Lecture 9 - Groundwater Pollution and Treatment.markup

CEE 3413 – Environmental EngineeringLecture 13 – Water Treatment, with

Groundwater as Source

INSTRUCTOR:Dr. Tania Datta

Office: Prescott Hall, Room No. 333 Email: [email protected]

Ground- vs. Surface Water

Groundwater• Constant composition• Low turbidity• Low color• Low or no DO• High hardness• High Fe, Mn

Surface water• Variable composition• High turbidity• Colored• DO present• Low hardness• Taste and odor

Types of Potable Water Source

• Primary objectives are to1.Remove hardness and other ions2.Eliminate pathogenic organisms

• Treatment technologies largely based on chemical precipitation

Groundwater Treatment:

Groundwater Treatment


• Depending on the characteristics of raw water, primary objectives of Groundwater Treatment are:

o Removal of Hardness

o Removal of Iron and Manganese

o Removal of other ions or heavy metals

o Filtration

o Disinfection

Total Hardness (TH)Groundwater Treatment

• Practically - the sum of the predominant polyvalent cations (Mg2+ and Ca2+ only) in consistent units (meq/L or mg/L as CaCO3)

22 CaMg CCTH

• Hardness (Ca2+ and Mg2+ ) is commonly removed in a process called “softening”

• In softening, these ions are removed according to these general reactions:

)(2

)(

22

323

2

sOHMgOHMg

sCaCOCOCa

Groundwater TreatmentSoftening

Ca2+ Removal

• If water naturally had enough carbonate, CO32-,

for all Ca2+ to precipitate, removal of hardness would be easy

• Since it typically doesn’t, bicarbonate (HCO3-) is

converted to carbonate (CO32-) by raising pH:

HCO3- + OH- = CO3

2- + H2O


Mg2+ Removal

• Will need to add OH-


Lime-Soda Softening

• Adjust chemistry of the water by adding lime (CaO) or soda ash (Na2CO3)

• Which chemical to add depends on the type of hardness to be removed

Drinking Water Treatment - Groundwater

Softening Chemistry1. Neutralization of free acid/H2CO3 removal

H2CO3 + Ca(OH)2 CaCO3(s) + 2H2O

2. Precipitation of CH due to calcium

Ca2+ + 2HCO3- + Ca(OH)2 2CaCO3(s) + 2H2O

3. Precipitation of CH due to magnesium

Mg2+ + 2HCO3- + 2Ca(OH)2 2CaCO3(s) + Mg(OH)2(s)+ 2H2O


4. Removal of NCH due to calcium:Ca2+ + Na2CO3 CaCO3(s) + 2Na+

5. Removal of NCH due to magnesium :Mg2+ + Ca(OH)2 + Na2CO3 Mg(OH)2(s)+ CaCO3(s) + 2Na+

Softening Chemistry


Softening Process Limitations• Due to kinetic limitations and the solubility of CaCO3 and

Mg(OH)2, lime-soda softening cannot produce water completely free of hardness

• Ca2+ limited to ~ 30 mg/L as CaCO3

• Mg2+ limited to ~ 10 mg/L as CaCO3

• To avoid slimy feel of soft water, normally aim for 75-120 mg/L as CaCO3.



Softening Process Limitations• Based on experience, more Ca(OH)2 must be provided than

that predicted by stoichiometry

– a minimum excess of 20 mg/L as Ca(OH)2 is used

• Also, Mg2+ in excess of 40 mg/L causes scaling. Therefore treatment plants only remove Mg2+ in excess of 40 mg/L

Ion-Exchange Softening


• Ion-exchange is a process that drives reversible exchange of ions between a solid and a liquid phase

• Calcium and magnesium in the water are exchanged for sodium from the resin granules.

SOURCE: http://water.me.vccs.edu; http://www.isws.illinois.edu/chem/psl/softeners.asp

Resins

Ion-Exchange Softening


• Ion exchange softening is effective at removing both carbonate and noncarbonate hardness

• Often used for waters high in noncarbonate hardness

• Disadvantages include:

o The Ca2+ and Mg2+ ions are replaced by sodium ions, which may cause health problems for people not supposed to eat any salt

o Ion-exchange resins also have to be regenerated, similar to the backwash concept of filters


Additional Reading Assignment:

• Softening: Textbook, section 6-3

• Adsorption: Textbook, section 6-8

Documents

Lecture 9 - Groundwater Pollution and Treatment.markup