Transcript Peninsula Technikon Faculty Of Engineering Department of

Simple chemical dosing systems
in small water works
G. Mwiinga
Cape Peninsula University of Technology
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1. Introduction
2. Simple chemical dosing systems – a brief
literature review
 Gravity driven solution feeders,
3. Chemical dosing applications in small water
works - South Africa
 Literature and plant visits,
4. A simple chemical dosing system developed
for small water works
 Gravity driven solution feeder
5. Concluding statements
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1. Introduction

Water treatment chemicals dosed in various forms:
–

dosing systems range from simple to complex types
–
–

influence complexity of dosing system.
Simple systems include those that pump predetermined
dosages directly from a solution tank and those that dose dry
or wet chemicals by gravity or volumetric measurements.
Complex systems include electronically controlled pump
metering systems that automatically adjust dosing rates based
on water flow rates and quality.
hydraulically and gravity driven systems are simpler
and are more appropriate as solution-feeders in small
water treatment plants.
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2. Simple chemical dosing systems
– a brief literature review
 Simplest dosing systems generally
appropriate for small plants are those that
dose chemical solutions by gravity:
–
–
–
–
–
–
predetermined amount of a solution drips into the
water at a point that has enough mixing energy.
usually have no moving parts and their
construction is possible from local materials
Key aspect is the control of dosing rates
Low operation and maintenance demands, but
operator training still essential
Calibration still very important
Solution preparation
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
The erroneous general practice reported in some
plants entails drip-dosing from a solution tank
without taking into consideration the decreasing
solution level and its concentration (see Figure).
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Examples of simple (gravity)
chemical dosing systems
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Constant - level box solution feeder
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Floating-arm type solution feeder
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Floating-bowl type solution feeder
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Detail of floating-bowl
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3. Chemical dosing applications in small
water works
Good and bad practices
A summary of problems:
– Lack of control of dosage
– malfunctioning or not in use due to breaking
down – lack of maintenance / replacement
– Although lack of adequate financial resources
contributes to lack of sustainability, technical
inappropriateness reported to be common.
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Electronic dosing systems at a plant in EC
(1 pump out of use at time of visit)
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Operation electronic dosing systems
at some plants in Western Cape
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Electronic
dosing system
at a plant in
Eastern Cape
(no standby
pump)
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•Electronic
dosing systems
replaced by drip
dosing at a Plant
in Eastern Cape
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Manual drip dosing control at
a plant in Eastern
Cape
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Solution tanks
that feed a dosing
tap by gravity at
the Western Cape
Volumetric
measurements
used to check
dosing rates
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Solution tank equipped with a dosing tap at
plant in the Western Cape
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Pump and flow meter dosing system at plant
in the Western Cape
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•Emergency gravity
dosing system at a
plant in the Eastern
cape (bin equipped
with a tap used );
•only option which
served the purpose,
but unsustainable /
ineffective
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4. A simple chemical dosing system
developed for small water systems
•
Requirements :
•
•
•
•
•
•
The dosing was to be achieved by gravity.
Constant dosing rate independent of solution level.
Easy adjustment of dosing rates/ calibration.
Easy chemical mixing
Sustainable materials
chemical dosing system that utilises a constant
head orifice was developed.
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
Main components :
 A constant-level inlet,
 A chemical solution tank,
 A constant-head orifice and dosing tube,
 A hand-mixer

Solution tank is supplied with a lid that has
provisions to hold the hand-mixer, constant-level
inlet system and dosing tube.
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Details of constant-head inlet
Inlet pipe
Flange
Float
Stopper
housing
Open end
Soft rubber stopper
fixed on end capping
outlet hole
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Details of lid for solution tank
Guide (provision)
for the hard
dosing tube
Mixer
Provision
for mixer
inlet pipe for
constant
level system
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Details of constant-head orifice and dosing tubes
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
Evaluation and results
 Influence of orifice/float position
 Dosing rates:
 consistence, reproductivity ,
calibration
 Operation - calibration
 Maintenance and materials
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Evaluated float positions relative
to outlet point
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0 o Position
Time
(min)
Run 1
45 o Position
90 o Position
135 o Position
180 o Position
Run 2
Run 3
Run 1
Run 2
Run 3
Run 1
Run 2
Run 3
Run 1
Run 2
Run 3
Run 1
Run 2
Run 3
0
7.42
7.19
7.13
7.50
7.75
7.28
7.64
7.72
7.62
7.63
7.62
7.69
7.62
7.47
7.36
30
7.08
6.80
6.37
7.39
7.52
6.75
7.52
7.17
7.50
7.59
7.64
7.59
7.64
7.64
7.25
60
6.69
6.76
6.50
7.13
7.15
6.70
7.49
7.75
7.51
7.74
7.39
7.61
7.10
7.17
7.12
90
6.56
6.56
6.38
6.37
6.90
6.45
7.37
7.51
7.36
7.62
7.39
7.61
7.63
7.04
7.16
120
6.20
6.16
6.43
6.50
6.63
6.09
7.15
7.41
7.25
7.50
7.48
7.40
7.50
7.24
7.40
150
5.20
6.16
6.41
6.38
6.64
5.60
6.68
6.82
7.01
7.51
7.39
7.34
7.48
7.27
7.28
180
5.30
5.73
5.68
6.43
6.57
5.25
6.65
7.17
7.00
7.39
7.28
7.36
7.38
7.19
7.29
210
5.34
5.72
6.53
6.41
6.43
5.77
6.67
7.15
6.99
7.36
7.41
7.38
7.37
7.17
7.17
240
5.67
5.83
6.32
5.68
6.67
5.96
7.04
7.08
7.12
7.63
7.39
7.53
7.28
7.16
7.40
270
5.73
5.52
6.77
6.32
6.77
5.83
6.86
7.16
7.14
7.50
7.47
7.37
7.40
7.15
7.27
300
5.77
6.44
5.72
6.77
6.71
5.97
7.03
7.28
7.22
7.63
7.59
7.36
7.59
7.27
7.28
330
6.72
6.55
5.50
5.12
4.51
5.21
6.79
7.18
7.16
7.50
7.37
7.26
7.50
7.27
7.28
360
4.36
3.94
4.17
3.50
2.74
3.34
6.90
7.27
7.55
7.44
7.40
7.39
7.38
7.17
7.33
390
1.58
3.22
3.69
2.17
2.27
2.16
5.20
4.83
5.52
7.61
7.65
7.27
7.14
7.14
7.41
420
0.29
2.38
2.27
3.69
1.67
1.45
4.42
5.12
5.31
7.50
7.47
7.16
7.52
7.04
7.39
4.29
4.40
4.84
4.17
4.17
4.39
2.85
2.74
3.05
3.57
3.34
6.08
450
480
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0 o position; 3 cm orifice head
8.00
7.00
6.00
Flow rate (l/h)
5.00
4.00
During this period level dropped by 30 cm from maximum starting point
3.00
2.00
1.00
0.00
0
30
60
90
120
150
180
210
240
270
300
330
360
390
420
450
480
510
540
570
Time (min)
Run 1
run 2
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Run 3
31
600
135 o position; 3 cm orifice head
9.00
8.00
7.00
Flow rate (l/h)
6.00
5.00
4.00
During this period level dropped by 45 cm from maximum starting point
3.00
2.00
1.00
0.00
0
30
60
90
120
150
180
210
240
270
300
330
360
390
420
450
480
510
540
570
Time (min)
Run 1
run 2
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Run 3
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600
Summary results
Parameter
0o
position :
retention time
= 330 min
45o
position :
retention time
= 300 min
90o
position :
retention time
= 360 min
135o
position :
retention time
= 420 min
180o
position :
retention time
= 420 min
Minimum (l/h)
5.20
5.25
6.65
7.16
7.04
Average (l/h)
6.25
6.55
7.20
7.48
7.32
Maximum
(l/h)
7.42
7.75
7.75
7.74
7.64
0.576
0.589
0.292
0.132
0.164
STDV
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Orifice Head (cm)
Calibration: Orifice head versus dosing rates
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30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
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36
38
40
42
44
46
48
50
52
54
56
58
60
62
64
66
68
70
72
74
76
78
80
82
84
86
88
90
92
94
96
98 100 102 104 106 108 110 112 114 116 118 120 122 124 126
Flow rate (l/h)
Actual
Theoretical
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Concluding statements

Appropriate is relative – simple is not always
equal to appropriate
–


Capacity to effectively use and maintain a given
technology determines appropriateness
Simple systems exist and must be
promoted, though may not be attractive
business wise
Electronic solution feeders not generally
appropriate in small water works
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
Gravity solution feeders have potential and
must be promoted
–
–


more appropriate in small and rural plants, as
an alternative to electronic dosing equipment or
stand-by dosing system where electronic
pumps are in use
Preparation of the stock solution,
determination of dosing rates and
calibration aspects very important
Knowledge sharing and technology
transfer important
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Stormsvlei village Western cape
Directly use high colour raw water from River
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Pilot plant at Stormsvlei village Western cape
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Pilot plant at Stormsvlei village Western cape
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Acknowledgements:

WISA and supporting
organisations
 Conference Organisers
 WRC / NRF for their support
THANK YOU
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Presenter’s Contact Details:
G. Mwiinga, Pr. Eng. (Civil / Water Eng.)
(Lecturer )
Cape Peninsula University of Technology
Faculty of Engineering
Civil Engineering Department (Bellville Campus)
email address: [email protected]
Tel: + 27 21 959 6663 (o), +27 822020826 (cell); Fax: + 27 21 9596660
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