Transcript Water Treatment

Water Treatment:
Disinfection Processes
Current Technology vs. Alternatives
Water Treatment
Why?
To minimize the amount potentially harmful
organisms in the water
To make the water aesthetically acceptable
(look and taste good)
Regulations
• Safe Water Drinking Act
– 1979, amended 1986
– Standard techniques
– Sets Maximum Contamination Levels for THMs
(100 ppm)
• Surface Water Treatment Rule (SWTR)
– 1989
– required disinfection and filtration of surface water
sources to remove pathogens
• Disinfection/Disinfection By-Product Rule
– lowers the MCL of THMs to 80 ppm
Parts of Water Treatment
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Coagulation
Flocculation
Sedimentation
Filtration
Disinfection
Methods of Disinfection
• Heat
• Boiling
• Radiation
• UV light
• Chemical
• Chlorine
• Alternatives
Factors Effecting Chlorination
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Concentration
Contact Time
Temperature
pH
& Foreign Substance
concentration
Chlorine
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Strong Disinfectant
Lasting “Residual”
THM Production
Treated water is “toxic”
Chlorine Reactions
Cl2 + H2O
Chlorine
HOCl
+
Water
Hypochlorous acid
HOCl
H+
Hypochlorous acid
+
Hydride ion
Means that in solution
it remains in Equilibrium
H+
+
Hydride ion
ClOHypochlorite ion
Cl-
Chloride ion
Chlorine Reactions
• Oxidation
– Cleaves molecules
• Substitution
– Replaces elements
in a molecule
RCHO + HOCl RCO2H + H+ + Cl-
OH
*
OH
Resorcinol
+ HOCl  *CHCl3+ other
Products
Chloroform
Figure 5 (Johnson, 1983)
Disinfection By-Products
• Most of the research and concern is about
THMs
• THMs (Trihalomethanes)
–
–
–
–
Chloroform (CHCl3)
Bromodichloromethane (CHBrCl2)
Dibromochloromethane (CHBr2Cl)
Bromoform (CHBr3)
THM Formation Equation
Free Chlorine
THMs
+
+
Precursors
(humic substances
and Bromide)
Other Halogenated
& non- halogenated
By-Products
THM Precursors
Source
• Plants
• Algae
• Man
Examples
• Fulvic and Humic Acid
Degradation Products
like resorcinol.
Plant pigments
• Algae Biomass
Amino acids (i.e..
uracil)
• Industrial Effluents
(i.e.. Phenols)
Table 2 (Johnson, 1983)
Alternatives to Chlorine
• Ozone
– O3
• Chlorine Dioxide
– ClO2
• Chloramines
– NH2Cl,NHCl2,
NH4Cl
Chlorine
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Strong disinfectant
Lasting “Residual”
THM Production
Treated water is “toxic”
Ozone
Powerful Oxidant
Lacks “residual”
Bromate formation
Need secondary
disinfectant, can be
expensive
Chloramines
• Weak disinfectant
• Little THM Production
• Treated water is “toxic”
Chlorine Dioxide
• Good disinfectant
• Must be prepared “on-site”
• by products: Chlorate and
Chlorite
• Lacks “residual”
• ClO2 presence reduces THM
formation
THM Control Methods
• Potassium Permanganate (KMnO4)
–
–
–
–
Strong oxidant
Effective for taste and odor problems
oxidizes iron and manganese
controls some THM precursors
• Enhanced Coagulation
– Modifications of conventional treatment like
pH and Coagulant dose
– Performs better in TOC removal than most
other methods
Conclusions
• Regulations result from ongoing research
• Changes in treatment occur when regulations
change
• Any kind of “treatment” is going to have some
kind of side effect, whether it be by-products or
limitations in efficiency
• Which of the side effects is worse? Until
everything about each possible alternative is
known, the true answer to that question will
remain definitively unanswered.
• Chlorine appears to be the most well rounded
choice for disinfection, and will most probably
remain the main disinfectant for most treatment
plants
THE END