polylactic acid cups versus paper cups: a composting efficiency

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


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.


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


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.


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

(%)


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


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


Appendix A

A biweekly photographic comparison of degradation in 5% PLA and paper treatments.


Appendix B


Mislabeled.

Should be day 98


Appendix C


Mislabeled.

Should be day 98


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.


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

polylactic acid cups versus paper cups: a composting efficiency

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