Chapter 16. Water Pollution and Waste:
Water Treatment Chemistry
Definition of water pollution
Pollutant is ‘a substance or effect which adversely alters the environment by changing the growth rate of species, interferes with the food chain, is toxic, or interferes with health, comfort, amenities, or property values of people’
Guidelines for the alteration of the environment
-Physical properties of T, Colour, Odour, and turbidity
-For general chemical classes of chemical properties such as pH, total dissolved solids (TSS), salinity, hardness, biological oxygen demand (BOD), detergents, and petroleum residues
-For specific elements, complex ions, and organic compounds; for radiological properties, that is levels of radioactivity due to particular isotopes; and
-For microbiological properties, that is, counts of specific organisms and groups of organisms
Community waste water = domestic wastes + industrial and other effluents
Major components of community waste water: being made up of human wastes, solid and dissolved forms of food wastes, soaps and detergents, and soil residues.
Typical measured properties of untreated sewage include the following
The underlined items are of major concern because they all upset the normal balance of aquatic life
16.1 Community waste-water treatment chemistry
Biological oxygen demand (BOD) 250mg L-1
Chemical oxygen demand (COD) 500mg L-1
Total solids (TS) 720mg L-1
Suspended solids (SS) 220mg L-1
Total phosphorus (TP) 8mg L-1
Total nitrogen (TN) 40mg L-1
pH 6.8
of animal and vegetative wastes via aerobic microbial reactions. In the process, organic wastes are converted into simple organic and inorganic compounds, and oxygen into carbon dioxide. The carbon dioxide, under the influence of light, takes part in photosynthesis and oxygen is returned to the water. This is
This cycle of self-purification is broken by the presence of excessive amounts of degradable amounts of OM (high BOD) producing anoxic conditions, by turbidity (high SS) that inhibits photosynthesis, and by unusually large conc of nutrients (most often, high TP) that stimulate plant and algal growth.
Waste water treatment plants offer primary, secondary, and tertiary treatment.
Primary treatment-pulverization to reduce large-sized solid material to smaller dimensions, followed by screening and flowing through clarifier
Secondary-aerobic biological process, e.g. activated sludge process (ASP). Oxygen is provided together with sludge, which is the previously activated biological mass. C, N, and P are taken up by microbes. Metals and other contaminants are removed by use of coagulants.
16.1 Community waste-water treatment chemistry
-to assist in the coagulation and flocculation processes, in order to maximize removal of very small solid particles of various compositions, and
-to react with and remove potential pollutant chemical species such as phosphorus in the water.
Stoke’s law
size.
specified the
of tank a in completely settle
to time sufficient have
will mm 0.1 of diameters particle
with
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time detention the
called is
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average an
takes
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designed been
has clarifier A typical
min.
7.4 about 440
) 10 4/(9 in
settle will dimensions e
with thes particles
sand 4m, of depth a
has tank settling the
Where
9 10
9
10 18
) 10 ( 8 . 9 ) 1000 2650
(
then , m kg 10 2.65 of density particle
assuming
diameter, in
m) (10 mm 0.1 sand, of particle spherical
a o relation t Stokes
apply the we
If
C.
20 and P at s m kg 10 1.0 water of
viscosity
and
; diameter/m particle
d
; s 9.8m g
C;
20 and P at m kg 10 1.0 water of
density
; m g particle/k of
density
; s m particles/
of velocity terminal
where
18 ) (
2 3
-
1 1
3
3
2 4 3
- 3
4 - 1
- 1 - 3
p 2 - 3
- 3
p 3 - p
1 - t
2
s
mms ms
v
v gd
t
p a p t
Coagulants: Alum (Al2(SO4)3), FeCl3, and hydrated lime (Ca(OH)2)
Rxn 16.6 plays a role in removal of small particles that cause turbidity.
Aluminum hydroxide precipitate carries a net positive charge, so neutralizes the negative charges of the SS. Neutralization of the double layer charge allows the colloids to approach each other and come together into larger aggregates that readily settle in the clarifier (the first function of coagulants)
16.1 Community waste-water treatment chemistry: Chemical coagulants
(16.6)
6 ) ( 3
) ( ) (
) ( 3
) ( ) (
(16.5)
) ( )
( ) (
) (
) ( ) (
) (
(16.4)
)
( )
( ) (
) (
) ( ) (
) (
(16.3)
)
( )
( ) (
) (
) ( ) (
) (
(16.2)
) ( )
( ) (
) (
) ( ) (
2 2
3 3
3 6 2
3 4
2 2 2
3 3
2
3 3
3 2 2
2 4
2
3 2
4 2 2
2 5
2
3 2
5 2 2
3 6 2
O H g
CO s
OH Al
aq HCO
aq O
H Al
aq O
H aq
OH O
H Al O
H s
OH O
H Al
aq O
H aq
OH O
H Al O
H aq
OH O
H Al
aq O
H aq
OH O
H Al O
H aq
OH O
H Al
aq O
H aq
OH O
H Al O
H aq
O H Al
phosphorus from the waste-water stream. N removal can be done through the reactions 16.10 or from 16.11 to 16.13
(16.13)
)
( )
( )
(
(16.12)
)
( )
( )
(
(16.11)
) ( )
( )
( )
(
. conditions tment
under trea species
ng chlorinati effective
the is acid us
Hypochloro mines.
trichlora and
di-, mono-, form
to
on, chlorinati subsequent
by effected is
nitrogen ammonium
of removal complete
More
. exchangers systhetic
or olite
- clinoptil exchanger
ion natural the
using exchange
ion by solution neutral
the from ions
ammonium remove
to is on modificati pH
involve not
does hat
approach t e
alternativ An
(16.10)
)
( )
( )
( NH
(16.9)
3 ) ( 6
) ( ) (
) ( 3
) ( ) (
5
(16.8)
)
( 2
) ( )
( )
( )
(
(16.7)
) ( )
( )
(
2 3
2
2 2
2
3 2
4
2 3
4
2 3
4 5
2 4 2
2 4
2 2
4 3
4 3
4 3
O H aq
NCl aq
NHCl aq
HOCl
O H aq
NHCl aq
HOCl aq
Cl NH
aq O
H aq
Cl NH aq
HOCl aq
NH
O H g
NH aq
OH aq
O H aq
OH s
PO OH
Ca aq
HPO aq
OH Ca
aq OH
s O H HPO
AlOH O
H aq
HPO OH
Al
s AlPO aq
PO aq
Al
