page 340 
Previous  1 of 9  Next 


Loading content ...
Statistical Quality Control Charts For Wastewater Evaluation JOHN M. FAIRALL, Chief Engineering and Sciences Staff Effluent Guidelines Division Office of Water Planning Standards U.S. Environmental Protection Agency Cincinnati, Ohio 45268 INTRODUCTION Random selection of a subset of samples from a large population provides the common basis for statistical evaluation of sample data. When evaluating several grab samples of water from a wellmixed tank, the value sought is the true value for the contents of the tank as estimated by the average value of the subset. Samples of wastewater collected from discharging industrial wastes are collected in sequence and not from a single large well mixed volume. The control chart method excels most methods for presenting data from sequential sampling (1). For the purpose of evaluating wastewater, a control chart for individuals using moving ranges of two observations with no standard given is recommended and presented as an example. DEVELOPMENT OF CONTROL CHART A control chart for individual values is prepared from the first 20 to 30 values. Table I lists 22 values for COD. The moving range, R, the difference in successive values of X or COD, is computed and listed as shown in Table I. The average value of X and R are computed. The upper control limit for X, UCLX; lower control limit for X, LCLX;and the upper control limit for R, UCLR, are computed as shown in Table I. The constants, 2.66 and 3.27, are based on setting the control limits at about three standard deviations from the average values (1), as plotted in Figure 1. All values within the control limits are interpreted as emanating from a constant cause system, which consists of production, control, treatment, sample collection, flow measurement, and the chemical analysis. Remove the values of X which fall outside UCLX and LCLX or cause values of R to exceed UCLR, and calculate new limits, thus discarding the high value of 123.3 as shown on the 21st. X = 61.1 UCLX = 86.0 LCLX = 36.2 R = 9.35 UCLR = 44.1 Dashed lines in Figure 1 depict these revised limits. With the establishment of new limits, additional elimination of values may be required (2). If these control limits will not protect the receiving stream, or meet the applicable effluent limitations, the system must be reviewed and an attempt made to reduce the variation and narrow the control limits or reduce the average discharges from the system by improved wastewater management. As stated previously, all values within the control limits are interpreted as being from a constant cause system. The chance variations about the mean or average line, sometimes 340
Object Description
Purdue Identification Number  ETRIWC197332 
Title  Statistical quality control charts for wastewater evaluation 
Author  Fairall, John M. 
Date of Original  1973 
Conference Title  Proceedings of the 28th Industrial Waste Conference 
Conference Front Matter (copy and paste)  http://earchives.lib.purdue.edu/u?/engext,23197 
Extent of Original  p. 340348 
Series  Engineering extension series no. 142 
Collection Title  Engineering Technical Reports Collection, Purdue University 
Repository  Purdue University Libraries 
Rights Statement  Digital object copyright Purdue University. All rights reserved. 
Language  eng 
Type (DCMI)  text 
Format  JP2 
Date Digitized  20090602 
Capture Device  Fujitsu fi5650C 
Capture Details  ScandAll 21 
Resolution  300 ppi 
Color Depth  8 bit 
Description
Title  page 340 
Collection Title  Engineering Technical Reports Collection, Purdue University 
Repository  Purdue University Libraries 
Rights Statement  Digital object copyright Purdue University. All rights reserved. 
Language  eng 
Type (DCMI)  text 
Format  JP2 
Capture Device  Fujitsu fi5650C 
Capture Details  ScandAll 21 
Transcript  Statistical Quality Control Charts For Wastewater Evaluation JOHN M. FAIRALL, Chief Engineering and Sciences Staff Effluent Guidelines Division Office of Water Planning Standards U.S. Environmental Protection Agency Cincinnati, Ohio 45268 INTRODUCTION Random selection of a subset of samples from a large population provides the common basis for statistical evaluation of sample data. When evaluating several grab samples of water from a wellmixed tank, the value sought is the true value for the contents of the tank as estimated by the average value of the subset. Samples of wastewater collected from discharging industrial wastes are collected in sequence and not from a single large well mixed volume. The control chart method excels most methods for presenting data from sequential sampling (1). For the purpose of evaluating wastewater, a control chart for individuals using moving ranges of two observations with no standard given is recommended and presented as an example. DEVELOPMENT OF CONTROL CHART A control chart for individual values is prepared from the first 20 to 30 values. Table I lists 22 values for COD. The moving range, R, the difference in successive values of X or COD, is computed and listed as shown in Table I. The average value of X and R are computed. The upper control limit for X, UCLX; lower control limit for X, LCLX;and the upper control limit for R, UCLR, are computed as shown in Table I. The constants, 2.66 and 3.27, are based on setting the control limits at about three standard deviations from the average values (1), as plotted in Figure 1. All values within the control limits are interpreted as emanating from a constant cause system, which consists of production, control, treatment, sample collection, flow measurement, and the chemical analysis. Remove the values of X which fall outside UCLX and LCLX or cause values of R to exceed UCLR, and calculate new limits, thus discarding the high value of 123.3 as shown on the 21st. X = 61.1 UCLX = 86.0 LCLX = 36.2 R = 9.35 UCLR = 44.1 Dashed lines in Figure 1 depict these revised limits. With the establishment of new limits, additional elimination of values may be required (2). If these control limits will not protect the receiving stream, or meet the applicable effluent limitations, the system must be reviewed and an attempt made to reduce the variation and narrow the control limits or reduce the average discharges from the system by improved wastewater management. As stated previously, all values within the control limits are interpreted as being from a constant cause system. The chance variations about the mean or average line, sometimes 340 
Resolution  300 ppi 
Color Depth  8 bit 
Tags
Comments
Post a Comment for page 340