Thermochemical pretreatment: a promising approach for bioplastic valorisation by codigestion in WWTPs

Thermochemical pretreatment: a promising approach for bioplastic valorisation by codigestion in WWTPs M. Lera*, M. Santonja**, J. Serralta** and N. Martí* * CALAGUA – Unitat Mixta UV-UPV, Departament d’Enginyeria Química, Universitat de València, Avinguda de la Universitat s/n, 46100 Burjassot, Valencia, Spain (E-mail: [email protected]; [email protected]) ** CALAGUA – Unitat Mixta UV-UPV, Institut Universitari d'Investigació d’Enginyeria de l’Aigua i Medi Ambient – IIAMA, Universitat Politècnica de Valencia, Camí de Vera s/n, 46022 Valencia, Spain (E-mail: [email protected]; [email protected]) Abstract Polylactic acid (PLA) represents a major waste fraction in waste management plants. Among various end-of-life scenarios, anaerobic digestion (AD) is increasingly recognized as a viable approach for bioplastics valorization. However, the anaerobic digestion of PLA is not recommended, because it exhibits low biodegradability (<10%) under typical industrial AD conditions. This work aims to enhance PLA’s anaerobic biodegradability through pretreatments for effective codigestion with wastewater treatment plant (WWTP) sludge. Biochemical methane potential (BMP) assays were performed on PLA pretreated at 30–90◦C under alkaline conditions (0.3-1M NaOH) for 48 hours. The hydrolysis of PLA decreases the pH due to the appearance of lactic acid monomers, indicating that maintaining a pH >10 is necessary for effective hydrolysis. A concentration of 0.6 M NaOH was sufficient to hydrolyse 40 g ·L⁻¹ of PLA. The higher the PLA concentration, the higher the NaOH concentration required for complete hydrolysis. Pretreated PLA with 0.6 M NaOH at 30ºC enhanced the BMP by up to 74%, suggesting that these conditions may be the most favourable. Furthermore, two lab-scale anaerobic digesters were operated during 15 weeks to evaluate the feasibility of the valorisation the pretreated PLA by anaerobic codigestion (co-AD) with WWTP sludge. The addition of PLA as a cosubstrate resulted in a 54% increase in methane production because of the hydrolysed PLA was fully degraded into methane. Keywords (maximum 6 in alphabetical order) anaerobic codigestion; biodegradability; bioplastics; hydrolysis; PLA; pretreatment. INTRODUCTION Plastic pollution has become a significant environmental issue. Every year, millions of tons of plastic waste accumulate in landfills and natural environments. The resistance of these plastics to natural degradation, owing to their chemical structure and the additives they contain, presents a considerable challenge 1 . Bioplastics, derived from renewable resources, are seen as a sustainable alternative. However, some bioplastics, such as PLA, do not break down easily and are not always compatible with current waste treatment methods 2 . Co-AD at WWTPs is an effective strategy for waste management and biogas production. In addition to biogas production, this process generates digestate, which can be used as an organic amendment. The valorisation of bioplastics through co-AD with WWTP`s sludge presents a promising solution. Lera et al. 3 reported the complete biomethanisation of PHB at lab-scale anaerobic digesters. However, PLA, which is one of the most widely used bioplastics in the plastic sector, does not decompose effectively in anaerobic digesters. Previous studies showed that PLA requires specific temperature and pH conditions that are incompatible with conventional anaerobic digesters, limiting its degradation to less than 10% under these conditions 4 . Research into chemical, physical, and biological pretreatments has shown potential for enhancing 1 Maddela, N. R., Kakarla, D., Venkateswarlu, K., & Megharaj, M. (2023). Additives of plastics: Entry into the environment and potential risks to human and ecological health. Journal of Environmental Management, 348, 119364. https://doi.org/10.1016/J.JENVMAN.2023.119364 2 Rosenboom, J. G., Langer, R., & Traverso, G. (2022). Bioplastics for a circular economy. Nature Reviews Materials, 7 (2), 117–137. https://doi.org/10.1038/s41578-021-00407-8 3 Lera, M., Ferrer, J. F., Borrás, L., Serralta, J., & Martí, N. (2024). Bioplastic’s Valorisation by Anaerobic Co-Digestion with WWTP Mixed Sludge. Water, 16 (22), 3293. https://doi.org/10.3390/w16223293 4 Cazaudehore G, Guyoneaud R, Evon P, Martin-Closas L, Pelacho AM, Raynaud C, Monlau F. (2022). Can anaerobic digestion be a suitable end-oflife scenario for biodegradable plastics? A critical review of the current situation, hurdles, and challenges. Biotechnol Adv.56. https://doi.org/10.1016/j.biotechadv.2022.107916 the biodegradability of PLA 5 . These methods aim to alter the structure of PLA, making it more accessible to microorganisms and improve methane production. In previous experiments carried out by the research group (data not shown), chemical pretreatment showed much better efficiency that physical or enzymatic pretreatments, therefore, it was deemed necessary to conduct a more in-depth investigation of this pretreatment to assess the impact of NaOH concentration and temperature on PLA biomethanisation efficiency and identify the optimum conditions for the process. This study aims to assess the potential of thermochemical pretreatment of PLA as a strategy to enhance its anaerobic digestion in WWTPs. BMP tests were conducted to compare the biodegradation rates of pretreated PLA at different NaOH concentrations (0.3-1M) and temperatures (30-90ºC). After determining the most favourable conditions for maximizing the biomethanisation of PLA, the feasibility of codigestion of the pretreated PLA with mixed sludge from WWTP was tested in two laboratory-scale anaerobic digesters and the increase in methane production was quantified. MATERIALS AND METHODS Pretreatments A series of pretreatments were carried out varying the NaOH concentration from 1 to 0.3 M and the temperature from 90 ºC to 30 ºC, maintaining a constant duration of 48 hours in all tests. Filmshaped PLA was used due to it is the most common type of bioplastic waste found in waste management facilities. In all the experiments a PLA concentration of 40 g·L-1, chopped into 2.5 cm x 2.5 cm fragments, was used according to ISO 15985 standards to assess the anaerobic biodegradability of plastics. BMP Test BMP tests were conducted using the Automated Methane Potential Test System II (Bioprocess Control). Anaerobic digested sludge from an urban WWTP (Valencia, Spain) was used as inoculum. Following the manufacturer's guidelines, BMP tests were performed with an inoculumsubstrate ratio of 2:1. The theoretical chemical oxygen demand (COD) of PLA was calculated using the stoichiometric formula (C3H4O2) resulting in 1.33 gCOD·g-1PLA. The tests were conducted for 30 days under mesophilic conditions (35ºC). Throughout the experiment, the AMPTS II system recorded methane production. Lab-Scale Anaerobic Digestion Reactors To assess the feasibility of valorising PLA through its codigestion with mixed sludge (MS), two conventional anaerobic digesters were operated at laboratory scale, one (digester 1) was fed with a mixture of mixed sludge and thermochemically pretreated PLA, and the other one (digester 2) was fed with mixed sludge. The influent PLA concentration was 8 g·L⁻¹, and the concentration of suspended solids in the mixed sludge was set at 30 g·L⁻¹. The operating conditions of both digesters are detailed in the following table: Table 1. Anaerobic digesters conditions Digester 1 MS+PLA Digester 2 MS Sludge retention time (d) 20 25 Reactor volume (L) 8 8 Influent flow rate (L·d-1) 0.40 0.32 Organic loading rate (g COD·L-1·d-1) 1.4 ± 0.1 1.1 ± 0.01 5 Vasmara, C., Cazaudehore, G., Ceotto, E., Marchetti, R., Sambusiti, C., & Monlau, F. (2024). Alkali, thermal, or thermo-alkali pre-treatment to improve the anaerobic digestion of poly (lactic acid)? Water Research, 258. https://doi.org/10.1016/j.watres.2024.121744 RESULTS AND DISCUSSION Biodegradability of PLA after the evaluated pretreatments The percentage of the hydrolysis achieved with the pretreatments and the percentage of the biomethanisation observed in the BMP tests are shown in Table 2. Table 2. Pretreatments effects on PLA hydrolysis and biomethanisation NaOH concentration Temperature Hydrolysis pH Total biomethanisation PLA hydrolysed biomethanisation 1M 90ºC 100% 12.7 38% 38% 1M 70ºC 100% 12.6 45% 45% 1M 50ºC 100% 12.0 78% 78% 1M 30ºC 96% 13.0 74% 77% 0.6M 90ºC 100% 12.3 75% 75% 0.6M 70ºC 100% 12.1 75% 75% 0.6M 50ºC 100% 11.0 79% 79% 0.6M 30ºC 94% 11.9 74% 79% 0.3M 90ºC 100% 11.6 74% 74% 0.3M 70ºC 51% 10.5 40% 72% 0.3M 50ºC 57% 10.4 45% 78% 0.3M 30ºC 53% 9.9 45% 85% As shown in Table 2, complete PLA hydrolysis was obtained in all the experiments except for those carried out with 0.3M and at 30, 50 or 70ºC. A NaOH concentration of 0.6M or higher ensures nearcomplete PLA hydrolysis (40 g·L-1) at all evaluated temperatures. Regarding the biomethanisation, the highest values (between 74% and 79%) were obtained in all the experiments carried out with NaOH 0.6M and in those carried out with NaOH 1 M at 30ºC and 50ºC. At 1 M NaOH, although the PLA was complete hydrolysed into lactic acid, the biomethanisation drops to 38% at 90 °C and 45% at 70 °C, indicating methanogenesis inhibition. Although further research is needed to elucidate the reasons of this inhibition, a NaOH concentration of 0.6 M and a temperature of 30ºC were selected as the optimal conditions for PLA pretreatment in the anaerobic codigestion experiments. A detailed economic study would be necessary to determine whether the increase in hydrolysis rate achieved by raising the temperature (which could reduce the reaction time and, consequently, the reactor volume) offsets the cost of increasing the temperature beyond 30°C. Codigestion of pretreated PLA with mixed sludge in lab-scale digesters Table 3 shows the characterisation of the MS, mixture of MS and PLA, and the two digestates. As can be seen in table, a significant reduction in the SS concentrations is achieved in both digesters. DS represent 8% and 7% of the TS in the MS and the digestate from digester 2, respectively, whereas in digester 1 it accounts for 22%, as the majority of the hydrolysed PLA fed (MS+PLA) corresponded to this fraction (30%). The low VFA concentrations indicate that no acidification problems have been observed. The addition of hydrolysed PLA as a co‐substrate increased the pH (7.6 vs. 7.1) and the alkalinity (6759 mg·L-1 CaCO3 vs. 2900 mg·L-1 CaCO3) of digester 1. Table 3. Characterisation of the influent and the digestate from both digesters Influent Digestate Parameter MS+PLA MS Digester 1 Digester 2 COD-T (mg·L-1) 49060 ± 3171 48025 ± 3171 20302 ± 1320 23051 ± 932 COD-S (mg·L-1) 14716 ± 1277 5095 ± 1277 752 ± 157 564 ± 164 TS (mg·L-1) 34142 ± 2805 32677 ± 2805 25126 ± 1237 23935 ± 390 SS (mg·L-1) 24064 ± 2389 30080 ± 2389 19509 ± 311 22180 ± 296 DS (mg·L-1) 10177 ± 731 2646 ± 731 6252 ± 521 2104 ± 441 VFA (mg·L-1 CH3COOH) ND 1718 ± 50 102 ± 10 99 ± 47 ALK (mg·L-1 CaCO3) ND 871 ± 225 6759 ± 437 2900 ± 115 pH 9.6 ± 0.1 5.5 ± 0.1 7.6 ± 0.1 7.1 ± 0.1 CH4 production (L·d-1) 33.9 ± 2.4 20.7 ± 3.5 Methane yield (L·g-1 CODinf) 0.25 ± 0.02 0.19 ± 0.02 COD-T: Total Chemical Oxygen Demand; COD-S: Soluble Chemical Oxygen Demand; TS: Total Solids; SS: Suspended Solids; DS: Dissolved Solids; VFA: Volatile Fatty Acids; ALK: Alkalinity; ND: Not Determined. Figure 1 shows the COD balance over the 15 weeks of the digester operation. Compared to the digester fed with MS (digester_2), the addition of hydrolysed PLA as cosubstrate resulted in a 54% increase in methane production. PLA hydrolysed to lactic acid represents 24% of the total COD in the influent of digester_1 and is completely biomethanised, as it is not detected at the digester outlet. In both digesters, the percentage of MS biomethanised is 53%. Figure 1. COD balance: digester 1 (MS+PLA; left) and digester 2 (MS; right). CONCLUSIONS The conclusions regarding the influence of pretreatments on the biomethanisation of PLA are presented below: • At a concentration of 40 g PLA·L⁻¹, NaOH concentrations above 0.6M ensure an alkaline medium (pH > 10), resulting in hydrolysis efficiencies close to 100%. At a temperature of 30ºC, the biomethanisation rate of PLA hydrolysed increased to 74% under typical anaerobic digester conditions. • Adding hydrolysed PLA as a co-substrate increased methane production by 54%. • Hydrolysed PLA is completely biomethanised, as it is not detected at the digester_1 outlet.