INDUSTRIAL AiYD ENGINEERI,VG CHEMISTRY
1i 4
to the Gurney plots. This value becomes 0.0081 square foot per hour (Table 11). The value of K , thermal conductivity, must be given a "dissociation" value to correspond with that found for aU2. This value is 0.625 (B. t. u. per hour per square foot per O F. per foot) and it must be used in computing the emissivity of limestone being dissociated into lime. Use of the Plots
As regards practicability Figures 6 and 7 are important. With these plots it is possible to estimate the time necessary for complete dissociation of spheres under different conditions of preheat and with different surface temperatures. Exampte-Estimate the time required t o dissociate completely a block of lime whose shape may be assumed to be spherical. Data: Diameter Initial temperature Surrounding temperature
= = =
6 inches
80°F. 2100' F.
Solution: In Figure 6 the 2100' F. line intersects the 6-inch line at 1.5. Then V% = 1.5
or
Vol. 20, No. 2
0 = (1.5)* = 2.25 hours
I n the same way the time for dissociating spheres of other dimensions for the same and for different surface temperatures above 1700" F. can be determined. A series of curves showing time as a function of surface temperature for different sizes could then be plotted as in Figure 9. Table I1 contains a list of constants for lime and limestone, together with units in which these constants are expressed. These data have been collected from various sources and are thought to be reasonably reliable. T a b l e 11-Thermal and Physical C o n s t a n t s for L i m e and L i m e s t o n e CONSTANT UNITS LIME AUTHORITY LIMESTONE AL-THORITY (lz = pKr Sq. ft./hr. 0.0188 0.045 a
-
(i
K
Tadokaro
S
Q C
B.t.u./lb./OF.
F.
ID
0.23
0,198 l65Ob
Tadokaro Smyth and Adams Tadokaro Knibbs
Calculated from other constants. b 1700° F. used in all calculations. a
Gases from Sewage Sludge Digestion' W. D. Hatfield, G. E. Symons, and R. R. Mills SANITARY DISTRICTOF DECATCR, ILL
The rate and volume of gas production from sludge Imhoff tank gases is shown in H E s e w a g e from the digestion in Imhoff tanks and the hydrogen sulfide Figure9 1and 2. High hydrocity of Decatur, Ill., is content of the gas are directly proportional to t h e gen sulfide content and large a mixture of 5 million temperature of digestion. volumes of gas are coincident g a l l o n s of domestic sewage Experimental laboratory data are given showing w i t h h i g h temperatures of per day from a population of the maximum gas rate at about 30' C. A t higher sludge digestion. 40,000, including the normal temperatures t h e rate is less. A t 30" to 35" C. from During the time that these industrial wastes from a city 40 to 50 per cent of t h e gas produced is from sludge data were being collected on of that size, and 5 million added during the last 24 hours. The per cent of gas the large-scale experiments on g a l l o n s of waste from the formed in the first 24 hours is given for lower temthe Imhoff tanks, considerStaley Corn Products Manuperatures. able data were being accumuf a c t u r i n g Company. The The Decatur Imhoff tanks demonstrate on a large lated in the laboratory, where mixture as received a t the scale t h e advantage of heating sludge-digestion chamconditions may be more caresewage plant is about five bers to increase rate of digestion and increase t h e fully controlled than on plant times as strong as a normal sludge capacity through rapid digestion. The chief o p e r a t i o n. The unusually city sewage. The minimum components of t h e Imhoff gas are: methane 70, carbon large volume of gas being temperature of the sewage in dioxide 20, and nitrogen 7 per cent. produced by the tanks was of the winter is about 20" C. interest to all who visited the and the maximum in the summer is about 40" C. This combination of unusual strength Decatur plant. The relation of temperature and starch wastes and high sewage temperature has made the operation of the was of particular interest because of a program of recovery of the starch waste undertaken by the Staley Company. Many Decatur sewage plant of unusual interest. Under these conditions rapid putrefaction takes place in preliminary tests were made by inoculating fresh sludge the sewage system and treatment plant, thus making a con- with digested sludge and incubating a t different temperatures siderable odor nuisance. The sewer gas, the Imhoff tank for various periods of time. Only the final set of experiments gas, and the air above all conduits and points of agitation is reported here. are charged with hydrogen sulfide. The Imhoff tank gases2 Experimental Procedure hal-e been completely trapped and are used to burn up the odors which are now collected by a suction fan from the covThe sludge was digested in a 2-liter, wide-mouth glass ered grit chamber and the covered conduits and by-passes. bottle fitted with a rubber stopper. Through the stopper A slight odor of sulfur dioxide is noticeable near the oven were three glass tubes-a delivery tube for collecting the gas for burning the gases, but it is not sufficient to cause com- over water, a tube through which daily additions of sludge plaint, even near by. were made, and a siphon tube for drawing off superThe relation of the temperature of the sludge digestion natant liquor. These experiments were made to approxichamber to the hydrogen sulfide content and volume of the mate conditions in the Imhoff tank and instead of seeding 1 Presented before the Division of Water, Sewage, and Sanitation the fresh sludge with old sludge in the proportions of 4 to 1, Chemistry at the 74th Meeting of the American Chemical Society, Detroit, as has often been done, about 250 cc. of well-digested Imhoff Mich., September 5 to 10, 1927. tank sludge (22 grams dry weight) were added to the 2-liter Hatfield, Public W'oorks, 68, 204 (1927).
T
2
INDUSTRIAL A N D ENGINEERING CHEMISTRY
February, 1928
Per r e n t by Volume H2S Figure 1
bottle and this was covered with 1600 cc. of supernatant liquor having a p H of 7.4. The bottles were then incubated for 4 days until gasification seemed negligible. Daily sludge additions were made with fresh sludge collected by settling 10 gallons of sewage for 1hour. The volume of sludge added was from 5 to 10 ml. per day and the amount of organic matter from 0.1 to 0.2 gram per day. There were approximately 9 grams of organic matter in the digested sludge in the bottles, thus making the proportion of 1 part of fresh organic matter to 90 parts of old organic matter. This proportion was satisfactory according to a recent publication by Rudolfs,3 but a t the time of our experiments it was taken as approximating our Imhoff tank conditions. The same amount of supernatant liquor was siphoned off each day as fresh sludge was added, thus keeping the water level the same and preventing the end products of digestion from accumulating and hindering the normal biological digestion. The bottles were shaken once each day. I n this set of experiments seven bottles were used. Nos. 111, IV, V, VI, and VI1 were each incubated a t almost confor 27 days stant temperatures. No. I was held a t 20" and then was changed to 10-12" C. No. I1 was started a t 11" C. and gradually increased to 20" C. during the 130 days of the experiment. No. I11 remained a t 20" C. throughout the experiments. No. VI1 differs from the rest in that
c.
&
f 8
75
+s 70
$,
9
% 65
3
E
Bo
k& 55 40 000
80 000 /ooooo Cubic f e e t 60s p e r 24 hours Figure 2
60 000
/20000
no digested sludge was used for inoculation, but daily additions of sludge were started in a bottle of clear supernatant liquor. No. VI1 was incubated a t 36" C. Data on Nos. I1 and VI1 are not comparable with the others because conditions were decidedly different. However, the information obtained from them is valuable and they have therefore been included. The daily data on No. I1 show t h a t the rate of gas production at 20" C. is greatly accelerated by allowing the sludge a New Jersey Agr. Expt. Sta., Report
of Sewage Substation, 1926.
175
to accumulate a t lower temperatures. The total gas produced in 90 or 130 days was greater then that produced by Nos. I11 or IV. This must have been due to certain liquefaction at low temperatures which made the organic matter more susceptible to gasification when the temperature was raised. The data on No. VI1 show that at high temperatures sludge digestion very rapidly becomes normal even without initial inoculation. A question which is not answered is-Is the lower rate and total gas production after 50 days of No. VI1 due to the absence of organic matter in the inoculation sludge used in the other bottles or to lack of proper inoculation? Table I contains the data on the first 90 days after sludge additions were started. Figure 3 shows the total volumes of gas produced in 90 and 130 days. A11 sludge additions were stopped on the 97th day. Figure 5 gives the average volume of gas formed during each 10-day period per gram of organic matter added. Curves 'I' and VI (Figure 5 ) show that during the first 30 days a t 30" to 35" C. large quantities of gas were given off by the inoculating sludge. I n curves I, 111, and IV, 30 days were required for the processes of digestion to come to equilibrium. All the curves after the 40th day become a relatively straight line. For this reason
I
n
Figure 3-Total
r
n l Y v z I Bottle No. Volume of Gas Produced i n 90 Day8
~
40 days are considered necessary for the establishment of equilibrium conditions and therefore the true gas rate is best indicated by discarding the first 40 days and studying the rate thereafter. I n this experiment from the 40th to the 90th day has been chosen as giving the most reliable data. These data are shown in Table I1 and Figure 4, as well as part of Figure 5 . The data presented in these tables may be summarized as follows: (1) Temperature has a direct influence on the rate of gas formation, which increases as the temperature approaches 3033 ' C. and then decreases as 35 O C. is reached. (2) Sludges accumulated during cool temperatures apparently break down into compounds which later at higher temperatures give a greater rate and volume of gas than sludges held at somewhat higher but constant temperatures during digestion. (3) The total volume of gas obtained in normal tank operation is proportional to the temperature and rate of gas production, even though the total gas production may eventually be the same a t all temperatures, because the sludge is drawn before gas production is complete. Sludge seldom remains in the Decatur tanks more than 90 days and averages less than that. (4) The temperature affects the acidity of the digestion chamber under the conditions of the Decatur sewage. The reaction in the Imhoff tanks is 6.8 to 7.0 pH. ( 5 ) The rapidity of gas formation is discussed below.
Rapidity of Gas Formation An interesting feature a t the Decatur plant is the great reduction in gas production only a few hours after the Staley Starch Company has shut down. This immediate effect
INDUSTRIAL AND ENGINEERING CHEMISTRY
176
Table I-Experimental I I1
Series Temperature, * C. Average, C. Organic matter, grams Total gas, ml. M1. gas per gram organic matter Av. ml. gas per day MI. per day, no sludge added Per cent reduction in 24 hours PH
20-11 13.3 10.14 4007 395 47.1 33 30 6.6
Data on Gas Collection for First 90 Day8 V IV 111
18-20 18.8 10.14 5163 510 57.4 37 35.5 6.7
11-20 16.3 10.577 5921 560 65.8 54 18 6.6
16-19 17.5 5.176 3671 710 74.5 54 27.5
10-12 11.45 4.249 1697 400 34 33 3
23-27 25.3 10.577 5470 518 60.8 40 34 6.8
30-32 31.6 10.577 6535 618 72.5 33 54.5 6.9
Data on Gas Collection for Last 50 Days I11 IV V
Table 11-Experimental I I1
Series Temperature, C. Average, C. Organic matter, grams Total gas ml. MI. gas der gram organic matter Av. ml. eaa Der dav MI. per iay,-no &dge added Per cent reduction in 24 hours
Vol. 20, No. 2
18-19 18.7 4.333 2162 500 43 37 14
24-27 25.3 5.176 2858 552 57.1 40 30
30-33 31.4 5.176 2960 572 59.2 33 46
VI
35-37 35.6 10.577 6448 615 71.6 25 65 7.0
VI 34-36 35.2 5.176 2930 566 58.6 25 57
VI1
35-37 36.3 10.987 5261 478 58.5 29 50.3 6.8
VI1
35-36 35.6 5.130 2752 530 65.0 29 53
could be due only to the drop in temperature of the tanks, which is slow, and the lack of food supply in sludge from the starch works. To test this out, occasionally the sludge additions to the bottles were omitted for one day and then resumed as usual. The average volume of gas produced during the 24 hours following after the omission of the sludge addition, and the percentage reduction of volume below that regularly obtained are shown in the last two lines of Table 11. At the temperature existing at the Decatur plant fully 50 per cent of the gas produced per day is from sludge deposited within the previous 24 hours. This rapid decomposition of the sludge accounts for the fact that no poorly digested sludge has ever been drawn from the digestion chambers, even though by mistake one tank was completely drained of its sludge. The volume of gas produced per gram of organic matter is higher than other investigators have reported. This must be due to the higher temperatures and the particular type
p
I
I
I
I
I
1
I
9
V 600
8
8& 500
s
DAYS
Figure 5-Rate
k. 400
B f 300
2
/4
/8 P2 26 30 34 TemperUfurc, Deqrces Centigrade
38
Figure 4-Temperature-Volume Relation during Last 50 Days of Sludge Additions
of organic matter settling from the starch waste-sewage mixture. The organic matter in the starch wastes is chiefly soluble matter from the corn and contains very little, if any, starch, gluten, or sugar. Acidity
The acidity of the sewage is about 6.5 pH. Table I gives the pH values of the supernatant liquor in each bottle after the first 40 days. Under the conditions in Decatur satisfactory digestion takes place a t a pH of 6.8 to 7.0, foaming conditions being accompanied by a pH of 6.5. Routine analyses of the gas collected from the Imhoff tanks have been made since October, 1926, and are summarized as follows: GAS Methane Carbon dioxide Hydrogen (?) Nitrogen Oxygen
of Gas Formation for 10-Day Period
A shutdown at the starch factory invariably decreases the carbon dioxide and increases the methane contents. Foaming on the tanks is always accompanied by an acid condition, high carbon dioxide, and low methane, but high carbon dioxide and low methane are not always accompanied by foaming. The hydrogen and nitrogen data were determined by the explosion method and are subject to-the usual error. Some may doubt the presence of hydrogen and consider the data above in error, but we are convinced that hydrogen is present in small amounts, probably varying from 1 to 3 per cent. Heating Value The heating value of the gas, as determined in a Junkers calorimeter and by calculation from the analyses, has averaged about 700 B. t. u. per cubic foot. The gas is being used to destroy odorous gases, to heat the various buildings, and to heat the air for the activated sludge plant before compression. Reports from the Department of Commerce are that the
FOAMING amalgamation efforts of the Heyl-Beringen A. G. have gone far beyond their original goal. It has extended its raw-material MINIMUMMAXIMUMAVERAGE TANKS basis by working with Gebruder Gutsbrod G., m. b. H., of Per cent Per cent Per cent Per cent
55 16 0
4 0
74 34 4 10 0
68 22 2 6 0
60-64 30-33 2 7 0
Frankfurt, and now wishes t o bring about an amalgamation of the principal German mineral-color producers with the intention of later founding a Deutschen Farbwerke A. G. with a capital of 15,000,000 t o 20,000,000 reichsmarks.
i
