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AN EXPERIMENTAL STUDY OF THE
ANEROBIC DIGESTEON OF MILK

PLANT BY-PRQDL’CTS

'l'hesis for the Degree of B

S.
MICHECAR STATE COLLEGE-
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An Experimental Study of the

Anerobic Digestion of Kilk Plant By-Produots

A Thesis Submitted to

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MICHIGAK STATE COLLEGE
of
AGRICULT’RE AED APPLIED SCIEKCE

:1, r
Kenneth S; ZacPherson

Candidate for the Degree of

Bachelor of Science
Car-,4 e.-twig-fines;Mum-«.1

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June 1959

THESIS?

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Acknowledgement

The writer wishes to express his sincere
Appreciation to Mr. E. F. Eldridge for
his guidance and helpful sug estions
which made possible the comnletion of

this work.

The Statement of the Problem
To determine:
(I) The feasibility of anaerobic digestion as a
means of dispOSml for milk plant by-products.
(2) The effect of seeding the by-products with
sewage sludge on the rate of digestion.
(3) The quantity and quality of gas produced and

its possible use in the milk plant for power or heat.

Introduction

The effective disposal of dairy by-products is une
of the most important and troublesomaprohlems to be solved
by sanitarians at the present time. Dairy by-products are
important due to their common occurance and to the fact
that they are of such composition as to cause disruption
of the ordinary processes of waste disposal unless in
very dilute form.

Before modern trends toward treatment of all indust-
rial wastes, it was common practice to dispose of dairy
by-products by merely discharging the waste into the
nearest body of water. There were many complaints about
the pollution in lakes and streams, but due to the fact
that no other effective met od of disposal was known
little was done about this condition.

Dairy y-products are especially troablesome due
mostly to their high organic solids content. In the east-
ern part of the United States there is not much trouble

from dairy by-aroducts because most of the dairy products

are used by the dense local population. In the middle west

he situation is quite different. So great a volumn of

dairy products are produced that all are not used by the
local population, and as a result there are many milk plants
that produce cheese, butter, casein, etc., for other sections
of the country. In the manufacture of these products there
are by-products of pure milk, kim milk, whey, washings, etc.,
which are high in orranic content and require treatment.

Kany of these plants are located on small streams in
which the flow of waste from the plant at certain times of
the year is equal to, or greater than the stream flow. The
resulting depletion of oxygen in the stream causes definite
polluted conditions which must be corrected if present san-
itary demands are to be satisfied. These plants, many of
which are located in small towns, also tax the sewage dis-
posal facilities of the town. Often the sewage disposal
plant must be designed for a population several times greater
than the actual population.

The composition of mi k is shown in the following table:

likdévaf' CZmawva/ .4725? Jugnmc,tflmévawvzflzcflkfiz
46P4vwv /¢4h»nwh //:7<(omv4zr /4CId’

./5§f: ‘fi%vfibfi'4fl¢rfiha: laud; l44llé?‘

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3.67. 3.1% ¢dZ .50: 5225-2
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(I) Butter ani cream are made from the fat.

(2) Casein and albumin are made from the protein.

(3) milk sugar, calcium lactate and lactic acid are
made from the lactose.

(4) Jhey contains all ash and lactose, and a small
amount (about.5?) of protein.

(5) Skim milk COnt&lLS all the ash, protein, and
lactose.

The first real development in sewage disposal was the
development of septic and Imhoff tanks, which operate as
anaerobic digestion chambers. However when milk by—produds
are put in these tanks, the whole system of digestion is
upset. As can be seen from the above table, all milk by-
products contain lactose, and this sugar causes the troubh.
This is because lactose is readily oxidizei to lactic acid,
which quickly inhibits the growth of most of tie anaerobic
species so helpful in reducing the volumn of precipitated
solids. Hhen the oxidation of lactose has been allowed to
proceed to a point where the pH value of 4 or 5 is indicated,
the action of most microorganisms is almost completely
inhibited. Some attempts have been made to raise the pH
into the alkaline range by the addition of lime or caustic
soda, which are the only chemicals cheap enough to be of
practical use, but the calcium or sodium reacts with the
lactic acid to form a calcium or sodium lactate, which is
a heavy precipitate, and almost as toxic to the growth of
anaerobic bacteria as t e free acid. These lactates have
some commercial value which may be capitalized upon to
pay for he treatment of the dairy by—products, but has not

been done to any extent as yet. These deposits of lactate

#—

in the di estion c ambers of Imhoff tanks have been known
to stay from 6 t o 9 months with no apparent decomposition.
Another uncesirable acid, butyric acid , is also formed in
theaImhoff and septic tanks ha dling dairy by-products.
Butyric acid is objectionable cliefly because of it's
disagreeable odor, which isnoticeable when only small quan-
tities of the acid are present.

Often in sewage plants an Imhoff ta k is followed by
a trickling filter. If dairy by-“roducts are allowed in
the tank, the resulting reactions give an acid effluent and
the tank acts merely as a settling basin. The highly acid
effluent is relatively stable due to the toxic effect of
the acid and is in a poor state for efficient treatment on
trickling filters. Treatment of dairy wastes by oxidation
on a trickling filter to remove the acid producing material
has ha some success. Work on this phase has been done by
Levine, Burke, Linton, Soppeland, Hatkins, Galligan and
Goresline at Iowa State College; Eldrid e at Kichigan State
College; and Ruf and Warrick in disconsin. At the present
time this method is accepted as the best developed althoug
it has definite limitations. These are chiefly that the
waste must not be over 25 to 3 hours old before treatment,
and that trouble is found with the filters clog ing.

The present trend toward waste disposal by the activated
sludge method offers a possibility of oxidizing the acid
forming compounds. The activated sludge process has been
found effective in lestroying these compounds, but very

large quantities of air were required, it was extremely

 

difficult to maintain the sludge in suitable conditions and
the effluents were not stable.

It therefore can be said that the common methods of
waste disposal by anaerobic processes are not applicable to
dairy by-products because: --

(I) The milk sugar is rapidly decomposed with the
production of acids that may curdle the waste.

(2) If the dairy by-products are sufficiently concen-
trated, the resulting acidity will inhibit or kill many of
the protein digesting anaerobic bacteria.

(3) The effluents from such anaerobic tanks are in
an undesirable state for most efficiency on filters.

(4) The high acidity and consequent relative sterility
of the contents of the anaerobic tanks receiving milk by-
products results in a relatively high stability of the eff-
luents.

Anaerobic treatment has the most success in milk by-
product disposal, but meets with the following difficulties:

(I) Trickling filters need very fresh influent and are
bothered with clogging.

(2) The activated sludfie method is expensive; uses a
large amount of air; is hard to control the sludge formation;

and gives an unstable effluent.

The object of this experiment is to determine:

(I) The feasibility of anaerobic digestion as a means
of disposal for milk plant by-products which consist of whey,
skim milk, washings, etc.

(2) The effect of seeding the by-products with sewag
sludge, on the rate of digestion.

(3) The quantity and quality of gas produced and its
possible use in the milk plant for power or heat.

To determine the above objects:

(I) Flascs were set up containing a known volumn of
whey and skim milk, and the digestion of each was measured.

(2) The by-products were seeded with sludge so that
mixtures contained at, 59, and 75% of sludge.

(3) The gas produced by each mixture was analysed in
a gas analysis apparatus.

The methods used in determining the above objects will
be discussed later.

The mixtures to be digested were first mixed in the de-
sired proportions so that a volumn of I500 ml. of each mix-
ture was available. This volumn was then transfered to two
liter flasks having a tight rabber stopper with a glass tube
connecting the flask to the gas collection apparatus. The
gas was collected in an inverted two liter graduate filled
with water and standing in a reservoir of water. The app-
aratus is shown in Figure I.

The milk by-products were gathered at the Hichigan State
College Dairy in East Lansing, Michigan. The skim milk was

24 hours old but had been kept in a refrigerator to hold back

any bacterial growth. The whey was fresh and was the by-
product from the casein removing process. The sludge was
gathered at the Lansing sewa5e treatment plant, Lansing,
Michigan. At this plant there are no grit chambers due to
he comparative absence of large inorganic solids. In place
of the grit chambers are two small sedimentation chambers
which remove the large particles of solids in the sewage.
These solids are then removed from the chambers and disposed
on a dump. From the first chambers, the sewage goes to a
large sedimentation basin where the finer settleable solids
are removed. This is the sludge used in the experiment, and
its analysis is given in Table I.

During the processing of milk for casein removal, th
casein is precipitated by the addition of hydrochloric acid
and heat, and the resulting by-product is distinbly acid,
usually having a pH of 4.2 to 5.5. In the case of the whey
used in this experiment, it was found necessary to add 1.05
grams of Ca(0H)2 per ml. of the whey to raise the pH to about
7.4. This caused a precipitate of calcium lactate, but it
was necessary to have this rather than have the pH around 5
because as has been previously discussed, the anaerobic bac-
teria are almost totally inhibited when the medium in which
they are living has a pH in this range.

The gas was removed from the gas collecting apparatus
by means of leveling tubes so that the gas was at atmospheric
pressure. The carbon dioxide in the gas was dissolved in
potassium hydroxide contained in the gas analysis apparatus.

The amount of methane was then assumed to be the remaining

 

I 'I‘lll- ! ’ ‘II in 1: I

gas. This is not an accurate assumption due to the fact that
the gas is apt to contain a few percent of oxyg n or nitrogen.
However, these other gasses occur in such small volumns that
they may be considered as negligable.

Th gas collection apparatus caused some error in the
results. Water was used to fill the inverted graduated cy-
linders that collected the has. Carbon dioxide in the gas
evolved from the flasks will dissolve in the water and will
therefore give high results for the percentage of methane in
the evolved gas. However, the temperature of the room in
which the experiment was run, remained fairly constant, so
after a few days the water should be sufficiently saturated
and very little more should be dissolved.

0n the first day of the experiment, the various mixtures
were made up and the pH of the mixtures were obtained. During
the running of the experiment the pH of the mixture in each
flask was determined at intervals as shown in Table 2. Also,
on the first day, the by-products and the sludge were analysed
for total and volitile solids; and the pH of the whey was

"H

raised by the addition of Ca(0H) , when sufficient gas had
been collected in the gas collecgor, it was analysed. This
data is shown in Table 3. The experiment was allowed to run
for 40 days and at the end of this time the mixture in eahh
flask was analysed for total and volitile solids. The exp-
eriment was run at room temperature of approximately 22 C.

The use of anaerobic digestion to dispose of milk plant
by-products has been tried many times and, except under unusual

circumstances, is not practical. Flasks #I and #2, containing

the whey or skim milk exactly as they would come from a milk
plant except that the pH of #I has been raised by the addition
of Ca(0H , and were allowol to digest for 40 days. The red-
uction ofzsolids in both flasks was very small and therefore
proves again that simple anaerobic digestion is not a pract-
ical method of disposal for milk by-products. As compared to
#5, containing only sludge, fiI produced only 20% as much gas
and had a reduction of total solids of 33% that of #5, while
#2 produced about 55¢ as much gas and had a reduction of total
solids of about 25¢ that of f}. It should be noticed that both
#I and #2 had almost completely stopped producing gas which is
a good measure of the bacterial activity going on. From Table
5 it may be seen that the gas evolved from #I and #2 was low
in methane gas compared to the gas from #5. This gas from #I
and #2 had a B.T.U. value of about 450 which is considerably
lower than is commonly used for heating or power.

Flasks #‘s 4, 6, and 8, contained 25, 50, and 75fi sludge
respectively, and the remainder of whey. Aiser a few days of
running it was discovered that #4 was digesting aerobicly due
to an error in laboratory procedure and the results were useless.
The data taken on this flask was discarded. Flask #6 showed a
definite increase in volitile solids reduction from flI, the
ratio being about 5 to I. #6 showed an increase of 46p i gas
production from #I, but the {as evolved was even lower tn
methane content. £8 produced a large tolumn of gas between
the first and fifth days, but the gas production stopped
almost completely after this initial spurt. The gas evolved

was at first of high methane content, which may be explained

by the absorption of CO by the water. The total gas evolved
rom #8 was greater thai that from #I, per gram of volitile
solids, by 166%. This leads to the conclusion that the seed-
ing of whey with sludge gives greater B.T.U. value. The in-
itial spurt of #8 cannot be explained and the same mixture of
whey and sludge should not be expected to act the same again
under similar conditions.

#‘s 5, 7, and 9 contain 25, 50, and 75p sludge respectively
and the remainder of skim milk. There is no relation between
the amount of seeding and the rate of digestion in these flasks.
The gas production remains about the same as for pure skim milk
and also the B.T.U. value of the gas evolved is little change;
from the 'as produced in $2.

From a study of the daily gas production, Table I, and
the pH values of the mixtures during the experiment, Table 2,
there is noted that the gas produced always occured when the
pH was above 5.6. In these mixtures this value seems to be
the limiting pH of anaerobic activity. It can also be obser-
ved that the closer the pH is to a neutral or slightly alkaline
SOlution, the faster the evolution of gas and therefore the
faster the rate of digestion. In 38 and {9 the 25$ of milk
by-product acidifies the whole mixture so quickly that mast of
the gas evolution occurs before the seventh day. This emph-
asizes the fact that only relatively small quantities of lactose
under proner bacteriological conditions, are necessary to so
acidify it's surrounding media in order to kill off or inhibit
all anaerobic bacteria. Control of the pH value is the most

important factor to be overcome in the anaerobic digestion of

milk by-products.

When considering the following conclusions, take into
account that the scope of this experiment is very limited
because of the following factors:

(I) Kanv errors enter into these results because of
the difficulty in getti.g represen ative samples of these
mixtures.

(2) The gas analysis is not strictly accurate.

(5) The temperature was not constant through out the
experiment.

(4) E0 corrections were made in the results for the
removal of small quantities (IO ml.) for pH determinations.

(5) The number and volumn of experiments run is so
small, due to limited apparatus and time, that too much
weight should not be put on any of the following conclusions
except #I.

Conclusions

(I) The anaerobic digestion of milk plant by-products
is not feasible due to the acid conditions which arise.

(2) Removal of lactose from milk plant by-products is
necessary before anaerobic digestion will successfully dispose
of them.

(3) Seeding the by-products with fresh sludge will
increase the rate of digestion and gas evolution but not in
direct proportion to the amount of seeding added.

(4) The gas evolved is of some value for heat or power
but it is doubtful that enough gas is produced to warrant the

expense of utilizing it.

Table 1 o
The Volumn of Gas from Each Flask in El.

(Totals corrected for vacume of collectinr apparatus.)

Days. Ilvsk
'1 2 3 4} ,3 L, ,
0-1 .3 72; 5e. 51; 571 2 3 333
1"} .-’ 4L '7 .3 OJ 2:52} 4. .~ ) 2 . u I L:
3-4 5 >- 25 23 53 79 2,33 925
4-5 25 553 123 153 13 153 2353 233
5-e 153 35 213 :3 133 1433
6-6 43 663 275 1463 233 23
8-5 53 365 253 1253 583
5-11 5 445 1to 25 2eo
11-12 553 e’ 5 283 23
12-14 45 673 233 13 eeo 13
14-15 43 113 553 ;0 23 543 23 55
15-17 413 5:3 eeo 255 43 543 15 25
17-16 253 513 443 173 13 553 75
1t-23 233 1133 543 225 13 553 423
23-21 153 543 333 233 23 13 13
21-22 53 453 443 135 3 43
22-24 210 1453 1853 513 53 53 53
24-26 123 553 2343 755 5 13
20-27 563 73 1533 553 15 13
>7-2. 733 533 2313 5 3
23-29 273 1333 253 13
29-51 553 1713 443
91-52 90 753 540 5
52-55 143 613 253 ;3
55-54 153 613 553 43 2o 13
54-54 73 443 233 13 5
55-58 533 1153 e43 153 53 23
50-40 50 410 113 250 10
Total 4348 7453 21554 7263 4552 5e16 7513 2832

Flask

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Taterial in

pH of

#

Flask

Days

2 C/ru/7Fn/ _.h/
0 O o O o O

.I 5/ C/r) 51/ 5

2 O 0/2 .9).—

r/plC/E/R/R/

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0 O O.

dice/Si)

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. O O C O .
rl/O 5 Us/ :/ .rn/

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o o o o o 0

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Table 5.

thane

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analys

Gas

% methane.)

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dioxide e~uals

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Table 4.

The Composition and Reduction of Mixtures

Compo- pH Total Volumn V Total Volumn
at Solids Solids Solids Solids

sition start % dry wt. % dry wt.

Whey f‘ ,
2 7.4 6.52 5.66 5.61 5.12

Skim
Hilk
Sludge 7.5 4.41 5.74 2.95 1.96

25%

Sludge 7.2 7.76 6.45 7.06 5.52
7 576

Sk . Ill-k .

50,6

Sludge 7.3 5.56 4.21 4.21 5.22
BUIJ

Whey

50/5
Sludge 7.2 6.64 5.21 5.25 4.27
50/0

Sk . 1:1]: 0

75%
Sludge 7.2 4.83 5.84 5.91 5.32

2 575

fihey

75.76 r
Sludge 7.2 5.52 5.93 4.27 5.49
25;»
Sk. Elk.

0\

Table 4 Continued

T18 Composition and Reduction of Hixtures

Reduction of Reduction of Gas in ml. per gm.
Total Solids Volital Solids Volitile Solids
% dry wt. fi dry wt. at start.
.71 .49 46.6
.94 .71 65.2
1.48 .78 582.5
.70 .53 72.2
1.15 .55 67.2
1.41 .54 65.5
.97 .82 124.0

.75 .49 46.7

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