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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 •
3.4 Buffering • • • • • • • • • • • •
3. 5 pH control Difficulty . • • • • • • • •
3. 5. 1 Titration curve generation •
3. 5. 2 Titration curve sensitivity •
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4. EXPE~NTALSETUPPROCEDURE. •
4 .I Apparatus • • • • • • • • •
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4. 1. 1 In-line Electrode • • • • • • • • •
4.1.2 pH Controller •
4. 1. 3 Metering Pumps
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4.1.4 Mixers • • • • • • • • • • • • • •
4.2 pH Sensing and Transmission • • • • • • • • • • •
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4. 3 Experimental Setup • • • • • • • • • • • • • • 29
4. 3. 1 Preparing reagent solutions. • • • • • • • • • • 31
4.3.2 pH sensor calibration • • • • • • • • • • • • 33
4.3.3 Pump calibration • • • • • • • • • • • • • 34
4. 4 Experimental Procedure • • • • • • • • • • • • • 34
5. RESULTS AND DISCUSSIONS • • • • • • • • • • • 36
5.1 Strong Acid versus Strong Base • • • • • • • • • • • 36
5.2 Strong Acid versus Weak Base • • • • • • • • • • • 40
5.3 Weak Acid versus Strong Base • • • • • • • • • • • 40
5.4 Weak Acid versus Weak Base • • • • • • • • • • • 45
6. CONCLUSIONS AND FUTURE RESEARCH • • • • • • • • 49
BffiLIOGRAPHY • • • • • • • • • • • • • • • • 50
APPENDICES
A. ECONOMIC JUSTIFICATION . • • • • • • • • • • 52
B. OPERATING MANUAL • • • • • • • • • • • • • 54
C. HAZARDOUS OPERABILITY STUDY. • • • • • • • • • 58
0
IV
LIST OF FIGURES
3 . 1 : Typical titration curve for strong acid and strong base system (McMillan 1984) • •
3.2: Typical titration curve for weak acid and strong base system (McMillan 1984) • •
3. 3 : Typical titration curve for strong acid and weak base system (McMillan 1984) • •
3.4: Typical titration curve for weak acid and weak base system (McMillan 1984) •
3.5: MultipleS-shaped curve • • • •
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4.1: In-line electrode • • • • • • •
4. 2: Schematic of experimental setup • • •
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5. 1 : Results of neutralization of hydrochloric acid with sodium hydroxide
5.2: Results of neutralization of sodium hydroxide with hydrochloric acid
5. 3: Results of neutralization of hydrochloric acid with sodium carbonate
5. 4: Results of neutralization of sodium carbonate with hydrochloric acid
5. 5: Results of neutralization of acetic acid with sodium hydroxide • •
5. 6: Results of neutralization of sodium hydroxide with acetic acid •
5. 7: Results of neutralization of acetic acid with sodium carbonate •
5. 8: Results of neutralization of sodium carbonate with acetic acid •
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CHAPTER 1
INTRODUCTION
Many wastewater streams are hazardous simply because they contain weak or strong
acids, bases, and/or their partially neutralized salts. Such wastewaters must be neutralized
before being discharged into the environment. The level of pH for any aqueous solution is
conventionally measured with a pH meter, and the wastewater is neutralized by the
addition of a strong inexpensive acid or a caustic base.
The Environmental Health & Safety (EH&S) Department of Texas Tech University is
responsible for collection of all chemical wastes produced on campus. Included in these
wastes are both acidic and basic wastewaters. The most typical acid wastes produced are
acetic acid, hydrochloric acid, and sulfuric acid. The composition of this wastewater can
vary considerably, but consists of small volumes ofliquids from various teaching and
research laboratories. Increased environmental regulations to encourage waste reduction
coupled with the rising cost for hazardous chemical disposal have encouraged the EH&S
Department to consider neutralization of wastewaters. Neutralization eliminates the
hazardous corrosive characteristics and allows the neutralized solution to be discharged to
the city sewer system.
Industrially pH control and/or adjustment have included a wide range of applications.
These include boiler water treatment, chemical and biological reactions, municipal
wastewater treatment, acid pickling and etching, cooling tower water treatment,
electrohydrolysis and coagulation/precipitation. However, the application of
neutralization to relatively small quantities of wastewater can be difficult because of the
unknown and non-stationary fluid composition, which must be brought to a pH = 7. 0.
At Texas Tech University, the wastewater is collected manually across campus from
the various teaching and research laboratories, and as noted is of a wide variety and
1
composition. Hence, the university's initial procedure for neutralization of such
wastewater was carried out manually by experienced technicians. Manual processes are
labor intensive requiring the generation of a sample titration curve followed by the actual
neutralization of the bulk wastewater. This study, however, reports on the findings of a
semi-automatic approach to the neutralization process designed to reduced labor
requirements.
There were two main objectives of this project. The first was to initiate a study of
cost-benefit ratio of an automated wastewater neutralization unit versus the old manual
procedure. The second objective was the assembly of a prototype, semi-automated
wastewater treatment unit. The prototype experimental setup was then assembled at the
Texas Tech University, hazardous waste treatment storage and disposal (TSD) facility.
This thesis describes the research and development process which created the pH
adjustment procedure. Hence, Chapter 2 discusses background information, such as the
definition of pH, rangeability and sensitivity, concept of titration curves, the various
possible titration curves, buffering, and the difficulty of pH control. Chapter 3 presents
some prior work, current practices, and a summary of the theoretical and the practical
work reported in the literature in this field. The experimental setup, a description of the
apparatus including the in-line electrode, pH controller, metering pumps, mixers and the
basic concepts of pH sensing and transmission are discussed in Chapter 4 along with a
detailed experimental procedure. Chapter 5 presents the results obtained with the system
together with a discussion. The conclusions and suggestions for the further developments
are embodied in Chapter 6.
2
CHAPTER2
LITERATURE SURVEY
Strong concentrations of acids and bases have a severe effect on aquatic life, and
these same solutions are hazardous to both human health and the environment. As a
consequence, pH adjustment/control has been an important unit operation in waste
treatment for many years. There has been a significant amount of research done on pH
controVadjustment and reported in the published literature. Even so, the solution for the
problems of a small quantity generator (SQG) of these wastes have not yet been
adequately addressed. In our case an attempt was made with different types of acidic or
caustic wastes based on our understanding with the basic principles of neutralization.
McMillan (1984) described the neutralization of various possible cases in some detail.
He discussed the neutralization of strong acid with strong base and weak base and
neutralization of weak acid with strong base and weak base in detail along with the typical
titration curves. These details were given in the following chapter, "Fundamentals of pH".
The pH chemistry was also discussed with clarity. The temperature effects on pH and the
effects which result when the solution to be neutralized is exposed to the air were
discussed. Wastewater exposed to the air can absorb enough carbon dioxide to lower the
pH. This is a particular problem for nearly pure water and caustic samples. The pH of the
pure water at 25 OC can change from a pH of7 to a pH of5.7 by exposure to air.
McMillan shows how the mixing equipment size, type and kind of agitation required can
be assessed by the titration curve.
F.G.Shinsky (1973) also describes the batch neutralization process. He describes the
advantages of batch neutralization as having the ability to retain an effiuent until its quality
meets specifications. According to Shinsky, economic incentives lean toward the batch
process whenever the plant production units are operated batch wise. Overshoot
3
problems and the remedy at neutral point are discussed in detail. Thus buffers can be used
to avoid the overshoot problem at the neutral point for neutralizing the strong acid with
strong base or vice versa. When the total flow rates or the hazard waste production are
reduced, then the batch process is an ideal option to consider since reservoirs used for
confinement are manageable.
The pH control/adjustment and basic components required for pH control were also
discussed by Ross (1989). The five basic components of pH control system were listed as
(1) Monitoring, controlling and record instrumentation,
(2) pH electrodes and holders,
(3) Effiuent holding tanks,
( 4) Chemical pumps and reagent storage tanks, and
( 5) Mixers I agitators.
According to Ross many of the problems encountered in a pH neutralization system
are centered around the pH electrode. Incorrect choice of electrodes, holders and
placement of electrodes are sources of common problems. He described the most
common problems experienced with pH electrode as
( 1) Oily and solid coatings requiring frequent removal for cleaning.
(2) pH bulb breakage or premature failure due to abrasives or solid materials in the
solution.
The pH error based on temperature is expressed as 0. 03 pH I pH unit I 1 0°C. For
example, between 1 S and 3 SOC with a working pH range between 6 and 8, the error
would only be ±0.03 pH.
The accuracy of the equipment used in the neutralization of wastewater is quite
important. Kalis (1990) in his article," How Accurate Is Your On-Line pH Analyzer?"
discussed the calibration techniques, maintenance, and temperature effects on pH probes
in detail with the clear pictures showing calibration errors. He described two calibration
4
techniques, the first being the grab sample method (process calibration), and the second
being the two point calibration (buffer calibration). He reported that using the grab
sample method, a 0. 02 pH accuracy is very difficult to achieve. A small sample from the
process is taken, and the pH is found by using a standard pH meter. Then the on-line
analyzer is adjusted to reflect the reference measurement from the lab. The advantage of
this process is that the sensor life is extended because the sensor need not be removed
from the process line. The disadvantage is the measuring error of the lab analyzer is added
to the process analyzer.
The two point calibration involves using the buffer solutions with pH 7, pH 4, and pH
10. The pH sensor is calibrated for a pH of 7 and then the slope control is adjusted with
buffer solutions with a pH 4 and pH 10. The temperature effects were not considered
because of the negligible effect of temperature over the range of 6 pH and 8 pH. He
found out the two point calibration is better and the instrument will be corrected for
sensitivity and offset voltage.
Horwitz (1993) described how pH control can be deceiving, strange, mystical, and
unpredictable. He suggested a four-step strategy for successful control. The four steps
being:
1. Equalize the flow to the control system. Equalizing of the flow is allowing the
solution to reside in the tank where reagent is added to the wastewater for some time for
better control of the system. The ability to equalize both flow and concentration variables
depends on the magnitude of the stream and the amount of time required to get proper
equalization. As flow streams get larger and equalization time increases, the size of the
necessary tankage may become excessive.
2. Providing good mixing of the solution for better control of the system.
3. Use a minimum of two stages. The number of the stages depends on the nature of
the range of pH variation and the steepness of the titration curve(s). Agitation is needed
5
for the first stage but is not necessary for the second stage. Third stage can also be used
when attenuation is necessary.
4. Characterize the controller with the titration curve.
6
CHAPTER3
FUNDAMENTALS OF pH
Neutralization processes and pH adjustments, involve the combined fundamentals of
thermodynamics, chemical reaction equilibria, reaction kinetics and solution chemistry.
Before attempting to study pH control or adjustment process as, it is important to
understand the basic principles of pH and neutralization. This chapter outlines the
fundamentals of pH, its applications and limitations, the concept of strong and weak
reagents, the neutralization characteristics of the different types of reagents and the
analysis and sensitivity of titration curves.
3. 1 The Definition of pH
The process variable pH is the negative logarithm ofbase 10 ofthe hydrogen ion
activity. The small p indicates that the mathematical relationship between the ion and the
variable as a power function; the H designates the ion as hydrogen (McMillan 1984).
Hydrogen ions exist in all streams that contain water, and can be visualized as a single,
positive-charged hydrogen nucleus, or proton (IP"). It is generally accepted that, in water
streams, this hydrogen ion is actually thought to be bound to a water molecule in the form
of a hydronium ion (H30+).
The mathematical definition of pH is:
pH= -log 10 (a H+) (3.1)
Where aH+ is the activity of hydrogen ions expressed in normality (g io~t). Activity
is related to concentration by the following equation.
a= yC (3.2)
Where activity, a, and concentration, C, are in the same units and y is the activity
coefficient. At high dilutions, the activity coefficient approaches unity, which gives the
7
familiar definition of pH. Since wastewater solutions are very dilute, the usage of
concentrations instead of activities should not make any significant difference.
pH= -log 10 (CH) (3.3)
where (CH) represents the concentration of hydrogen ions in the solution in g-ions
/liter. The measurement of pH gives the dissociation of acid or base molecules into ions.
pH is thus, a measure of the acidity or alkalinity of an aqueous electrolyte solution.
For non unity activity coefficients, the activity coefficient is calculated in the following
way. The activity coefficient of an ion is closely related to the ionic strength of a solution.
The ionic strength (Jl), which is a measure of the total concentration of ions in a solution,
is defined as 1
J.1 = -:rc· z~ 2 I I (3.4)
where Ci is the concentration of the ith ion and Zi is the charge on the ith ion. The
exact relationship between y and J.1 is given by the extended Debye-Huckel equation
(Harris 1987) as: -0.51z2JP
logy = r.: a-vJ.J
1 + ( 305 )
(3.5)
where a is the effective hydrated radius ofthe ion in picometer (10-12 m). This
equation works well for J.1S 0.1 M. The activity coefficients of different ions as a function
of solution ionic strength is listed in a tabular form on page 91 of Quantitative Chemical
Analysis (Harris 1987). From that table, a values for H+ and OH- ions are 900 picometers
and 350 picometers, respectively.
The positive values of pH correspond to hydrogen ion concentrations less than unity
and negative values of pH correspond to concentrations greater than unity. Pure water
has a pH of 7 at 24 °C, 0.1 N hydrochloric acid has a pH of 1.0 at 25 OC and 0.1 N
sodium hydroxide has a pH of 13.0 at 25 OC (CRC Handbook). However, the pH meter
8
scales have a maximum range of only 0 to 14 pH. There are two reasons for this. First,
the set points of feed back pH loops are all well within the 0 to 14 range, since biological
processes and equipment cannot tolerate concentrations outside of this range. Secondly,
even though some feed forward pH measurements could fall outside this range, the pH
electrode's accuracy rapidly deteriorates at the extremities of this range. The hydrogen ion
concentration decreases by a factor of 10 for each unit increase in pH. The product of
hydrogen ion and hydroxyl ion concentration is a constant for water at a given
temperature. The hydroxyl ion consists of a hydrogen bound to a oxygen ion with a net
negative charge of unity. Thus, the charge of the hydroxyl ion is equal in magnitude and
opposite in sign to the hydrogen ion. The 7 pH point is termed as neutral point. For
water at 25 °C the product of hydrogen ion concentration and hydroxyl ion concentration ,
is a constant and the product ofthem is equal to 10·14.
A dissociation constant is used to define the relationship between the activities of the
components. For example consider the dissociation reactions in water:
HCI <=> H+ + Cl
NaOH <=> Na+ + OH- .
(3.6)
(3.7)
The dissociation constant for acid {K3 ) for equation 3. 6 is given by
K = [W][C/-] (3.8) a [HC/]
A weak acid is one that is only partially dissociated in water. This means that Ka is
small for a weak acid. The dissociation constant for base <Kt, ) for equation 3. 7 is given
by [OJr][Na+]
K., = [NaOH] . (3.9)
The K values provide a measure of the strength of the acid or base. The dissociation
constant "K" typically falls numerically in the same range as the hydrogen ion
9
concentration, so that it is convenient to express it as a negative logarithm like pH; where
the small p designates the power function.
pK = -logK. (3.IO)
For example, for water the dissociation constant is given as Kw and it is given as '
H20 <=> H+ + OH- (3.II)
Kw = [H+] [OH·]
log Kw = log [H+] +log [OH·]
-log Kw= pH+ pOH = I4.00 at 25 OC.
The dissociation constants vary with temperature, and, hence, the pH of the solution
varies with temperature. The error caused by operating between 40 and 80 Of over a
range from pH 6 to 8 without temperature compensation is only ±0.03 pH units (Shinsky
1973). Even in the case of high or low pH, the temperature effect on the pH is small, and
can be neglected because the error at the neutralization point is negligible.
3. 2 Rangeability and Sensitivity
The pH scale corresponds to a hydrogen ion concentration ranging from I oO to I o-14
g-moles per liter. Measuring electrodes can respond to changes as small as O.OOI pH, so
instruments can track hydrogen ion concentration changes as small as Sxi0-10 moles per
liter at 7 pH. No other measurement has such tremendous sensitivity (McMillan I984).
The implications of such great rangeability and sensitivity is illustrated by Hoyle
( 1972). The reagent flow should essentially be proportional to the difference between the
hydrogen ion concentration of the process fluid and the set point. Neutralization from a
pH of 6 to a pH of 7 requires only one unit of reagent, and to neutralize from pH I to pH
7 requires 1 00,000 units of reagent. This is because of the logarithmic relation between
pH and hydrogen ion concentration. The flow control device should have enough
rangeability to bring the pH of solution to 7 by delivering the required amount of reagent.
IO
3. 3 Titration Curves
The titration curve is the primary unit of information required for pH control.
Simply, this curve expresses the pH versus the ratio of reagent to influent ratio
relationship, and, thereby, allows us to determine how much reagent is required to
neutralize an influent solution. It is also called the "characteristic curve" or the
"equalization curve." The titration curve is the single most important piece of information
for the design, operation and troubleshooting of pH control systems.
3. 3 . 1 The strong acid and strong base neutralization
The strong acid and strong base curves as shown in Fig. 3 .1 (McMillan 1984) is
distinguished by its vertical slope throughout most of the pH scale range. Titration curves
rarely consist of straight lines; and the enlargement of a straight line usually will reveal
another curve. A titration curve without a clearly defined abscissa is worthless, because
the shape will change with the abscissa range. The neutral point occurs at a pH equal to
one half of the pKw (dissociation constant of water which is given as 10 -14 g-moleslliter),
so that its location depends on solution temperature. K w is a weak function of
temperature of the form 1 o-f(T), where ftT) at 25° C is the familiar 14. K w varies from
14.94 at 2730 K to 13.02 at 333° K (McMillan 1984). Since, the dissociation constants
vary with temperature, the pH of the solution varies with temperature. The neutral point
on the titration curve is the point at which pH of the solution is 7. The equivalence point
on the titration curve is the point at which the quantity of base added is the exact amount
necessary for stoichiometric reaction with the acid or vice versa.
In the neutralization of any strong acid with strong base, there are three regions of the
titration curve of concern. They are:
11
1~
t3
12
11
10
9
8
7 -
6
5
~
3
2
· 1
0
0 0 .2 0.~ 0.6 0.8 1.0 1.2 1.~ 1.6
Ratio of Reagent to Influent Flow
Figure 3.1 Typical titration curve for strong acid and strong base system (McMillan 1984)
12
1.8 2 .0
I . Before reaching the equivalence point. In this region the solution concentration
is high in hydrogen ion concentration. The pH is calculated by using the hydrogen ion
concentration at any point of time before the equivalence point is reached.
2. At the equivalence point. At the equivalence point the amount of base added is
the exact amount necessary for stoichiometric reaction with the acid. Enough OH- ions
had been added to react with all the H+ ions in the solution. The pH at the equivalence
point in the neutralization of any strong acid (or base) with strong base (or acid) will be 7
at 25 °C. The equivalence point coincides with the neutral point for a strong acid and
strong base system. The slope of the titration curve is steepest at the equivalence point for
a strong acid strong base neutralization. The strong acid and strong base system appears
to be symmetrical about the equivalence point.
3. After the equivalence point. After the equivalence point more base is added.
Hence the solution is rich in OH- ions. The pOH of the solution is determined analytically
by calculating the OH- concentration. The pH can be calculated from pOH by the relation
pH+pOH=l4.
3. 3. 2 The weak acid and strong base neutralization
The weak acid and strong base system in Fig. 3.2 (McMillan 1984) is distinguished by
its steep slope in the upper pH scale range. The center of this steep slope is the
equivalence point location, and it is dependent upon the dissociation constant of the weak
acid. When compared to the strong acid and strong base titration, the curve in this case
starts at much higher pH, increases more rapidly at first, then flattens out before it
becomes vertical. The vertical part of the curve is only about half as long as in the case of
strong acid and strong base neutralization.
13
... 13
12
11
·o I 9 I s~
I 7 .!.
I pH i
6..L
sl ~
3
2
1
0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Ratio of Reagent to Influent Flow
Figw-e 3.2 Typical titration curve for weak acid and strong base system (McMillan 1984)
14
1.8 2.<
case starts at much higher pH, increases more rapidly at first, then flattens out before it
becomes vertical. The vertical part of the curve is only about half as long as in the case of
strong acid and strong base neutralization.
Before any base is added, the solution contains just HA in water. HA is weak acid
and the pH of the solution is determined by the equilibrium.
From the equation 3.6, it is given that [H+][A-]
Ka = [HA] .
From equation 3.11, it is given that
Kw = [H+] [OH·].
(3.12)
(3.13)
(3.14)
From equations 3.13 and 3.14, the pH ofthe solution is determined by calculating the
hydrogen ion concentration. The hydrogen ion is considered to be dissociated both from
the weak acid as well as water. Before the equivalence point the neutralization reaction is
given as follows.
(3.15)
Before the equivalence point, there is a mixture of both HA and A- , which is a buffer.
Hence to calculate the pH in this region, the Henderson-Hasselbalch equation is used.
This equation is given as
pH= pK8 +log ( [A·] I [HA] ). (3.16)
Based on this equation when pH equals the pKa, the negative acid ion activity is equal
to the acid molecule activity, and, hence, the dissociation is at the midpoint.
At the equivalence point, the quantity of the base added is exa~ly enough to consume
HA. At the equivalence point HA is converted to A-, a weak base. To calculate pH at
this point the equation is written for the weak base with water.
(3.17)
15
The concentration of A- is no longer the initial concentration of HA, because it has
been diluted with the base. The concentrations ofHA and OH-are equal. Hence, since
the pOH can be calculated, pH can also be calculated. The equivalence point pH will
always be above 7 for the neutralization of strong base and weak acid, since the acid is
converted to its conjugate base at the equivalence point.
3. 3. 3 The strong acid and weak base neutralization
The strong acid and weak base system as shown in Fig. 3. 3 (McMillan 1984) is
distinguished by its steep slope in the lower pH scale range. The center of this steep slope
is in the lower pH scale range, and is the equivalence point. It is always below pH 7, and
it is dependent upon the dissociation constant of the weak base. The procedure to find the
pH at any point in the neutralization process and the equivalence point can be calculated
by using the procedure described in section 3.3.2.
3. 3. 4 The weak acid and weak base neutralization
The weak acid and weak base system in Fig. 3. 4 (McMillan 1984) is distinguished by
the lack of a steep slope throughout the range. The titration curve slope is greatest at the
equivalence point, but it is still relatively small compared to the previous systems. The
equivalence point ordinate depends upon both the acid and base dissociation constant.
Considering the stoichiometry for the neutralization of weak acid and weak base
K = Ka 1 K- -
- Ka(for BH+) -
16
(3.18)
Kb (3.19) Kw
14
13
12
11
10
9
8 t
flH I
7 --
6
5
• 3
2
1
0 -0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Ratio of Reageot to tn1tuent F"""'·
Figure 3.3 Typical titration cwve for strong acid and weak base system (McMillan 1984)
17
1.8 2.1
t.C
13
12
11
10
9
e -I
oH
: t 5 I ~
3
2
1
0
0 02 0.~ 0.6 0.8 1.0 1.2 1.4 1.6
Ratio of Reagent to Influent Flow
Figure 3.4 Typical titration curve for weak acid and weak base system (McMillan 1984)
18
1.5 2 .0
Combining the equations 3.18 and 3.19 it can be written as
HA+B<::>BH++A- K = KaKb. Kw
(3.20)
For weak acids and weak bases, K of the above reaction can be large or small. There
are three cases of concern.
Case 1: K is large ( K>>1).
If K is large the reaction goes to completion. When the reactants are mixed, they will
proceed to make products until one of the reactants is consumed. This case can be treated
as if one reactant is a strong acid or a strong base.
Case 2: K is not large.
In this case the reaction does not goes to completion because the value ofK is not
large. A substantial quantity of unreacted starting material will be in equilibrium with
products.
Case 3: Equimolar mixture ofHA and B, regardless of the size ofK.
An equimolar mixture of weak acid and weak base behaves as the intermediate form
of a polyprotic acid. The pH ofthe equimolar mixture should be very nearly midway
between pK3 for HA and pK3 for BH+.
A common misconception is that a titration curve consists of a single S-shaped curve.
In Fig. 3.5 (McMillan 1984), a weak acid with pKa dissociation at 2.5 and 5.5 pH is
titrated with a weak base with pKb dissociation constants at 8.5 and 11.5 pH. It is noted
that three S-shaped titration curves are formed with an equivalence point at the point of
steepest slope for each S. The dissociation constants have to be more than 2 pH units
apart to create multiple S-shaped curves. When ever an acid (base) is neutralized with a
base( acid), when exact amount of base (acid) necessary for stiochiometric reaction is
added the acid (base) is said to be neutralized and that particular point on titration curve is
19
14
13
12
11
10
9 pKa=6.5
6
pH 7
6
s
4
3
2
1
0
0 02 0.4 0.6 0.8 1.0 1.2 1 .... 1.6 1.8 2.0
Ratio of Reagent to Influent Aow
Figure 3.5 MultipleS-shaped curves (McMillan 1984)
20
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
37
14
12 2 0 +-------~----~~----~-------+------~------+-------~----~~----~
0 0.
5 1
1.5
2 2.
5 3
3.5
4 4.
5
Tim
e in
min
Fig
ure
5.
I R
esul
ts o
fNeu
tral
izat
iono
fHyd
roch
lori
c A
cid
wit
h S
odiu
m H
ydro
xide
w
\0
13
11 9
:g_
7 5 3 1
0 2
Neu
tral
Lin
e
4 6
8 10
12
14
16
18
20
22
24
26
28
30
32
Tim
e in
min
Figu
re 5
.2
Res
ults
of N
eutr
aliz
atio
n o
f Sod
ium
Hyd
roxi
de w
ith H
ydro
chlo
ric
Aci
d
neutral point can be managed, and it is possible to keep the wastewater solution pH well
within the EPA mandated limits.
5.2 Strong Acid versus Weak Base
This case was also tested for both possibilities, one neutralizing strong acid with weak
base and the other neutralizing the weak base with strong acid. The strong acid and weak
base typically obtained from the research and academic laboratories were hydrochloric
acid and sodium carbonate.
Figures 5.3 and 5.4 represents the results obtained for both cases of neutralization.
The results shown in Figure 5. 3 demonstrate that the pH of the hydrochloric acid solution
was brought well within limits ofEPA and the pH of the final solution was 6.6. The
results show that the time taken to neutralize hydrochloric acid by sodium carbonate is
approximately 38 minutes whereas in the other case only 4.5 minutes are required. Figure
5.4 demonstrate that the pH of the final basic solution was 7. Neutralization requires
more time to neutralize the strong acid with weak base. However, for our system, time is
not an important constraint. The technician can leave the system on and then come back
at a convenient time and to drain the wastewater when neutralization is completed.
5.3 Weak Acid versus Strong Base
This case was also tested for both possibilities, one neutralizing weak acid with strong
base and the other neutralizing the strong base with weak acid. The weak acid and strong
base selected were acetic acid and sodium hydroxide. The same experimental procedure
as discussed in section 4.4 was repeated for both the cases, and the results are plotted in
Figures 5. 5 and 5. 6. Figure 5.5 shows the results of the acetic acid neutralization with
sodium hydroxide. Figure 5. 6 shows the results of the sodium hydroxide neutralization
with acetic acid.
40
13
11 9
=a 7
~ -
5 3 1
0 2
4 6
8
Neu
tral
Lin
e
~
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
Tim
e in
min
Figu
re 5
.3
Res
ults
ofN
eutr
aliz
atio
n o
f Hyd
roch
lori
c A
cid
with
Sod
ium
Car
bona
te
13
-
11
-
9 -
:I:
0..
7
~
I 5 ~
/ N
-~
Neu
tral L
ine
I 3
-
I 1
I I
I I
I I
I I
0 0.
5 1
1.5
2 2.
5 3
3.5
4 4.
5
Tim
e in
min
Figu
re 5
.4
Res
ults
ofN
eutr
aliz
atio
n o
f Sod
ium
Car
bona
te w
ith H
ydro
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cid
13
11
I N
eutra
l Lin
e '
9
::X:: 0..
7
I ~
w
5 3 1
0 2
4 6
8 10
12
14
16
18
20
22
24
26
Tim
e in
min
Figu
re 5
.5
Res
ults
of N
eutra
lizat
ion
of A
cetic
Aci
d w
ith S
odiu
m H
ydro
xide
13
+
11
-
9
:t
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7
1
~
I 5 +
~
I 3
-
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0
-7
Neu
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Lin
e
i I I
1 2
I 3
Tim
e in
min
I 4
Fig
ure
5.6
Res
ults
of
Neu
tral
izat
ion
of
Sod
ium
Hyd
roxi
de w
ith A
cetic
Aci
d
l I
5 6
5.4 Weak Acid versus Weak Base
This case also tested for both possibilities, one neutralizing weak acid with weak base
and the other neutralizing the weak base with weak acid. Acetic acid and sodium
carbonate are considered for this experiment. The curves in Figures 5. 7 and 5. 8 show the
results obtained for both cases. The time taken for neutralizing the acetic acid was
approximately 24 minutes whereas the time taken for neutralizing the sodium carbonate
required approximately 6 minutes. The curves looks like the portions of typical titration
curve of weak acid versus weak base.
The main difficulties observed with this kind of experimental setup are as follows.
The pH reading that is displayed on the display board of pH controller is different from the
actual solution pH in the tank. The reagent is pumped to and mixed in the tank almost
instantaneously. It takes few seconds for the solution in the tank to reach the sensor. It
was observed that there was a probable error of approximately± 0.5 pH to ±0.1 pH
depending upon the kind of waste that is being treated. There are also some variations
caused by contamination of the solution by absorption of carbon dioxide from air, and
imperfect mixing. These source of variation can hypothetically effect the pH of the
solution to some extent.
The dissociation constants vary with temperature, and, hence the pH of the solution
varies with temperature, as a consequence the pH of the solution will also vary with
temperature. The dissociation constant is a weak function of temperature. The error
caused by operating between 40 and 800 F over the range from pH 6 to 8 without a
temperature compensation is only ±0. 03 pH units. This is hardly a significant error
because the LO relay setpoint and m relay setpoint are well within the range from 6 to 8
pH units. The error in this experiment will be smaller because the pH sensor that was used
has a temperature compensator. The pH sensor can be operated efficiently in the range of
45
~
0'\
:X::
0.
14
12
10
Neu
tral
Lin
e
8 6 4 2 0 +-----+-----+-----+-----+-----r-----r-----~--~----~----~-----+----~
0 2
4 6
8 10
12
Tim
e in
min
14
16
18
Fig
ure
5.7
Res
ults
ofN
eutr
aliz
atio
n o
f Ace
tic a
cid
with
Sod
ium
Car
bona
te
20
22
24
13
11 9
::t:
0.
7
I "'
~
I /
'-l
5 t
Neu
tral L
ine
I 3 1
0 0.
2 0.
4 0.
6 0.
8 1
1.2
1.4
1.6
Tim
e in
min
Figu
re 5
.8
Res
ults
of N
eutra
lizat
ion
of S
odiu
m C
arbo
nate
with
Ace
tic A
cid
32 to 185° F. Hence temperature does not affect the pH readings significantly in these
wastewater neutralization studies, because the experimental setup is in a temperature
controlled room.
Overall, this semi-automated batch system for the neutralization of wastewater
produced by Texas Tech University worked well for acidic and basic solutions tested.
Economically and time-wise, this system solves many ofthe problems of hazard waste
neutralization for acidic and basic wastewater. The system is simple to operate and
maintain. The results obtained demonstrate for SQG's the effectiveness of this system to
economically neutralize wastewater . A simplified economic analysis of the system was
made and compared with the cost incurred by conventional third party disposal methods in
order to estimate the cost-benefit ratio. It is observed that semi-automated batch process
could save the expenditure of wastewater neutralization of Texas Tech University by
$18,000 over a five-year period oftime.
48
CHAPTER6
CONCLUSIONS AND RECOMMENDATIONS
Semi-automated batch process techniques were applied to the controVadjustment of
pH processes. This technique was successfully demonstrated on a bench scale with an in
line pH controller. The model was tested for several kinds and concentrations of mineral
and organic acids and caustic wastes. The system successfully neutralized the mineral or
organic acids and caustic wastes using inexpensive and easily available reagents.
The system was setup in the TSD facility for EH&S Department's use. After it was
tested, the system is ready to function at the designed level. However, the system could
be made more sophisticated by incorporating some small changes. These changes could
not be implemented at this point of time because of financial constraints. Another pH
sensor and controller along with a metering pump would eliminate the problem of
changing reagent tanks as per the requirement. This second in-line controller should be
able control another metering pump, which is connected to dedicated acid or caustic
reagent. This addition would solve the problem of changing the suction end tube of
metering pump from acid to base reagent whenever there was a change in the type of
wastewater being treated. The electrodes still need to be checked from time to time. A
spare set of electrodes should be available to replace a faulty electrode.
Environmental regulations change from time to time. With inevitable change, future
requirements may mean that a computer will be needed between the controller and
metering pump along with AID or D/ A converter. This will provide the technician with
the ability to inventory all the wastes that are being neutralized, and to store other details
such as the final pH and quantity of the of the solution left in the holding tank.
49
BffiLIOGRAPHY
"Programmable controllers cut industrial wastewater treatment costs," Pollution Engineering, V 21, pp. 84, May 1991.
"The automated wastewater plant," Chemical Engineering, V 100, pp. 147, January 1993.
Armour, M.A. :Hazardous laboratory chemicals. Disposal guide, CRC Press, Boca Raton, FL, 1991.
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.
LMI B711-915, Metering pump Instruction manual, Liquid Metronics Division, Acton, MA, 1993.
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.
57
APPENDIX C
HAZARDOUS OPERABILITY STUDY
58
Tab
le C
. I H
azar
dous
Ope
rabi
lity
Stud
y
Item
Li
ne o
r E
q-P
roce
ss
Dev
iati
on
Poss
ible
Cau
ses
Poss
ible
con
sequ
e-A
ctio
n re
quire
d
uipm
ent
para
met
er
nces
1 H
oldi
ng ta
nk
Leve
l M
ore
I. M
ore
addi
tion
of re
agen
t I.
Spill
s in
to th
e sp
ill t
ank
1. St
op th
e m
eter
ing
pum
p
2. N
eutr
aliz
atio
n of
conc
en-
2. D
ilute
the
was
tew
ater
bef
ore
trat
ed w
aste
wat
er
neut
raliz
atio
n
3. p
H s
enso
r bro
ken
3. C
heck
/repl
ace
the
sens
or
4. p
H c
ontr
olle
r bro
ken
4. C
hedc
/repl
ace
pH c
ontr
olle
r
Les
s 1.
VI
open
I.
Env
ironm
enta
lly h
azar
dous
1.
Clo
se v
alve
VI
2. P
ossi
ble
leak
in p
ipin
g 2.
Lea
ks in
to th
e sp
ill ta
nk
2. R
epla
ce p
ipin
g
3. P
ossi
ble
leak
in p
ump
3. R
epla
ce p
ump
Con
cent
ra-
As
wel
l as
I. pH
sen
sor b
roke
n I.
No
good
mix
ing
I. C
heck
/repl
ace
the
sens
or
"" tio
n 2.
pH
con
trol
ler b
roke
n 2.
Pre
ssur
e ris
e in
pip
ing
2. C
heck
/repl
ace
the
cont
rolle
r \0
3.
V2
is c
lose
d 3.
Ope
n va
lve
V2
pH
Hig
her
l. F
ast a
dditi
on o
f rea
gent
I.
May
cro
ss s
etpo
int
1. R
educ
e th
e %
stro
ke o
f m
eter
ing
pum
p
Low
er
l. S
low
add
ition
of r
eage
nt
l. C
onsu
mes
tim
e l.
Inc
reas
e th
e %
stro
ke o
f
2. R
ecirc
ulat
ory
pum
p is
off
m
eter
ing
pum
p 2.
Mak
e su
re th
e pu
mp
is o
n
2 V
alve
VI
Flow
N
o 1.
Val
ve V
I is
clo
sed
I. Pr
essu
re in
crea
se in
the
I. O
pen
valv
e V
I
2. P
ump
is b
roke
n pi
ping
2.
Rep
lacd
fix
pum
p
3. P
ump
is c
logg
ed
2. C
once
ntra
tion
rise
in ta
nk
3. C
lean
pum
p
4. P
ipe
is c
logg
ed
3. N
o dr
ain
faci
lity
4. C
lean
pip
e
S. V
alve
is c
logg
ed
4. N
o go
od m
ixin
g S.
Cle
an v
alve
VI
Tab
le C
. I H
azar
dous
Ope
rabi
lity
Stu
dy
3 V
alve
V2
Flow
N
o 1.
Val
ve V
2 is
clo
sed
I. Pr
essu
re i
ncre
ase
in t
he
1. O
pen
valv
e V
2
2. P
ump
is b
roke
n pt
pmg
2. R
epla
ce/f
ix p
ump
3. P
ump
is c
logg
ed
2. C
once
ntra
tion
rise
in ta
nk
3. C
lean
pum
p
4. P
ipe
is c
logg
ed
3. N
o dr
ain
faci
lity
4. C
lean
pip
e
S. V
alve
is c
logg
ed
4. N
o go
od m
ixin
g S.
Cle
an v
alve
V2
4 R
ecir
cula
tory
Fl
ow
No
1. P
ipe
is c
logg
ed
I. C
once
ntra
tion
rise
in ta
nk
I. C
lean
/rep
lace
pip
e
pum
p 2.
Pum
p br
oken
2.
Rep
lace
pum
p
3. P
ump
turn
ed o
ff
3. M
ake
sure
pum
p tu
rned
on
4. N
o w
aste
wat
er in
tank
4.
Tra
nsfe
r w
aste
wat
er in
to
hold
ing
tank
/ turn~ff th
e pu
mp
0\
S M
eter
ing
Flow
N
o I.
pH
of w
aste
wat
er is
I.
Pres
sure
ris
e 1.
Dra
in th
e ne
utra
lized
was
te
0 pu
mp
betw
een
6.5
and
7.S
2. p
H m
ay n
ot r
each
set
poin
t 2.
Cle
an/r
epla
ce p
ipin
g
2. P
ipe
is c
logg
ed
3. R
efill
the
reag
ent
3. N
o re
agen
t avi
alab
le
4. R
epla
ce th
e co
ntro
ller
4. p
H c
ontr
olle
r is
bro
ken
Mor
e I.
% s
trok
e is
hig
h 1.
Fas
t pH
cha
nge
1. R
educ
e th
e %
str
oke
Less
1.
% s
trok
e is
low
1.
Slo
w p
H c
hang
e 1.
Inc
reas
e th
e %
str
oke
6 pH
sen
sor/
pH
Fa
ster
1.
Fas
t add
ition
of r
eage
nt
1. M
ay c
ross
set
poin
t 1.
Red
uce
%st
roke
Con
trol
ler
2. H
igh
% s
trok
e 2.
Neu
tral
ize
in b
ulk
3. L
ess
quan
tity
of w
aste
-w
ater
com
pare
d to
the
flow
ra
te o
f rea
gent
0\ -
pH s
enso
r I
Con
trol
ler
pH
Slow
er
Tab
le C
. I H
azar
dous
Ope
rabi
lity
Stu
dy
I. Sl
ow a
dditi
on o
f rea
gent
2.
Low
% s
trok
e 3.
Con
cent
rate
d w
aste
wat
er
in h
oldi
ng ta
nk
I. S
low
neu
tral
izat
ion
2. M
ay o
verf
low
the
tank
I.
Inc
reas
e th
e %
stro
ke
2. D
ilute
the
was
tew
ater
be
fore
neu
tral
izat
ion
