Environ. Sci. Technol. 1992,26,193-198
Biodegradability of Modified Plastic Films in Controlled Biological Environments Larry R. Krupp and Willlam J. Jewell” Department of Agricultural and Biological Engineering, Cornell University, Ithaca, New York 14853
The disposal of over 10 million tons of plastic per year in the United States has raised the demand for “degradable” or “biodegradable” plastics as a means of reducing the environmental impact of these materials. Unfortunately, the promotion of the “degradable”concepts has evolved in the absence of standard tests and definitions. The biodegradabilities of 12 modified plastic films were tested by exposing the films to controlled and highly efficient microbial degrading anaerobic and aerobic bioreactors. Extended biochemical oxygen demand (BOD) testing was also used to assess film biodegradability. Two photodegradable films were exposed to sunlight prior to BOD ,testing. Mass loss after bioreactor exposure and BOD exertion, which were strongly correlated, were used to assess biodegradability. Films containing starch showed no evidence that anything other than the starch component was biodegraded, despite, in two cases, disintegration caused by prooxidant activation. The mass of the starch addition and BOD exertion were strongly correlated. Sunlight exposure of photodegradable films caused significant disintegration of the films, but did not render the film fragments biodegradable. Only one test film, a poly(hydroxybutyrate)poly(hydroxyvalerate) copolymer, showed evidence of substantial biodegradation.
Introduction The current crisis in municipal solid waste disposal has underscored the growing concern of the adverse effects of plastic waste on the environment. Plastics in municipal solid wastes represent 5-8% of the dry weight, but up to 20% of the waste volume, depending on the density of the mixture (1).In recent years, several plastics manufacturers and others have developed plastic films which have been claimed to be biodegradable or degradable (2-7). These new materials are being offered as part of the solution to the plastic waste disposal crisis. The current trend toward environmentally “friendly” products, combined with landfill restrictions in certain regions and pending legislation @), suggests that there may be a strong market incentive for certain biodegradable materials. Whether these materials are truly biodegradable or degradable, however, is currently a subject of much debate and uncertainty (1,9-11). The controversy regarding these materials is due to several factors, including the definitions used for the terms “biodegradable”and “degradable,”the lack of standard test methods for assessing biodegradability of these films, and the scarcity of published scientific biodegradability studies. Some studies, for example, have assessed the “biodegradability”of some of these new films by measuring changes in physical properties, or by observation of microbial growth or physical deterioration, after exposure to biological or enzymatic environments (2,12-17). In many of these studies, the biological environments were poorly defined (e.g., soil or compost) or otherwise limited (e.g., one microbial species). For purposes of this study, biodegradation is defined as the conversion by microorganisms of a material to carbon dioxide, water, and other trace inorganic pr ucts under aerobic conditions or to carbon dioxide, meth e, and other
2
0013-936X/92/0926-0193$03.00/0
inorganic products under anaerobic conditions. Biodegradation is therefore considered a subset of “degradation”, which is defined here as any physical or chemical change in a material caused by any environmental factor, including light, heat, moisture, wind, chemical conditions, or biological activity. While respirometry studies have been used to measure the biodegradation of many organic materials under both aerobic and anaerobic conditions, few such studies have been applied to modified plastic films. The few respirometry studies that have been reported have examined a limited number of films (18-20). In this study, two techniques were used to assess film biodegradability. First, the films were exposed to wellcontrolled and highly efficient anaerobic and aerobic bioreactors and the change in film mass was measured. The bioreactors were intense biological environments that consistently biodegraded nearly all of the complex, solid, organics added as substrate (sorghum and cellulose mixture). The intensity of the biological activity in the bioreactors made them excellent accelerated exposure test environments. Second, the biochemical oxygen demand (BOD) test was used as a respirometric test of film biodegradability.
Materials and Methods Test Films. The test films, described in Table I, included most of the “biodegradable” films that were commercially available in 1989. Film samples were uniformly exposed to bioreactors by attaching samples to a supporting frame and submerging the frame in the reactor. Samples (2 X 10 cm) were bolted across cut-out windows in a cylindrical supporting frame (0.03-in. polycarbonate sheeting, Ithaca Plastics, Ithaca, NY)using nylon nuts and bolts (Product Component Co., Martinez, CA). Some samples were enclosed in nylon mesh netting (less than l-mm mesh size) prior to attachment to the frame. Bioreactors. The bioreactors, made of cylindrical polyethylene tanks fitted with gas-tight covers, were aerobic and anaerobic semicontinuously mixed and fed microbial reactors. Both reactors were fed a substrate composed of a 1:l (volatile solids basis) mixture of a-cellulose and field-dried, milled sorghum (ATx623xRIO)and were fed twice per week at a loading rate of 1.42 g of volatile solids (VS) per kilogram of total reactor contents per day. The compound formula for the substrate mixture was approximately C6H9.604.1N0.07, and its maximum biodegradability has been measured to be approximately 85% of the VS fraction (W,26). Both reactors were maintained at a total dry solids concentration of 4.5% and a hydraulic retention time of 115 days. The relatively low loading rate and long retention time were used to maximize the efficiency of biodegradation. The anaerobic reactor was operated for 350 days at 40 kg of total reactor contents at 58 “C. Biogas was collected in gas collection bags and analyzed twice per week for volume and CHI and C02 concentrations. The aerobic reactor was operated for 185 days at 30 kg of total reactor contents and 37 “C. Wet air was supplied to the aerobic reactor at approximately 1 ft3/h and was recycled at 1.6 ft3/min to achieve sufficient oxygen transfer.
0 1991 American Chemical Society
Environ. Sci. Technol., Vol. 26, No. 1, 1992
193
Table I. Test Films Included in This Study description Amko polyethylene (PE) containing 6% ADM starch; opaque, white, shiny, smooth Polar PE containing 6% ADM starch; natural color, slightly grainy texture D-Grad PE containing 6% ADM starch; natural color, slightly grainy texture Naturegrad Plus (NG+) PE plus 10-12% St. Lawrence Ecostar starch; natural color, slightly grainy texture, cornlike smell Bio-Guard PE plus 10-12% St. Lawrence Ecostar starch; rust colored, slightly granular texture, slight corn smell USDA-20, USDA-40 PE, ethyleneacrylic acid (EAA), urea (5%), and gelatinized starch; 20% and 40% starch formulations were tested starch/PMA Starch/poly(methyl acrylate) graft copolymer (48% starch) PHB/PHV poly(hydroxybutyrate)/poly(hydroxyvalerate) copolymer PVA Hefty Webster kraft paper cellulose film
-
nondeeradable PE
poly(viny1 alcohol) (water soluble) Hefty degradable trash bag (photodegradable) Good-Sense degradable trash bag (photodegradable) brown kraft paper sack cellophane film, coated on each side with nitrocellulose PE trash bag, contains no bio- or photodegradable additives
Twice per week both reactors were analyzed for temperature, pH, total and volatile solids, ammonia nitrogen, alkalinity, volatile fatty acids (VFA, anaerobic reactor), and dissolved oxygen (aerobic reactor). Periodic elemental analysis of the content of both reactors was also performed. Supplemental additions of nitrogen, trace nutrients, and alkalinity were made to maintain sufficient concentrations in the reactors. A VS mass balance approach was used to assess the efficiency of substrate biodegradation of each reactor. A more complete characterization of reactor substrates, operation, and analyses has been provided elsewhere (21). Water Bath. A pure water bath was maintained at 58 "C as a control environment for the bioreactors. The water bath tank was acid-washed and filled with hot tap water every 2 weeks during the study. Sunlight Exposure. Films were uniformly exposed to sunlight by attaching them to a flat polycarbonate frame as described above and placing the frame horizontal outdoors. The films were exposed to sunlight and the elements 24 h/day during the summer in Ithaca, NY. No attempt was made to characterize or quantify the ultraviolet radiation. Mass Changes. Film masses were measured to 0.1 mg using a Mettler AE163 digital balance. Before exposure, films were weighed without modification. After exposure, films were rinsed with water and then dried at 58 "C for 48 h prior to weighing. Preexposure masses were corrected for the effect of drying before mass changes were calculated. BOD. The standard 5-day BOD test is most commonly used to measure the oxygen demand of wastewater. It is an indirect and nonspecific measure of biodegradation &e., mineralization). By extending the time period of exposure, it was possible to increase the range of the test and to ensure that bioavailable materials would be measured. BOD testa were conducted for 60 days at 20 "C. Dilution water contained 2 mL of raw settled sewage and 10 mg of 2-chloro-6-(trichloromethyl)pyridine(TCMP) (nitrification inhibitor) per liter (24). Three replicate bottles were run per film sample per trial and nine blank bottles were run per trial. Dissolved oxygen (DO) concentration in each bottle was measured every 5 days using a YSI 5749 electrochemical BOD stirring probe connected to a YSI 54RC 194
Environ. Sci. Technol., Vol. 26, No. 1, 1992
source Amko Plastics, Inc., Cincinnati, OH 45246 Polar Plastics, Inc., North St. Paul, MN 55109 Manchester Packaging Co., St. James, MO 65559 Petoskey Plastics, Inc., Petoskey, MI 49770 Eco-Matrix, Ltd., Brookline, MA 02146 USDA, Midwest Area, Northern Regional Research, Center, Peoria, IL 61604 same Dr. James A. Romesser, E. I. du Pont de Nemours & Co., Inc., Wilmington, DE 19880-0228 same P&C Supermarket, Ithaca, NY 14850 Webster Industries, Peabody, MA 01960 Bancroft Bag, Inc., West Monroe, LA 71291 7th Generation, Inc., So. Burlington, VT 05403 P&C Supermarket, Ithaca, NY 14850
dissolved oxygen meter. During DO measurement, bottle contents were completely exposed to the atmosphere for approximately 30 s. BOD was calculated as the milligrams of oxygen consumed per milligram of plastic film added to the BOD bottle (mg of 02/mg of PL). Soluble fractions of films were prepared for BOD testing as follows: 25 mL of distilled water was autoclaved in the BOD bottle, after which a known mass of the test material was added. The bottle was stoppered and incubated at 58 "C for a duration corresponding to the time required for solubilization, as determined by previous testing. The solid film sample was then removed and the BOD test initiated. Blanks for these trials were prepared identically, excepting sample addition. BOD "integrity" testing was also conducted to determine the effect on oxygen concentrations in the bottles after repeated opening of the BOD bottles. Theoretical BOD. The theoretical BOD values for complete oxidation of many of the test films were calculated from the stoichiometry of the general equation for aerobic biodegradation (22): C,H,ObN,
+ (n +
-
- 2 ) 0 2-,
This equation shows that the complete oxidation of 1 mol of the organic compound C,H,ObN, would consume n + a/4 - b / 2 - 3c/4 mol of O2 This number, divided by the compound molecular weight and multiplied by the molecular weight of oxygen, represents oxygen consumption in the BOD units used in this study, mg of Oz/mg of PL. The theoretical BOD for starch (CH20),therefore, is 1.07 mg of 02/mg of starch, while that of polyethylene (C2H,) is 3.07 mg of 02/mg of PE. A film containing P E and 40% starch, for example, would exert a BOD of 0.43 mg of Oz/mg of PL (0.40 X 1.07) if all of the starch and none of the PE was biodegraded. Experimental Program. The bioreactors were operated for several months until stable and highly efficient conditions were maintained and documented. Films were then exposed concurrently to the anaerobic reactor, aerobic reactor, and hot water bath for 4 weeks. Where possible, five replicates were tested per sample. To test the effect of prolonged exposure, films were replaced in the anaerobic
Table 11. Results of Bioreactor Exposure and BOD Testing of Films" test material (starch content) *special exposure
BOD of material
D-Grad (6%) *ext hot air Polar (6%) *ext hot air Amko (6%) Bio-Guard (10-12%) NG+ (10-12%) USDA-40 (40%) USDA-20 (20%) starch/PMA (48%) PHB/PHV PVA starch (100%) bioreactor feed cellophane kraft paper Webster *lO-week UV Hefty *lO-week UV nondegradable PE
0.010 0.080 0.010 0.060 0.006 0.032 0.048 0.44 0.11 f 0.03 0.28 1.12 0.22 f 0.04 0.73 f 0.11 0.53 0.30 f 0.02 0.54 0.005 0.014 0.005 0.014 0.006
aerobic reactor BOD of mass remaining loss, % film
anaerobic reactor BOD of mass remaining loss,b % film
hot water bath BOD of BOD of remaining soluble film fraction
mass loss, %
0.5
0.006
-0.3 (-0.5)
0.015
-3.2
0.033
0.007
0.5
0.009
-0.2 (-0.8)
0.014
0.1
0.008
NA
0.2 1.2 1.6 44.9 17.4 45.0 100
0.004 0.010 0.012 0.011 0.025 0.035
-0.1 (-0.4) 0.7 (-0.1) 1.5 (1.5)
0.010 0.011 0.013 0.035 0.016 0.010
-0.3 0.7 1.1 45.7 8.8 f 1.2 12.7 2.3 100
0.006 0.008 0.010 0.010 0.012 0.15 f 0.01 1.08
NA
NP 21.2 (22.0) 45.9 (46.2) 91.4 f 6.7
32.6 f 6.5 93.5 f 6.3 0.1
NA NA NA NA NA NA 0.005
71.8 f 6.8 -0.3 (-0.7)
0.1
0.005
-0.2
0.007
NA NA NA
NA NA NA NA NA NA
0.009 0.013 0.40 f 0.025 0.11 2~ 0.018 0.010 0.004 f 0.012
NA NA NA
C
0.71 0.51 f 0.05
0.14 f 0.013 0.017
0.017
9.5 f 1.7 0.3 -0.2
NA
NA
-0.1 (-0.8)
0.010
-0.1
NA
NA
-0.7 (-1.0)
0.015
0.3
0.007
NA
NA NA NA 100
NA NA
NA NA
"All BOD values are the BOD exerted at day 60. Units are mass of O2 consumed per mass of sample. Errors are the population standard deviations (SDs) of the replicates tested per sample. SDs have been included only if they are 25% of the average or greater than 0.010 (1.0 for mass loss data). NA, not applicable; test not run. NP, not possible to measure mass loss due to embedded material. *Number in parentheses is mass loss after extended anaerobic reactor exposure (17 or 25 weeks). Negative mass loss indicates mass gain. E Soluble BOD same as that for unexposed film sample.
reactor for up to 25 weeks. The Hefty and Webster films were also exposed concurrently to sunlight for up to 10 weeks. BOD testing was performed with unexposed films and with the portions of the films remaining after exposure to the bioreactors, the water bath, and sunlight. For samples that solubilized to a degree, BOD testing was performed on the soluble fraction.
1.2 ,
$
:::, 0.9
0.8
0.7 0.6 0.5
Results and Discussion The postexposure mass losses and the 60-day BODS of the films are shown in Table 11. With the exception of the USDA-40 film after anaerobic bioreactor exposure and the water-soluble PVA film, minimal rinsing of the films in water rendered them clean enough to measure mass changes. BOD curves for some unexposed films are shown in Figure 1. Bioreactors. Both bioreactors were highly efficient biodegrading environments that consistently biodegraded approximately 95 % of the biodegradable sorghum/cellulose substrate added. For the anaerobic reactor, the VS removal efficiency (VSRE) was consistently approximately 80%, as calculated from three different measurements: the mass of biogas produced (VSRE = 80.7 f 4.8%), influent and effluent VS concentrations measured on a fixed solids (ash) basis (77.3 f 4.7%), and influent and effluent VS masses (82.5 f 3.7%). The pH was 7.2-7.4 and VFA concentrations were 33 f 33 mg/L, indicating good stability of the biological system. The VSRE of the aerobic reactor was 72.3 f 7.7% based on ash-based VS measurement and 78.3% based on VS measurement. Reactor pH was 6.8-7.0 and dissolved oxygen was maintained at 1-5 mg/L. As a further check of aerobic reactor performance, for one feeding period, O2consumption and COP production were traced. For both bioreactors, the efficiency of VS biodegradation was higher than the VSRE because some of the substrate VS was converted to bacterial cells. The lower VSRE of the aerobic reactors may be explained by greater cell yield
0.4
0.3 0.2 0.1
0.0 0
10
20
30
40
50
60
70
Time (days)
Figure 1. BOD curves for test films. Unless otherwise noted, curves are for unexposed film: (A) PHBIPHV, (B) soluble starch, (C) krafl paper, (D) USDA-40, (E) PVA, (F) D-Grad after prooxidant activation, (G) Bio-Guard, (H) D-Grad.
(28). Given that the maximum biodegradability of the substrate was approximately 85% , both reactors removed a very high percentage (>95%) of the biodegradable substrate added. This was the intent of the experimental design. Since most plastics are disposed of in landfills, it is of interest to compare conditions in the anaerobic bioreactor to those in a typical landfill. This may be done by comparing the effect of the two environments on paper. In the anaerobic reactor, kraft paper lost significant mass after a 4-week exposure. By contrast, excavations of some landfills have unearthed undegraded paper after up to 30 years burial (I). Therefore, although extrapolation of bioreactor exposure to "real-world" exposures like landfills is complicated by many factors, it is clear that the anaerobic reactor used in this study was significantly more intense biologically than a typical landfill. Short-term bioreactor exposure may approximate many years of landfill burial. Environ. Sci. Technol., Vol. 26, No. 1, 1992
195
1.2 y = 0.0103~- 0.0109
t
1.o
0.8
r = 0.980
0.6 0.4
Cellophane 0.2
USDA-20
0.0 -10
0
20
10
30
40
50
60
70
80
100
90
110
Mass Loss After Aerobic Exposure (%)
Figure 2. Regression of 60day BOD of unexposed film vs mass loss of film after a 4-week exposure to the aerobic reactor.
Mass Loss vs BOD. As shown in Table 11, the magnitude of film mass loss after bioreactor exposure corresponds to the magnitude of BOD exertion by the film. A regression of BOD of the unexposed film vs mass loss after aerobic reactor exposure is shown in Figure 2. The high correlation ( r = 0.980) validates the experimental methodology and supports the use of the BOD test as a test of film biodegradability. In most cases, the extended BOD test resulted in reproducible results with low variability. For example, unexposed films with a starch content of 10-12% (Naturegrade Plus) showed an oxygen consumption equivalent to 45% of the theoretical oxygen demand of the starch with a standard deviation of 2.1 % of the observed value. After exposure to the bioreactors, the observed oxygen demand decreased to the following values for this material: SD, %
bioreactor exposure
BOD, % of theoretical initial O2demand
of obsd value
postaerobic postanaerobic
11.2 12.0
h15.4
f8.3
In most cases, the standard deviation was less than 10% of the observed values for unexposed and exposed samples. The lack of complete biodegradation of the starch in the bioreactors may have been related to physical protection of the starch particles by the plastic polymer. The change in BOD due to removal of soluble components was examined by exposing all samples to hot water. Post-water exposure of the films and testing of the solubilized components confirmed the leaching of biodegradable components. In some cases, the hot water exposure had an effect similar to the bioreactor exposure-for example, with the high starch content materials. The water-soluble fraction of USDA-40 exerted a BOD equal to 91% of that of unexposed films, and the films exerted a drastically reduced BOD equal to that of the bioreactor-exposed films after water exposure. This confirmed that a component was rapidly solubilized from the film by hot water treatment and that the soluble product(s) was(were) nearly 100% biodegradable. Although the hot water removed some of the components from the starchimpregnated films, this treatment was highly variable and incomplete compared to the bioreactor-exposed samples. Control Films. Results for the control films confirmed the test methodology. As expected, the biodegradable control films, paper and cellulose film, both lost substantial mass after bioreactor exposure but minimal mass after exposure to the water bath, suggesting biodegradability (Table 11). A portion of the cellulose film (9.5%) was soluble in hot water. BOD data confirmed the biodegradability of the materials, as each film exerted a sub196
Environ. Sci. Technol., Vol. 26, No. 1, 1992
stantial BOD. The BOD curve for paper is shown in Figure 1. Also as expected, the nondegradable PE control film did not lose mass after exposure to the bioreactors or the water bath and did not exert a significant BOD. The slight mass gains measured for this film were likely due to attachment of trace quantities of reactor contents. Starch-Containing Films. The three PE films containing 6% Archer Daniel Midlands Co. (ADM) starch (D-Grad, Polar, and Amko) were similar to the nondegradable PE control film. The films lost no or negligible mass after bioreactor exposure, even after up to 25 weeks in the anaerobic reactor. BOD testing of these films similarly failed to indicate biodegradability. The BOD curve for the D-Grad film is shown in Figure 1. Although the Polar and Amko films were unaffected by hot water, the D-Grad film became yellowed, gained 3.2% mass, and became very brittle after hot water exposure. Yellowing, embrittlement, and 3.2% mass gain were also observed for samples of both D-Grad and Polar films that were inadvertently exposed to hot (58 "C)air for 1-2 months. These effects are consistent with those caused by activation of the prooxidant additive in these films (14,23). Prooxidant activation increased BOD exertion (0.08 vs 0.01 for D-Grad, 0.06 vs 0.01 for Polar). Given that the theoretical BOD of the starch component of these films is 0.064 while that of the PE component is 2.9, these BOD results suggest that prooxidant activation enhanced biodegradation of the starch component. The BOD curve for the activated D-Grad film is shown in Figure 1. Like the ADM starch films, the PE films containing 10-12% St. Lawrence Starch Co. starch (Bio-Guard and NG+) did not show any evidence that anything other than a fraction of the starch component was biodegraded. Extended bioreactor exdosure caused only 0.7-1.6 % mass loss, while the unexposed films exerted BODSof 0.03-0.05 mg of 02/mg of PL, which is equivalent to 25-50% of the theoretical BOD of the starch component only. The BOD curve for Bio-Guard is shown in Figure 1. The lack of substantial BOD exertion by the ADM and St. Lawrence starch films was not caused by microbial toxicity, as the BOD exerted by samples spiked with soluble starch was greater than or equal to the BOD exerted by soluble starch alone. BOD testing with samples that showed no net O2 consumption showed that microorganisms present in those BOD bottles at day 60 were capable of biodegrading control films. Therefore, the lack of BOD exertion by these starch-containing films was not caused by the absence of viable microorganisms. The three starch-containing films fabricated by the USDA (USDA-40, USDA-20, and starch/PMA) showed greater starch availability than the ADM and St. Lawrence films, but showed no evidence that anything other than the starch component was biodegraded. The mass loss of these films after bioreactor exposure was equivalent to their starch contents, while the BOD exerted by the films was equal to or less than that expected based on biodegradation of only the starch component (Table 11). For example, the USDA-40 material, which contained 40% starch and 5% urea, lost 45% of its mass after bioreactor exposure and exerted a BOD of 0.44 mg of 02/mg of PL (Figure 1). The USDA-40 film also lost 45% of its mass after hot water exposure, indicating solubility. The BOD of the soluble fraction (0.40 mg of Oz/mg of PL) was similar to that of the unexposed film, indicating that the biodegradable fraction of the film was also the soluble fraction. Although BOD results with the starch/PMA film indicate less than total starch consumption (0.28 mg of 02/mg of PL measured vs 0.51 theoretical), BOD was still
-E
90
--
y = 0 . 7 3 3 ~ 2.910
80
--
r = 0.970
'E
.
40
B
20
BOD Test Integrity. Results did not indicate that repeatedly opening the BOD bottles caused any adverse effects on DO measurement. In tests conducted with BOD blanks for 35 days, the DO in bottles opened every 5 days was not statistically different than that in control bottles opened only once. Nitrification occurred in all BOD bottles, as shown by a marked increase in O2consumption in all bottles at days 20-30. This corresponds to the duration of nitrification inhibition obtained using 10 mg/L TCMP (29). The effect of nitrification on BOD values was negligible; since the test films were not significant sources of nitrogen, nitrification occurred to the same degree in sample bottles as it did in blank bottles.
~
USDA-40
H
30i
H starCNph4.A
USDA-20
10
0 0
10
20
30
40
SO
60
70
80
90
100
Starch Content of Material (% w/w)
Flgure 3. Regression of percent of unexposed film biodegraded vs starch content for starch-containing films. Curve is based on BOD exertion and assumes that a BOD exertion was a result of starch consumption only.
being exerted at day 60 for this film. As suggested by the discussion above, starch content was correlated with BOD for the starch-containing films tested. Assuming that only the starch component was biodegraded in the BOD test, a regression of the percent of unexposed films biodegraded in the BOD test vs starch content (Figure 3) shows high correlation ( r = 0.970). This correlation supports a conclusion that only the starch component was biodegraded. Although the slope of this curve would not likely reach the theoretical value of 1.0 due to cell yield, the slope of the curve is reduced by the starch/PMA and pure starch values, both of which were low due to sample loss (starch) and incomplete oxidation at 60 days (starch/PMA). Other researchers have also reported that the non-carbohydrate portion of the starch-containing plastics is not readily biodegraded (27). Other Biodegradable Materials. The PVA film exerted a BOD of 0.22 mg of 02/mg of PL (Figure l),which is 12% of the theoretical BOD for complete oxidation of partially (88%) hydrolyzed PVA, indicating that PVA was partially biodegraded. Because PVA is water soluble, it was not exposed to the bioreactors. The PHB/PHV film was substantially biodegraded. Mass loss after bioreactor exposure was 90-loo%, compared to only 2.3% after water exposure, despite enclosure of the film in 1 mm2 pore size nylon mesh. The film exerted a BOD of 1.10 mg of 02/mg of P L (Figure l), which is approximately 61% of the theoretical BOD of a 1:l PHB/PHV formulation. These data indicate that PHB/PHV was readily biodegraded. BOD testing with pure starch and with the sorghum/ cellulose bioreactor substrate showed both materials to be readily biodegradable. Starch exerted a BOD of 0.73 mg of 02/mg of PL (68% of theoretical) and the sorghum/ cellulose exerted a BOD of 0.53 mg of 02/mg of PL. Actual BODS of these materials may be higher than those measured due to error in measuring mass of the small quantities of materials used (
