Flotation and Sedimentation

gayoldMécanique

21 févr. 2014 (il y a 3 années et 4 mois)

73 vue(s)

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2
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WATER TREATMENT
MECHANICAL PROCESSES
6
1
2
4
B
C
9
BASIC KNOWLEDGE
Di
sso
lved air
f
l
o
tati
on
D
i
ss
olved air
f
lotation u
s
e
s
the
f
act
that the solubilit
y
o
f
air in water
increases as t
h
e pressure rises
at constant temperature.
S
ome of
t
h
e treate
d
water i
s

s
aturate
d
wit
h

air under pressure (rec
y
cle water).
T
h
e re
cy
cle water is t
h
en in
je
cte
d
into the
f
lotation tank throu
g
h a
sp
ecial valve t
h
at causes an instan
-
taneous re
d
uction in pressure
(
relief valve
)
. The sudden relief to
atmosp
h
eric pressure causes t
h
e
d
issolve
d
air to precipitate as a clou
d
of
small bubbles. A scra
pe
r clears the
f
loat
f
rom the
s
ur
f
ace o
f
the water.
To improve the per
f
ormance o
f
the
process, coa
g
ulants and
f
locculants
ma
y

b
e a
dd
e
d
to t
h
e raw water. T
h
is
he
lp
s to o
pt
imise the size o
f
the
s
oli
ds

s
o t
h
at more air
b
u
bb
le
s
can
b
e attac
h
e
d
to t
h
e
s
oli
ds
.

Ap
pl
i
cat
i
on exam
pl
e
s
Indu
s
trial water treatmen
t

P
ap
er in
d
ustr
y
Foo
d
in
d
ustr
y
Oil refineries
Plastics in
d
ustr
y
D
ome
s
t
i
c water treatment
S
econdar
y
clarification, if the

activate
d
slu
dg
e se
d
iments
ver
y
s
l
ow
S
up
pl
ementi
ng
or re
pl
acin
g
primar
y
clari
f
icati
on
Fundamental principle of dissolved air flotation:
1 air bubbles, 2 solids, 3 relief valve, 4 recycle water, 5 pump, 6 float, 7 scraper
A raw water, B compressed air, C treated water
FLOTATION
Suspended solids with a density close to or less
than that of water can’t be removed by sedimenta-
tion. Such solids would sediment only very slowly or
would remain suspended. The aim of flotation is to
increase the buoyancy of the solids. This is done by
forming small gas bubbles that attach to the solids.
This makes them rise to the surface of the water
where they can be skimmed off. It is required that
the solids should be hydrophobic. That means that
they are more wettable with air than with water. The
separated solids are termed float. The key factor
influencing flotation is the size of the gas bubbles.
The smaller they are the less will be their rate of rise.
This is compensated by larger numbers of small gas
bubbles attaching to the solids than large bubbles.
The main process used in water treatment is
dissolved air flotation. Another flotation variant is
electro-flotation. The two processes differ primarily in
the way the gas bubbles are produced.
In term
s
o
f
the
se
p
aration o
f

solids
f
rom
g
ases
the phrase

dust
separation” is also use
d
. T
h
e soli
d
p
h
ase ma
y
, on t
h
e one
h
an
d
,
b
e a
u
sa
b
le material
,
on t
h
e ot
h
er
h
an
d,
it may be an unwanted material (
g
as
purification). In
g
ravity separators
the
g
as
f
low is routed at slower
v
elocity t
h
rou
gh
a separator c
h
an
-
nel. On their wa
y
, the particles sink
an
d
ar
e
c
o
ll
e
ct
e
d.
In practice t
h
e
se
p
aration of soli
d/
li
qu
i
d m
i
xture
s
(suspensions)
takes
pl
ace in se
d
imentation tanks
t
h
rou
gh
w
h
ic
h
t
h
e suspension
c
ontinuousl
y

f
lows. The shape o
f
the
b
ase may
b
e rectan
g
ular or circular.
In rectan
g
ular tanks t
h
e suspensio
n
f
low
s
in on one
s
ide and
f
low
s
out

over t
h
e rim on t
h
e opposite si
d
e.

On the wa
y
, the solid particles si
nk

t
o
th
e
b
o
tt
o
m
of
th
e
tank. Th
e
tan
k
f
loor is positioned at an an
g
le to

aid dischar
g
e o
f
the solid material.

T
h
ere are also
d
evices
by
w
h
ic
h
t
h
e
settled solid (slud
g
e) can be cleare
d
f
rom the tank bottom.
S
edimenta
-
tion tanks are mostl
y
use
d
in wate
r
t
r
e
a
t
m
e
n
t.
T
he

s
ett
li
n
g
ve
l
oc
i
ty
of
the particles
is the key variable in the desi
g
n o
f
se
d
imentation tanks an
d
separator
ch
annels. It is
d
irectl
y
relate
d
to
t
h
e particle size, t
h
e particle s
h
ape
(f
low resistance
)
and the difference
in densit
y
between the
f
luid and
solid. I
f
the particles in a suspen
-
sion are ver
y

f
ine, or i
f
the di
ff
er
-
ence in densit
y
between the
f
luid and
soli
d
is sli
gh
t, t
h
e settlin
g
velocity is
v
er
y
low. A technicall
y
use
f
ul sepa
-
r
ation b
y
means o
f
sedimentation is
t
h
en not possi
b
le. Anot
h
er varia
b
le
in
f
luencin
g
the settlin
g
velocity in
l
iquids is the concentration o
f
solid
particles. At
h
i
gh
concentrations,

se
d
imentation is
h
in
d
ere
d
. As t
h
e
c
oncentration increase
s,
t
h
e so-calle
d

c
luster settlin
g
velocity
b
ecomes less

than the velocity o
f
the sin
g
le parti
-
cl
es
.
Sedimentation tank:
1 wastewater inlet, 2 sludge extractor, 3 sludge hopper, 4 clean water overflow
5 cart for sludge clearing
BASIC KNOWLEDGE
Mechanical process engineer-
ing in many cases utilises
gravity to separate different
phases. Gravity can be used
to separate a solid phase
off from a fluid. When solid
particles are suspended in
a fluid, gravity causes them
to sink. For this to happen,
the density of the solid must
be greater than that of the
fluid. The process is termed
sedimentation. Fluid is the
umbrella term for gases and
liquids. It is used because most
physical laws apply equally to
both.
SEDIMENTATION