Group Title: Citrus Station mimeo report - Florida Citrus Experiment Station ; 57-6
Title: Determination of the chemical oxygen demand of citrus waste water
Full Citation
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 Material Information
Title: Determination of the chemical oxygen demand of citrus waste water
Series Title: Citrus Station mimeo report
Physical Description: 4 leaves : ill. ; 28 cm.
Language: English
Creator: McNary, Robert R
Dougherty, Marshall H
Wolford, R. W
Citrus Experiment Station (Lake Alfred, Fla.)
Florida Citrus Commission
Publisher: Florida Citrus Experiment Station :
Florida Citrus Commission
Place of Publication: Lake Alfred FL
Publication Date: 1956
Subject: Citrus fruit industry -- By-products -- Florida   ( lcsh )
Citrus fruit industry -- Waste disposal -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
Bibliography: Includes bibliographical references (leaf 4).
Statement of Responsibility: R.R. McNary, M.H. Dougherty and R.W. Wolford.
General Note: Caption title.
General Note: "October 11, 1956."
 Record Information
Bibliographic ID: UF00072383
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 74906860

Full Text

Citrus Station Mimeo Report 57-6
October 11, 1956

Determination of the Chemical Oxygen Demand of Citrus Waste Water
R. R. McNary, M. H. Dougherty and R. W. Wolford

During investigations carried out in this laboratory on the treatment of
citrus wastes, the 5-day B.O.D. and total, fixed and volatile solids have been
used as a means of evaluating the thoroughness of the treatment. This was quite
satisfactory on a laboratory scale where the raw waste was made up by diluting
citrus juices a predetermined amount, thereby producing a uniform raw waste. In
later work on a pilot plant scale where actual processing plant wastes were being
treated, the great variability of the strength of these wastes made it highly
desirable to determine their strength quickly in order to adjust treatment op-
erations to the current situation. The use of a biological procedure for this
purpose is not suitable because of the time required to obtain the results.
Several chemical oxygen demand procedures have been published that might be suit-
able. Probably the most rapid procedure is that of Porges et al. (1). This
procedure has several disadvantages. The method of heating the reaction mixture
is tricky and requires the constant attention of the analyst. In the authors'
hands it was found difficult to heat the mixture uniformly to 1650 to 1700C. in
five to six minutes, especially in the summer time. With open window, varying
air currents change the rate of heating continually. Another disadvantage stems
from the fact that the potassium dichromate is dissolved in a mixture of sulphuric
and phosphoric acids. This solution is viscous and it is difficult to measure out
the correct amount volumetrically. The authors found that blank determinations
varied appreciably for this reason.

The procedure of Moore et al. (2, 3, 4) has the disadvantages of the somewhat
lengthy reflux period of two hours and the fact that the titrating solution must
be standardized each day. Otherwise this procedure is satisfactory. The di-
rections for this procedure state that the reflux period can be shortened when it
can be demonstrated by experiment that a shorter period will give satisfactory
results with the particular type of waste being analyzed. The time required in
the case of citrus waste water has not been published heretofore. The minimum
reflux time was therefore investigated by determining the change in chemical
oxygen demand of two synthetic waste water samples made from canned orange juice
diluted with tap water when the reflux time varied from 5 to 120 minutes. The
results are shown graphically in Fig. 1. It was apparent that citrus wastes
were quickly oxidized by the dichromate. In these two instances 97 and 98 percent
of the final, two hour, C.O.D. value already appeared in the first five minutes.
It was arbitrarily decided, however, to standardize on a reflux period of ten
minutes for citrus wastes since an error in timing the period would then be less
likely to affect the result.

Milk wastes are not quite so rapidly oxidized. A sample of diluted fresh
milk required at least 15 minutes reflux time (Fig. 1).

Three titrations are necessary for the determination of the chemical oxygen
demand of a single sample of waste by the Moore method. One titration is re-
quired to determine the normality of the ferrous ammonium sulfate solution and

Florida Citrus Experiment Station
and Florida Citrus Commission,
Lake Alfred, Florida.
710a 10/11/56 RRMc


this must be repeated each day. The second titration is required for the blank
and the third for the sample itself. Some of this manipulation might be avoided
if the result could be obtained by colorimetry rather than by titration. To
determine whether the color change when the dichromate solution is reduced is
adequate for colorimetric analysis, a solution corresponding to the blank deter-
mination of the Moore procedure was placed in a Beckman DU spectrophotometer and
the absorption spectrum in the visible region determined. The absorption spectrum
was also determined on another potassium dichromate, sulfuric acid, water mixture
in which sufficient sucrose was added before refluxing to completely reduce the
dichromate present. The spectral transmission curves of these two solutions are
shown in Figure 2. At 650 millimicrons the transmission of the reduced chromate
was 52 percent of the transmission of the unreduced chromate. That this difference
was ample for use in the colorimetric ,assay of the strength of citrus waste waters
was demonstrated by adding increasing amounts of the waste to the reflux mixture
and reading the transmission in a photoelectric colorimeter. These results are
shown graphically in Fig. 3.

To translate the percent transmission obtained from photometer readings to
oxygen demand values a standard curve was prepared (Fig. 4). A solution of sugar
containing exactly 2.000 grams per liter was made from a sample of National
Bureau of Standards sucrose that had been dried for a week in a vacuum desiccator.
From 1.00 to 25.00 ml. of this solution were measured by burette into a round
bottom, 500 ml. flask, diluted to 50.00 ml. with distilled water also measured in
a burette. The usual quantities of dichromate solution and sulfuric acid were
added and the mixture was refluxed for ten minutes. The flask was removed from
the reflux apparatus and cooled to room temperature in a running-water bath. A
portion of the liquid was placed in a cuvette and the optical transmission at
650 millimicrons determined in a Fisher Electrophotometer. A blank sample con-
taining no sugar was used to adjust the instrument to read 100 percent trans-

To eliminate the necessity to prepare a blank determination each time a
sample is analyzed, a blank solution was sealed in one of the glass cuvettes used
in the Fisher Electrophotometer. As far as can be determined, this solution has
not changed its light transmission at 650 millimicrons since the day it was
sealed six months ago. Using this permanent blank, the complete oxygen demand
determination can be carried out in about 20 minutes.

Since the sealed blank was apparently stable, it appeared possible that a
set of comparison standards could be prepared which would also be stable. If
successful, the determination of the chemical oxygen demand could then be made
by visual comparison of the color of the refluxed mixture with the standards.
The necessity of having a colorimeter would be removed. To test this hypothesis,
a solution of sucrose containing exactly 2.000 grams per liter was prepared as
before. Theoretical quantities of this solution were measured out with a burette
to give a series of samples containing from zero to 1500 p.p.m. C.0.D. in incre-
ments of 50 p.p.m. Sufficient water was added to bring the volume up to 50 ml.,
then 25 ml. of potassium dichromate solution and 75 ml. of sulfuric acid were
added and the mixture was refluxed for ten minutes. After cooling, the resulting
solutions were sealed in 25 mm. diameter test tubes. The colors ranged from the

Florida Citrus Experiment Station
and Florida Citrus Commission,
Lake Alfred, Florida.
710b r 10/11/56 RRMc

Table 1
Comparison of Oxygen Demand Analyses of Citrus Waste Water and
Citrus Activated Sludge Effluent

Raw Citrus Waste Citrus Activated Sludge Effluent
5-Day Standard Colori- Visual 5-Day Standard Colori- Visual
B.O.D. methods metric comparison B.O.D. methods metric comparison
C.O.D. C.O.D. C.O.D. C0.D. C..OD. C.O.D. C..D.
1295 2290 2450 2375 4.0 45 55
1330 2310 2500 2375 6.7 55 70 -
1400 2295 2350 2250 5.0 66 70 50
1630 2330 2475 2355 6.1 52 70 50
1705 2285 2500 2355 5.7 42 65 50
1565 2240 2375 2375 6.6 51 70 50
1385 2300 2490 2375 3.4 34 30 50
1410 2270 2375 2375 3.6 26 60 50
1220 2175 2350 2250 5.8 20 60 25
1680 2300 2350 2375 3.0 31 40 25

1462 2280 2422 2346 5.0 42 59 44

Table 2
Ratios of Oxygen Demand Analyses Obtained by Different Procedures
Raw Activated
waste sludge
Ratio, Standard methods C.O.D. to B.O.D. 1.56 8.40
Ratio, Colorimetric C.O.D. to B.O.D. 1.66 11.80
Ratio, Visual comparison C.O.D. to B.O.D. 1.60 8.80
Ratio, Colorimetric C.O.D. to Std. methods C.O.D. 1.06 1.40
Ratio, Visual comparison C.O.D. to Std. methods C.O.D. 1.03 1.05

Florida Citrus Experiment Station
and Florida Citrus Commission
Lake Alfred, Florida.
710 10/11/56 RRMc

- 2600

- 2400

2200P Citrus Waste

- 2000

S1800 -

1600 Q

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S1000 0


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30 g

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Lake Alfred, Florida. 710e 10/11/56 RRMc

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Citrus Experiment Station and Florida Citrus Commission, Lake Alfred, Fla. 710f 10/11/56 iRMc




% Transmission vs. Theoretical Oxygen
Demand of Sucrose Solution

Measured in Fisher Colorimeter at 650 mu

Oxygen Demand, p.p.m.

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Florida Citrus Experiment Station and Florida
Lake Alfred, Florida. .710g --.10/11/56- R4Mc

Citrus Commission,





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amber of the zero oxygen demand tube to a blue-green of the 1500 p.p.m. chemical
oxygen demand tube. Against a white background and with good light, such as
provided by a fluorescent titration illuminator, the differences between adjacent
tubes can be readily distinguished with one exception. The color difference be-
tween the 50 p.p.m. tube and the zero p.p.m. tube was slight and extra care must
be taken to provide uniform lighting in this region.

The set of sealed, color-comparison tubes has been in existence only 2 1/2
months. Thus far no changes have been noticed in the colors and it is hoped that
they will prove to be permanent.

A series of ten citrus waste samples have been analyzed for 5-day B.O.D. and
for C.O.D.; the latter by three methods, namely the procedure of Moore with the
full two hour refluxing and titration, the quick method proposed herein with ten
minute refluxing and colorimetry, and the quick method with ten minute refluxing
and visual comparison with sealed standards. The data are shown in Table 1. The
ratios of the averages of these ten samples when analyzed by different procedures
is given in Table 2.


The object of this investigation was not to develop an analytical procedure
for determining the strength of citrus wastes that would replace the procedures
already available. There is no substitute for the 5-day B.O.D. determination and
none of the chemical procedures measure the same substances in the same manner
that it does. There is however, a definite place for the C.O.D. test, particularly
when one is concerned with only one type of waste. Excellent discussions on the
relationships between B.O.D. and C.O.D. data may be found in the literature (4, 5).
Citrus wastes, being predominantly carbohydrate in nature, are readily oxidized by
either chemical or biological means. The results determined by the two procedures
are considerably different. The C.O.D. values obtained by the Moore procedure on
the average were 1.56 times the B.O.D. values. As long as a citrus waste sample
is not contaminated with another kind of waste, this factor may be used to esti-
mate the B.O.D. value from the C.O.D. analysis with fair accuracy. When a
different type of waste is being analyzed, different ratios must be used as
illustrated by the effluent ratios in Table 2.

Ten minutes of reflux time is adequate for the determination of the C.O.D.
of citrus wastes.

When some of the accuracy of the Moore procedure may be sacrificed in favor
of greater speed and less manipulation, the colorimetric procedure described here
can be used. The colorimetric results given here were higher than the titration
results by a factor of 1.06. This factor could be used when one wishes to compare
results obtained by the two procedures. The difference arises from use of the
potassium dichromate solution as a primary standard in the titration procedure
and the use of the sucrose solution as primary standard in the colorimetric pro-
cedure. The authors used the Fisher Electrophotometer for the preparation of the
standard reference curve and for the analyses mostly because it was felt that a

Florida Citrus Experiment Station
and Florida Citrus Commission,
Lake Alfred, Florida.
710c 10/11/56 RRMc


filter-type instrument of this sort would more likely be used by industrial
laboratories. A few trials indicated that greater precision could have been ob-
tained in a prism-type instrument, such as the Beckman DU, and possibly by other
instruments also.

When no instrument is available, a fairly good approximation of the C.O.D.
value can still be obtained by visual comparison of the refluxed liquor with
standards sealed in glass. It is assumed that these standards will be stable
over a long period of time. Periodic checking will be necessary to confirm this

Whether the analytical results are obtained by titration, colorimetry, or
visual comparison, the presence of chlorides and of aliphatic organic acids
affects the results in the same manner. The reader is referred to the literature
(2, 3, 4) for a discussion of this subject. In the analysis of citrus waste
waters in Florida, usually, these effects are not important.


1. Porges, N., Pepinsky, J. B., Hendler, N. C., and Hoover, S. R. Biochemical
Oxidation of Dairy Wastes. I. Methods of Study. Sewage and Ind.
Wastes 22, 3, 318 (1950).

2. Moore, W. A., Kroner, R. C., and Ruchhoft, C. C. Dichromate Reflux Method
for Determination of Oxygen Consumer. Anal. Chem. 21, 953 (1949).

3. Moore, W. A., Ludzach, F. J., and Ruchhoft, C. C. Determination of Oxygen
Consumed Values of Organic Wastes. Anal. Chem. 23, 1297 (1951).

4. Standard Methods for the Examination of Water, Sewage, and Industrial
Wastes. 10th Ed., Amer. Pub. Health Assn., New York, N. Y. (1955).

5. Ettinger, M. B. Evaluation of Methods Currently Used to Determine B.O.D.,
C.O.D. and Odor for Industrial Wastes. Sewage and Ind. Wastes 28, 9,
1116 (1956).

Florida Citrus Experiment Station
and Florida Citrus Commission,
Lake Alfred, Florida.
710d 10/11/56 RRMc

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