1
Chapter 8
WATER TREATMENT
Environmental Chemistry, 9th Edition
Stanley E. Manahan
Taylor and Francis/CRC Press
2010
For questions, contact:
Stanley E. Manahan
[email protected]
1
2
8.1 Water Treatment and Water Use
Three Major Categories
• Purification for domestic use
• Treatment for specialized industrial applications
• Treatment of wastewater to make it acceptable for
release or reuse
3
8.2 Municipal Water Treatment
인산염
Figure 8.1 Schematic of a municipal water treatment
plant
Water Purification
• The water is aerated to
increase the amount of
dissolved oxygen and
promote oxidation of
organic impurities.
• Ozone or chlorine is used
to disinfect the water
before it is sent out to
consumers.
Fe3+,
Al3+
Cl2 + H2O → HClO + H+ + ClClO- + H2O → HClO + OH-
미생물산화
3차
응집제처리
활성탄처리
공기 주입
석회처리 (Ca5(PO4)3)5OH)
중금속제거(황이온)
질소화합물제거(탈기법) NH4+, NO3-, CN→
염소처리(소독)
N2, NH3
Cl2 + H2O → HClO + H+ + ClClO- + H2O → HClO + OH-
8.3 Treatment of Water for Industrial Use
Examples of industrial uses requiring water treatment
Cooling water
• Minimal treatment required
Boiler feedwater
• Removal of corrosive and scale-forming substances
Food processing
• Must be free of pathogens and toxic substances
Improper industrial water treatment may cause problems
• Corrosion • Scale formation • Reduced heat transfer
• Reduced water flow with increased pumping costs
• Product deterioration and contamination
• Equipment failure or reduced performance
9
10
External treatment of entire water supply by measures such
as aeration and filtration to remove gases and solids
Internal treatment for specialized applications
• Reaction of dissolved oxygen with hydrazine or sulfite
• Chelating agents to prevent CaCO3 deposition
• Addition of precipitants, such as phosphate for calcium
removal
• Dispersants to inhibit scale
• Inhibitors to prevent corrosion
• pH adjustment
11
8.4 Sewage Treatment
Sewage contains many things such as organics, grit, grease
Three phases of treatment
• Primary
• Secondary
• Tertiary or advanced
A major objective of all three levels of treatment is removal
of biochemical oxygen demand, BOD
12
Primary sewage treatment
Removes constituents that are readily separated physically
• Larger solids • Grit • Grease • Scum
Several kinds of physical treatment
• Primary sedimentation (settling)
• Grit removal
• Surface skimming to remove grease
• Grinding to break up solids
• Filtration
Secondary Waste Treatment by Biological
Processes
Major objective is to remove BOD, {CH2O} that would
consume oxygen in receiving waters
Biochemical reaction is: {CH2O} + O2  CO2 + H2O
Devices with fixed films of active biomass
• Rotating biological contactors (rotating disks)
Figure 8.2 Trickling filter
13
Figure 8.3 Activated Sludge Process
The membrane bioreactor is an alternative
to activated sludge in which water is
withdrawn through a membrane filter
14
15
Anaerobic Digester to Treat Biosolids (Sewage
Sludge) from Secondary Waste Treatment
Methane generated can be used for power in treatment plant
Figure 8.4 Pathways for the Removal of BOD in
Biological Wastewater Treatment
16
Problems with nitrification/denitrification
• Excessive aeration in secondary waste treatment can
produce nitrate which can undergo denitrification producing
N2 bubbles that prevent biosolids settling
4NO3- + 5{CH2O} + 4H+  2N2(g) + 5CO2(g) + 7H2O
17
Tertiary (Advanced) Waste Treatment
Performed on effluent from secondary waste treatment
Generally removes
1. Suspended solids
• Responsible for most BOD in effluent
2. Dissolved inorganics
• Generally algal nutrients, N and P
3. Dissolved organics
• Toxic substances • Personal care products
• Pharmaceuticals and their metabolites
Pathogens may require removal
See also reuse and recycling of wastewater on a later slide
Figure 8.5 A Complete Physical-Chemical Wastewater
Treatment Plant
18
19
8.6 Removal of Solids
Relatively large particles removed by settling and filtration
Colloidal particles usually require coagulation
• Filter alum, Al2(SO4)3•18H2O, most commonly used
• Forms gelatinous Al(OH)3 precipitate
Al(H2O)63+ + 3HCO3-  Al(OH)3(s) + 3CO2(g) + 6H2O
Bridging species such as
the Al(III) dimer are likely
involved in coagulation
Iron salts that form Fe(OH)3 may also be used as coagulants
Natural and synthetic polyelectrolytes aid coagulation
Coagulation-filtration is very effective in removing solids
Figure 8.6 Dissolved Air Flotation
20
21
Table 8.5 Membrane Processes in Water Treatment
Increasing pressure, decreasing size removed
Microfiltration, removes suspended solids, emulsified
components, bacteria, protozoa
Ultrafiltration, removes macromolecules above 5,000100,000 molecular mass (function of pore size)
Nanofiltration, removes molecules above 200-500
molecular mass (function of pore size)
Most solutes and ions, used for desalination

8.7 Removal of Calcium and Other Metals
Ca2+ and Mg2+ are water hardness
• Form precipitates with soap making it ineffective
• Detrimental to synthetic detergents
• Produce scale in plumbing, hot water systems, boilers
Deposit formation in hard water
• Occurs when water is heated driving off CO2 gas
Ca2+ + 2HCO3-  CaCO3(s) + CO2(g) + H2O
22
Softening water (removal of Ca2+ and Mg2+)
23
Lime softening
• Ca2+ + 2HCO3- + Ca(OH)2  2CaCO3(s) + 2H2O
Softening by addition of sodium carbonate when calcium is
present as a salt other than carbonate
• Ca2+ + 2Cl- + 2Na+ + CO32-  CaCO3(s) + 2Cl- + 2Na+
Addition of sodium carbonate can provide high pH to
precipitate Mg2+
• Mg2+ + 2OH-  Mg(OH)2(s)
Recarbonation with CO2 lowers pH, prevents precipitates
• OH- + CO2  HCO3• CaCO3(s) + CO2 + H2O  Ca2+ + 2HCO3• Mg(OH)2(s) + 2CO2  Mg2+ + 2HCO3Aggressive water (too much CO2) is corrosive
Chemically stabilized water on verge of precipitating CaCO3
Precipitation of calcium with orthophosphate
• 5Ca2+ + 3PO43- + OH-  Ca5(PO4)3(s)
Figure 8.7
24
Regeneration of ion exchanger with NaCl adds salinity to
wastewater
Chelation (sequestration) with EDTA to prevent preciptates
25
Removal of Iron and Manganese
• Soluble in water as Fe2+ and Mn2+
• Oxidation to insoluble Fe(OH)3 and MnO2
• Removal complicated by metal ion chelation
Removal of Heavy Metals
• Lime softening remove heavy metals
• Coprecipitated with Fe(OH)3 and MnO2
• Precipitated as sulfides such as PbS
• Cementation
Fe(s) + Cd2+(aq) Fe2+(aq) + Cd(s)
• Electrodeposition
• Activated carbon
8.8 Removal of Dissolved Organics
Many kinds including
• Disinfection byproducts
• Humic substances
Standard removal process is with activated carbon
• Activated by high-temperature reaction with H2O (steam)
or CO2
Some organic resins remove dissolved organics
Oxidation
• Oxidizing agents such as ozone, O3
• May be augmented photochemically with ultraviolet
radiation, hn
26
27
Removal of Dissolved Inorganics
Distillation
Ion Exchange (NaCl removal)
• H+-{Cat(s)} + Na+ + Cl-  Na+-{Cat(s)} + H+ + Cl• OH-+{An(s)} + H+ + Cl-  Cl-+{An(s)} + H2O
Figure 8.9
Electrodialysis
removal of ionic
solutes
Reverse Osmosis, Figure 8.10
28
Phosphorus Removal
Most commonly by precipitation with Ca2+
• 5Ca2+ + 3PO43- + OH-  Ca5(PO4)3(s)
See other precipitants for phosphate in Table 8.2
Nitrogen Removal
• From ammonia by chlorination
NH4+ + HOCl  NH2Cl + H2O + H+
2NH2Cl + HOCl  N2(g) + 3H+ + 3Cl- + H2O
• By nitrification/denitrification
6NO3- + 5CH3OH + 6H+ (denitrifying bacteria) 
3N2(g) + 5CO2 + 13H2O
29
8.10 Sludge
30
Sludge consists of solids left over from water treatment
• Biosolids is a term given to sludge from biological
treatment
• High water content (typically 85%)
• Disposal is a problem
• Secondary wastewater treatment sludge usually treated
by anoxic biodegradation, which generates methane used
for fuel
• Sometimes pathogens in sludge cause problems
Chemical sludges contain residues from the addition of
Ca(OH)2 (lime) and also contain Al(OH)3 or Fe(OH)3 added as
precipitants
31
8.11 Water Disinfection
Most commonly from addition of chlorine or hypochlorite
salts such as Ca(ClO)2
• Cl2 in water produces HClO and ClO- ion
• Free available chlorine = HClO + ClO• Combined available chlorine consists of chloramines
produced by reaction of Cl2 and hypochlorite with NH4+:
NH2Cl, NHCl2, NCl3
Chlorine dioxide, ClO2, for water treatment to avoid
formation of trihalomethanes, such as HCCl3
• Produced on-site by reaction
2NaClO2(s) + Cl2(g)  ClO2(g) + 2NaCl
Ozone and Other Oxidants
Figure 8.11 Generation of O3 for water treatment
Ozone is a green water
disinfectant
Ferrate, FeO42-, is a good oxidant
Oxidation augmented by ultraviolet radiation, a
green, reagentless technology
32
8.12 Natural Water Purification Processes
33
Soil is a natural filter for wastes and wastewater
Physical, chemical, biological characteristics of soil enable
waste water treatment
• Detoxification • Biodegradation
• Chemical decomposition • Physical and chemical fixation
Bacteria, actinomycetes, and fungi in biodegradation
Industrial Wastewater Treatment by Soil
• Acclimated microorganisms
• Very effective for petroleum wastes
8.13 Green Water
Much of the world suffers from water deficiency
Water may be available from desalination of sea water or
brackish groundwater
• 70% of Saudi Arabia water
• Multistage flash evaporation
• Reverse osmosis (Section 8.9 and Figure 8.10)
Water reuse and recycling
• Recycling before water is ever discharged
• Reuse when another consumer uses discharged water
Current major systems for water reuse
• Irrigation
• Cooling water
• Groundwater recharge
34
Figure 8.12 System for water reuse
Current state-of-the-art water
renovation system is the
Groundwater Replenishment
System in Orange County,
California
35
36
Figure 8.13 Injection of reclaimed water to underground
aquifers to prevent seawater intrusion
37
13.14 Water Conservation (using less)
1.
2.
3.
4.
5.
6.
Indoor and household water conservation practices
Water-conserving devices and appliances
Outdoor and landscaping water conservation
Efficient irrigation practices
Conservation in non-irrigation agricultural practices
Water-efficient manufacturing
13.15 Protecting Water Supplies from Attack
Potential contamination by pathogens such as
• Shigella dysenteriae • Vibrio cholerae
• Cryptosporidium parvum
• Prevented by maintaining disinfectant chlorine residual
Potential contamination by toxic substances
• Biological toxins such as botulinus
• Chemical toxins such as cyanide
38
39
40
41
X
X
X
X
X
X
X
X
42