Transcript Drinking Water Treatment and Disinfection

Drinking Water Treatment – Chapter 25
Class Objectives
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Be able to define the possible components of a water
treatment train and their functions
•
Be able to differentiate between rapid and slow filtration
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Identify the components of a water treatment train that
are best for a virus. A protozoa.
•
List the possible detrimental effects of microbial biofilms
in water distribution systems
•
Differentiate between dissolved organic carbon and
assimable organic carbon
•
Describe the AOC test
Where does drinking
water come from?
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•
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Rivers
Streams
Lakes
Aquifers
Drinking water treatment
processes
• Water treatment processes provide barriers between the
consumer and waterborne disease
• One or more of these treatment processes is called a
treatment process train
Typical Water Treatment Process Trains
•
Chlorination
•
Filtration (sand or coal)
•
In-Line Filtration
involves a coagulation step (additive
that allows aggregation of
suspended solids, e.g., alum, ferric
sulfate, and ferric chloride,
polyelectrolytes)
•
Direct Filtration
involves a flocculation step where
the water is gently stirred to
increase particle collision thereby
forming larger particles
•
Conventional Treatment
involves a sedimentation step which
is the gravitational settling of
suspended particles
Filtration Processes Used
• Rapid filtration
– used in United States
– fast filtration rates through media (sand or anthracite)
– backwashing needed
• Slow sand filtration
– common in United Kingdom and Europe
– slow filtration rates through media (sand and gravel)
– removal of biological layer needed
– higher removal rates for all microorganisms
Coagulation, Sedimentation, Filtration: Typical Microbial Removal
Efficiencies and Effluent Quality
Organisms
Coagulation and
sedimentation
Rapid filtration
(% removal)
(% removal)
Slow sand
filtration
(% removal)
Total coliforms
74–97
50–98
>99.999
Fecal coliforms
76–83
50–98
>99.999
Enteric viruses
88–95
10–99
>99.999
Giardia
58–99
97–99.9
>99
90
99–99.9
99
Cryptosporidium
Removal efficiency is dependent on microbial type:
• Giardia and Cryptosporidium
– filtration is best
• large size
• resistant cyst and oocyst
• Enteric viruses
– disinfection is ultimate barrier
– filtration and coagulation also help via adsorption to particles
• dependent on surface charge of virus
Water Distribution Systems
Treated drinking water may go through miles of pipe to reach
a consumer. The quality of the water is impacted by several
things:
Dissolved organic compounds in finished drinking water is
responsible for:
• enhanced chlorine demand
• trihalomethane production
• bacterial colonization of water distribution systems
 Increases resistance to disinfection, e.g., E. coli is 2400 X more resistant to
chlorine when attached to surfaces
 Increases frictional resistance of fluids
 Increases taste and odor problems, e.g., H2S production
 Can result in colored water (iron and manganese oxidizing bacteria)
 Can cause regrowth of coliform bacteria
 Can cause growth of pathogenic bacteria, e.g., Legionella
Bacterial growth in distribution systems is influenced by:
• Concentration of biodegradable organic matter
• Water temperature
• Nature of the pipes
• Disinfectant residual
• Detention time within distribution system
How do you determine biodegradable organic carbon in a water
distribution system?
One way is to determine Assimilable Organic Carbon (AOC)
• This test is used to determine amount of organic carbon
capable of being oxidized by microbes
• Measurements of bacterial activity in the test sample are
determined over time by plate counts, ATP, turbidity, or direct
cell counts
AOC Test
• Performed with a single bacterial species, Spirillum NOX or Pseudomonas
fluorescens P-17
• A water sample is pasteurized by heat to kill the indigenous microflora and then
inoculated with the test bacterium in stationary phase
• Growth is monitored (7 to 9 days) until stationary phase is reached
• Growth is determined and compared to standard growth on acetate (AOC
concentrations are then reported as acetate-carbon equivalents)
AOC can be calculated as follows:
AOC (μg carbon/liter) = (Nmax x 1000)/Y
Nmax = CFU/ml
Y = yield coefficient in CFU/μg carbon
where:
When using P. fluorescens strain P-17, Y = 4.1 x 106 CFU/μg acetate-carbon
Thus, if the final yield of the test organism is 5 x 106 CFU/ml after 9 days of
incubation:
AOC = 5 x 106 CFU/ml x 1000 ml/L = 1.22 μg acetate-carbon equivalents/liter
4.1 x 106 CFU/μg acetate carbon
Comparison of Concentrations of DOC and AOC in Various Water Samples
Dissolved organic
carbon
(mg carbon/L)
Assimilable organic
carbon (mg carbon/L)
River Lek
6.8
0.062–0.085
River Meuse
4.7
0.118–0.128
Brabantse Diesbosch
4.0
0.08–0.103
Lake Yssel, after open storage
5.6
0.48–0.53
River Lek, after bank filtration
1.6
0.7–1.2
Aerobic groundwater
0.3
<0.15
Source of water