Disinfection Byproducts in Drinking Water: Past, Present, and Future

Philip C. Singer University of North Carolina at

Chapel Hill

Tsinghua University October 2012

Disinfection Byproducts in Drinking Water: Past, Present, and Future

The PastChlorine has been used widely for the purification of drinking water since 1906.

The most common oxidant and disinfectant used for the treatment of drinking water:

o Oxidation of reduced Fe and Mn

o Destruction of color

o Oxidation of taste and odor-causing organics

o Control of nuisance aquatic growths

o Inactivation of microbial pathogens

The Dark Side of Chlorine

In 1974, it was discovered that chlorine applied to water forms disinfection by-products (DBPs) that may have adverse human health effects.

Consumption of chlorinated drinking water was linked to cancers of the digestive and urinary tracts.

DBPs have been regulated in finished drinking water in the US since 1979.

DBPs from Chlorination

HOCl + NOM ---> chlorinated DBPs

HOCl + Br- -----> HOBr + Cl- (Fast Reaction)

HOBr + NOM ----> brominated DBPs _____________________________________

HOCl + Br- + NOM ----> chlorinated, brominated, and mixed bromo-chlorodisinfection by-products

Presence of bromide shifts DBP speciation to the Br-containing species

More recently, iodinated DBPs have been reported.

Disinfection By-Products from Chlorination Trihalomethanes [THMs (4)] – CHCl3, CHBrCl2, CHBr2Cl, CHBr3 Haloacetic acids [HAAs (9)] – ClAA, Cl2AA, Cl3AA, BrAA, Br2AA, Br3AA, BrClAA, BrCl2AA, Br2ClAA Only 5 are regulated in the US [HAA5] .

ClH

Cl Cl

BrH

Cl Cl

BrH

Cl Br

BrH

Br Br

Trichloromethane (chloroform)

Tribromomethane (Bromoform)

Dibromochloromethane

Bromodichloromethane

THMs

Hundreds of halogenated DBPs have been identified. Haloacetonitriles, haloacetamides, halonitromethanes, cyanogen halides, haloketones, halogenated furanones. DBPs identified to date comprise only about 50% of the total organic halides (TOX) produced from chlorination. From a weight (µg/L) standpoint, trihalomethanes (THMs) and (haloacetic acids) are the major classes of halogenated DBPs found in finished drinking water.

Symons et al, JAWWA, 1975

DBP Regulations in the US • Nov 1979 THM Rule

– 0.10 mg/L (100 µg/L) maximum contaminant level (MCL) for total trihalomethanes (TTHMs).

– Based on system-wide running annual average • Sample collection at 3 locations with mean

water age and one remote location – MCL basis: chronic (long-term) health concerns

Control of Halogenated DBPs Most common techniques have been: • Source water control measures, such as control of

algal activity and minimizing salt water intrusion.

• Coagulation of natural organic material (NOM) and moving the point of chlorination from raw water to settled or filtered water.

• Use of chloramines as residual (secondary) disinfectant in distribution system.

• Use of alternative oxidants and primary disinfectants, e.g. O3, ClO2, UV irradiation.

The Present

DBP Regulations in the US • Stage 1 Disinfectants/Disinfection By-Products

(D/DBP) Rule (1998) – 0.08 mg/L (80 µg/L) MCL for Total THMs (THM4) – 0.06 mg/L (60 µg/L) MCL for HAA5

• ClAA, BrAA, Cl2AA, Br2AA, Cl3AA – Based on system-wide running annual average

• Sample collection at 3 locations with mean water age and one remote location

– MCL basis: chronic (long-term) health concerns

DBP Regulations in the US • Forthcoming Stage 2 Disinfectants/Disinfection

By-Products (D/DBP) Rule (2013-2014) – Same MCLs for THM4 and HAA5 as Stage 1

– Compliance based upon a location running annual average (LRAA) instead of a system-wide RAA

• Sample collection at locations with maximum levels of THMs and HAAs based on initial distribution system evaluation

– Intended to be more protective of all water users

– Intended to reduce variations in DBP levels, such as those associated with summertime peaks in DBP concentrations (lower acute exposure)

• Information Collection Rule (July 1997-December 1998)

• All large utilities in the US (serving > 100,000 people) monitored raw and finished drinking water

• Provided information on plant operations and design:

-Monthly water quality and treatment data

-Quarterly DBP data

• 296 utilities, 500 treatment plants.

• Data was to be used to support Stage 2 Rule development

Occurrence of DBPs in Finished Drinking Water in the US

McGuire et al, AwwaRF, 2002

1975 1997/8

Control of Halogenated DBPs • Coagulation of natural organic material (NOM)

and moving the point of chlorination from raw water to settled or filtered water.

• Use of alternative oxidants and primary disinfectants, e.g. O3, ClO2, UV.

• Use of chloramines as residual (secondary) disinfectant in distribution system.

Coagulation of Natural Organic Material and Moving the Point of Chlorination

• Coagulation removes UV-absorbing components of dissolved organic carbon – UV-absorbing substances are the primary precursors

of halogenated DBP formation

0

500

1000

1500

2000

2500

0.000 0.100 0.200 0.300 0.400 0.500 0.600

TOX

Form

atio

n Po

tent

ial [

ug/L

]

UV Absorbance [cm-1]

RawCoagulated

Archer and Singer, JAWWA, 2005

Use of Alternatives Primary Disinfectants Do not produce THMs or HAAs Ozone Strong oxidant and disinfectant but produces bromate

when applied to bromide-containing waters MCL for bromate is 10 ug/L Chlorine Dioxide Good oxidant and disinfectant but chlorite is generated

as a byproduct MCL for chlorite is 1.0 mg/L UV Irradiation Effective disinfectant, but requires a secondary

disinfectant

Use of Chloramines for DBP Control In anticipation of the Stage 2 D/DBP Rule, many

utilities in the US have switched, or are switching to chloramination for secondary disinfection.

HOCl + NH3 NH2Cl

Essentially stops formation of THMs and HAAs

Obolensky et al, J Env Eng, 2007

ICR Database: Who is Using Chloramines, and Why? 1997-1998

ICR Database: Who is Using Chloramines, and Why? 1997-1998

Obolensky et al, J Env Eng, 2007

Potential Problems Associated with Chloramination

• Nitrification • Enhanced Formation of Some DBPs • Nitrosamine Formation • Mobilization of Lead

Nitrification • Excess NH3, warm temperatures

• NH3 + O2 NO2- ammonia-oxidizing bacteria

• NO2- + O2 NO3

- nitrite-oxidizing bacteria • Nitrite catalyzes NH2Cl decomposition • Nitrification leads to

– depletion of combined residual – biogrowth in system – potentially high levels of nitrite – reduction in pH – potential increase in corrosion

Auto-

decomposition NH4+ NO2

- AOB

NO3-

NOB

+

NH2Cl

NH2Cl HOCl

NH3

+

NH4+

NH4+

Iron Tubercle Nitrifying Biofilm

Organic Nitrogen

NOM

Nitrification

After Ferguson and DiGiano, JAWWA, 2005

Nitrification Control • Need to develop a rigorous monitoring program for free

ammonia, nitrite/nitrate, combined chlorine residual • Need to optimize Cl2/N ratio and minimize free ammonia

levels at the point of entry to the system – Cl2/NH3-N ratios 4.5 - 5.0 optimal – < 0.2 mg/L free ammonia preferable

• Increase the combined chlorine residual, if necessary, to maintain 2.0 mg/L NH2Cl throughout the system

• Employ and optimize booster chlorination • Decrease water age in the system

– e.g., turn over storage tanks, flush system periodically • Periodically switch to free chlorine?

0

1

2

3

4

5

6

7

0 0.4 0.8 1.2 1.6Free Ammonia - mg N/L

Tota

l Chl

orin

e - m

g C

l 2 /L

Pilot scale data - nitryfying locations Louisville nitrifying locations

Glendale nitrifying locations Bangor nitryfying locations

Pilot scale data - non nitrifying locations Louisville non-nitrifying locations Glendale non-nitrifying locations Bangor non-nitrifying locations Nitrification Potential Curve

Nitrification Courtesy of G. Harrington

Enhanced Formation of Some DBPs by Chloramination

• Formation of some DBPs is greater with combined chlorine than with free chlorine – Cyanogen chloride – Iodinated DBPs – Nitrosamines

Chloramination and Nitrosamines

Nitrosamines – Class of well-known (proven) chemical carcinogens – General Structure R2N-N=O – N-nitrosodimethylamine (NDMA) is most common

nitrosamine

USEPA plans to regulate nitrosamines as a group in the near future

Chloramination and Nitrosamines NDMA

– Formed during chlorination/chloramination of wastewater – Found in treated drinking water

• 25% of US systems had detectable NDMA concentrations (UCMR2)

– By-product of chloramination (a chloramination DBP) • 35% of US systems using chloramination had detectable

NDMA levels (UCMR2) – Also associated with use of certain cationic polymers

(DADMACs) and anion exchange resins – Other nitrosamines found, but at lower concentrations – Concentrations increase with increasing water age

NDMA Formation • Formed from reaction between dimethylamine and other

amine precursors and dichloramine, in the presence of dissolved oxygen

• Low levels of dichloramine typically present in equilibrium with monochloramine

• Dichloramine also formed during breakpoint reactions

Schreiber and Mitch, Env Sci Tech, 2007

NDMA formation after 1 h (▪) and 6 h (□).

Chloramine speciation after 6 h:  NH2Cl (▪), NHCl2 (□), free chorine (gray markers). Note break in x-axis and scale change.

Schreiber and Mitch, Env Sci Tech, 2007

NDMA and Breakpoint Chlorination

•Formed from reaction between amine precursors and NHCl2 •Low levels of NHCl2 present in equilibrium with NH2Cl •NHCl2 formed during breakpoint reactions

NDMA Health Effects/Regulation • 10-6 cancer risk level for NDMA – 0.7 ng/L • 10-6 cancer risk level for bromodichloromethane – 600

ng/L • MCLs being considered by USEPA

– CA Dept of Health has a Notification Level of 10 ng/L – Mass Department of Environmental Protection has a

Guideline of 10 ng/L – Ontario, Canada Ministry of the Environment has a

Maximum Allowable Concentration of 9 ng/L for NDMA

Nitrosamine Control For systems using chloramines as a secondary disinfectant

– Nitrosamines: • Likely to be regulated as a group in the near future • Need to ascertain the presence of nitrosamines and

nitrosamine precursors in raw and finished water • More likely to be a concern in raw waters sources

located downstream of chlorinated wastewater discharges

• Recommend nitrosamine formation potential testing • If nitrosamine precursors are found in the raw water,

need to determine the degree to which they are destroyed by chemical oxidation by primary disinfectants (chlorine, ozone)

Chloramination and Lead Release

• Recent literature suggests that the higher pH values and lower redox potentials associated with the use of chloramines compared to free chlorine can solubilize lead deposits (precipitates) that have accumulated on pipe walls.

• With free chlorine -- Pb(IV)O2 deposits

• With combined chlorine -- Pb(II) deposits

• Pb(II) is more soluble than Pb(IV)

pE-pH Diagram for Pb (Lytle and Schock, JAWWA, 2005)

Current DBP Status in the US

• Utilities seeking compliance with Stage 1 DBP Rule

• Planning for compliance with Stage 2 DBP Rule • Engineering optimized coagulation, use of

alternative primary disinfectants, chloramination, while minimizing downside risks

• Considering alternative precursor removal technologies, e.g. GAC, biofiltration, nanofiltration, anion exchange

The Future • Emerging DBPs

– Hundreds of other DBPs formed when water is chlorinated • Halocetonitriles, halopicrins, cyanogen halides,

haloketones, haloacetamides, halofuranones

– These other DBPs tend to be formed at concentrations 1-2 orders of magnitude lower than regulated THMs and HAAs

– However, some of these other DBPs are orders of magnitude more harmful than regulated DBPs, particularly the N-DBPs

Relative Toxicity of DBPs (after Plewa et al., 2008)

1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06

haloacetamides

halonitromethanes

haloacetonitriles

haloacetic acids

trihalomethanes

geno- and cytotoxicity index (log-scale)

DBP

chem

ical

cla

ss

cytotoxicity

genotoxicity

THMs and HAAs

Other DBPs

The Future

• Emerging DBPs • Nitrosamines

– USEPA plans to regulate nitrosamines as a group in the near future • Byproduct of chloramination • 35% of US systems using chloramination

had detectable NDMA levels (UCMR2) • Nitrosamines are proven carcinogens

The Future

• Emerging DBPs • Nitrosamines • Enhanced precursor removal technologies

– GAC, biofiltration, nanofiltration, anion exchange – Multiple benefits – Precursors for some of these DBPs (e.g. N-

containing natural organic material) are different than those for THMs and HAAs and are not removed as effectively as THM and HAA precursors

The Future

• Emerging DBPs • Nitrosamines • Enhanced precursor removal technologies • Brominated and iodinated DBPs

– As lesser quality waters are used for potable water supply, bromide (and iodide) concentrations are likely to increase

– DBP speciation will shift to brominated and iodinated forms

– Br-DBPs and I-DBPs are more harmful than their fully chlorinated counterparts

Conclusions

• DBPs can be harmful • Initial regulations have led to lower exposures

to DBPs • Water utilities need to address “emerging” DBPs • Investments need to be made in the future to

lower the exposure of the public to DBPs of specific public health concern without sacrificing disinfection effectiveness

phil_singer@unc.edu