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1
Biodegradation of Compostable Plastics in
Green Yard-Waste Compost Environment
Joseph Greene, Ph.D.
Department of Mechanical Engineering Mechatronic
Engineering and Manufacturing Technology
California State University, Chico
Chico, California, USA 95929-0789
Abstract
Compostable plastic materials, produced from Poly Lactic
Acid (PLA), corn starch, or sugar cane, degraded in a green
yard-waste compost environment. The compostable plastics
claim to meet ASTM D6400 standards for biodegradation,
sustainable plant growth, and eco-toxicity. Biodegradation
was measured by disintegration studies over 20 weeks. The
commercially available compostable products, made from
PLA, sugar cane, or corn starch, biodegraded while in a
commercial compost facility with other common yard waste
compostable items. The PLA container, cup, and knife
completely degraded in 7-weeks at a rate similar to the
Avicell micro-cellulose control. The cornstarch-based trash
bag and sugar cane plate degraded at a similar rate as the
Kraft paper control. The three materials degraded between
80% and 90% after 20 weeks.
Introduction
Plastics are seemingly ubiquitous in our world
today. Plastics products are used by the consumer and then
either collected for recycling or thrown away with the trash.
Waste disposal companies usually collect the plastics with
other recycled products. Plastics, metals, and glass are
sorted from the refuse and sent to recyclers. The solid waste
can be recycled or sent to an incinerator or landfill. As
reported in a Statewide Waste Characterization Study,
approximately 350,000 tons of rigid plastic packaging
containers (RPPC) were disposed of in California during
2003 which represents approximately 1% by weight of the
overall waste stream. Plastic trash bags comprised 1% and
plastic film comprised 2.3% of the waste stream. [1] The
commercial sector generated approximately 50% of the
waste, the residential sector generated approximately 30%
of the waste, and the self-hauled sector generated
approximately 20% of the waste. In 2003, plastics
contributed to 12% by weight of the waste stream for the
commercial waste, 9.5% of the waste from residential waste,
and 3.9% of the waste stream in self-hauled waste. [2] The
use of biodegradable and compostable plastics in California
can reduce the amount of plastics in the landfills.
Composting is a promising waste management option for
degradable plastics because the composting process is
designed to degrade wastes. There are, however, obstacles
make many communities reluctant to accept plastic bags for
composting. [3] Degradable plastic bags that are effective in
compost environments should break down, but also hold
moisture, not be lighter than composting feedstock and
begin to degrade after several days. [4]
Background
Compostable plastics degrade in composting
facilities and break down into water, methane, carbon
dioxide and biomass. Micro-organisms in the soil or
compost degrade the polymer in ways that can be measured
by standard tests over specified time-frames. Compostable
plastics are defined according to the ASTM D6400 standard
as materials that undergo degradation by biological
processes during composting to yield carbon dioxide, water,
inorganic compounds, and biomass at a rate consistent with
other known compostable materials and leave no visible
distinguishable or toxic residue. Biodegradable plastic is
defined according to the ASTM D6400 standard as a
degradable plastic in which the degradation results from the
action of naturally occurring microorganisms such as
bacteria, fungi, and algae. Biodegradable plastics can be
made into different commercial products, including, trash
bags, food containers, packaging trays, plastic utensils, and
packaging containers and bags. The use of biodegradable
polymers is increasing at a rate of 30% per year in some
markets worldwide. [5]
Several organizations are involved in setting
standards for biodegradable and compostable plastics,
including, US Composting Council (USCC), American
Certification System of Biodegradable Products Institute
(BPI), Environment & Plastics Industry Council, American
Society for Testing and Materials (ASTM), European
Committee for Standardization (CEN), Japan’s GreenPla
program, and British Plastics Federation. The standards
from these organizations have helped the industry create
biodegradable and compostable products that meet the
increasing worldwide demand for more environmentally
friendly plastics. [6] If a biodegradable polymer does not
meet the requirements listed in ASTM D6400 or EN13433,
then it is not considered compostable. It must degrade in a
specified time frame without leaving any residuals in the
compost. [7]
Biodegradable polymers are those that are capable
of undergoing decomposition into carbon dioxide, methane,
water, inorganic compounds or biomass by the actions of
microorganisms. The rate of decomposition, residuals, and
by-products can be measured in standardized tests.
Compostable polymers are those that are degradable under
compositing conditions, which include actions of
microorganisms, i.e., bacteria, fungi, and algae, under a
mineralization rate that is compatible with the composting
process. Polyethylene plastic bags that are produced with
starch additives are not certified as compostable plastics
since they do not meet he ASTM D6400 standards. The
plastics do disintegrate but leave small plastic fragments in
the compost, which violates the ASTM D 6400 standards.
The ASTM D6400 standard differentiates between
biodegradable and degradable plastics. Some synthetic
polymers, e.g., Low Density Polyethylene (LDPE), can
erode over time if blended with an additive to facilitate
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degradation. These polymers break down into small
fragments over time but are not considered biodegradable
since they do not meet the ASTM D6400 standards.
Bioerodable polymers, photodegradable polymers, and
water-soluble polymers break down in environments
different from the biodegradable and compostable polymers
and as such are outside the scope of the research.
The Biodegradable Products Institute (BPI)
provides important criteria for valid full-scale testing of
compostable plastics.[8] The BPI Logo Program is designed
to certify and identify plastic products that will biodegrade
and compost satisfactorily in actively managed compost
facilities. [9] The Biodegradable Products Institute and US
Composting Council (USCC) use ASTM D6400 standards
to approve products for their compostable logo. The ASTM
standards are the result of eight years of intensive work to
identify plastic and paper products, which disintegrate and
biodegrade completely and safely when composted in a
municipal or commercial facility. The approved products
with a compostable logo include compostable bags and film,
food service items, and resins.
Experimental Work
Materials
The materials are all commercially available
plastics that are made from corn, polylactic acid, potato, or
sugar cane. The compostable materials that were added to
compost in the laboratory experiment were representative
samples of a plate made from sugar cane, a tray made from
potato-starch, a trash bag made from corn starch, and a cup,
fork, knife, straw, and clear clamshell container made from
NatureWorks polylactic acid (PLA). The positive control
materials include cellulose filter paper, Kraft paper and
Avicell micro-cellulose. The micro-cellulose is also used as
control in experiments in Australia and Europe. No ASTM
standards exist for compost testing at commercial facilities.
The compost soil at composting facilities is active
composting with thermophilic bacteria. The compost has
substantial background carbon dioxide in the soil from
degrading organic materials and would thus mask the
degradation of the plastic sample materials making
measurement of carbon dioxide produced from the
degrading plastic difficult. Typically, degradation is
measured by disintegration of the compostable material. A
negative control, e.g., a polyethylene bag, was not used in
the university compost experiment. It is well known that
polyethylene bags are unaffected by soil and does not
degrade in the soil over a three-month time frame.
Polyethylene bags can take many years to degrade in soil
and sunlight.
Experimental Set-up
The experimental set-up is similar at the CSU,
Chico University Farm and the Chico municipal compost
facility. The compostable products and compost were placed
in a perforated plastic agricultural bag and placed in the
compost mound. The temperature and moisture of the
compost in the bag were measured and the ambient
temperature and weather conditions were recorded. The
compost mounds were turned several times a week to mix
the compost. The plastic sample bags were removed from
the compost before the turning operation and then were
placed back in the compost after the turning. Compost
maturity was measured with compost analysis kits from
Solvita. The mass of the plastic samples was also recorded
at weekly or bi-weekly intervals.
Compost Facilities
The city of Chico municipal compost facility is
located on a 10-acre site that produces 500,000 cubic yards
of compost each year via aerobic windrow compost. The
compost is mixed with a large machine called a windrow
turner. The turning machine straddles a windrow of
approximately eight feet high by 13 feet across. Turners
drive through the windrow at a slow rate of forward
movement. A steel drum with paddles turns the compost
rapidly. As the turner moves through the windrow, fresh air
(oxygen) is injected into the compost by the drum/paddle
assembly and waste gases produced by harmful bacteria are
removed. The oxygen feeds the beneficial composting
bacteria and thus speeds the eventual composting process.
This process is then extended by windrow dynamics.[10]
The facility accepts green yard waste, which includes lawn
clippings, leaves, wood, sticks, weeds, and pruning. Testing
in commercial compost facilities allows the compostable
plastics to be exposed to active compost that should have a
high degree of enzyme activity and high temperatures that
mimic typical composting conditions in a traditional
compost facility.
Biodegradation Results
The degradation of the compostable plastics was
measured by monitoring the mass of the plastic over time.
The presence of carbon dioxide and ammonia indicates the
level of maturity of the compost soil. Compost maturity
index was measured at weekly intervals as well as the
temperature and at the compost sites. The testing in
commercial compost facilities allows the compostable
plastics to be exposed to active compost which should have
a higher degree of enzyme activity and higher temperatures
that mimic the most likely conditions that the compostable
plastics will be exposed to in real life.
Disintegration can be measured by measuring the
mass of the sample over time as it degrades. The bags were
removed from the compost mound and the contents were
screened with a 2-mm sieve to separate the compostable
sample from the compost. The samples were shaken to
remove the dirt and then collected and weighed. The
disintegration results for the municipal compost site are
listed in Table 1 for the City of Chico Municipal Compost
Facility. At the Chico Compost Facility the degradation of
the compostable samples varied between compostable
materials. Some of the materials were fully degraded in 7
weeks, including the Avicell microcellulose control, and the
PLA knife, PLA cup, and PLA clamshell container. Thus,
the PLA materials had disintegration rates comparable to the
cellulose control material. The Kraft paper control had
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similar disintegration rates as the corn-starch based trash
bag and the sugar cane plate. The three materials degraded
88%, 84%, and 78%, respectfully, after 20-weeks.
Regulated Metal Testing
The degraded materials should not leave any heavy
metals in the compost soil after degradation. The compost
soil was tested for lead and cadmium. The acceptable limit
is 30 mg/kg for lead and 0.3 mg/kg for cadmium. The
compost soil for each sample was put into solution and the
heavy metal in the compost soil was measured with
Fisherbrand [11] hollow cathode single-element 2 inch
diameter lamps with elements for lead and cadmium. The
results for cadmium were delayed because of a 7-week
back-order on the lamp.
Lead and cadmium were measured by flame atomic
absorption spectrometry using a Jarrell-Ash Model. Lead
and cadmium absorption was measured at 283.3 nm and
228.8 nm respectively. The background correction was
measured at 281.2 nm for Lead and at 226.5 nm for
cadmium. The detection limits are 0.02 ppm lead and 0.005
ppm cadmium in the analytical solution. For a 1-g sample
the detection limits are 0.2 ppm Pb and 0.05 ppm Cd.
The soil samples that were used during the
phytoxicity testing were also used to measure the lead and
cadmium levels. Approximately 10 g of compost soil from
each sample was dried for 24 hours at 105 °C. The average
moisture loss was about 30%. About 3 g of each sample
was weighed into a 150 mL beaker to which 50 mL of 8 M
HNO3 was added. The samples were digested for 4 hours at
85 °C with occasional stirring. After 4 hours, 50 mL of
deionized water was added to each sample followed by
vacuum filtration through a Whatman GF/A glass filter with
1% (v/v) HNO3. The filtrate was quantitatively transferred
to a 250-mL volumetric flask and filled to the mark with
1% (v/v) HNO3. The resulting samples all had a relatively
intense orange-red appearance.
Sample preparation included adding a 0.8239 g
sample of Pb(NO3)2 to a 500-mL volumetric flask, dissolved
and diluted to the mark with 1% (v/v) HNO3 yielding a
1099.5 ppm Pb2+ solution. Various standard solutions in the
range of 0.220 to 1.10 ppm Pb2+ in 1% (v/v) HNO3 were
prepared along with a 1 M HNO3 solution.
Standard solutions were prepared by dissolving
0.2460g Cd in approximately 3mL of 6M HCl and
approximately 2 mL of 8M HNO3 in a 250 mL volumetric
flask and diluted to the mark with 1% HCl (v/v) yield on
984 ppm Cd solution. Various standard solutions including
a blank from mature compost alone were prepared from
0.0984ppm to 9.840 ppm Cd in 1% HCl.
Results The standard solutions and eight sample solutions
were analyzed using a ThermoElectron S Series Flame
Atomic Absorption Spectrophotometer using an air-
acetylene flame and equipped with a Pb hollow-cathode
lamp detecting at 283.3 nm and a Cd hollow-cathode lamp.
The sample solutions gave absorbances at or very near the
lowest standard employed which was just above the
detection limit of the instrument. Using 0.220 ppm Pb2+ as
the detection limit leads to an upper limit of 20 ppm Pb2+ in
the original soil samples. The 20 ppm value equates to 0.02
mg/kg for Pb. The Cd concentrations were lower than 1ppm
which equates to 0.001 mg/kg Cd. All of the soil samples
from the compostable materials had lead concentrations
lower than the limit of 30 mg/kg Pb and Cd concentrations
lower than the limit of 17 mg/kg Cd.
Phytotoxicity Testing
The compostable materials must not release toxic
materials into the compost soil after degrading. The compost
soil can be tested to assess phytoxicity, or poisonous to
plants. The germination of tomato seedlings in the compost
soil was evaluated after a 10-day duration. The
phytotoxicity test was based upon the ISO 11269 standard.
The tomato seeds are a “Tiny Tim” variety form Vaughans
Seed Company. The tomato variety is one that is used in the
Biology classes on campus and is known to grow quickly
and is robust. The tomato seed is of a 1994 variety. 10 to 12
seeds were planted in small beverage cups (280 ml) that
were filled with approximately 50 grams of compost from
each of the 24-samples.
The sample containers were watered frequently
while in a greenhouse. The green house was warm and
moist with a temperature of 25C and relative humidity of
80%. After 10-days in the green house with ambient light,
the number and length of shoots were recorded for each
sample. The lack of emerging seedlings would indicate
phytotoxicity. The percentage of seeds that germinated and
the average length of the seedlings are listed in Table 6. Ten
seeds were placed in each container. A germination index is
determined by taking the product of percent germination
and the average length and dividing by 100.
All of the samples had seedlings grow except the
sugar-cane samples. The test was repeated with 30
additional seeds, but no growth occurred after an additional
10 days of exposure in the greenhouse. The sugar-cane
samples failed the test. The pH of the materials that
supported growth ranged from 8.5 to 9.1, whereas the pH of
the three samples that did not support seedling growth was
between 8.2 and 8.3. Thus, the degradation of the sugar
cane plates resulted in a compost soil that was slightly more
acidic than the control compost soil. This might be due to
the presence of acetic acid, which might be a byproduct of
the fermentation of sugar. Further analysis can be done in
the future as the testing is outside the scope of this research
project.
Conclusions
The biodegradation results of municipal compost
facility demonstrate the compostable plastics degrade in the
time frame of the ASTM D6400 standards. The compostable
plastics degraded within the specified time frame and left no
residue in the compost soil.
The disintegration results at the municipal compost
facility demonstrate that the compostable materials degrade
under moist green-waste compost. The PLA container, PLA
cup, and PLA knife degraded at a similar rate as the Avicel
4
cellulose control and were degraded completely in 7-weeks.
The corn starch-based Biobag trash bag and sugar cane plate
degraded at a similar rate as the Kraft paper control. The
three materials degraded between 80 and 90% after 20
weeks.
Acknowledgements
The author would like to thank the following people who
have helped develop this research work: Mr. Mike Leaon
(CIWMB), Mr. Edgar Rojas (CIWMB), Dr. Cindy Daley
(CSU, Chico), Dr. Ken Derucher (CSU, Chico), Dr.
Gregory Kallio (CSU, Chico), Dr. Randy Miller (CSU,
Chico), and Mr. Peter Natale (CSU, Chico)
Appendix
Table 1. Material Degradation Results for Compostable
Samples at the Municipal Compost Facility.
Initial 28-
Jul 2 weeks 7 weeks 12 weeks 14 weeks 20 weeks
Item Hole No Mass, g %
de
g
rade %
de
g
rade %
de
g
rade %
de
g
rade %
de
g
rade
Avicel
cellulose
control 1 28.3 29 100 100 100 100
Cup- PLA 6 13.983 28 100 100 100 100
Knife-
PLA 3 3.876 48 100 100 100 100
Container
-
PLA 4 22.642 12 100 100 100 100
Kraft
Paper
Control 7 20.9 28 52 69 73.4 88
Trash
bag- corn
starch 2 18.863 20 31 65 70.79 84
Plate-
Sugar
Cane 5 23.418 15 19 37 41.88 78
Table 2. Phytotoxicty of Compost Soil.
Material Average
Germination
%
Avera
g
e
Len
g
th,
mm
after
10-days
Average
Germination
Index
Average,
pH
Compost 50.00 12.33 7.43 8.73
Cellulose 46.67 8.00 3.20 8.7
Kraft Paper 30.00 9.00 3.00 8.93
Polyethylene23.33 12.00 2.80 8.6
Trash Bag 36.67 12.00 4.40 8.93
PLA
Container 26.67 8.00 2.20 8.8
Sugar Cane 0.00 0.00 0.00 8.27
PLA Cup 26.67 7.00 2.30 8.97
Figure 1. Temperature of the air and compost during the
duration of the Municipal Compost experiment.
References
[1] “Statewide Waste Characterization Study,” Cascadia Consulting Group, California Integrated Waste Management
Board, December 2004
[2] ibid
[3] J. Garthe and P. Kowal, “Degradable Plastics”, < http://www.age.psu.edu/extension/factsheets/c/C15.pdf >
January 2005
[4] G. Chapman, “Compostable bag differences,” BioCycle (May 1999)
[5] “Make Way for the New Breed of Biodegradable Plastics, Tech Talk” Environment and Plastics Industry
Council, <http://www.plastics.ca/staticcontent/staticpages/epic/pdfs/techtalk2Q2001.PDF > (June, 2001)
[6] Nayaran R., Pettigrew C., “ASTM Standards Help Define and Grow a New Biodegradable Plastic Industry,”
ASTM Standardization News, (December, 1999).
[7] Jakubowica, I., “Evaluation of degradable polyethylene (PE),” Polymer Degradation and Stability, V 80, N 1, p
39-43, (2003).
[8] R. Narayan and S. Mojo, “Summary of ASTM D6400-99 Test Methods and Correlation to Composting Trials”, <
http://www.bpiworld.org/BPI-Public/News/Article.html > (October 2005)
City of Chico Municpal Compost Experiment
Start Date is July 28, 2005 End Date is December 14, 2005
0
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40
50
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0 102030405060708090100110120130
Days
Temperature, C
Air Temperature
Compost Tempera ture
5
[9] “BPI Logo Program”, < http://www.bpiworld.org/ > (October 2005)
[10]“Compost windrow turner,” November 1, 2001, <http://en.wikipedia.org/wiki/Compost_windrow_turner>
(December 2005).
[11]“Fisherbrand* Hollow Cathode Single-Element 2 in. dia. Lamps with Elements Aluminum to Platinum,” n.d.,
<https://www1.fishersci.com/Coupon?cid=1328&gid=181949> (December 2005).
