WASTEWATER NEUTRALIZATION USING A SEMI-AUTOMATED BATCH SYSTEM FOR SMALL QUANTITY GENERATORS by PANEENDRA S. TIRUPATHI, B.Tech. A THESIS IN CHEMICAL ENGINEERING Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE IN CHEMICAL ENGINEERING Approved Accepted May, 1994
66
Embed
WASTEWATER NEUTRALIZATION USING A SEMI-AUTOMATED …
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
WASTEWATER NEUTRALIZATION USING A SEMI-AUTOMATED BATCH
SYSTEM FOR SMALL QUANTITY GENERATORS
by
PANEENDRA S. TIRUPATHI, B.Tech.
A THESIS
IN
CHEMICAL ENGINEERING
Submitted to the Graduate Faculty of Texas Tech University in
Partial Fulfillment of the Requirements for
the Degree of
MASTER OF SCIENCE
IN
CHEMICAL ENGINEERING
Approved
Accepted
May, 1994
~o~ gr:zqCJj/J
~t2 1~ 11 ~) lf ACKNOWLEDGMENTS ~/l-~ rjj1/9~
' ) )-:J. 77 I am deeply indebted to my research advisor, Dr. Richard
Tock for his valuable guidance, patience, constant
encouragement and financial support during the course of
this research. I also express a sincere appreciation to Dr.
R. Russell Rhinehart, my other committee member, for his
suggestions and criticisms throughout this work.
I would like thank Mr. Randy Nix and Mr. Richard
Whitehead of the Environmental Health and Safety Department
of Texas Tech University for their timely help and support
in conducting the experiments.
I wish to dedicate this work to my parents. I also wish
to express my gratitude to my brothers and sisters, back
home in India for their unflagging support and
encouragement.
.. 11
TABLE OF CONTENTS
ACKNOWLEDGMENTS • • • • • • • • • • • • • •
LIST OF FIGURES • • • • • • • • • • • • • • • •
CHAPTER
1. INTRODUCTION • • • • • • • • • • • • • • •
2. LITERATURE SURVEY • • • • • • • • • • • • •
3. FUNDAMENTALSOFpH • • • • • • • • •
3. I Definition of pH . • • • • • • • • • •
3.2 Rangeability and Sensitivity • • • • • • • •
3. 3 Titration Curves • • • • • • • • • •
3 0 3 0 1 The strong acid and strong base neutralization •
3. 3. 2 The weak acid and strong base neutralization •
3.3 .3 The strong acid and weak base neutralization. •
3. 3. 4 The weak acid and weak base neutralization •
called as equivalence point. Hence, in a mixture of acids, the acids are neutralized one by
one depending upon the value of their dissociation constants, the higher the dissociation
constant the earlier it gets neutralized. Hence, there forms multiple S-shaped curves and
multiple equivalence points. The curves are symmetrical because the concentrations and
the distances between the dissociation constants are equal. Normally pH titration curves
are not symmetrical.
3.4 Buffering
Buffering is defined as the capacity of a solution to resist changes in pH. Examples of
buffer solutions include mixtures of acetic acid and sodium acetate, ammonium nitrate and
ammonia, and the sodium salts of dihydrogen phosphate and mono hydrogen phosphate.
Any weak reagent discussed above buffers by holding a reserve of undissociated reagent.
The buffer consists of an acid and its conjugate base. To buffer effectively, there must be
a source of unionized agent available and a source of companion ions. For example, acetic
acid, though a weak acid, has very little buffering capacity above pH 6 where it is almost
completely ionized. However, at low pH values it has great capacity to absorb hydroxyl
ions. Similarly, a mixture of weak acid and its strong-base salt or a weak base and its
strong-acid salt will show a great capacity to resist pH changes by absorbing ions.
Commercial buffer solutions are usually prepared using the combinations.
For example, consider an acetic acid and sodium acetate buffer.
We have
CH3COOH --+- CH3COO- + H+
NaOH --+- Na+ + OH-.
(3.17)
(3.18)
From equation 3.17, the dissociation constant of acid can be written as
[C H3COO-][Ir]
Ka= [CH3COOH]
21
The dissociation constant of acetic acid is given as
Ka = 1.75 x Io-s.
The extent of the dissociation of acetic acid is very small when , ~ompared to the
original acid concentration because the concentration of acetic acid is much larger than Ka and because the acetate ion furnished by the sodium acetate represses the dissociation of
acetic acid. So ifO.l N concentration of acetic acid and sodium acetate is used the pH of
the solution would be 4.76. The pH of this buffer changes by 0.1 pH if 10 ml of sodium
hydroxide ofO.l N is added.
3. 5 pH Control Difficulty
Generation of the titration curve either analytically or experimentally, and the
sensitivity of titration curve are the two most important difficulties associated with pH
control. Samples exposed to the air can absorb enough carbon dioxide to lower the pH.
This is a problem peculiar to nearly pure water and caustic samples. The pH of absolutely
pure water at 25° C can change from a pH of 7 to a pH of 5 simply by exposure to air.
The errors at the high and low ends of the pH scale depend upon the type of pH
measurement electrode used. This is particularly a problem for pH measurement above I 0
pH when the sample or reagent contains the sodium ion or other strong alkali ions. If the
titration curve changes with time, separate samples should be gathered over a
representative period and individually titrated. The samples should not be combined for
titration.
3. 5. 1 Titration curve generation
The analytical titration curves considered thus far involved only single, monoprotic
acids or bases. If a mixture of strong and weak, monoprotic and multiprotic reagents is
involved, then generation of an analytical titration curve becomes a difficult task though
22
not impossible. One way to circumvent this problem is to generate the titration curve
experimentally in a laboratory sample. Once the titration curve is known, analytically or
experimentally, the titrant flow can be calculated and used for pH control. One of the
most important applications of pH control is continuous, large scale eftluent wastewater
neutralization. In this particular study, however, application of pH control was desired for
small quantities of wastes in a batch process. The acidic or caustic wastes generally
generated by the academic practices contain a wide variety of acids or caustics from
different sources. If the identification, concentration, and the equilibrium constant for all
the species in wastewater were known then the titration curve could be, in principle,
analytically determined. Determination of analytical titration curve is practically
impossible in the case of highly variant wastewater pH control. Experimental procedures
appear to be the best solution for the generation of titration curves for mixtures of
corrosive wastewater.
The titration curve generation method involves taking a small process sample in a
beaker and adding aliquots of a fixed concentration of the reagent to change pH. The
values are recorded as a curve of pH versus the amount of reagent added. The level of
reagent addition is reduced near the equivalence point in order to have a clear titration
curve which correctly gives the details of titration.
3 . 5. 2 Titration curve sensitivity
The relationship between pH and all possible combinations of strong and weak acids
and bases are explained in the above sections. The typical pH electrode can respond to
changes as small as 0. 001 pH, which means pH measurement can track changes of Sx 1 o-
10 at 7 pH. No other commonly used measurement is known to have this level extreme
sensitivity (McMillan 1984 ). The reagent demand varies in response to the titration curve
for each situation. Hence the titration curve will give an idea how much of reagent is
23
needed. In a particular case at pH= 6, the amount of reagent needed relative to the
amount of reagent needed at pH = 7, is just one unit, whereas the reagent needed from pH
= 2 to pH= 7 requires 10,000 units (Hoyle, 1972).
24
CHAPTER4
EXPERIMENTAL SETUP AND PROCEDURE
A semi-automated batch process was selected to neutralize the wastewater produced
by the research and teaching laboratories at Texas Tech University. This was based on the
understanding that the wastes are typically of various composition and concentration. The
batch processing approach offers several advantages. The most important advantage is
the ability to retain the treated solution until its quality meets the required specifications
for release. When treating extremely corrosive wastes, this safe guard is mandatory.
Also, with the batch system, a single mixing vessel can be used to add the reagent needed
to neutralize the wastewater over a period of time. Batch treatment can also be used to
collect and treat small quantities of wastes that neutralize each other. This latter approach
represents a control method with obvious cost savings.
For small quantity generators (SQGs), neutralization of wastewater does not always
represent a viable option because of limitations in manpower or expertise. The simplicity
of the proposed system and its favorable economics should encourage SQGs to consider a
semi-automated batch system for neutralization. A brief description of various
components used with the experimental setup is given below. This will be followed by an
explanation of experimental procedure.
4.1 Apparatus
4. I. I In-line Electrode
The electrode used was a Signet's in-line (model2710) electrode. The electrode is
depicted in Fig. 4.1 (Signet's manual for pH sensor). The electrode is a double junction
electrode, which significantly reduces the migration of poisoning ions to the reference
25
~Oaring .. ~ (SensOr/Housing)
@~~ . ....__. @SeN«--~~---~
Figure 4.1 In-line Electrode
26
electrode, and thereby results in an increased electrode life. A silver/silver chloride
electrode in this pH sensor permits its use over a wide temperature range.
This electrode with its unique flat surface design is ideal for application in
wastewaters, such as those produced with lime slurries, pulp and paper, electroplating,
and other installations where coating, abrasion, breakage, or reference junction fouling
problems exist. The sensor must be installed with the electrode pointing downward to
insure adequate electrolyte flow. Therefore, for optimum performance, it was installed
vertically in the experimental setup. Where physical constraints exist, it can be installed at
a maximum 450 angle and still be functional.
4 .1. 2 pH controller
For pH measurement process control application, Signet's pH controller (Model
9030) was used. The modular "plug-in" input/output option cards allow the pH controller
to be customized to the needs of the user. All the functions are accessed and controlled
from the front panel, so there are no mechanical potentiometers which require adjustment.
The two line LCD features a 4.5 digital main display line plus an eight character,
alphanumeric line which is used to prompt the user through all calibration and output
control parameters. It can be operated between 32° to 1300 F. Display accuracy is plus
or minus 0. 02 of the pH scale. The gain is defined as the ratio of the change in the output
to the change in the input. The pH controller used is an on-off controller. The Lo relay
setpoint was set at 6. 5 and Hi relay setpoint was set at 7.5. So the pH controller switches
on the device connected to it as soon as the pH falls below 6.5 or above 7.5.
4.1.3 Metering pump
A metering pump (LMI b711-915) was installed which could be energized by the
signal from the pH controller. This pump responds to any dry switch closure. An
27
adjustable electronic pressure control is another unique, standard feature on this pump.
Both the pump and the connecting tubing which was used are resistant to both acidic and
basic solutions. The pump has a capacity range of0.19 to 38.4 gaVday with a maximum
insertion pressure of 1Mpa.
In operation, the pump transfers the acidic or basic reagent when it is energized by
the pH controller. With this experimental design, the pH controller energizes the pump
when relay 1 ( LO relay setpoint) or relay 2 (HI relay setpoint) are on. Thus the pump is
automatically switches to the on mode whenever the pH falls below 6.5 or rises above
7.5; 6.5 and 7.5 being LO relay and Ill relay of pH controller. The flow rate that was
used in the lab scale experiments is 27 gals per day.
4 .1. 4 Mixers
In neutralizing the wastewater solution either in continuous streams or batch systems,
the wastewater needs to be well mixed with the reagent, so as to enable the neutralization
reactions to take place. The mixer assures good mixing of the wastewater solution with
reagent. In our system assembled in the treatment storage and disposal facility (TSD) of
Texas Tech University, a gear pump for recirculation was used to enhance the mixing of
the solution. The gear pump was connected to the bottom of the stainless steel holding
tank with thick walled PVC piping. The unthrottled flow rate of the gear pump was
measured at 125 gal/hr. An in-line pH sensor was placed in the discharge pipe from the
gear pump. The sensor reads the pH of the solution and relays the information to the pH
controller. The controller activates the metering pump which transfers the reagent needed
from the reagent tank to the holding tank in the neutralization process.
28
4.2 pH Sensins and Transmission
A pH measuring and control device consists of a sensing part and a transmission part.
The pH probe, or sensing elements used in our system, was an in-line pH sensor which
was mounted vertically in the pipe line as explained earlier.
The transmission was accomplished with a pH transmitter. In this instance the small
m V current developed by the sensing glass electrode and reference electrode is converted
into pH units for recording. The pH sensor also sends the signal to the controller through
a cable.
4.3 Experimental Setup
Fig. 4.2 depicts the experimental setup that was assembled for a batch process, semi
automated wastewater neutralization system. A holding tank with capacity of 3 5 gal was
used for the neutralization process of corrosive wastewater at the treatment storage and
disposal facility (TSD). A recirculating pump was connected to the bottom of holding
tank as shown in Fig. 4.2. The discharge line from the pump extends into the holding
tank, and has an in-line pH sensor. All the piping consists of thick walled poly vinyl
chloride (PVC) pipe.
The sensor cable was connected to the input terminal of the pH controller. The
controller was fitted with an output card, which was connected to the metering pump
switching circuit. Basically, the output card in the pH controller activates the metering
pump according to preset conditions. The pH controller is an on-off controller. These
preset conditions were selected as pH 6.5 for the LO relay setpoint and a 7.5 pH as them
relay setpoint. In this case, the control of the on-off switch of the reagent metering pump
is controlled by the pH reading. Two separate containers were used to store the
inexpensive acid and caustic reagents. The suction tube end of the metering pump is
immersed in the acid or caustic reagent according to the requirement to neutralize the
29
~----------------------, I I I . I
Metering Pump
P2
Base T2B
Holding Tank Tl
V2
Pump Pl
pH Controller C 1
T I I
pH Sensor Sl
Figure 4.2 Schematic of Experimental Setup.
30
VI
Yorain
corrosive wastewater solution. The stroke of the metering pump, and, hence the flow rate
of the metering pump, can be adjusted with a knob on the front panel of the reagent pump.
The flow rate of the metering pump was set at 27 gals per day. The discharge end of the
tubing from the reagent pump is arranged in such a way that the flow of the reagent is
directed into the holding tank. If necessary a timer can be used to keep track of elapsed
time for the process, and, thereby provide a plot or curve of pH versus time.
4.3.1 Preparing reagent solutions
Acidic or basic reagents were used as to neutralize the wastewater. The acid reagent
used for neutralizing basic wastewater was hydrochloric acid (strong acid), and the basic
reagent used for neutralizing acidic wastewater was sodium hydroxide(strong base).
These solutions were prepared in the concentration range 0.01 N to 0.03 N, which is an
approximate concentration range for many wastewater neutralizing reagents. However,
the results can be extrapolated to higher or lower concentrations. The reagents are
prepared with deionized water.
The various calculations that are needed in preparing the solutions are described as
follows. Most of the commercially available acids or bases are in the form of their highly
concentrated solutions. The solutions are available in glass bottles with labels containing
relevant information, such as density, specific gravity, normality, and weight percent. To
prepare a required dilute solution it is necessary to know the relationship between
normality and the weight percent. This relationship is given as
(4.1)
Where
N Required normality of the dilute solution,
31
Pc : Density of the concentrated solution in gm/ml,
V c Volume of the concentrated solution in ml,
X Weight fraction of the component,
E : Equivalent weight of the component in gm/gm equivalents,
V : Estimated volume of the dilute solution in liters.
For example, it is necessary to calculate the volume of30% by weight of sodium
hydroxide solution needed to prepare 30 liters ofO.Ol N sodium hydroxide solution. By
using the above equation
Vc = 0.01 X 40 X 30 = 30_2?ml. 1.3215 X 0.30
Equation 4.1, therefore, is useful for conversion of normality to weight percent or
vice versa. However, if the normality of the concentrated solution is known, then the
following formula can be utilized to calculate the volume needed based on the
concentration requirements for the dilute solution.
(4.2)
Where
N8 : Normality of the dilute solution,
Nb : Normality of the concentrated solution,
V a Volume of the dilute solution in ml,
V b Volume of the concentrated solution in mi.
The volume of the concentrated hydrochloric acid solution of 5 N required to prepare
50 liters ofO.Ol N dilute solution is
- 0.01 X 50000- IOOml Vb- - · 5
32
Some of reagents are available in the form of pellets and powders which are actually
cheaper than the concentrated solutions. In this case it becomes necessary to calculate the
weight of reagent required to prepare a dilute solution. The basic definition of normality
is given by
Where
N= W ExV
N : Normality of the dilute solution,
W : Weight of the sample in gm,
V : Volume of the dilute solution in liters,
(4.3)
E : Equivalent weight of the component in gm/gm equivalents.
For example, the quantity of sodium hydroxide pellets or powder required to make
0. 0 1 N 50 liter solution is
w 0.01 X 50 X 40= 20gms.
It is important to note that the number of replaceable hydrogen ions before
neutralization are two in case of phosphoric acid even though it is a triprotic acid. In case
of phosphoric acid solution preparation, the equivalent weight is taken as half of molecular
weight even though in general equivalent weight is one-third of molecular weight.
4.3.2 pH sensor calibration
The pH sensor should be calibrated from time to time to assure accurate results. The
pH sensor is typically calibrated with the help of the buffer solutions. Buffers of pH 7. 0
and pH 4. 0 were utilized to calibrate the pH sensor for the acidic solutions, whereas
buffers of pH 7. 0 and pH 10.0 are used to calibrate the pH sensor for the basic solutions.
It was observed that the pH calibration procedure can still have an error up to ± 0. 5 units
33
from the true value. This error arises from various factors like the age of the probe,
temperature changes, and electronic drift, etc.
4.3.3 Metering pump calibration
The metering pump needs to be calibrated from time to time. The calibration process
for the pump is as follows. The pump should be primed. The pressure control knob is
turned fully clockwise. Place the valve in a graduated container with a volume of 1000
mi. The pump is then switched on. Using a timer tum the pump on for a measured
amount of time. Then calculate the pump output theoretically (maximum output X the %
stroke) and compare the results with the volume displaced in the graduate. If there is any
deviation adjust stroke estimating required correction and the above procedure is
repeated.
4. 4 Experimental Procedure
The experimental procedure will be explained with the help of Fig. 4.2 on page 28.
The wastewater that requires neutralization is emptied into the holding tank. For strong
acidic or basic wastes, the maximum volume of wastewater that could be neutralized is
slightly less than half the capacity of the holding tank. The initial pH of the wastewater
should be measured either with a simple pH meter or by switching off the reagent pump
and noting the reading displayed on the pH controller. Once the pH of wastewater
solution is known, the suction end of the metering pump is placed in the corresponding
reagent required to neutralize the wastewater in the holding tank.
Enhanced mixing of the wastewater is assured with the recirculating pump which can
recirculate at a flow rate of 125 gal/hr. This pump has a capacity to empty the tank 3.59
times an hour. The recirculating pump is switched on after opening the valve to allow the
flow of wastewater into the holding tank. The pH controller was calibrated when it was
34
used for the first time, and, for good performance, it should be calibrated from time to
time with buffer solutions of pH 7. 0 and pH 4. 0 for acidic solutions and pH 7. 0 and pH
10.0 for basic solutions. The pH controller is then switched on. The metering pump is
switched on after making sure that the suction end and discharge end of the metering
pump are in the appropriate containers.
Once the pH electrode relays the pH of the wastewater solution to the pH controller,
the pH controller activates the metering pump if the pH of the wastewater falls outside the
pH range of relay set points. The pH controller thus operates the metering pump and
transfers the reagent required to neutralize the wastewater in small quantities. Relay set
points are set points that represent the pH at which each relay is energized. In LO relay
operation (lower limit), the relay is energized when the pH drops below the setpoint and is
de-energized when the pH rises above the setpoint. In Ill relay operation (upper limit),
the relay is energized when the pH rises above the setpoint and is de-energized when the
pH falls below the setpoint.
A timer was used to keep track of elapsed time and pH of the wastewater solution
was recorded. Normally process reagent was continuously added in small quantities by
the metering pump until the pH was brought into limits. The pH readings were observed
at the beginning of the experiment on the pH controller for a while to make sure that pH
value of the wastewater solution is rising or falling as per the case to make sure that the
pump suction end of the tubing is in the right reservoir. The pH controller then
automatically de-energizes the metering pump once the pH of the solution is within LO or
In relay setpoints, and the process is stopped. A plot of pH versus time can be made to
see the exact results of the neutralization process. These curves typically look like a
titration curve with experimental constraints. After treatment and final pH check by the
operator, the wastewater is ready to be transferred to the drain and the city sewer system.
35
CHAPTERS
RESULTS AND DISCUSSION
In Chapter 4, the design and development of the batch system neutralization
procedure was discussed in detail. This chapter presents the actual performance results
obtained with the non-linear controller for a variety of waste acids and bases. The test
runs were carried out on the bench scale setup described in Chapter 4. The samples
selected were typical wastes from research and teaching laboratories. Some of the
wastewater was supplied by the EH&S Department of Texas Tech University. The
experiments were run for all possible scenarios that might arise during the process of
neutralization of typical wastewaters.
The reagents were selected on the basis of cost effectiveness. They are hydrochloric
acid of concentration O.OIN as acidic reagent and sodium hydroxide of concentration
O.OIN as basic reagent. Reagents like sodium carbonate of0.01N and acetic acid O.OIN
were also tested for cases which required neutralizing strong acid with weak base and
strong base with weak acid. These weak reagents were also used to see the results of
weak acid weak base neutralization. Section 5.1 describes how the system works for
hydrochloric acid and sodium hydroxide. Section 5.2 deals with the working procedure
for a strong acid and weak base, i.e., hydrochloric acid and sodium carbonate. Similarly,
Sections 5.3 and 5.4 describes the procedure and the results obtained for a weak acid
against a strong base and a weak base.
5.1 The Strong Acid versus Strong Base
There were two types of experiments performed in this instance: neutralization of a
typical strong acid with a strong basic reagent and the neutralization of a strong base with
a strong acidic reagent.
36
In the fist case hydrochloric acid, a typical acidic waste produced across campus, was
emptied into the holding tank. The recirculating pump was switched on to recirculate the
waste acid. As soon as the controller receives a signal from the probe it displays the pH of
the wastewater solution. When the wastewater pH falls below pH 6.5, the metering pump
is activated and starts transferring the strong basic reagent. The strong basic reagent is
added incrementally until the solution in the tank attains a pH of6.5. As soon as the pH
of the solution reaches 6.5 the pump automatically shuts off flow of the basic reagent.
The last few drops of reagent, after good mixing by recirculating pump, will make the
solution pH, approximately 7.0. The final pH level is the reason for setting up the LO
relay setpoint at 6. 5 and m relay setpoint at 7. 5.
Figure 5.1 shows the results obtained when sodium hydroxide of concentration 0.01N
was used to neutralize hydrochloric acid. The results are in the form of a curve pH versus
time in minutes. The results obtained by the semi-automated batch system for wastewater
treatment demonstrate that the wastewater pH was brought well within the limits specified
by EPA prior to discharge into a publicly owned water treatment (POW).
The above procedure was repeated for the neutralization of strong base, i.e., sodium
hydroxide, with hydrochloric acid. The results are shown in Figure 5 .2. The results show
that the pH of the solution was brought well within limits of EPA. The experimental
constraints being the time lag in the reading received on the pH controller display board
and the actual pH of the solution. The final pH of the wastewater solution was 7.1 0.
The strong acid versus strong base titration curve has a tremendous gain· at the
neutral point. This characteristic makes the neutralization of such wastes highly
challenging. But with the semi-automated batch system the problem is not
insurmountable, because the amount of wastewater to be neutralized is large when
compared with the reagent flow rate. In this way the problem of sudden rise in. pH at the
Freeman, H.M. : Standard handbook of hazardous waste treatment and disposal, McGraw Hill Company, New York, 1989.
Goldman, Jr. J.C. and P.T. Bowen: "Exploring wastewater treatment: A treasure chest oftechnologies," Pollution Engineering, V 24, pp. 56-62, September 1992.
Gray, D.M. and J. Marshall :"How to choose a pH measurement system," Pollution Engineering, V 24, pp. 45-47, November 1992.
Gustafsson, T.K. and K.V. Wallaer: "Myths about pH and pH control," AICHE Journal, V 32(2), pp.335-337, 1986
Gustafsson, T .K.: "Calculation of the pH value of mixture of solutions - An Illustration of the use of chemical reaction invariants," Chemical Engineering Science, V 37(9), pp.1419-1421, 1982.
Harris, D.C.: Quantitative chemical analysis (2nd ed.), W.H.Freeman and Company, New York, 1987.
Horwitz, B.A.: " pHrustrations of a process engineer," Chemical Engineering Progress, V 89, pp. 123-125, March 1993.
Hoyle, D. C.: "The effect of process design on pH and pion control." Proceedings of 18th ISA-AID Symposium, San Francisco, CA, 1972.
Jacobs, O.L.R., W.A. Bardan and C. G. Proudfoot: "Computer-aided design of systems for regulating pH." Chemical Engineer, pp.19-21, March 1984.
Kalis, G.: "How accurate is your pH analyzer?" Intech, V 37, pp. 5~58, June 1990.
Kalis, G. and N.Nichols: "Combine monitoring techniques to advance pH control," Power, V 135, pp. 64-65, September 1991.
50
Kaufinan, J. A. : Waste disposal in academic institutions, Lewis Publishers, Inc, Chelsa, MI, 1990.
Kendrick, A.: "Tight pH control key to waste treatment process," Process Industries Canada, V 68, November 1984.
Mahuli, S.K., R.R. Rhinehart and J.B. Riggs: "pH control using a statistical technique for continuous on-line model adaptation," Computers & Chemical Engineering, V !1, pp. 309-317, April 1993.
Mahuli, S.K.: "Non-linear model-based control of pH," Master's thesis, Texas Tech University, August 1991.
McMillan, G.K.: "pH control: A magical mystery tour," Intech, Volume 31, pp. 69-76, September 1984.
McMillan, G.K.: pH control, Instrument Society of America, 1984.
Peters, D. G., J.M. Hayes, G.M. Hieftje: A brief introduction to modem chemical analysis (Saunders Golden Sunburst Series), W.B. Saunders Company, 1976.
Piovoso, M.J. and J .M. Williams : " Self-tuning pH control: A difficult problem, An effective Solution," Intech, V 32, pp 45-49, May 1985.
Powers, P. W. : How to dispose of toxic substances and industrial wastes, Noyes Data Corporation, Park Ridge, NJ, 1976.
Rittenhouse, R. C.: " Wastewater management rises top priority," Power Engineering, V 96, pp.21-26, October 1992.
Ross, M.: "pH control from practical point ofview," Metal Finishing, V ll pp. 47-48, November 1989.
Shinskey, F.G.: pH and pion control in process and waste streams, Wiley-Interscience, New York, 1973.
Signet 2712, pH Sensor Instruction manual, George Fischer Signet Inc, Tustin, CA, 1992.
Signet 9030 lntelek Pro, pH Controller Instruction manual, George Fishcer Signet Inc, Tustin, CA, 1992.
Vetrovec, F.: "Controlling pH automatically," Instruments & Control Systems, V 59, pp. 59-60, January 1986.
51
APPENDIX A
ECONOMIC JUSTIFICATION
An economic analysis was made to estimate the cost-
benefit ratio for a semi-automated batch system desiqned to
neutralize the wastewater produced by teachinq and research
laboratories of Texas Tech University. The EH&S Department
currently expends $20 per kiloqram for disposal of corrosiv~
wastewater. The University produces approximately 200
kiloqrams of hazardous wastewater. Approximately $20,000
has been spent for third party disposal over the last five
years.
The total cost incurred for the development of the
semi-automated batch system for neutralization is as
follows.
Meterinq Pump
pH Controller
pH Sensor
Total
• •
• •
• •
• •
$ 662.00
$ 635.00
$ 285.00
$1582.00
This total plus additional reaqent costs and
miscellaneous expenses could run to only $2000 over five
years. The listed equipment has a workinq life in excess of
five years. Hence, at present generation rates, the use of
neutralization for hazardous waste reduction by the semi-
52
automated batch process would save the University $
18,000.00 over a five year period.
Hence, this or a similar semi-automated batch process
can be adopted by any small quantity generator as a means of
disposal for the corrosive wastewater in compliance with EPA
regulations. This approach is currently more cost effective
(10/1) than third party disposal.
53
APPENDIX B
OPERATING MANUAL
Nomenclature
V1, V2 Manual operated valves
C1 pH controller
P1 Recirculatory pump
P2 Metering Pump
S1 pH sensor
T1 Holding Tank
T2A Acid Reagent Tank
T2B Base Reagent Tank
The schematic is shown in Fig. 4.2 in page 30.
Initial Conditions
1. V1 is closed.
2. V2 is open.
3. P1 turned off.
4. P2 turned off.
5. All the power switches turned off.
operating Procedure for Acidic Wastewater Solution
1. Transfer the wastewater solution into the holding
tank(T1). The maximum quantity of wastewater solution at
one time should not exceed 17 gals(60 1).
2. Make sure V2 is open.
54
3. Place the suction end of metering pump from the water
tank to the basic reagent tank(T2B).
4. Turn on the recirculatory pump(P1).
5. Turn on the power supply to the pH controller(C1) and
metering pump(P2).
6. Adjust the knob for desired stroke (%80 optimum).
7. Monitor the pH on the pH sensor(S1), making sure the
pH of the wastewater is rising.
8. When the metering pump(P2) stops transferring the
reagent, check the pH of the wastewater solution on the pH
controller display board (it should be between 6.5 and 7.5
pH).
9. Check the pH of the wastewater solution with the hand
pH meter for a second confirmation.
Initial Conditions
1. V1 is closed.
2. V2 is open.
3. P1 turned off.
4. P2 turned off.
5. All the power switches turned off.
Operating Procedure for Basic Wastewater Solution
1. Transfer the wastewater solution into the holding
tank(T1). The maximum quantity of wastewater solution at
one time should not exceed 17 gals(60 1).
2. Make sure the valve V2 is open.
55
3. Place the suction end of metering pump from the water
tank to the acidic reagent tank(T2A).
4. Turn on the recirculatory pump(Pl).
5. Turn on the power supply to the pH controller(Cl) and
metering pump(P2).
6. Adjust the knob for desired stroke (tao optimum).
7. Monitor the pH on the pH sensor (Sl) and making sure
the pH of the wastewater is dropping.
8. When the metering pump(P2) stops transferring the
reagent, check the pH of the wastewater solution on the pH
controller display board (it should be between 6.5 and 7.5
pH).
9. Check the pH of the wastewater solution with the hand
pH meter for a second confirmation.
Shut Down Procedure
1. Turn-off the power supply to pH controller(Cl).
2. Turn-off the power supply to the metering pump(P2).
3. Close valve V2.
4. Open valve Vl.
5. After neutralized solution is drained out, turn-off
the recirculatory pump (Pl).
6. Close valve Vl.
1. Place the suction end of the metering pump(P2) in
water tank.
56
Emergency Shut oown Procedure
1. Turn-off the power supply metering pump.
2. Turn-off the power supplies.
Safety Requirement
1. Wear safety goggles, gloves and lab apron when
preparing reagent solution as well as when running the batch
system.
2. When emergency happens, turn-off the metering pump
first and then turn-off all the power supplies.
3. All chemicals must be labeled.
4. Floor must be kept dry and clean. Any spill should
be cleaned up immediately.
5. Flammable and highly corrosive materials are not to
be neutralized in this batch system.
6. Final pH check before draining out the wastewater to
confirm the wastewater pH is well within limits of EPA.
7. Do not open V1(drain valve) at any time except when
the wastewater is neutralized and ready to drain out.
Chemical used
1. 0.01-0.03 N Sodium hydroxide as basic reagent.
2. 0.01-0.03 N Hydrochloric acid as acidic reagent.
3. 0.01-0.03 N Acetic acid as weak acidic reagent.
4. 0.01-0.03 N Sodium carbonate as weak basic reagent.