rate-dependent losses. To

reduce the energy needed for desalination, the rate of entropy gen-

eration Wmust be minimized because the energy consumed by irre-

versible processes is ,TW,where Tis the absolute temperature. Wcan

be expressed as JvDp1JdDp 1JaDa,where Jvis the total volumetric

flux of water plus solute, Jdis the flux of solute relative to the water,

Dpis the pressure difference from frictional losses ( Dp/ Jv),Dp is

the osmotic pressure difference across the membrane, and JaDais the

entropy generation from any active ion pumping. The higher the

flux, the higher the salt concentration will be at an RO barrier,

increasing Dp and Dp. Interestingly, for the same separation per-

formance (that is, the same JdDp ), if JaDais sufficiently small, the

entropy generation using active membranes could be less than for RO

for a water recovery of much more than 50%, because the fluid flux in

RO will be higher than in active transport ( JvjRO ? Jvjactive ), and the

Chemical potential, µ End

End

Start

End

Start

salt concentratesat wall with non-zero water flux

Diffusive saltbackflow

Chemical potential, µ

Power input Ion rejection

Diffusive saltrejection

Minimum energy with flux

Hydratedsalt flux +

–

+ +

–

dactive dRO

No flux

Waterflux

Saltrejection

Source, µs

Product, µp

Concentrate, µc

Concentrate, µc Source, µs

Product, µp

Figure 5 |Reverse osmosis and active desalination membrane processes. Concentration gradients in RO ( a) and active ( b) desalination membranes. The energy levels marked with ‘start’ and ‘end’ correspond to the evolutionof each process. The darker blue colour denotes higher concentration. Insetsdepict different mechanisms of salt ion separation. The active process withenergy input shows a conceptual strategy for overcoming the Born barrierwith fixed charges.

NATURE jVol 452 j20 March 2008 REVIEWS

of the separation.

Membranes with a uniform pore distribution and a more per-

meable separation layer can potentially maintain or improve salt rejec-

tion while increasing the flux in RO. Recent research on the transport

of water through hydrophobic double-walled carbon nanotubes is

promising, demonstrating water fluxes that are over three orders of

magnitude higher than those predicted from continuum hydrodyn-

amic models (refs 86–89; also O. Bakajin, personal communication,

23 October 2007). The high flux may be due to the carbon nanotubes’

atomically smooth, hydrophobic walls allowing considerable slip of

water through the pores. The preliminary work of ref. 89 reported

unusually high water flux through microfabricated membranes com-

prised of aligned carbon nanotubes ,3mm long with an inner dia-

meter of ,1.6 nm. Further measurements with these membranes

reveal 90salt rejection coefficients that match or exceed those of com-

mercially available nanofiltration membranes, while exceeding their

flux by up to four times. But such membranes may be difficult and

costly to manufacture, prone to defect formation, and might have a

high propensity for fouling given their hydrophobic nature.

The high performance of membranes based on carbon nano-

tubes 86,87,89 , however, reveals an important pore characteristic shared

by biological ion channels: hydrophobic pores ,1 nm in diameter.

These cores allow the ion hydration shell to remain intact, thereby

reducing the enthalpic translocation energy to be closer to the entropic

loss for confining an ion in a pore. Decreasing the pore diameter much

below 1 nm creates a large free-energy barrier, which arises from strip-

ping the hydration shell off the ion and water molecules that need to

overcome a Born energy barrier. Modifying the surfaces of the mem-

brane, as discussed for nanofiltration membranes, can alter the surface

properties, and thus potentially decrease the energy barrier.

Technological challenges to incorporating carbon nanotube mate-

rials include the functionalization of the mouth of the pores to

increase selectivity and potentially reduce hydrophobicity at the sur-

face, integration of the active layer with robust support substrates,

scaling up the fabrication of the ion channel and carbon-nanotube-

based membranes and increasing the pore density per area of the

active layer, and decreasing the cost of membrane fabrication. Still,

the costs of such membranes could eventually be affordable with

future improvements in carbon nanotube synthesis and membrane

processing.

Aquaporins (water channels) and ion channels of biological cells

have also motivated the search for alternative approaches to engin-

eering membranes with high water flux and selectivity 91,92 .De novo

synthesis of ion channels 93and the development of low-molecular-

weight anion transporters is an emerging topic in supramolecular

chemistry 94. Based on an array of aligned carbon nanotubes with

hollow graphitic cores embedded within a solid polymer film, the

first biomimetic protein channel controlled by the same mechanism

of phosphorylation/dephosphorylation that occurs in nature has also

been recently reported 95. However, much work remains to incor-

porate these futuristic materials into large-area membranes at com-

petitive costs.

Even if a perfect membrane could be created, with no pressure

drop required for complete salt rejection, the increase of flux rates

for RO is ultimately limited by the concentration polarization layer at

the membrane (see Fig. 5a), which constitutes an additional impe-

dance to fluid flow. The higher the flux of water, the higher the

gradient in solute concentration on the rejection side. The polariza-

tion impedance can be reduced via tangential fluid flow, but can

never be eliminated. What is worse, the transmembrane chemical

potential difference increases along the direction of the tangential

flow (from ‘start’ to ‘end’ in Fig. 5a), while the transmembrane pres-

sure difference decreases because of pressure losses, resulting in addi-

tional irreversible losses with higher fluxes.

To see how active systems might compare to RO systems, we can

extrapolate from the energetics of existing ion channels. Biological

channels transfer 10 7ions per pore per second, and measurements

corroborated by systematic computer simulations reveal that the free-

energy barrier for biological potassium channels is 2–5 kcal mol 21

(depending on the type of K 1channel), which corresponds to a spe-

cific energetic requirement of 2.55–6.4 kW h m 23of water produced

from sea water with a salt concentration of 32,000 parts per million.

For potassium ions, the lower bound is near the current energetic costs

for RO, but is still much higher than the theoretical minimum.

However, these channels are ion- and charge-specific, and there is a

significant energetic cost for the exclusion of the other ions. If active

nanopores of dimensions greater than 1 nm are created that pass a

multiplicity of anions and cations, as depicted in Fig. 5b, the energetics

can potentially drop by more than a factor of two below that of

biological channels.

Bio-inspired systems for active transport provide another route

towards improving the energetics of desalination. In contrast to con-

ventional desalination whereby water is ‘pushed’ through an RO

membrane by a pressure gradient, or in electrodialysis whereby

hydrated anions and cations are forced through their respective

ion-selective membranes by electrokinetic action, active ion separa-

tion involves pumping of both hydrated anions and cations through

the same membrane via modulation of pore potentials, against a

chemical potential, leaving desalinated product water. As Fig. 5b

illustrates, membranes that actively ‘pull’ hydrated ions through

the barrier reverse the direction of the concentration polarization

layer, and should not suffer the same decrease of performance with

increasing flux as does RO. Nature also provides a solution to the

problem of lowering the cost of overcoming electrostatic barriers in

engineered systems, which typically involve dielectric membranes.

The Born energy barrier 96to move an ion of charge qfrom water

to the low-dielectric-constant membrane ( e<2 for typical biological

membranes) is DG

by the energy liberated if the penetrating ion meets a counter-ion

buried inside the membrane ( DG95 –q2/edH), or by the pump, as

depicted in the insets of Fig. 5. It has been suggested 97that for bio-

logical ion pumps the energy cost of burying the counter ion is

paid for by actively manipulating charged groups in the proteins

within the pumps. However, whether passive or active, high-

permeability membranes with high resistance to fouling are needed,

as well as new strategies for synthesizing membranes with multiple

functions to screen small molecules, and to resist stresses and che-

mical degradation.

Conclusion

The work highlighted here, plus the tremendous amount of addi-

tional research being conducted on every continent that could not be

mentioned, is sowing the seeds of a revolution in water purification

and treatment. We believe that advancing the science of water puri-

fication can aid in the development of new technologies that are

appropriate for different regions of the world. That said, the sheer

enormity of the problems facing the world from the lack of adequate

clean water and sanitation means that much more work is needed to

address the challenges particular to developing nations, which suffer

a diversity of socio-economical-political-traditional constraints, and

require a broader approach incorporating sustainable energy sources

and implementing educational and capacity building strategies.

Consortiums of governments at all levels, businesses and industries,

financial and health organizations, water and environment associa-

tions, and educational and research institutions need to focus increas-

ing attention towards solving these water problems. While better

water resource management, improved efficiencies, and conservation

are vital for moderating demand and improving availability, it is our

belief that improving the science and technology of water purification

can help provide cost-effective and robust solutions.

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Acknowledgements We acknowledge the US National Science Foundation Science and Technology Center, WaterCAMPWS , Center for Advanced Materials for the Purification of Water with Systems.

Author Information Reprints and permissions information is available at www.nature.com/reprints. Correspondence and requests for materials should beaddressed to M.A.S. ([email protected]).

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