WTP Operator Training:WTP Operator Training:

Water ChemistryWater Chemistry

Missouri Water and Wastewater Conference

Dr. John T. O’Connor, PETom O’Connor, PE

Elements of Water ChemistryElements of Water Chemistry

• Molecular Properties of Water

• Acids and Bases - pH, pOH, Alkalinity

• Solubility Equilibria - Precipitation

• Oxidation-Reduction Reactions

The AtomThe Atom

Water MoleculeWater Molecule

+ +

- -

Hydrogen Bonding - HHydrogen Bonding - H1818OO99

HydrophobiaHydrophobia

Ionization of HIonization of H22OO

What are Acids & Bases?What are Acids & Bases?An Acid is a Proton (H+) Donor

A Base is a Proton (H+) Acceptor

H2O + H2O = H3O+ + OH-

[H+] = [OH-], moles/litre = 10-7 M

pH + pOH = pKw constant

7 + 7 = 14

8 + 6 = 14

Chemical ShorthandChemical Shorthand

pH = - log [H+]pOH = - log [OH-]

pK = - log [K]

e.g., [10 -7 M] = 7

HCl + HHCl + H22O O H H33OO++ + Cl + Cl--

Acids and BasesAcids and BasesAcid pK Base

HCl -3 strong Cl -

H2SO4 -3 SO42-

HNO3 -1 NO3 -

H2CO3 6.3 HCO3 -

H2S 7.1 HS -

NH4 + 9.3 NH3

HCO3 - 10.3 weak CO3 2-

Carbonate EquilibriaCarbonate EquilibriaHH22COCO33 HCOHCO33

--COCO332-2-

AlkalinityAlkalinity

Acid Neutralizing Capacity of Water

(Ability to accept protons)

= HCO3- + 2CO3

- + OH- - H+

Titration End-Point: H2CO3 (pH≈ 4.5)

Hydrogen SulfideHydrogen SulfideHH22S S HS HS-- S S2-2-

Ammonium Ion Ammonium Ion AmmoniaAmmonia NHNH44

+ + NHNH33

Precipitation & SolutionPrecipitation & Solution

Precipitation of Iron, Manganese

Formation of Coagulant Floc

Corrosion of Copper, Lead

Calcium Carbonate Stability

Coagulation withCoagulation withFerric Hydroxide Ferric Hydroxide

Inorganic CoagulantsInorganic Coagulants

• Aluminum Sulfate ( Filter Alum) Aluminum Sulfate ( Filter Alum)

AlAl22(SO(SO44))33 · 14H · 14H22OO Al(OH)Al(OH)3 3

• Ferric Sulfate Ferric Sulfate

FeFe22(SO(SO44))33 · 9H · 9H22OO Fe(OH)Fe(OH)33

Metal SolubilityMetal Solubility

• OH- : Zn(OH) 2Cu(OH) 2 Cd(OH) 2

• CO32- : ZnCO3 CuCO3 CdCO3

• SS2- 2- :: ZnS ZnS CuS CdS CuS CdS

Metal Hydroxide Solubility vs pHMetal Hydroxide Solubility vs pH

At.Wt. Influent Effluentg/mole µg/l µg/l

Zinc 65.4 5730 690

Copper 63.5 2760 297

Cadmium 112.4 94 15

Lead 207.2 100 0

Nickel 58.7 37 13

Iron 55.8 1700 100

Metals Removal in WWTPMetals Removal in WWTP

Oxidation-ReductionOxidation-Reduction

Oxidation is the loss of an electron

Fe2+ (ferrous ion) - e- Fe3+ (ferric ion)

Reduction is the gain of an electron

Clo (chlorine) + e- Cl- (chloride ion)

Rapid to glacially slow reaction rates

Oxidation-ReductionOxidation-Reduction

Oxidizing AgentsOxidizing Agents

Oxygen O°2 - 4e- O 2- (H2O)

Chlorine Cl°2 - 2e- 2Cl-

Potassium Permanganate

KMnO4 - 3e- MnO2

Hydrogen Peroxide

H2O2 - 2e- O2 + H2O

Reducing AgentsReducing Agents

Hydrogen Sulfide H2S + 2e- S0

Nitrite Ion NO2- + 2e- NO3

-

Ferrous Ion Fe2+ + e- Fe3+

Carbon C° + 4e- CO2

ChlorinationChlorination

Cl2 + H2S Cl - + S0

Cl2 + NO2- Cl - + NO3

-

Cl2 + 2Fe2+ 2Cl- + 2Fe3+

Rapid Reaction - Immediate Chlorine Demand

OzonationOzonation

O3 + H2S O2 + S°

O3 + NO2- O2 + NO3

-

O3 + 2Fe2+ O2 + 2Fe3+

Rapid Reduction: O° O2- - 2e-

The Human ElementsThe Human Elements

Groundwater ConstituentsGroundwater ConstituentsAll water is the same.

It is only the stuff in it that is different.

• INORGANICINORGANIC

• ORGANICORGANIC

• GASGAS

Inorganic ConstituentsInorganic Constituents

Major Ions: Ca2+, Mg2+, Na+ ; HCO3- , SO4

2-, Cl-

Nonionic: SiO2

Minor Ions: NH4+, K+ ; F-, PO4

3-

Trace Constituents:Al, As, Ba, B, Br, Cd, Cs, Cr, Co, Cu, I, Fe, Pb, Mn, Hg, Mo, Ni, Ra, Ru, Se, Ag, Sr, Sn, Ti, U, Zn

Electroneutrality ConditionsElectroneutrality Conditions

Missouri River

Dec. 2000

Kansas City

Well Water

Organic ConstituentsOrganic Constituents

Total Organic Carbon (TOC) =

Purgeable organic carbon (e.g., methane)+ Non-Purgeable organic carbon (NPOC)

Dissolved (natural color) (DOC)+ Particulate (bacteria, algae)

TOC in Missouri WatersTOC in Missouri Waters

Source Water TOC (avg)

Lakes 4.8

Rivers 3.6

Wells < 100 ft. 1.2

Wells > 100 ft. 0.2

Dissolved Gas CompositionDissolved Gas CompositionAbundance in Ground WatersAbundance in Ground Waters

Methane CH4

Nitrogen N2

Carbon dioxide CO2

Hydrogen sulfide H2S

Radon Rn

MethaneMethane

• up to 77 mg CH4/l in Illinois Groundwaters

• early methane wells used for home heating,

cooking

• wellhouse explosions

•flaming the tap

Physical & Inorganic Water Qualities - Constant

Cations: Hardness (calcium, magnesium), sodium, potassium, ammonium ions

Anions: Alkalinity (bicarbonate), chloride, sulfatefluoride, arsenite ions; little or no nitrate

Temperature: 12 ± 2 °Celsius; density, viscosity constant

Groundwater CharacteristicsGroundwater CharacteristicsMidwestern United StatesMidwestern United States

GASES DO absent (converted to carbon dioxide)H2S: 0 to 1 mg/l (sulfate converted to sulfides)Methane: 0 to ~ 70 mg CH4/l

METALS Iron, manganese, lead, copper, cadmium, zinc concentrations limited by carbonate, sulfide Arsenic (AsO2

-) - commonly 0 to 50 µg /l

NITROGEN Ammonium Ion: 0 to > 5 mg N/lNitrate Ion: low (converted to nitrogen gas, N2)Nitrite Ion: low

Microbially-Mediated Microbially-Mediated Reducing Conditions Reducing Conditions in Midwestern U.S. Ground Watersin Midwestern U.S. Ground Waters

Historical TOC DataHistorical TOC DataMissouri Finished Drinking Water SuppliesMissouri Finished Drinking Water Supplies

Missouri Finished Drinking Water SuppliesMissouri Finished Drinking Water Supplies

Metals in Municipal WatersMetals in Municipal WatersMissouri Finished Drinking Water SuppliesMissouri Finished Drinking Water Supplies

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Missouri Finished Drinking Water SuppliesMissouri Finished Drinking Water Supplies

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