INDUSTRIAL AND ENGINEERING CHEMISTRY
Tune 1954
stants (C values) for each of the three carbons are correlated by lines having the familiar Arrhenius-type formula:
C
= Ae-EIRT
where A and E are constants depending upon the carbon type. Carbon Type Hardwood charcoal Metallurgical coke Ceylon graphite
A , Fraction C Gasified/ Min. Atm. Oz Partial Pressure 1 . 2 x 104 4 . 5 x 106 8.1 X 108
E, B.t.u./Lb. Mole 28,600 52,200 88,500
Nearly nine tenths of the data lie within 2 ~ 5 0 %of the values predicted by the above equations and constants. The tendency of the data to scatter a t the lower end of each line is probably due to analytical difficulties caused by low concentrations of the reaction products in the off gas. The effect upon the reaction rate constant of gas velocity and of fraction carbon burned off was also investigated by suitable cross plots of the data. There was a considerable scatter among the points for both plots. There was, however, some indication that a fivefold increase in gas velocity halved the reaction rate constant, C. This decrease may have been caused by less efficient gas-solid contact a t the higher gas flow rates. No correlation was obtained between the cumulative per cent carbon burned off and the rate constant, C.
1331
bon type and temperature between 300' and 950' F. The fraction of carbon gasified as carbon monoxide, CO/(CO COz), averaged 0.24. This fraction tended to rise as the oxygen partial pressure was reduced below 0.2 atmosphere. The rate of carbon gasification per unit weight was proportional to the oxygen partial pressure.
+
ACKNOWLEDGMENT
The authors wish to thank the Standard Oil Development Co. for the financial support which made this work possible. LITERATURE CITED
(1) Bolland, C. B., and Cobb, J. W., J. SOC. Chem. Ind., 52, 1953-91
SUMMARY AND CONCLUSIONS
(1933). (2) Cobb, J. W., Chaleur et ind., 15, 377-86 (1934). (3) Gilliland, E. R., and Mason, E. A., IND. ENG.CHEM.,41, 1191-6 (1949). (4) Kullgren, K. F., Gas u. Wasserfuch, 67, 226-9 (1924). (5) Lambert, J. D., Trans. Faraday SOC.,32,452-62, 1584-91 (1936). (6) Letort, M., et al., J. chim. phys., 47,548-56 (1950). (7) Lewis, W. K., Chem. Eng. News,25,2815-18 (1947). (8) Lewis, W. K., Gilliland, E. R., and McBride, G. T., IND. ENG. CHEM.,41, 1213-26 (1949). (9) Paxton, R. R., Sc.D. thesis, Massachusetts Institute of Technolonv. 1949. (10) Rhead:y. F. E., and Wheeler, R. V., J. Chem. Soc., 101,84&60 (1912); 103, 461-88 (1913).
Both carbon monoxide and carbon dioxide were primary products of the low temperature oxidation of carbon. The ratio of their production was relatively independent of car-
RECEIVEDfor review October 16. 1953. ACCEPTEDFebruary 24, 1954. Presented before the Division of Gas and Fuel Chemistry, Symposium on Properties a n d Reactions of Carbons, a t the 124th Meeting of the AXERICAN CHEMICAL SOCIETY, Chicago, Ill.
Wastes from Potato Starch Plants TOMMY W. -4MBROSEl AND CASTLE 0. REISER2 University of Idaho, Moscow, Idaho
A
LTHOUGH many references may be found which give the approximate volume and strength of industrial wastes, such data for plants processing potato starch are not readily available. These data are helpful to public health authorities and others in estimating the approximate pollution attributable to a plant. Furthermore, if the nature of the various wastes i s known, the economic recovery of valuable materials from these wastes may follow. The wastes from two starch plants which were discharged into a stream used by the Presque Isle Air Base in Maine during World War I1 are reported to have created a serious pollution problem. U. S. Army Engineers made a study of this condition and the results were given in a report published April 4, 1945. Although copies of this report are not readily available, it covered possible methods of treating the waste pulp and protein water ( l a ) . Consideration was given to dewatering and saving the waste pulp by means of vacuum filtration and pressing. Studies on the protein water included chemical coagulation Kith and without aeration, heat coagulation, and biofiltration. A high rate biofilter for treatment of the protein water was recommended but was economically unattractive. White potato starch has been manufactured in Idaho, Maine, and to a small extent in Minnesota. Sweet potatoes have been processed for starch in some of the southern states (9). In 1949, Idaho was reported to have taken the lead in white potato starch production with a daily capacity of 192 tons (3). A 1 Present 2
address, General Eleotrio Co., Richland, Wash. Present addreas, Food Machinery and Chemical Corp., San Jose, Calif.
plant of approximately 30 tons capacity has been built a t Idaho Falls since. Most of the Idaho plants are located along the Snake River or its tributaries where the stream flow is large and other industries are small. Hence the pollution of the river by these plants does not appear too serious. This investigation was undertaken to determine the order of magnitude of the contamination and the nature of the streams wasted. DESCRIPTION OF STARCH PROCESS
Starch production in Maine by both batch and continuous settling processes has been described by Howerton and Treadv,-ay (6). In general, the Idaho plants have a larger capacity and operate with a combination batch and continuous settling process. The manufacture of starch from potatoes is mainly physical in nature. It involves the grinding of the whole potato followed by water extraction of the soluble materials. Nonstarch solids are separated from starch by screening and selective settling of water slurries. Generally, sulfur dioxide gas is added to the slurry to aid in preventing undesirable chemical and biological actions. Hypochlorite solutions may be used also to sterilize the equipment and improve the product quality. Processing conditions differ among the various plants and some operating data that have been accumulated as a result of experimentation are withheld as trade secrets. Although the composition of potatoes varies with type, age, and locality, an average composition would show approximately
1
5Fyi
Vol. 46, No. 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
1332
Hzlo,
PO STORAGE j/T/
110-Mesh
WASHER
GRINDER
GRINDER
S0 2
FLUME WATER
S0e-l H 2 0 4
1st. WASH
1 PROTEIN WATER EXHAUST AIR I
WATER
J
BROWN S T A R C H ~ S T A R C HT A B L ~ WATER
2 nd. WAS H WATER
+
STARCH PRODUCT I I AIR CYCLONE DRYERS Figure 1. Flow Sheet of Typical I d a h o Potato S t a r c h Plant
14 to 17% starch, io to SO% water, 2.17, nitrogen compounds, 1.0% ash, 1% fiber, and minor amounts of organic acids and fats. The important steps in starch plant operations generally used In Idaho may be described briefly with the aid of Figure 1. Cull or surplus potatoes are conveyed from storage to a washer by flumes using river water, which is recirculated for re-use in some plants. From the washer, the potatoes are conveyed to a rasp or hammer mill for grinding. Process water and sulfur dioxide are added to this finely ground material as it passes to an 80-inch mesh screen. With the aid of a water spray, starch is washed through the screen and the pulp, consisting mainly of skin and fibrous material, passes off the end. After regrinding, the pulp is passed over a 110-mesh screen for further removal of starch. The starch slurry which passes through the shaker screens is fed to a continuous centrifuge to separate the protein Tater containing solubles extracted from the potato. Process m t e r is added to the starch, and the slurry is passed over a 120-mesh refining screen for further removal of pulp. The starch is then passed to settling vats in series, where, in a period of approximately 9 hours, starch settles to the bottom. Impurities concentrate in a r a t e r and brown starch layer a t the top. After draining off the water, the brown layer is removed by scraping and suspending in water. The final removal of white starch takes place when it is passed over settling troughs. White starch from the tables and the bottom of the second settling tank is suspended in water for transport to a filter or centrifuge, where the moisture content is reduced to about 40%;. By contacting with hot air in a series of cyclone dryers, this starch is dried to a final moisture content of about 17%. It is then screened and bagged. Plants processing starch in this manner have the following waste streams: 1. Flume water that contains mostly dirt and may be recirculated.
STAR CH WATER
2. Potato pulp that has about 96% moisture and contains the peel and fibrous material plus some starch. 3. Protein water containing most of the soluble ingredients from the potato. 4. Starch wash water from the first and second settling vats that also contains some solubles. 5. Brown starch water from the settling troughs that contains fine particles of brown starch and solubles. 6. Starch water filtrate from the final starch slurry. SAXIPLISG AND ANALYSIS
In order to obtain data on the nature of the wastes from a potato starch plant, five typical plants were visited in the fall of 1950 to get samples and flow rates of the various streams. Because of the limited time available for plant inspections, only spot and manual samples and flow rates mere obtained The absence of instruments for indicating and controlling flow rates made it difficult to obtain average values. However, there is usually little variation in processing conditions after steady operation is obtained. The determination of flow rates vias complicated by the absence in the plants of meters on individual streams. IJ7ater consumption of the entire plant was metered when city water was used. In measuring the flow of flume water, a V-notched weir was used. The quantity of starch water was determined by measuring its volume in starch settling tanks. Protein water, brown starch vater, and starch water flow rates wcre determined by collecting and measuring samples for a given time interval. Pulp production was obtained from average plant data. The standard &day B.O.D. test ( 1 ) for the examination of sewage and water was used to measure the pollutional characteristics of waste streams. Samples were seeded with sewage. In some samples, the high pollution concentrations required dilutions in the order of 5OOO:l.
June 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
1333
tities of starch and fiber, which are probably biologically stable, but are oxidized chemically. The best agreement in these values is shown in the protein water because of its lower content of such cellulosic material. Also, of the protein waters analyzed, the best agreement in the oxygen demand tests is shown for plants not previously adding sulfur dioxide. Accordingly, it appears that sulfur dioxide addition exerts an inhibiting effect on bio: logical activity which lowers the 5-day B.O.D. A comparison of 5-day and 20-day B.O.D.'s showed about 65% of the 20-day demand occurred in the shorter period. This value is in good agreement with the 68% generally assumed. A study of the protein water showed that the average solids content of about 1.7% is probably too dilute for economical recovery by evaporation. In addition, the material darkens and oxidizes to an unpalatable product in ordinary evaporation. Solids removed from the solution were found to contain about 38% nitrogen compounds, 3% starch, up to 12% sugar, 15 to 20% ash, and organic acids. The exact composition, especially the sugar content, varies with the type of potatoes, age, and storage conditions, as well as the processing conditions. Brautlecht ( 8 ) found all the major mineral nutrients for biological growth present in potatoes and most of these would be extracted in the protein water. Values of 8.4% sugar, 1.7% phosphorus pentoxide, and 10.6% potassium chloride were obtained for the 8olids composition of a protein water prepared from potatoes that had been stored 10 months. Good yeast growth has been obtained in the protein water without the addition of any nutrients and offers a possible method for its utilization (11).
In order to obtain a measure of the oxygen-consuming power of these wastes if completely oxidized, the dichromate modification of the oxygen consumed test proposed by Ingols (8) was also used. After this work was completed, a modification proposed by Eldridge was reported as being the most satisfactory method for analyzing the C.O.D. of milk wastes (IO). In the case of the more highly concentrated wastes, where utilization and recovery of the contaminant may appear desirable, the samples were analyzed for their chemical constituents. The percentage of nitrogen compounds was calculated from the nitrogen content by multiplying by a factor of 6.25. CHARACTER OF WASTE STREAMS
The character of waste streams from the starch plants inspected is shown in Table I. In order to aid in the comparison of these values, flow rates and pollution values have been computed on a common basis of one ton of potatocs being processed. From the data given, it may be seen that most of the waste contamination occurs in the protein water and pulp, which account for about 95% of the total pollution. The only other stream of any significance is the first starch wash water which, on the average, is responsible for a little less than 4% of the total value. Chemical oxygen consumed values, which are referred to as C.O.D., are multiplied by 0.68 to correct them to a 5-day basis and are reported for comparison with the B.O.D. values. I t is apparent that the correlation between these two tests is poor, especially in the case of streams containing appreciable quan-
TABLE I. CHARACTERISTICS OF STARCH PLANT WASTES (iill oxygen demand values are on 5-day basis: rates are per ton of potatoes processed) Starch Plants Flume Water B.O.D., p.p.m. C.O.D. p,p.ni. Flow r i t e , gal./ton B.O.D., lb./ton C.O.D., lb./ton
I
I1
111
IV
47 333 600 0.2 1.6
98 320 173" 0.1 0.5
32 50 1920 0.5 0.8
250 407 150" 0.3 0.5
V
Average 100 260
50 178 2700 1.1 4.0
...
0.4 1.5
Protein Water Solids content, wt. % Protein in solids, wt. % B.O.D., p.p.m. C.O.D. p . p . m , Flow rite, gal./ton B.O.D. lb./ton C.O.D.: Ih./ton
1.91 44.8 7375 9030 670 41.2 50.4
0.97 34.0 3420 471 5 865 24.6 34.0
1.31 37.2 2424 3940 1220 24.6 40.0
2.40 38.0 6936 10200 508 29.4 43.3
1.91 38.7 6837 7578 542 30.8 34.2
1.70 38.5 5400 7090 670 30.1 40.3
First Starch Wash Water Solids content, wt. % Protein in solids, wt. % B.O.D., p.p,m. C . O . D . , p p.m. Flow rate, gal./ton B.O.D. lb./ton C.O.D.: lb./ton
0.32 32.6 1770 2100 210 3.1 3.7
0.36 33.0 2055 2030 160 2.7 2.7
0.20 22.5 I005 2604 81 0.7 1.8
0.90 31.0 2181 5650 94 1.7 4.4
0.52 36.4 1385 2200 227 2.6 4.1
0.46 31.1 1680 2920 155' 2.2 3.3
Second Starch Wash Water B.O.D.. u.u.m. C . 0 . D ' p.p.m. FIOW rEte, gal./ton B.O.D., Ib./ton C.O.D., lb./ton
397 1090 210 0.7 1.9
225 410 160 0.3 0.5
379 696 81 0.3 0.5
424 493 77 0.3 0.3
... ... ... ... ...
360 670 135' 0.4
Brown Starch Water Solids content, w t . %, B.O.D., p.p.m. C.O.D., p.p.m. Flow rate al./ton B.o.D., li.$ton C.O.D., lb./ton
1.78 9 GO 1548 16 0.1 0.2
0.50 134 754 30 0.03 0.2
0.14 329 810 25 0.1 0.2
...
1124 2950 44 0.4b 1.1
...
0.81 640 1520 30 0.2 0.4
13 126 17 0.00 0.02
59 293 19 0.01 0.04
143 142 46 0.05 0.05
399 612 20 0.07 0.10
150 290 25 0.0 0.0
0.495 1.010
0.248 0.873
0.446 1.021 55.5 24.8 56.8
Starch Water B.O.D., p.p.m. C.O.D., p.p.m. Flow rate al./ton B.o.D., li.$ton C.O.D., lb./ton
rl
a D (I
a
Pulp (Dry Basis) 0.595 0,446 0.446 B.O.D. lb./lb. 1.024 1.024 1,190 C.O.D.' lb./lb. 52 59 Averaee lb./ton 23.2 26.3 B 0 D lb./ton 53.3 ... 60.4 C:O:D:: Ib./ton a Water reoiroulated. b Brown starch layer settled in vats; other plants use tables. C Computed from average B.O.D. values.
.. .. ..
Q
a 0 ll
a
...
...
...
... ...
...
0.8
INDUSTRIAL AND ENGINEERING CHEMISTRY
1334
TABLE 11.
C O M P O S I T I O ~O F
DRIEDP O T l T O
PULP
Weight,
70
Protein Fat Fiber Bsh Nitrogen-free extract
3.3 0.1
10.4 4.8 81.4 103.27
~
Laboratory investigations ( 7 ) have shown that about 75Y0 of the nitrogen compounds in potatoes will be extracted in the juice upon grinding without the addition of wash water. The amount extracted is increased to about 9170 with the addition of wash water a t a 3:l m-eight ratio of water to potatoes. Little advantage is shown for using more water, and in countercurrent washing a smaller ratio mould be desirable. Idaho starch plants use an average weight ratio of about 2.2:l. By recirculating the wash \later from the centrifugal separator where starch is removed from the slurry, smaller quantities of xater could be used with a resultant decrease in the volume of the protein water and increased solids concentrations. About 40% of the nitrogen compounds in the protein Tater can be coagulated by heating to 85' C. and 60% can be coagulated by acidifying to a pH of 3.25. However, the drying of the coagulated protein is a difficult process and mould probably require spray drying.
Vol. 46, No. 6
shown for the entire plant and on the basis of a ton of potatoes processed. Population equivalents were calculated from the standard factor of 0.167 pound of S d a y B.O.D. per capita per day. If pulp is not wasted, the maximum population equivalent per ton of potatoes proceesed mas found to be 271 with an average value of 200. K i t h pulp considered, the maximum value was 420 and the average was 348. On this basis a 30-ton staich plant obtaining a 12.5y0 starch yirld would have a n averagr population equivalent of 84,000 with and 48,000 without pulp wastage. Sctual values mill probably be somevc hat higher than this as microorganisms capable of utilizing starch and cellulosic material accumulate in the effluent stream. Also, these test8 were made early in the fall when potatoes contain less sugar and EOlUbk?Sthan those processed later. SU4IM4RY 4ND CONCLUSIOhS
B.O.D. measurements indicate protein wash water and pulp contribute about 95% of the pollution attributed to a starch plant. The pulp can be dried for feed. Utilization of the pulp ail1 reduce the population equivalent to the oxygen demand from about 348 to 200 people per ton of potatoes processed or from about 2800 to 1600 people per ton of starch produced. Examination of the protein water shows that it contains about 1.7% solids analyzing approximately 38.5% nitrogen compounds. Variable quantities of starch, sugar, ash, and organic acids, and all the mineral nutrient necessary for yeast groffth are present. Recirculation of some TABLE 111. POLLUTION RATESO F STARCH P L a S T S of the wash water might be Potatoes Processeda, Tona/Day used to increase the conccn(I) 200, (11) 250, (111) 150, (IV) 62.5, (V) 120, Average t r a t i o n of d i s s o l v e d solids. 10 hr. 24 hr. 24 hr. 8 hr. 180 hr. Total pollution of starch plants Chemical and heat coagulation Total without pulp 5-day B.O.D., lb./ton 45.3 27.7 26.2 31.7 36.0 33.3 ill separate 6070 of the disB.O.D. population equiv.,'ton 271 1GF 158 190 210 200 solved proteins, but a product B.O.D. population equii7. of 54,200 41,500 23,700 11,900 plant 26,200 ... is obtained that is difficult to 37.9 &day C.O.D., lb./ton 67.8 43.3 48.3 43.5 46.3 345 227 C.O.D. population equiv./ton 259 289 260 277 process. C.O.D. population equiv. of plant
69,200
66,800
38,800
18,100
31,200
Total including pulp &day B.O.D. lb./ton 70.1h 52.5 51 . o 56.5b B . O . D . population equiv./ton 420 314 305 338 B.O.D. population equiv. of plant 84,000 78,500 45,800 21 ,200 a Potatoes processed a t time of inspection; maximum plant capacity may be greater. b Based on average pulp value.
The starch wash water from the first settling tank is similar t o protein water from the centrifugal separator, but has only
about one fourth the solids content. Its average B.O.D. of 1680 is, however, about eight times that of domestic Eewage. About 4% of the pollution of the average starch plant may be attributed to this stream. Less than 1% of the pollution is attributable to the remaining liquid wastes consisting of the second starch wash water, brown starch water, and starch water. The pulp from a starch plant contributes about 43% of the total pollution of the plant. It contains approximately 96% moisture and consists of fibrous material containing about 4Oy0 starch on the dried basis. Although common pressing methods generally will not lower the moisture content below SO%, one investigator has been able to obtain a pressed product of about 65y0 moisture (6). This was accomplished by treating the pomace with approximately 0,3'% lime for some 5 minutes prior to dewatering in a Davenport press. The composition of the pulp from this process is shown in Table 11. Eskew and coworkers a t the Eastern Regional Research Laboratory have also reported ( 4 ) utilization of pulp. The pollutional characteristics of Idaho starch plants, in terms of oxygen demand, are summarized in Table 111. Values are
6Q.8h 358
. . I
68,l 348
ACKNOWLEDGMENT
This investigation was supported by a research grant from the National Institutes of Health, P u b l i c H e a l t h Service. The cooperation of officials of the starch plants visited and the Idaho Department of Public Health is gratefully ackno~ledged. 13,000
...
LITERATURE CITED
Am. Public Health Assoc.. New York, "Standard Methods for the Examination of Water and Sewage," 9th ed., p. 129, 1946. Brautlecht, C . .%., IND.EXG.CHEX.,32, 893 (1940). Chem. Eng., 56, No. 3 , PPI-28 (1949). Eskew, R. X., Edwards, P.W., and Redfield, C. S., of Agr., Bur. Agr. and Ind. Chem., AIC-204 (1948 Highlands, 3.2. E., personal communication. 1951. Howerton, IT. W., and Treadway, R. H., IND. EX. 1402 (1948). Humphrey, A. E., M.8. thesis, University of Idaho, 1950. Ingols, R. S., and hlurray, P. E., Water & Sewage U'o~ks, 95, 113 (1948). II.,and Balch, R. T., IND.ENG.
., and Hoover. S. R., Proc. 6th I n d . W a s t e Cons., Purdue University, p. 367 (February 1981). Reiser, C. O., J . Agr. Food Chem., 2, 70 (1954). Weaver, E. A , , Heisler, E. G., Porges, N., McClennan, 1'1. S., Treadway, R. H., Howerton, Mr.JV., and Cordon, T. C., U. S. Dept. of Agr., Bur. Agr, and Ind. Chem., AIC-350 (1953). RECEIVED for review- August 24. 1953.
A C C E P ~ E January D 21, 1034.
