Gezira j. of Eng. & applied. Sci.  12-(1): 68-86(2017)

 

Construction of Control Charts for Drinking Water Treatment in Medani Station of Water Purification, Gezira State, Sudan

 

Sara Ibrahim Mohamed Ahmed1

ElnourKamaledin Abusabah2

 

1.Department of Chemical Engineering and Chemical   Technology, University of Gezira,Faculty of Engineering and Technology

 

2.Department of Chemical Engineering and Chemical   Technology, University of Gezira, Faculty of Engineering and Technology

 

Abstract

 

Water  is one of the important natural resources of the earth, one of these resources is surface water. The objective of this study is to construct variable control charts, on data from Wad Medani Station for Water Purification (WSWP), to test the quality of five properties of tap water namely, pH, turbidity, total hardness, residual chlorine and Total Dissolved Solids (TDS). The samples were selected randomly to cover all the year days. Variable control charts (  charts and  charts) were constructed for the five properties,and then fresh 20 samples were taken and analysed, and then plotted on the charts. The average of pH was 8.1, one point of the pH was above the action limit in pH  charts, two points were located below the lower limit, and five points were located above and below the warning limits and the other points were located within the stable limits. The average of turbidity was 35, in turbidity  chart, it was found that the turbidity values varied by an unexpected amount, The average of residual chlorine was 0.2, one point of the residual chlorine was above the action limit in  charts, two points were located above the warning limit and all the other points were located within the stable limits, this indicates that the process is under control. The average of TDS was 170,in  Chart for TDS, most of points were located between upper and lower warning limits, three points were located above the warning limit and two points were located below warning limit, so the process is considered under control. The average of Total Hardness was130, two points of the Total Hardness was above the action limit in  charts, two points were located below the warning limit and the other points were located between upper and lower warning limits, so the process is considered under control.The study recommends the use of quality control charts beside the laboratories tests to monitor the process and study the seasonal change of the quality of the water in the untreated water from the river to the sedimentation basin.

Key words:  Control Charts, Quality, Water Treatment, Surface Water.

 

 

1. Introduction:

Water is an important natural resource of the earth and has been essential for the existence of all living things, one of these sources is surface water, and this source is easily contaminated by animal and human wastes(Imafuku, 1999). Control charts are usually used quite extensively at the beginning of a new program, process or product, for all important operations and processes. Charts are removed as they are found to be unnecessary or the process stabilizes. Control charts are examples of the primary techniques of statistical process control(Oakland, 2003). Control charts (also called process chart or quality control chart) are graphs that show whether a sample of data falls within the common or normal range of variation.The objective of this study is to construct variable control charts, on data from Wad Medani Station for Water Purification (WSWP), to test quality of five properties of tap water namely, pH, turbidity, total hardness, residual chlorine and Total Dissolved Solids (TDS).(Control charts are used to control the water treatments process so as  of not to be produce water that is not conforming with standards of good quality water)conforming water by controlling the treatment A control chart is a graph that contains a centreline, upper and lower control limits. The centreline represents the process average. The control limits represent the upper and lower boundaries of acceptability around the centreline. The horizontal axis represents sample numbers or points in time, and the vertical axis represents measurements from samples, as shown in the Fig bellow, Upper Warning Limit (UWL) and Lower Warning Limit (LWL) may also be added. Generally action is required if a result is beyond either of the control limits. The UWL and LWL are set at a level so that most of the results will fall between the lines when a system is running in control (Walter A. Shewhart in1920’s). 

 

 

 

 

 

 

Fig (1) Statistical Basis of the Control Chart

Source :( Oakland,2003)

 

1.2Uses of Control Charts:

 

Control charts are useful for analysing and controlling repetitive processes as they help to determine when corrective actions are needed, because they display running records of performance, control charts provide numerous types of information to management.(Montgomery, 2009).they are a useful tool for studying variation. The limits quality gives the control chartan  analytical power to enable its user to determine whether a process can be considered stable and thus, predictable, or unstable, and  unpredictable.

1.3 Equations Used for Construction of the Charts

 

1.3.1Construction of Average charts or  charts

Average charts areusedfor quantitative quality e.g. weights, volumes, compositions etc. If the measurements are normally distributed with a mean  and standard deviation σ for a sample of (n) items then:

-              Estimated Standard Deviation (S) =  The Range (R)/ The factor  (dn)    (1)    

σ= R/dn (2)

 

-       dnis a factor which depends only on the sample size n as shown on table(2)

(For n = 5, dn=2.33)                                                                              

The limits of  charts:

  -  Upper limits      (3*  ) (3)                                                              

 - Lower limits      (2*  )  (4)                                                              

1.3.2 Construction ofRange charts or  charts

 

The limits of  charts

-         Upper control limits     *. (5) 

-         Lower control limits      . (6)   

-         Upper warning limits       .                                          (7)

-         Lower warning limits       . (8)

The multiplier D is obtained from samples table (2). There are tables of pairs of values

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table (1) Fresh 20 samples for the Five Properties of the Tap

WaterCollected from Wad Medani Station of Water Purification

Samples

No.

pH

Turbidity

Residual Chlorine

TDS

TH

1.

8.30

33

0.12

180

115

2.

8.25

35

0.20

179

130

3.

8.20

37

0.25

168

100

4.

8.30

33

0.58

16

110

5.

8.25

98

0.15

187

128

6.

8.42

37

0.20

184

138

7.

8.15

27

0.34

154

110

8.

8.20

18

0.28

167

147

9.

8.07

26

0.08

146

121

10.

8.16

28

0.09

199

150

11.

7.75

31

0.22

180

148

12.

8.00

31

0.29

160

100

13.

7.83

30

0.21

161

110

14.

7.85

29

0.10

195

147

15.

8.18

29

0.17

175

137

16.

8.00

32

0.20

156

157

17.

7.99

29

0.10

169

146

18.

7.96

30

0.06

147

144

19.

7.95

45

0.10

160

145

20.

7.65

36

0.36

165

143

 

Table (2) Relationship between Standard Deviation and

Range and   Control Chart Limits for Sample Range

Source: (Oakland, 2003)

Sample size

For lower limits

For upper limits

 

(n)

Sample size N

Factor

2

0.00

0.04

2.81

4.12

2

1.13

3

0.04

0.18

2.17

2.98

3

1.69

4

0.10

0.29

1.93

2.57

4

2.06

5

0.16

0.37

1.81

2.36

5

2.33

6

0.21

0.42

1.72

2.21

6

2.53

7

0.26

0.46

1.66

2.11

7

2.70

8

0.29

0.50

1.62

2.04

8

2.85

9

0.32

0.52

1.58

1.99

9

2.97

10

0.35

0.54

1.56

1.94

10

3.08

 

2.0Materials:

 

In this study a25 and 50 ml burettes, were used graduated in 0.1ml.25ml Conical flask 250 ml.,100 ml graduated cylinder. ,Wash bottle with distilled water.,  in this study also used the chemicals and reagents, were mainly brought from central laboratory of Medani Station for Water Purification and they are, Potassium chromate indicator solution., Standard silver nitrate titrant., Standard EDTA titrant., buffer solution., Ammonia Ammonium pupurate (murexide).,Erio Chrome Blank T (ECB), and ammonium chloride.

 

2.1 Apparatus:

 

 pH meter [HACH (2000)], Turbidity meter [HACH (2100 N IS)], and TDS meter [HACH (2000)]  were used in this study .

 

2.2Methods:

 

The data of five properties of tap water namely pH, Turbidity, Residual chlorine, Total Hardness, TDSs were collected from (WSWP), the sample size was equal to 5 observed data and the subgroups were equal to 10. Variable control charts (  charts and  charts) were constructed for the five properties, and fresh 20 samples were taken and analysed, then plotted on the charts, using Mat lap software program.The charts were used to observe and evaluate the behavior of the process overtime and corrective action taken if necessary.

The methods include measurement of properties which were chosen for the study and the experimental work was carried out in central laboratory of Wad Medani Station for Water Purification (WSWP), The methods are explained below:

 

2.2.1 Total Hardness test:

 

25 ml of sample was put in 50 ml graduated cylinder and diluted to a total volume of 50ml, then the sample was poured into 250ml conical flask, and1ml of the pH buffer solution was added to the sample and adjusted to 10 with ammonium chloride/hydroxide solution eriochrome black was added as indictor and a red color resultedfollowed by titration versus  EDTA disodium salt and the end point the colorchanged from red to blue, the volume of EDTA used to reach the end point was recorded, then the result in mg /l calcium carbonate was recorded.

 

2.2.2 Chlorine Test:

 

For chlorine test, 1ml of potassium chromateindicator solution was to the sample added and titrated with standard silver nitrate titrant to a pinkish yellow as the end point then the volume of silver nitrate was recorded.

 

2.2.3 TurbidityTest:

 

The method of analysis of turbidity was done using HACH (2100 N IS) turbidity meter and the results were reported in nephlometric turbidity unit (NTU).

 

2.2.4pHand TDS Tests:

 

The pH and TDS tests of water sample were measured using pH and

TDSmrters.

 

3.0 Results and Discussions:

 

The tables below show the control chart limits for  and charts which were calculated from equations in section 1.3

 

 

Table (3)The Control Chart Limits which were used to construct the charts Calculated by Equations shown in section (3)

 

The  Average  (X)

The Range (R)

Standard Deviation  (S)

 

UCL

 

LCL

 

UWL

 

LWL

pH

8.09

0.45

0.191

1.05

0.07

0.81

0.17

Turbidity

35.20

15.67

6.73

36.98

2.50

28.36

5.97

Residual chlorine

0.20

0.33

0.012

0.78

0.05

0.59

0.12

Total dissolved solid

168.40

40.80

17.5

96.28

6.53

73.85

15.09

Total hardness

132

38.57

16.55

91.00

6.17

69.80

14.27

               

 

UCL

LCL

UWL

LWL

pH

9.25

8.73

8.99

882.00

Turbidity

44.22

26.18

41.22

29.18

Residual Chlorine

0.34

0.07

0.29

0.12

Total Dissolved Solid

191.92

144.90

233.08

103.77

Total Hardness

154.20

109.70

146.80

117.19

 

 

 

 

 

 

 


9

8.5

pH 8.1

7.5

7

 


10   20      30    40      50      60     70     80     90    100    

Sample number

 

Fig (3.1):  Chart for tap water pH

 


1.2

0.9

pH 0.6

0.3

0

 


10   20      30    40      50      60     70     80     90    100    

Sample number

 

Fig (3.2):  Chart for tap water pH

 

Source: Own calculations based on secondary sample from (WSWP)

 

 

 

One point of the pH was above the action limit in pH  charts, two points were located below the lower limit, five points were located above and below the warning limits and the other points were located within the stable limits. This means the cause for violation points must be searched for, then must check the process line shift to find out the reason behind this deviation and must be removed.

 

2

4

6

8

10

12

14

16

18

20

7.6

7.7

7.8

7.9

8

8.1

8.2

8.3

8.4

8.5

pH

Sample number

 

 

Data

Violation

Center

LCL/UCL

Fig (3.3)  chart of pH for Tap water

 

Source: Own experimental work

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Control charts for Turbidity of Tap Water:

 


50

40

Turbidity 35

30

25

 


 10   20      30    40      50      60     70     80     90    100 

Sample number

Fig.(3.4):  Chart for tap water turbidity

 

Source: Own calculation based on secondary sample from (WSWP)

 


40

30

Turbidity 20

10

0

 


10   20      30    40      50      60     70     80     90    100 

Sample number

 

Fig.(3.5):  Chart for tap water turbidity

 

Source: Own calculation based on secondary sample from (WSWP)

 

 

In turbidity  chart, it was found that the turbidity values have been varied by an unexpected amount.

Fig (3.6)  chart of Tap water turbidity

 

Source: Own experimental work

 

 

 

 

 

 

 

 

 

 

 

 

Control charts for Residual Chlorine:

 

0.30

0.25

Residual chlorine 0.20

0.15

0.1

 


           10  20      30    40      50      60     70     80     90   100    

Sample number

 

Fig.(3.7):X- Chart  for  residual chlorine

 

Source: Own calculation based on secondary sample from (WSWP)

 


0.4

0.3

Chlorine0.2

0.1

0

 


10   20      30    40      50      60     70     80     90    100 

Sample number

 

Fig.(3.8):  Chart for tap water chlorine

 

Source: Own calculation based on secondary sample from (WSWP)

 

 One point of the residual chlorine was above the action limit in residual chlorine  charts, two points were located upper the warning limit and all the other points were located within the stable limits this is indicates that the process is under control.

Fig (3.9)  chart of Residual Chlorine

 

Source: Own experimental work

 

 

 

 

 

 

 

 

 

 

Control charts for Total Dissolved Solids (TDS):

 


200

190

TDS 170

150

130

 

10   20      30    40      50      60     70     80     90    100    

Sample number

 

Fig.(3.10):X- Chart for TDS

 

Source: Own calculation based on secondary sample from (WSWP)

 


100

75

TDS 50

25

0

 

10   20      30    40      50      60     70     80     90    100    

Sample number

 

Fig.(3.11):R- Chart for TDS

 

Source: Own calculation based on secondary sample from (WSWP)

 

 

 In  Chart for TDS, most of points were located between upper and lower warning limits, three points were located upper the warning limit and two points were located below warning limit, so the process considered under control.

 

Fig (3.12)  chart of TDS

 

Source: Own experimental work

 

 

 

 

 

 

 

 

 

 

Control charts for Total Hardness:

 

150

140

Total Hardness 130

120

110

 


          10   20      30    40      50      60     70     80     90    100 

Sample number

 

Fig.(3.13):X- Chart for total hardness

 

Source: Own calculation based on secondary sample from (WSWP)

 


80

60

Total Hardness 40

20

0

 


          10   20      30    40      50      60     70     80     90    100 

Sample number

 

Fig.(3.14):R- Chart for Total Hardness

 

Source: Own calculation based on secondary sample from (WSWP)

 

 

 Two points of the Total Hardness was above the action limit in Total Hardness  charts, two points were located below the warning limit and the other points were located between upper and lower warning limits, so the process considered under control.

Fig (3.15)  chart of Total Hardness

 

Source: Own experimental work

 

 

 

 

 

 

 

 

 

 

 

 

References:

 

American Society for Quality Control (2002), ISO 2002, Statistics vocabulary and symbols, Milwaukee.

 

Ebrahim Soltani, (2007) Control and Total Quality, University of Kent, UK Pei – Chun Lai.

 

Hart, M. K. @ Hart, R. R. (2007). Introduction  to Statistical process Control Techniques. Statis software, Inc, 1128NE 2ndst, ste108, Corvallis, Oregon. Department of Business Administration Royal Military College of Canada Kingston, Ontario, Canda.

 

Imafuku(1999) Kurita Handbook of Water Treatment, Masato (et al) 4.7. Nishi – Shinjuku. 3chome, Second Edition, Tokyo 160 – 838, Japan.

 

International Organization for Standardization (2002). ISO 9000-2000, Quality Management Systems Fundamentals and Vocabulary, Geneva.

 

Masato, 1999 Introduction to Water Treatment, University of Peen. Britain.

 

Montgomery, D. C. (2005). Introduction to Statistical Quality Control 6th edition. John Wiley, New York.

 

Oakland J. S. (2003) Statistical Process Control. Fifth Edition. Business Excellence and Quality management, University of Leeds Business School. Britain.

 

Shewart, W.A. (1924). Economic Control of Quality of Manufactured Product, D. Van.