polylactic acid cups versus paper cups: a composting efficiency
TRANSCRIPT
Polylactic Acid Cups versus Paper Cups: A Composting
Efficiency Comparison
May 2012
Student Investigator: Sarah Kogler
Faculty Supervisor: Dr. R. Michitsch
University of Wisconsin-Stevens Point
UNIVERSITY OF WISCONSIN SYSTEM SOLID WASTE RESEARCH PROGRAM Student Project Report
S. Kogler, PLA vs. Paper Composting Efficiency Study 2
Introduction
Prior to fall 2009, University Dining Services (UDS) at the University of Wisconsin-Stevens
Point (UWSP) used disposable Styrofoam food service ware across the campus. At that time
UDS switched to a biobased alternative plastic as part of a broad sustainability initiative. The
replacement selected was made of polylactic acid (PLA), which is made from corn - an annually
renewable resource. PLA is marketed as compostable, which was the primary reason for
switching from less expensive Styrofoam products. Although the UWSP Student Government
Association (SGA) saw the purchase of PLA as a stimulus for starting a composting program on
campus, nearly three years later no such program exists.
In fall 2011, a different method of reducing PLA waste was implemented. A source separation
system in dining areas collected PLA plastic ware for chemical recycling. Sponsored by the
Wisconsin Institute for Sustainable Technology, in the 2011-2012 academic year this recycling
approach was used as a demonstration project. However, this method of disposal is located off-
site, which incurs a collection cost. With source separation already in place, SGA-endorsed
composting needed to be further evaluated before it was selected as a viable option for UWSP to
handle the PLA waste. Having a local, large-scale composting program that uses compostable
food wastes as well as PLA would reduce collection costs of wastes. It is also likely the compost
from this process would offset landscaping costs on the UWSP campus. It would also provide an
opportunity for UWSP students to learn about sustainable practices and ways to reduce waste
that would otherwise be destined for landfills.
Before composting at UWSP can be deemed feasible on a large scale, it is necessary to study
the efficiency of composting PLA waste. In order for a composting program to be successful,
there must be enough organic feedstock to efficiently degrade the amount of campus PLA waste,
and knowing efficiencies will determine whether initiating a program would benefit a campus
the size of UWSP. According to a recent study done by the Advanced Solid Waste Management
class at UWSP, 48% of the waste (by weight) from the Dreyfus University Center could be
composted (Hull, 2012). While PLA makes up a minor part of that waste (1.25%, by weight), it
is still important to know if it is practical to compost the PLA with the other compostable wastes
at UWSP (Hull, 2012). The compostability of PLA is dependent on moisture, temperature, and
pH conditions. These primarily affect the initial step in the composting process; a chemical
depolymerization process known as hydrolysis (Danyluk et al., 2010). This process requires high
heat and high moisture, and the end products are oligomers, small molecules that can be
consumed by native microbial populations. By the end of the composting process, the byproducts
of PLA are water and carbon dioxide.
If the PLA waste generated on campus is not composted (or recycled), it is likely not
economically prudent to continue purchasing the PLA products. Disposable paper service ware is
a possible alternative to using PLA because it is also thought to be compostable. Although the
opacity of disposable paper service ware is less attractive for food presentation, the decreased
S. Kogler, PLA vs. Paper Composting Efficiency Study 3
cost is desirable. It is thought that the cost effectiveness and the assumed compostability of paper
products would make it a strong competitor to the PLA. The effects and efficiency of composting
disposable paper service ware needs to be studied if it is to be composted at UWSP at any time in
the future. Knowing the degradation efficiency also allows for a direct comparison of
composting paper product waste versus composting PLA waste.
Objectives
The intent of this experiment was to evaluate the composting efficiency of polylactic acid
(PLA) cups in a controlled environment by taking weight measurements and visual observations.
The composting efficiency of paper cups was evaluated in the same fashion. Information from
this study will be accessible to the UWSP Sustainability Task Force for future purchasing and
waste management decision making.
Methods
PLA is marketed for industrial composting conditions, which differ greatly from those found in
the backyard. Active composting in an industrial setting is efficient because it considers
parameters including the carbon to nitrogen ratio of feedstock, moisture content, particle size of
feedstock, aeration, temperature, and pH (Dougherty, 1999). For efficient and safe degradation,
the aforementioned concerns are both monitored and managed throughout the process. Current
standards for maximized degradation of compost recommend a carbon to nitrogen ratio of 30:1.
A balanced C/N feedstock structure is recommended, with diverse materials to provide uniform
moisture, ideally between 45-65%. Having a uniform particle size increases surface area exposed
to microbial activity. Aeration is prescribed to maintain a 5% oxygen concentration and pH
should be within 5.5-8.
Initial Start Up
Industrial composting conditions were simulated in a laboratory setting. Compost vessels were
5.68L stainless steel buckets with lids, which contained the compost feedstock: leaves, grass,
sawdust, peat moss, coffee grounds, and chicken feed. Feedstock materials were proportioned to
maintain a 30:1 carbon to nitrogen ratio. Initial pH of the feedstock materials ranged from 3.88 to
5.29. Buckets were incubated at 55°C ± 2ºC for 12 weeks and 16 weeks, respectively, for the
Figure 1. Composting vessels
in incubator set to 55°C.
S. Kogler, PLA vs. Paper Composting Efficiency Study 4
PLA and paper treatments at UWSP. The compost was manually turned weekly for aeration.
Moisture was maintained at 60% through weekly sampling and additions of deionized water.
In addition to the feedstock mixture (378g), separate treatments (5%, 10%, 20%, and 30% by
weight) of PLA cold cups (ECO-Products®) and paper cold cups (Dixie®, lined with leak-
resistant coating, not marketed as compostable) were mixed in. Cups were cut into 3cm × 3cm.
There was also a control treatment that contained only feedstock. Each treatment had four
replicates.
Compost Maintenance
The compost did not undergo an active phase in which feedstock was added continuously;
instead it underwent a 12-week maturation process (PLA) or a 16-week maturation process
(paper). The paper treatments were extended from 12 to 16 weeks when there was little visual
change to the paper pieces. Weekly moisture regulation and aeration occurred, with biweekly
photography of randomly selected pieces of paper or plastic to provide a visual timeline of
degradation. Post composting, all material was separated using a 2 mm sieve.
Nutrient Analysis
The final compost was sampled and tested for C, total N, NH4+, NO3
-, P, and K. The purpose of
nutrient analysis was to determine if the compost would pose any detriment in macronutrients to
soil, should the compost be used as a soil amendment. All analyses were performed by the
Environmental Microbial Analysis and Research Laboratory on the UWSP campus according to
standard methods.
Results and Discussion
At the beginning of the experiment, both the PLA and paper cups were cut up to represent the
shredding that takes place in industrial composting operations. The cups were cut down to 3cm ×
3cm squares to make the pieces large enough for visual observations during the composting
process.
Initial weights of the PLA and paper added varied by treatment percentage and were based on
the initial starting weight of the feedstock, 378g. This amount was determined by the space
constraints from the composting vessels. Final weights were determined after manually sieving
with a 2mm sieve. Final weights represented pieces of PLA or paper that were larger than 2mm.
Composting was extended from 12 weeks to 16 weeks for the paper treatments because after
12 weeks, minimal degradation had occurred. The efficiency of composting was determined by
comparing the average percentage of treatment weight loss. Table 1 displays these weight losses.
All four of the PLA treatments lost over 99% of their initial weight. The four paper treatments
varied in average weight loss and ranged from -19.40% (indicating a weight gain) in the 5%
treatment to 68.15% in the 30% treatment. Efficiency, in the context of this experiment, can be
S. Kogler, PLA vs. Paper Composting Efficiency Study 5
thought of as: the amount of degradation that various amounts of PLA/paper cups undergo from
the same amount of compost feedstock. It is clear from this data that the PLA cups degraded
more efficiently than the paper cups. In the paper treatments, the weight loss increased as the
initial amount of paper increased. An explanation of this is that within the composting
environment, more paper allowed for more food for a larger microbial population and, with a
higher microbial population, more degradation occurred. In similar future experiments, microbial
biomass could be tested to determine if this was the case.
Visual observations were recorded every two weeks and documented with photographs. A
complete visual comparison of the photographs by treatment can be found in Appendices A
through D. Degradation of the PLA cup pieces followed documented trends. The PLA first
changed color from transparent to an opaque cloudy white. Then the pieces condensed and
shrank to nearly half the original size. After 2 weeks, the pieces began to crack when manually
mixed in the composting vessels. Visible hairline cracks, all oriented in the same direction, could
be seen on many of the pieces. During week 3, nearly all the original pieces had broken down
into small particles and, in week 4, the pieces showed further breakdown. By week 5, in the 5%
PLA treatment, the PLA pieces were barely visible. In other treatments there were still pieces
that were large and continued to break down. Breakdown continued until week 12 when the PLA
treatments were sieved.
Visual observations of the paper treatments noted that the paper cups absorbed moisture
immediately around the cut edges. After week 1, the polyethylene coatings had separated from
the paper layer occasionally. Gradually, many of these coatings peeled away and fell off entirely.
Only after the paper layer was exposed was it able to be degraded. This was evident as all the
paper degraded from the outer edges inward. During the sieving process, many small paper
pieces (~0.5cm × ~0.5cm) were found. However, many large paper pieces, nearly intact, were
also found in all treatments. The plastic coatings were also sieved from the compost. These were
very light weight, but, as expected, did not show any signs of degradation. Overall, the paper
Treatment Initial Wt Final Wt % Wt Loss
5% 18.90 1.86 99.90
10% 37.80 6.73 99.82
20% 75.60 15.51 99.79
30% 113.40 10.92 99.90
5% 18.90 22.57 -19.40
10% 37.80 28.21 25.38
20% 75.60 24.28 67.89
30% 113.40 36.12 68.15
PL
AP
ap
er
Table 1. Average Weight Loss of PLA after 12
weeks composting and paper after 16 weeks
composting, by treatment.
S. Kogler, PLA vs. Paper Composting Efficiency Study 6
treatments showed little visible degradation differences except for the presence of many more
plastic coatings in the 20% and 30% treatments. Toward the beginning of the experiment, mold
was observed in several composting vessels on a regular basis. It is unclear whether the mold had
any effect on the degradation of those treatments. This is an area of possible future research.
Nutrient analysis was performed on samples of all compost treatments to determine the
potential soil amendment status of the compost containing the degraded PLA and paper. Primary
nutrients (carbon, nitrogen and hydrogen) found in the compost treatments were compared to the
average concentration found in plants that are grown in Wisconsin soils (Schulte, et al., 2005;
Table 2). This information is valuable to anyone that would apply this compost to soil. The
values for the carbon and hydrogen were comparable to normal Wisconsin soil levels. The
nitrogen in the compost is low compared to the average soil concentration. The difference is due
to comparing a compost to soil nutrient standards. The low nitrogen content shows that adding
this compost would increase the nitrogen levels slightly, but would not be problematic from a
nutrient perspective. Overall, there is little difference between any of the treatments for any of
the primary nutrients, which suggests that these amounts of PLA or paper have little effect on
these nutrient values.
Plant essential nutrient data was also analyzed, which includes nitrogen, phosphorus and
potassium. The aforementioned nutrients are important for soil quality and plant health, so it is
desirable to know how much of each primary nutrient is in the compost. The values from all
compost treatments were compared to nutrient content of a fertile silt loam, a common
Wisconsin soil (Schulte, et al., 2005; Table 3). These nutrient values varied much more by
treatment than the primary nutrient values. The blank treatment contained no paper or PLA, so it
is a baseline for the other compost treatments. For the ammonium (NH4+), the PLA 5%, 10% and
5% 46.423± 2.672 1.820± 0.372 5.760± 0.397
10% 48.218± 1.136 1.680± 0.139 6.130± 0.220
20% 48.465± 0.864 1.505± 0.091 6.105± 0.114
30% 47.793± 0.984 1.635± 0.212 5.763± 0.190
5% 47.783± 1.152 2.115± 0.351 6.160± 0.249
10% 46.683± 4.418 2.038± 0.470 5.873± 0.653
20% 48.768± 1.126 2.473± 0.274 5.788± 0.271
30% 47.935± 1.108 2.378± 0.149 5.955± 0.130
48.063± 1.515 2.043± 0.425 6.138± 0.151
Avg Plant
Concentration45 43 6
Nitrogen
(%)
Blank
Treatment
Table 2. Average primary nutrients by treatment,
including average concentration of plants grown in
Wisconsin soils.
Incre
asi
ng
PL
A
Incre
asi
ng
Pap
er
Carbon
(%)
Hydrogen
(%)
S. Kogler, PLA vs. Paper Composting Efficiency Study 7
20% treatments were below the value of the blank compost (9.638 ± 0.686). The 30% treatment
was much higher than the blank compost. The large difference between the PLA 20% and 30%
treatments could indicate a tipping point of ammonium within the nitrogen cycle. The 10, 20 and
30% paper treatments also contained high levels of ammonium. The nitrate levels in all of the
compost treatments were very low. The lowest value for nitrogen, 0.006 mg/L, corresponds with
to the highest value of ammonium, 42.367 and are both found in the 30% PLA treatment.
Phosphorus values were all within the range common for a Wisconsin fertile silt loam soil, which
indicates that it would not be problematic to apply this compost to this type of soil. Potassium
values are lower than the given range for a Wisconsin fertile silt loam soil; however, the compost
would still be able to be applied as a soil amendment. Potassium values for the 30% PLA and 20
and 30% paper treatments were much higher than the blank compost. This suggests that some
potassium may have been supplied to the treated composts from the treatment amendments.
Limitations
Although the nutrient data seems to indicate all the compost treatments could be applied to
soil, the final pH of all the treatments was extremely low (Table 4). The desired pH for compost
is between 5.5 and 8.0. All but one (30% paper) of the compost treatments did not meet the
minimum of 5.5. The uniform consistency, availability and common use of the original feedstock
materials for composting were factors taken into higher consideration than the pH of each of the
materials (initial pH of the feedstock materials ranged from 3.88 to 5.29). It is unusual that the
pH of each compost treatment did not increase after the composting process. It was expected by
the researcher that this pH would increase and likely not be a problem.
Since a large scale composting operation for a campus would not take place under completely
ideal conditions, the laboratory conditions for this experiment are limiting as to the extrapolation
of the resulting data. The materials used for feedstock (chosen for their uniform consistency,
5% 8.333± 0.597 0.101± 0.071 41.317± 4.797 42.507± 12.565
10% 8.238± 0.461 0.053± 0.027 39.200± 3.689 37.080± 4.896
20% 7.413± 1.145 0.030± 0.021 37.000± 4.106 32.235± 2.719
30% 42.367± 1.186 0.006± 0.008 33.917± 8.124 70.059± 11.912
5% 9.563± 0.333 0.069± 0.012 40.363± 1.392 41.615± 3.858
10% 21.725± 13.829 0.048± 0.025 36.450± 2.863 48.894± 8.060
20% 42.075± 3.120 0.057± 0.032 40.188± 3.101 63.783± 4.405
30% 30.450± 3.018 0.043± 0.012 32.650± 1.512 65.420± 3.931
9.638± 0.686 0.065± 0.023 39.475± 4.060 36.726± 3.689
Not available 20-50
Blank
WI Silt Loam
NO3-
mg N L-1
Phosphorus
mg P L-1
Potassium
mg K L-1
Incre
asi
ng
PL
A
Incre
asi
ng
Pap
er
NH4+
mg N L-1
Treatment
Table 3. Average plant essential nutrients by treatment, including
average for Wisconsin fertile silt loam soil.
100-150
S. Kogler, PLA vs. Paper Composting Efficiency Study 8
availability and common use for composting) are also not representative of the entire
compostable waste stream, which varies greatly from day to day at UWSP.
Summary and Recommendations
Overall, the efficiency of composting PLA was successful. The efficiency of composting paper
cups was not successful. If paper cups were to be considered as a compostable alternative to PLA
for UWSP Dining Services, actual compostable paper cups would need to be purchased. It is
likely that paper cups marketed as compostable would be equally as expensive as the PLA
products currently purchased. Aside from the cost, PLA is also more desirable than paper
products for presentation of food and beverage products.
Further research is necessary regarding the composting efficiency of PLA waste in an outdoor
campus-sized or large in-vessel composting operation. Future research regarding the composting
efficiency of PLA waste should include actual waste taken from campus dining facilities. The
results of this study will be made available to the UWSP Sustainability Task Force for future
discussion regarding campus composting operations.
Final pH
5% 3.64
10% 3.47
20% 3.35
30% 5.20
5% 3.98
10% 4.61
20% 5.09
30% 5.58
3.98
Table 4. Average final pH
of compost treatments.
PL
AP
ap
erBlank
Treatment
S. Kogler, PLA vs. Paper Composting Efficiency Study 9
Appendix A
A biweekly photographic comparison of degradation in 5% PLA and paper treatments.
S. Kogler, PLA vs. Paper Composting Efficiency Study 10
Appendix B
A biweekly photographic comparison of degradation in 10% PLA and paper treatments.
Mislabeled.
Should be day 98
S. Kogler, PLA vs. Paper Composting Efficiency Study 11
Appendix C
A biweekly photographic comparison of degradation in 20% PLA and paper treatments.
Mislabeled.
Should be day 98
S. Kogler, PLA vs. Paper Composting Efficiency Study 12
Appendix D
A photographic comparison of degradation in 30% PLA and paper treatments.
Mislabeled.
Should be day
56.
Mislabeled.
Should be day
56.
Mislabeled.
Should be day
84.
S. Kogler, PLA vs. Paper Composting Efficiency Study 13
References
Danyluk, C., Erickson, R., Burrows, S., Auras, R. 2010. Industrial Composting of Poly(Lactic
Acid) Bottles. Journal of Testing and Evaluation. 38(6): 717-723.
Dougherty, M. 1999. Field Guide to On-Farm Composting. Natural Resource, Agriculture, and
Engineering Service, Ithaca, NY.
Hull, H. 2012. Dreyfus University Center Waste Audit. Advanced Solid Waste Management
Class.
Schulte, E., and Walsh, L. (2005) Management of Wisconsin Soils. UW extensions A3588.
www.soils.wisc.edu Retrieved: 5/31/12