Food waste and biopolymers, plastics derived from plants, are unexploited sources of energy when discarded in landfills without energy recovery. In addition, polylactic acid (PLA) and food waste have complimentary characteristics for anaerobic digestion; both are organic and degrade under anaerobic conditions. Lab-scale reactors were set up to quantify the solubilization of pretreated amorphous and crystalline PLA. Biochemical methane potential (BMP) assays were performed to quantify CH4 production from both treated and untreated PLA in the presence of food waste and anaerobic digested sludge. Amorphous and crystalline PLA reached near-complete solubilization at 97% and 99%, respectively, when alkaline pretreatment was applied. The PLA that received alkaline treatment produced the most of CH4 throughout the run time of 70 days. The PLA without treatment resulted in 54% weight reduction after anaerobic digestion. Results from this study show that alkaline pretreatment has the greatest solid reduction of PLA and maximum production of CH4 when combined with food waste and anaerobic digested sludge.
Biopolymers such as polylactic acid (PLA) are made from biobased feedstocks, many of which are biodegradable [
Organic waste streams often include PLA and food waste that have negative environmental impact when disposed of in landfills and compost facilities. PLA in food applications has the distinct advantage that it can be composted alongside food waste; however, compost facility managers and studies report that the PLA biopolymer does not fully degrade in industrial composting facilities [
The GHGs that are emitted from food waste have led to many states in the US and European countries to limit the amount of food waste that can go to landfills [
Previous studies have already successfully codigested food waste with anaerobic digested sludge (ADS) inoculum: e.g., municipal wastewater [
Recent studies have demonstrated the ability to digest biopolymers with municipal sludge in mesophilic conditions and yield CH4 that can be used for combined heat and power (CHP) [
PLA is a particulate solid, and hence, its availability for microbial hydrolysis is often rate limiting [
This study seeks to assess alkaline pretreatment for accelerating the solubilization of PLA and to quantify the CH4 production from codigesting both treated and untreated PLA with food waste. Alkaline pretreatments of amorphous and crystalline PLA were performed with sodium hydroxide (NaOH) to analyze the solubilization of PLA. CH4 production was assessed for food waste, PLA (untreated and treated), and anaerobic digested sludge (ADS) to determine conditions for enhanced methanogenic yield. Codigestion of food and PLA waste creates the potential to (1) redirect a significant fraction of waste entering the municipal waste stream, (2) reduce or offset GHG emissions from landfills, and (3) produce renewable energy.
Food waste was collected from Clemson University’s catering service. The food waste was a mix of the following foods: string beans, lima beans, edamame, parsley leaves, potatoes, chickpeas, chicken, and pork. To form a heterogeneous mixture for this study, the food waste was prepared by mixing the whole food waste by hand, followed by chopping and grinding food waste with 500 mL of water in food processor (Black and Decker model FP1140BD, USA; 450 Watts) for 10 minutes on setting 2, which resulted in a paste. The food waste paste was blended (model Black and Decker BL1120SG, USA; 550 Watts) with 700 mL of water for 10 minutes on setting 4 to create a food waste slurry concentration of 107 g of food waste/L.
Thin-film amorphous PLA bags and crystalline PLA cups manufactured by NatureWorks LLC and produced by EarthFirst and Repurpose Compostables, respectively, were used in this study. Both PLA products report that they are 100% plant based and consist of the proprietary resin, Ingeo™, derived from PLA. The thin-film bags and cups were cut into 2 × 2 cm and weighed. Solubilization assessment of thin-film and crystalline PLA was performed in 250 mL serum bottles; the parameters of which are given in Table
Experimental composition of each solubilization test.
PLA | DI H20 (mL) | Initial pH | Final pH | 10 M NaOH (mL) |
---|---|---|---|---|
Alkaline amorphous | 100 | 13.9 | 13.5 | 10 |
Control amorphous | 100 | 7.1 | 8.9 | 0 |
Alkaline crystalline | 100 | 13.6 | 13.06 | 16 |
Control crystalline | 100 | 6.65 | 6.41 | 0 |
Lab-scale biochemical methane production (BMP) tests were used to determine the biodegradability and CH4 production of cosubstrate (i.e., food waste, crystalline PLA, and ADS) (Figure
BMP assay setup consisting of food waste, PLA (treated or untreated), and anaerobic digested sludge (ADS).
Treated BMP tests were created by cutting crystalline PLA into 2 × 2 cm fragments and adding 100 mL of deionized water and 16 mL of NaOH. The treated PLA was incubated at 12.96 pH for 15 days. Treated PLA was neutralized to 7.17 with 2.0 M hydrochloric acid (HCl). 0.2 L of solubilized PLA was added to treated serum bottles along with H2O, food waste, and ADS.
The amount of PLA used for untreated experiments was determined based on the density of PLA at 1.24 g/mL and the volumetric ratios of PLA, NaOH, water, and HCl used in treated experiments. Untreated crystalline PLA was cut into 2 × 2 cm and weighed. Each 200 mL serum bottle consisted of 0.18 L of ADS, food waste, and H2O, and 1.1 g of PLA was added to the untreated serum bottle.
Treated and untreated BMP tests were performed in triplicate; the experimental parameters are given in Table
Volumes and mass of polylactic acid (PLA) and mass of acetate used for experiments with food waste (FW) and anaerobic digested sludge (ADS).
BMP test | FW (L) | ADS (L) | H2O (L) | PLA | Acetate (g) |
---|---|---|---|---|---|
Treated | 0.02 | 0.08 | 0.06 | 0.2(L) | 0.0 |
Untreated | 0.02 | 0.08 | 0.06 | 1.1 (g) | 0.0 |
Negative control | 0.02 | 0.08 | 0.06 | 0.0 (L) | 0.0 |
Positive control | 0.0 | 0.08 | 0.1 | 0.0 (L) | 0.75 |
All analytic tests were performed in triplicate, and the following analyses were performed: total chemical oxygen demand (TCOD), semisoluble chemical oxygen demand (SSCOD), total solids (TS), volatile solids (VS), and pH. Biogas production was measured daily with a frictionless glass syringe (Perfektum, NY), and contents were analyzed using an Agilent 7890B gas chromatograph with thermal conductivity detection (GC-TCD) (Shanghai, China). Data were reported at 35°C at 1 atm. Initial and final values of BMPs, TCOD, and SSCOD were measured by filtering sample through 1.2
In the 15-day incubation period, both alkaline-treated amorphous and crystalline PLA reached near complete solubilization at 97% and 99%, respectively (Figure
Weight of alkaline pretreated and untreated PLA at day 1 and day 15 (left) compared to SSCOD at day 15 (right).
PLA dissolving in high alkaline solution has been seen previously, and the results report that hydrolysis of aliphatic polyester cleaves the ester bonds [
Codigestion of food waste and treated and untreated PLA demonstrated satisfactory results and is supported by biotransformation to CH4. The initial pH values for treated BMP test are within the ideal range of 6.8–7.2, while untreated was not. The untreated tests were performed without pH adjustments to show realistic expected outcomes at wastewater treatment plants. SSCOD was higher for treated compared to untreated, suggesting that treated test has more organic material readily available for conversion to CH4.
The final pH values for treated and untreated suggest that the digester was in good condition since organisms produce alkalinity as they consume protein-rich organic matter [
There were some limitations in measuring TS and VS of the untreated PLA. Due to the size of the PLA, the TS and VS measurement could not be performed accurately and yield a fair comparison between treated and untreated PLA after experimental runs (Table
Characteristics of starting and ending mixtures of PLA (treated or untreated), food waste, and ADS.
Characteristics | Initial | Final | ||
---|---|---|---|---|
Treated | Untreated | Treated | Untreated | |
TCOD (g/L) | 38.2 ± 2.2 | 27.8 ± 0.7 | 10.6 ± 0.5 | 10.2 ± 0.7 |
SSCOD (g/L) | 12.3 ± 2.8 | 3.4 ± 0.8 | 0.1 ± 0.0 | 0.7 ± 0.5 |
TS (g/L) | 5.7 ± 0.7 | 7.7 ± 0.9 | 0.1 ± 0.0 |
|
VS (g/L) | 3.3 ± 0.5 | 2.5 ± 0.2 | 0.0 ± 0.0 |
|
VS : TS | 0.6 ± 0.2 | 0.3 ± 0.0 | 0.0 ± 0.0 |
|
pH | 7.1 | 7.4 | 8.9 | 8.7 |
Comparison of untreated crystalline PLA before and after BMP tests.
The nutrient-rich digested solid from the treated BMP test could be potentially used for land application in the US if it meets the Environmental Protection Agency regulations for safe application [
Figure
Methane production of food waste and PLA (treated or untreated). Error bars in graph represent standard deviation (methane production of PLA at day 70 adjusted to subtract gas produced from negative controls).
The treated PLA BMP test produced the highest CH4 production throughout the duration of the test (Figure
The treated PLA BMP test demonstrates three very distinct phases in Figure
The remaining untreated solubilized PLA would be landfilled. Kolstad et al. [
The main objective of this study was to determine whether pretreating PLA could increase CH4 production, enhance food waste anaerobic digestion, and enhance PLA destruction in batch BMP assays. The results of this study show that alkaline treatment solubilizes PLA. After a 15-day incubation period under alkaline pretreatment, near complete solubilization was achieved for both amorphous (97%) and crystalline (99%) PLA. This study also showed that crystalline PLA has more organic nutrients. Pretreated crystalline PLA had a high organic content and SSCOD, making it a likely candidate that could produce increased methanogenic yield during anaerobic digestion. In addition, untreated PLA experiences weight reduction of 54% after the BMP test. The structure of the PLA was unstable, indicating that degradation had occurred and that anaerobic digestion assists in solid destruction and CH4 production. In addition, untreated crystalline PLA produced less than 1% of the total gas production on day 70, indicating that gas production is coming to an end and that anaerobic digestion is not capable of completely reducing PLA solids without pretreatment.
This study showed that alkaline-treated crystalline PLA produced the most CH4, compared to untreated PLA. After day 48, treated BMP test began to produce more CH4 than untreated BMP test. Treated PLA and untreated produced 1021 and 756 mL of CH4, respectively, within 70 days and graphically displayed a biphasic curve, highlighting the complexity of multiple substrates in the test. Overall, alkaline pretreatment of PLA may enable it to be codigested with food waste in anaerobic digestion systems. In addition, alkaline pretreatment of PLA may enhance the ability of these AD systems to produce CH4.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This work was supported by the National Science Foundation under CBET (grant nos. 1066658, 1553126, and 1246547). Shakira Hobbs was supported by an IGERT-SUN fellowship funded by the National Science Foundation (grant no. 1144616) and Environmental Research and Education Foundation (EREF). Special thanks are due to Swette Center for Biotechnology at Arizona State University.