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CEE 3413 – Environmental EngineeringLecture 13 – Water Treatment, with
Groundwater as Source
INSTRUCTOR:Dr. Tania Datta
Office: Prescott Hall, Room No. 333 Email: tdatta@tntech.edu
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
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
Groundwater Treatment
Mg2+ Removal
• Will need to add OH-
Groundwater Treatment
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
Drinking Water Treatment - Groundwater
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
Drinking Water Treatment - Groundwater
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.
Drinking Water Treatment - Groundwater
Drinking Water Treatment - Groundwater
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
Drinking Water Treatment - Groundwater
• 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
Drinking Water Treatment - Groundwater
• 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
Drinking Water Treatment - Groundwater
Additional Reading Assignment:
• Softening: Textbook, section 6-3
• Adsorption: Textbook, section 6-8