Drinking Water Treatment and Disinfection

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21 Φεβ 2014 (πριν από 3 χρόνια και 8 μήνες)

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Drinking Water Treatment


Chapter 25

Class Objectives


Be able to define the possible components of a water
treatment train and their functions



Be able to differentiate between rapid and slow filtration



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?




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
(% removal)

Rapid filtration
(% 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

Cryptosporidium

90

99

99.9

99


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


Removal efficiency is dependent on microbial type:

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

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


How do you determine biodegradable organic carbon in a water
distribution system?


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 (
μ
g carbon/liter) = (
N
max

x 1000)/Y where:

Nmax

= CFU/ml

Y =

yield coefficient in CFU/
μ
g carbon


When using
P. fluorescens
strain P
-
17, Y

=

4.1 x 10
6

CFU/
μ
g acetate
-
carbon


Thus, if the final yield of the test organism is 5 x 10
6

CFU/ml after 9 days of
incubation:



AOC =
5 x 10
6

CFU/ml x 1000 ml/L

= 1.22
μ
g acetate
-
carbon equivalents/liter





4.1 x 10
6

CFU/
μ
g acetate carbon

AOC Test

Comparison of Concentrations of DOC and AOC in Various Water Samples

Source of water

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