EE_lec_6_-_water_processing

lameubiquityMechanics

Feb 21, 2014 (3 years and 3 months ago)

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Environmental
Engineering

Lecture
6


Sources of Drinking Water


Rivers: upland and lowland


Lakes and reservoirs


Groundwater aquifers


Sea water (Desalination)

WATER TREATMENT PROCESSES

WATER
TREATMENT
PROCESSES

Selection of Treatment Process

PRE
-
TREATMENT OF WATER


Steps required before standard treatment
processes


Screening


Storage, equalization


Aeration


Chemical pre
-
treatment: pre
-
chlorination

PRE
-
TREATMENT OF WATER

Screening


Coarse Screening


typically inclined bars of
25
mm diameter and
100
mm spacing


prevent large floating material


Raking is facilitated by the inclination of the bars.


Velocities are usually limited to about
0.5
mls
through the screens


Fine screens


fine screens are fitted after the coarse screens if storage is not
provided,


If there is storage then fine screens are placed at the outlet of the
storage tanks.


typically mesh with openings about
6
mm diameter


circular drum type or a traveling belt


Micro screening.


the mesh openings :range from
20
to
40
µ
m.


used only as the main (physical) treatment process for relatively
uncontaminated waters.

PRE
-
TREATMENT OF WATER


Storage and Equalization


Storage


Serve as a safety line in the event of pollution.


Also serve as reservoirs in time of low supply


Should be equivalent to
7
to
10
days of the average water
demand


This period is good for settling and adequate to reduce most
pathogens by exposure to daylight


Storage time of about
12
h is commonly used to reduce
pumping costs and balance the demand


Equalization


Provide an 'equal' (or consistent) flow to the plant


Equalization Example

hour
flow
(
m
3
/
s
)
flow
(
m
3
/
h
)
cumulative
flow
(
m
3
)
equalized
flow
(
m
3
/
s
)
equalized
flow
(
m
3
/
h
)
cumulative
flow
(
m
3
)
required
storage
(
m
3
)
1
0.13
468
468
0.139
500
500
32
2
0.12
432
900
0.139
500
1000
100
3
0.11
396
1296
0.139
500
1500
204
4
0.10
360
1656
0.139
500
2000
344
5
0.08
288
1944
0.139
500
2500
556
6
0.06
216
2160
0.139
500
3000
840
7
0.08
288
2448
0.139
500
3500
1052
8
0.10
360
2808
0.139
500
4000
1192
9
0.12
432
3240
0.139
500
4500
1260
10
0.14
504
3744
0.139
500
5000
1256
11
0.16
576
4320
0.139
500
5500
1180
12
0.18
648
4968
0.139
500
6000
1032
13
0.20
720
5688
0.139
500
6500
812
14
0.19
684
6372
0.139
500
7000
628
15
0.18
648
7020
0.139
500
7500
480
16
0.17
612
7632
0.139
500
8000
368
17
0.16
576
8208
0.139
500
8500
292
18
0.15
540
8748
0.139
500
9000
252
19
0.16
576
9324
0.139
500
9500
176
20
0.17
612
9936
0.139
500
10000
64
21
0.18
648
10584
0.139
500
10500
-84
22
0.16
576
11160
0.139
500
11000
-160
23
0.13
468
11628
0.139
500
11500
-128
24
0.10
360
11988
0.139
500
12000
12
0.139
Design the size of
an equalization
tank to balance
flow rates from a
municipal

wastewater as
given in columns
(
1
) and (
2
)


Aeration is the supply of oxygen from the atmosphere to water to
effect beneficial changes in the quality of the water.


It is a common treatment process for groundwater and less
common for surface waters. Aeration is used :

1.
To release excess H
2
S gas which may cause undesirable tastes and
odors.

2.
To release excess CO
2
which may have corrosive tendencies on
concrete materials.

3.
To increase the O
2
content of water in the presence of undesirable
tastes due to algae (fishy smell),

4.
To increase the O
2
content of water which may have negative taste,
color and stain properties due to the presence of iron and manganese
in solution. The addition of oxygen assists the precipitation of iron and
manganese.


Aeration can be a simple mechanical process of spraying water
into the air and allowing it to fall over a series of cascades
(waterfalls), while absorbing or desorbing (stripping) oxygen in its
journey.

PRE
-
TREATMENT OF WATER


Aeration


Chemical pre
-
treatment is used to remove undesirable
properties of water (bacteria, algae or excess color) is a
more expensive process than chemical post
-
treatment.


Pre
-
chlorination is used on low turbidity water with a
high coliform count.


The chlorine is injected into the water stream and over
the period that it stays in the settling tanks,


it oxidizes and precipitates iron and manganese.


It also causes pathogenic kill and reduces color.


Doses as much as
5
mg/l are used,


Water authorities tend to use pre
-
chlorination at times of
the year when the surface water supply is likely to be
polluted from agricultural or industrial sources or when
excess organic matter is transported

PRE
-
TREATMENT OF WATER


Pre
-
chlorination


Standard treatment is the set of unit
processes that reduce color, turbidity and
particulate impurities to acceptable levels


Standard treatment consists of the following
unit processes:


Sedimentation


Coagulation and flocculation


Sedimentation of flocculent particles


Filtration

PRIMARY
-
TREATMENT OF
WATER

PRIMARY
-
TREATMENT OF WATER

Sedimentation


Sedimentation by definition is the solid
-
liquid separation
using gravity settling to remove suspended solids'
(Reynolds,
1982
).


In water treatment, sedimentation processes used are:


Type L:

to settle out discrete non
-
flocculent particles in a
dilute suspension.


Type n: to settle out flocculent panicles in a dilute
suspension



Type L

Settling tanks are of two types:


Rectangular


length width ratio of
2
and a depth of the order of
1.5
to
6
m.


Circular


Dimensions typically are
10
to
50
m in diameter and
2.5
to
6
m
in depth

Rectangular
Sedimentation
Tank

Circular
Sedimentation
Tank

Sedimentation tank design


Key parameters and typical values in
the design of settling tanks:


Surface overflow rate:


20
-
35
m
3
/day/m
2


Detention time:


2
-
8
hours


Weir overflow rate:


150
-
300
m
3
/day/m

Sedimentation tank design


Stokes law for settling velocity

Sedimentation tank design


Stokes law for settling velocity

Sedimentation tank design

Sedimentation tank design

Surface overflow rate,

Same for circular tanks

Surface area

Sedimentation tank design

example

Sedimentation tank design

example

Sedimentation tank design

example

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