Biochemical and Chemical Oxygen Demands of Carbohydrateswith Different Degrees of Polymerization Stig A. Larsson,' Birgit A-L. Ostman, and Ernst L. Back* Swedish Forest Products Research Laboratory, Box 5604, S-114 86, Stockholm, Sweden
The biochemical and chemical oxygen demands of carbohydrates depend on the degree of polymerization. This is illustrated by a comparison of xylose with a xylan from birch (DP 200) and galacturonic acid with polygalacturonic acid (DP 80). The polymers have a lower and slower biochemical oxygen demand and a lower chemical oxygen demand, especially if the latter is determined by the permanganate method. The dichromate method gives a higher degree of oxidation, less dependent on the molecular weight. A rapid biochemical degradation of carbohydrates-i.e., a high biochemical oxygen demand (BOD) in effluents from, for example, pulp and paper industries, may be a disadvantage in that it leads to a rapid lowering of the oxygen content of the water system near the mill. On the other hand, a rapid degradation may be an advantage from the point of view of efficiency and time of treatment when aerated ponds or activated sludge treatment are used. In addition, dissolved high-molecular carbohydrate material can be recoagulated before the sedimentation tank, whereas low-molecular carbohydrates can not. The rate of biochemical or chemical degradation of carbohydrates seems to be dependent on their degree of polymerization (DP). Monosaccharides are degraded more rapidly and have higher BOD5 per unit mass than polysaccharides ( 1 ) . A comparison of disaccharides and their monosaccharide constituents have shown an apparent lag in the BOD, due the existence of the bond holding the two monosaccharides (2). Among the most important hemicelluloses dissolved from the wood in, for example, thermomechanical pulping, are xylan and glucomannan. These have an original DP of 50-200-i.e., they consist of 50-200 monosaccharide units in the original wood. Some data ( 2 ) indicate that the BOD5 for these hemicelluloses is only one sixth of the BOD5 for the corresponding monosaccharides. The chemical oxygen demand (COD) determined by permanganate oxidation, is higher and more rapid for monosaccharides than for polymeric saccharides (3). The object of this note is to illustrate these effects in more detail by an analysis of the biochemical and chemical degradation of some pure components of pulp and paper effluents. In this report COD stands for chemical oxygen demand in general. When either oxidation by dichromate or permanganate is meant, this is mentioned. Experimental Substances. The BOD and COD have been determined for the following substances: Xylose-Manufacturer Merck; biopur (99% as analyzed by dichromate oxidation); m.p. 144-148°C. Galactose-Manufacturer Merck, puriss. Xylan from birch-Wood meal from Betula uerrucosa was extracted with 24% KOH and purified with Cl2 and IPresent address, AB Casco, Box 11010, S-10061, Stockholm, Sweden. 160
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by repeated precipitation with ethanol ( 4 ) . Analysis of the carbohydrate composition gave almost 100% xylose. The uronic acid content was 10.8%, the lignin content 0.9%, and the mean DP, about 200 ( 4 ) . Galacturonic acid, monohydrate-Manufacturer Sigma Chemical Co.; about 99%; the carboxyl content, analyzed by titration according to the manufacturer, was 98.8% of the theoretical value. Titration a t our laboratory gave a carboxyl content which was 101% of the theoretical in the dry substance. Polygalacturonic acid-Manufacturer Sigma Chemical Co.; made by de-esterification of pectin from citric fruits; 85%, the rest being mainly monosaccharides. The D P is, according to the manufacturer, 38-100, and analysis a t our laboratory by measuring the viscosity in copper ethylenediamine (CED) solution ( 5 ) gave a limiting viscosity, [SI, of 63, which corresponds to a mean D P of about 80. The carboxyl content was found by titration to be 93% of the theoretical value based on the substance with 85% purity. All substances were dried in an evacuated desiccator over phosphorus pentoxide for at least 3 days before weighing. Methods. The BOD was determined by a manometric method (6) in a Voith Sapromat (Sapromat AP6, manufacturer, J . M . Voith GMBH, Heidenheim, Germany). The BOD for galacturonic and polygalacturonic acids were determined after partial neutralization to pH 6. After addition of a seed solution, the pH for the galacturonic acid was 7.3 and for the polygalacturonic acid 6.7. Municipal wastewater coming to the Loudden purification plant, Stockholm, was used as seed. The seed was kept in contact with air for one day and then filtered through cotton. In preparing the seed solution 10 ml of nutrition solution A, 10 ml of solution B, 5 ml of C, and 5 ml of D were first mixed, then the mixture was diluted to 1liter with the filtered seed substance. The nutrition solutions were: A-34.0 grams of KH2P04, 87.0 grams of KzHP04, and 133.6 grams of Na2HP04.7H20 per liter of solution. B-216 grams of KNOBper liter of solution. c-22.5 grams of ihfgS04.7HzO per liter of solution. 0-0.25 gram of FeC13.6HzO and 27.5 grams of CaClz per liter of solution. (The salts were dissolved separately.) The COD was determined by dichromate oxidation (7, 8) and by permanganate oxidation ( 9 ) , and is given in each case as the mean value of t'wo analyses. The COD for a certain organic substance is dependent, among other things, on the method of oxidation. Oxidation by permanganate is considered to give about 40%, and oxidation by dichromate, from 90-100% of the theoretical oxygen demand. Details in the experimental procedure may also be important. Oxidation by dichromate, according to the procedure in Lindberg and Borno ( 8 ) , is said to give lower values (less oxidation) than oxidation according to the procedure in Standard Methods, 1946 (9). Results The biochemical oxygen demand after 21 days, expressed as grams of 0 2 per gram of substance, is much higher for xylose and galactose than for xylan from birch,
i
as can be seen in Figure 1. These substances were analyzed simultaneously and with the same seed solution. Xylose and galactose were oxidized to about 75% and 8W0, respectively, but the xylan was oxidized only to 33% during this three-week period. The oxidation rate during the first stage is more rapid for xylose, and to some extent galactose, than for xylan. There is a similar effect with galacturonic and polygalacturonic acids as illustrated in Figure 2. The degradation rate during the first days is more rapid for the monomer. Galacturonic acid was oxidized to 85% and polygalacturonic acid to 70% after 20 days. Both acids were neutralized before this analysis, which was repeated in the same way a t a somewhat later date and with a new seed solution from the same purification plant. The repeatability is fairly good, as Figure 2 shows. The following equation is considered to be generally valid for biochemical oxidation reactions ( I ) :
dY $ = K(L,
0.80 g 0,lg
0.60-
%p
galactose
time in Voith's Sapromat, days 5
10
15
20
Biochemical oxygen demand in grams of O2 per gram of substance as function of time for two monosaccharides and one polysaccharide. Measurements have been carried out simultaneously and with same seed solution Figure 1.
-
where Kalog e = rate constant
La = total latent BOD at the time t
= 0 (approx = BODzo) yt = total oxygen demand till the time t (in days)
BOD, g 0 , I g substance
The rate constant gives a measure of how rapidly the BOD of a certain substance reaches its maximum value. The experimental values for the oxygen demand during the first 10 days fit the equation reasonably well and the rate constant for this period is for xylose, 0.090; xylan, 0.084; galacturonic acid, 0.155; and polygalacturonic acid, 0.156. Table I summarizes the BOD and COD for the same substances. The COD is determined by oxidation with dichromate and permanganate. The degree of oxidation as a percentage of the theoretical maximum is also tabulated. It can be seen that the COD depends on the degree of polymerization-i.e., that the polymeric substances have a lower oxygen demand than the corresponding monomers. This effect is most pronounced for oxidation with permanganate. The BOD values are corrected for the lag phasei.e., the adaptation period before the oxygen demand starts.
_ _ _ _theoretical _ _ - _ _ _ _ _ _ oxygen _ _ _ _ _ _ _demand _____________ for polygalacturonic acid for galacturonic acid
~~____~~~____~___________________ Dolvaalacturonic acid . .-
0.60-
0.40-
Discussion Effluents from pulp and paper mills contain both polymeric and monomeric carbohydrates. If these effluents are hydrolyzed, a more rapid BOD and a higher COD can be expected. In mills where the white water system has a
5
10
20
15
Biochemical oxygen demand in grams of 0 2 per gram of substance as function of time for galacturonic acid monohydrate (circles) and polygalacturonic acid (triangles). Two curves for each substance are given. These duplicates were carried out at different times and with different seed solutions Figure 2.
Table I. BODand COD of Some Monomeric and PolymericCarbohydrate Substances in Percent of Theoretical Value Method (ref) Xylose Xylan from Galacturonic acid Polygalacturonic acid, DP 80 birch, DP 200
Theoretical, g O ? / g substance
1.97
1.22
0.76
0.91
Oxygen Demand in % of Theoretical Value Bi oc he m ica I BOD,
BOD; BOD20
Chemical By dichromate S t a n d a r d Methods, 1946 (9) Lindberg a n d B O r n O , 1967 (8) By permanganate Standard Methods, 1965 (7) (SCAN-W 1 ~ 6 6 1966) , (10)
35 49 73
17 24 33
62 67 84
61 66
97 88
89 81
100 95
54 87
61 58
30 40
70 62
20 10
-
44 60 70
49 62
-
Volume 9, Number 2 , February 1975
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rather low pH and high temperature, for example a mill using thermomechanical pulping, these properties become more pronounced the more closed the water system is, and may lead to a hydrolsis of the polymeric carbohydrates. When the pulp yield in thermomechanical pulping for fiber building board production is increased, it is probable, that the molecular weight of the dissolved substances decreases. That means that the BOD, in grams of 0 2 per gram of substances increases with increasing pulp yield in an Asplund defibrator process, although the total BOD, per ton pulp decreases, as has been demonstrated (11).It was shown in the same paper that the COD determined according to the permanganate method is almost constant. This may be due to change in the composition of the dissolved substance-e.g., to a higher content of lowmolecular acids. It is known that permanganate does not oxidize volatile acids (3, 12). In this investigation the standard procedure for determining the BOD in wastewater was used; that means the samples were seeded with domestic wastewater. Even though the model compounds used in this investigation are nontoxic, it must be considered probable that the use of acclimated seed would give a higher BOD, especially for the polymers. In a stream, however, where the pollutants constantly are diluted with unpolluted water, only a lower degree of acclimation of microorganisms takes place. To use a seed acclimated especially to a certain polymeric carbohydrate cannot be regarded as comparable to the conditions in the receiving water.
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Environmental Science & Technology
It is also commonly believed, that the mechanism for degradation of polymeric carbohydrates includes an extracellular hydrolysis of the polymer before the degradation of the monomer can take place, which means an advantage for the BOD test sample, where the enzymes for the hydrolysis can reach a much higher concentration than in the receiving water.
Acknowledgment The authors wish to thank Marianne Bjorklund for valuable technical discussions. Anthony Bristow has revised the translation,
Literature Cited (1) Nylander, G., Suen. Pupperstidn. 67,565 (1964). (2) Varma, M., Hall, L., WuterSewage Works, 113, 102 (1966). (3) Meissner, B., Wusserwirtsch. Wussertech., 8 ( l ) , 14 (1958). (4) Hansson, J-A, Hartler, N., Suen. Pupperstidn., 71, 358 (1968). ( 5 ) SCAN-C 15:62, ibid., 65,921 (1962). (6) Liebmann, H., Offhaus, K., Abwussertechnik, 3, IV (1966). (7) “Standard Methods for Examination of Water and Wastewater,” 12th ed., Am. Publ. Health Ass., 1965. (8) Lindberg, A., Borno, Ch., Vutten, 3,208 (1967). (9) “Standard Methods for Examination of Water and Sewage,” 9th ed.. Am. Publ. Health Ass.. 1946. (10) SCAN-W 1:66, Suen. Pupperstidn., 69,673 (1966). (11) Back, E. L., Larsson, S. A., ibid., 75,723 (1972). (12) Nylander, G., ibid., 67, 170 (1964).
Receiued for review January 21, 1974. Accepted September 12, 1974. Work supported by the Swedish Enuironmentul Cure Project ( S S V L ) .
