Advanced Reduction Processes for Water and Wastewater Treatment

Ahmed Abdel‐WahabProfessor, Chemical Engineering Program

Director, Qatar Sustainable Water and Energy Utilization Initiative (QWE)Texas A&M University at Qatar

World Water Day WorkshopWater Research Center, Sultan Qaboos University 

Water Supply/Use versus Water Consumption

SupplyConsumptive use• Evaporated during the process• transpired by plants • incorporated into products or crops • consumed by people or livestock

Water Supply/Use versus Water Consumption

Supply

Consumptive use

Lost/Wasted• Returned to source• Lost by other means

Inefficient, driven by demand

• Network losses• Infiltration• Disposal of treated wastewater• Disposal of industrial wastewater• Loss/evaporation due to inefficient use

Supply Consumption

Ideal

100% water efficiencyImpossible

CostPracticality

Uncontrolled losses

Water Supply/Use versus Water Consumption

Consumptive use

Lost

EfficientHolistic approach to resources managementMaximized reuse/recycleMinimum lossesOptimized water consumptionOptimized energy consumption Are we there yet??

Example of an efficient system with a holistic approach management

Treatment

Wastewater

Urban channelsDischarge

Organic solid waste

Fuel or electricity

Biogas

Energy

Food and Fibre

Biosolid

Harvested plants

Sludge

UrbanCenter

Water supply

Dis

trict

coo

ling

Land

scap

ing

Indu

stry

Biosolid

Areas of Improvement

Don’t put all your efforts on the backend!

• Fragmented management• Risk-free approach• Specific sectorial goals• Lack of holistic approach to

resources management• Subsidizing the tariffs• Inefficient irrigation systems• Lack of regulatory

framework/incentives for wastewater recycle/reuse

• Public perception with respect to wastewater reuse

Success Story- Singapore NEWater Project• Introduced in 2002• 4 NEWater plants can

meet 30% of Singapore’s water needs

• NEWater to meet 50% of Singapore’s water demand by 2060

• This project is supported by significant research effort for continuous advancement

http://www.pub.gov.sg

Opportunities/Motivation• Cost of wastewater treatment is

less than one third of desalination cost

• Clustered industrial facilities offers the potential for macroscopic industrial water management

• Solar Energy is a sustainable source of energy for water production and treatment

• Hybrid systems can significantly reduce desalination/treatment cost

• Human capacity building and public awareness are key elements in achieving the sustainability goals

Advanced Reduction Processes for Water and Wastewater Treatment

• Oxidation-reduction reactions are the primary method for destroying environmental contaminants.

• Every oxidation-reduction reaction must be feasible thermodynamically if it is to occur.

• However, the requirement to achieve desired levels of destruction within a reasonable time is often not met.

• Therefore, the main limitation in developing redox treatment processes is normally related to the process kinetics.

• Although a multitude of reactions are possible, only a few occur at sufficiently rapid rates to be used efficiently for treatment.

Introduction

• An example of a group of redox treatment processes that are able to meet kinetic limitations are Advanced Oxidation Processes (AOPs).

• AOPs have been applied to a number of water treatment problems where oxidation of contaminants is required.

• AOP is based on the formation of hydroxyl radicals (OH˙) that act as oxidants for the target contaminants.

• The hydroxyl radical is an effective oxidant because it reacts rapidly with a large number of compounds by removing electrons from them.

Advanced Oxidation Processes (AOPs)

• Applications of free radical chemistry have been limited almost entirely to applying oxidizing free radicals.

• However, reductive free radicals can also be formed and applied to treatment problems that require reductions.

• Examples of oxidized contaminants that are destroyed by reductive treatment include:

chlorinated organics, perchlorate, chromate, nitrate, nitrite, arsenate, selenate, bromate, chlorate, and a number of radionuclides.

• Advanced Reductive Processes (ARPs) are based on an approach that is similar to that of many AOPs, i.e. combining reagents and activating methods to produce reactive free radicals.

• However, ARPs produce reducing, rather than oxidizing, free radicals.

Advanced Reduction Processes (ARPs)

Advanced Reduction Processes (ARPs)

Reducing Agent

Reducing Agent

Activating method

Activating method

ReductantRadical

ReductantRadical

SulfiteSulfite

DithioniteDithionite

SulfideSulfide

Ferrous ironFerrous iron

UV light

E-Beam

Ultrasound

Microwave

SO2•-

SO3•-

HS•-

eaq-

H•

• Dithionite:• Dithionite (S2O4

-2) is known to have a long, weak S-S bond that can be broken to produce two sulfur dioxide radical anions

S2O4-2 = 2 SO2•-

• It has an absorption peak in the ultraviolet near 315 nm

• It is a high production volume chemical with low price

• Sulfite:• Sulfite solutions absorb UV light with a maximum near 276 nm.

• UV irradiation of sulfite solutions has been found to produce sulfite radical anion.

• The hydrated electron is another strong and rapid reductant and it has been found in sulfite solutions irradiated with UV light.

Advanced Reduction Processes (ARPs)

• Sulfide:• Sulfide solutions absorb UV light with a maximum at 230

nm.

• Sulfide irradiation with UV has promoted formation of sulfide radicals and hydrogen.

• Ferrous Iron:• Solutions of ferrous iron absorb UV light with a maximum at

220 nm and UV irradiation promotes formation of hydrogen and aqueous electrons .

Advanced Reduction Processes (ARPs)

Previous and ongoing ARP projects1. “Advanced reduction process for hazardous waste treatment”, National

Priorities Research Program (NPRP), Qatar National Research Fund (QNRF), 2009 - 2012.

2. “Disinfection by products removal from water using advanced reduction process”, NPRP, QNRF; 2012 - 2015.

3. “Reductive immobilization and removal of arsenic and selenium from contaminated water using advanced reduction process”, NPRP, QNRF; 2014 - 2017

4. “Solar-driven advanced reduction processes for destroying persistent contaminants in water”, NPRP, QNRF; 2016 - 2019.

18

Discovery Project (NPRP 08 - 172 - 2 –049)

SO32-

S2O42-

Fe(II)

S2-

UVL/UVM/UVB

Microwave

Ultrasound

Electron Beam

Targets

VC

1,2-DCA

• waste streams from PVC and vinyl product manufacture facilities

• intermediate accumulation from reductive biodegradation of PCE/TCE

• accidental release

• Maximum Contamination Level (MCL) for drinking   water    MCL of VC=2 μ g/L MCL of 1,2-DCA= 5 μ g/L

VC and 1,2-DCA

Results of screening tests

SO32-

S2O42-

Fe(II)

S2-

UVL/UVM/UVB

Targets

VC

1,2-DCA

Direct Photolysis of VC • Direct photolysis with UV-L with a

peak at 254 nm wavelength resulted in VC destruction.

• VC degradation kinetics under direct photolysis was assumed to follow a pseudo-first-order decay model:

���

• At a UV light intensity of 2400 µw/cm2 and initial VC concentration of 0.5 mg/L, the rate constants were found to be 0.012, 0.011, and 0.018 min-1 at pH 3, 7, and 10, respectively.

Direct Photolysis of VC• There are two possible pathways that could describe the transformation

from VC to chloride.

Direct Photolysis of VC

0 50 100 150 200 250 300

0.2

0.4

0.6

0.8

1

1.2

1.4

Irradiation Time (min)

VC

con

c. (m

g/L)

VC+UV no bufferModel VC+UV no buffer

pH 8.55 pH 7.93

pH 7.90

pH 7.84

pH 7.31

• VC degradation by various reagent/UV combination

Reagents screening VC

• 1,2‐DCA degradation by various reagent/UV combinationReagents screening 1,2-DCA

dithionite sulfite

sulfide ferrous iron

Reagents screening Summary • Most  ARPs (reagent/UV combination) are effective in 

degrading VC and 1,2‐DCA

• pH has great effect on the degradation rates

• The rapid degradations caused by reactive species that are produced when the reducing reagents receive UV irradiation dioxide radical (SO2

•-) sulfite radical (SO3•-)

hydrated electron (eaq-) hydrogen atom (H)

• Dechlorination efficiency: the fraction of chlorine atoms in  VC or 1,2‐DCA that was degraded and converted to chloride ions

• Rdech = (CCl , chloride ion released)/ (CCl , in initial VC or 1,2‐DCA‐CCl, in final VC or 1,2‐DCA)

• Major influence factor: Solution pH

• At low or neutral pH (≤7), chloroethane (C2H5Cl) is the major organic product

• At higher pH (≥8.2), non‐chlorinated hydrocarbon (probably propane C3H8) is the major organic product

• At pH 11, >90% dechlorination obtained (all organic Cl releases as Cl‐)

Dechlorination efficiency

• Reactive species in the sulfite/UV ARPaqueous electron (eaq

-) and sulfite radical (SO3•- )

which one is the major species that causes the degradation?

• Use eaq‐ scavengers to test the mechanism

• eaq‐ scavengers: NO3‐ and N2O

reaction rate of eaq- and NO3

- / N2O >> reaction rate of eaq

- and VC / 1,2-DCA

• NO3‐ or N2O does not scavenge SO3

•‐

Degradations mechanisms

Results

Degradations mechanisms

VC + sulfite/UV + NO3- Degradation kinetics is not affected

VC + sulfite/UV + N2O Degradation kinetics is not affected

1,2-DCA + sulfite/UV + NO3- Degradation is completely inhibited

1,2-DCA + sulfite/UV + N2O Degradation is completely inhibited

eaq‐ is the major reactive species causing  1,2‐DCA degradation

SO3•‐ is the major reactive species causing VC degradation

• Bromate occurrence in high concentrations in desalinated water and in reject brine from desalination plants is a major concern in the GCC.

• High concentrations of bromide (~76 mg/L) promotes the formation of high concentrations of bromate. 

• Published reports indicated that tests of drinking water samples from a GCC country showed bromate levels around 10 times the WHO's recommended guidelines.

• Chlorate is a disinfection byproduct resulting from the use of chlorine dioxide as a disinfectant.

• Once bromate or chlorate ions are formed in water, they are relatively stable in environmental conditions and very difficult to remove 

Disinfection Byproducts Removal from Water Using Advanced Reduction Process (NPRP 4 - 1174 - 2 – 458)

Bromate reduction – Direct photolysis

Figure 3. Mechanisms of bromate reduction by sulfite/UV ARP. [14]

The effect of sulfite dose on bromate removal. Conditions: [bromate]0 = 4 μM, UV-M irradiance ~ 3,500 μW/cm2, and UV-L ~ 4,900 μW/cm2

Time (min)

0 20 40 60 80 100 120 140 160

Nor

mal

ized

Con

cent

ratio

n (C

/C0)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

only bromatesulfite 1.56 mg/Lsulfite 3.1 mg/Lsulfite 7.8 mg/Lsulfite 15.6 mg/Lsulfite 31.3 mg/L

(c) UV-L

0 20 40 60 80 100 120 140

Bro

mat

e C

once

ntra

tion

(M

)

0

1

2

3

4

5

Only bromateSulfite 1.56 mg L-1 (5 times)Sulfite 3.1mg L-1 (10 times)Sulfite 4.78 mg L-1 (15 times)Sulfite 7.8mg L-1 (25 times)

(a)

UV-L (254 nm) UV-M (200-700 nm)

Bromate removal by sulfite/UV ARP

33Ref. B. Jung et al. Chemosphere 117 (2014) 663-672

Chlorate (ClO3-)

UV-B280-320 nm

UV-B280-320 nm

UV-M320-380 nm

UV-M320-380 nm

UV-L254 nm

UV-L254 nm

Ref. B. Jung et al., Int. J. Environ. Sci. Technol. 2016, DOI 10.1007/s13762-016-1132-y

S2O42- + hν 2SO2

-

Dithionite rapidly decomposes at acidic pH and forms various products.

2S2O42- + H2O 2HSO3

- + S2O32-

2HSO3- S2O5

2- + H2O

Decomposition products:Sulfite (SO3

2-), Bisulfite (HSO3-),

Thiosulfate(S2O32-),

Metabisulfite(S2O52-)

Therefore, a decomposition product that is being activated by UV could be responsible for chlorate photodegradation.

34

Dithionite/UV ARP

SummaryUV-L UV-M UV-B

Dithionite VC1,2-DCA

NitrateBromate1,2-DCA

Chlorate1,2-DCA

Sulfite

NitrateVC

1,2,-DCABromate

ChlorateBromate1,2,-DCA

1,2-DCA

SulfideNitrate

VC1,2-DCA

1,2-DCA

Ferrous iron VC

35

Advanced Reduction Processes (selenium & arsenic)

2S2O42- + H2SeO3 + H2O → Se + 4HSO3

-

S2O42- + HSeO3

- → Se + 2HSO3- + OH

Solar-Driven Advanced Reduction Processes

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

0 20 40 60

Con

cent

ratio

n of

chl

orat

e (m

g/L)

Time (min)

chlorate+TiO2+solar chlorate+dithionite12mM+TiO2+solar

Time (min.)

% TCE removalTiO2 + sunlight TiO2 + sulfite + sunlight

30 45% 72%60 63% 89%

-0.56

+2.58

V vs. NHE @ pH = 7

3.2 eV

1.3 eV

Bi2S3TiO2

e-

h+

TiO2

Ti3+

CB

VB

Ox.

Red.

Red.

Ox.

TiO2

Red.

Ox.

CB

VB

CB

VB

Fe(III)

TiO2

N

Red.

Ox.

CB

VB

VisVis Vis

Vis

3.2 eV

MWCNTTiO2

e-

h+

CB

VB

Ox.

Red. Vis

(A) (B) (C) (D) (E)

VLR-TiNP@Fe3O4

Magnetic core

VLR-TiSD

Spindle

MWCNT+VLR-TiNTMWCNT@VLR-TiSD

(A) (B) (C) (D)

Solar-Driven Processes in the Dark!

• A holistic approach to water resources management is a key for maximizing water efficiency 

• More effort is required for maximizing wastewater reuse

• Advanced oxidation and reduction processes can destroy persistent contaminants and eliminate them from the environment  

• Abundance of solar energy in the region makes it an attractive energy source for water and wastewater treatment

Concluding Remarks

Acknowledgments

THANK YOU