Alkaline-Thermal Hydrolysis Prior to Anaerobic Digestion Affects Fate of Microplastics

1. INTRODUCTION Considering continuous generation of wastewater sludge in vast amounts and beneficial constituents, many countries have been using sludge on land. Nevertheless, land application of insufficiently treated sludge raises concerns due to transferring pathogens and trace pollutants, including toxic organics and microplastics (MPs) into the environment. MPs constitute a large portion of pollutants in sludge because of inseparable and versatile use of plastics in our daily life. Due to their small sizes (<5 mm), MPs can easily transport across environmental compartments and reach WWTPs in great amounts. WWTPs with modern technologies can remove MPs from wastewater with >90% efficiency but concentrate them in sludge. Anaerobic digestion of sludge is a widely used management option that stabilizes the sludge and reduces the amount to be disposed of. Various pretreatment methods have been developed to improve the efficiency of digestion such as thermal hydrolysis process (THP), chemical, and mechanical techniques. Recent studies have revealed that MPs can affect the efficiency of anaerobic digestion process.1 Furthermore, corresponding deteriorative effects of stabilization on MPs have been superficially addressed.2 Despite their stiff nature, exposure to abiotic factors such as heat, light, water, chemicals, and mechanical impacts may initiate structural changes and induce biodeterioration. For example, polyethylene terephthalate (PET), can hydrolyze to a certain degree in the presence of alkaline solutions due to saponification process in ester linkages. In addition, they can hydrolyze in wet conditions at high temperatures as in the case of THP.3 For the first time in literature, this study investigates whether alkaline pretreatment, THP and combined alkaline and thermal hydrolysis pretreatment (ATHP) processes applied for sludge disintegration and biogas enhancement can concurrently deteriorate PET MPs and lead them biodegrade during anaerobic digestion. Moreover, the study investigates the effects of different concentrations of MPs on digestion efficiency. 2.MATERIALS AND METHODS WAS and anaerobically digested sludge (ADS) were taken from a conventional WWTP serving a population of 4 million with a capacity of 765,000 m3/day. The model PET MPs were obtained by cutting water bottles into square shapes using scissors. Then, the particles were passed through a series of sieves to obtain MPs in sizes of 250-500 µm. WAS solubilization efficiency of three different disintegration techniques (i.e., alkaline, THP, and ATHP) were tested in varying conditions by monitoring soluble COD (sCOD) of WAS, which was spiked with 3 mg PET/g TS. Alkaline pretreatment was carried out for 5 days by dosing WAS with 0.5 M alkali using KOH and NaOH at 0.10 and 0.40 M, respectively. THP was performed at 127 °C and 1.7 bar for 30, 60 and 120 min using an autoclave. In ATHP, WAS samples (three sets) which had been alkaline pretreated (0.5 M) for 2, 4 and 5 days were autoclaved at 127 °C for 120 min. PET MPs collected from pretreated WAS were analyzed using SEM and FTIR to observe the changes they experienced during pretreatment. Biochemical methane potential (BMP) reactors of pretreated and unpretreated sludges spiked with 0, 1, 3, 6 mg PET MPs/g TS were set up in triplicates at a food-to-microorganism ratio of 1, and a total solids content of 2%. Reactors were coded as R0, R1, R3 and R6, with the letter P added for disintegrated reactors. All reactors were operated for 60 days under mesophilic conditions. Reactors were monitored for gas production throughout their operation. At reactor termination, digester performance and plastic characteristics were analyzed. 3.RESULTS AND DISCUSSION As shown in Table 1, disintegration degree (DDCOD) reached up to 81% by alkaline pretreatment in 5 days. Considering the most evident increase (71%), two days alkali pretreatment was selected. As the duration of separate THP was prolonged, solubilization increased only slightly and reached to a DDCOD of 19% at a maximum, comparable to literature.4 Although there was no notable difference beyond 30 min, 120 min was selected as the main purpose was exposing MPs to stress conditions before anaerobic digestion. Figure 1 shows that solubilization achieved in two days of alkaline pretreatment was improved with the addition of THP. However, the contribution of THP became relatively small beyond two days of alkaline application. Two days of alkaline application and a subsequent 120 min THP was selected for ATHP, which altogether resulted in DDCOD of 79%. MPs extracted from pretreated sludges were analyzed for changes in physical and chemical structure by evaluating their FTIR spectra and SEM micro-photographs comparatively with control samples. PET exposed to different pretreatment processes experienced varying levels of changes in different regions of the polymer structure, which differentiates the impacts of thermal and chemical stress factors. Both alkaline pretreatment and THP caused an overall decrease in the absorption intensity of PET. Crystallinity of the polymer increased the most with THP and followed by ATHP. Alkali treatment did not cause a significant change. Therefore, heat application might be the mechanism of increasing the crystallinity of ATHP pretreated PET. This is possibly due to the formation of new crystalline zones in polymer's structure from the breakdown of branches in backbone and bending on themselves. Partial melting and the recrystallization of the amorphous region of the polymer can be the reason of increase in crystallinity. It is evident from Figure 3(a) that MPs exposed to alkaline environment experience a strong surface peeling in comparison to untreated samples. Furthermore, THP seems to cause the corners of the square-shaped MPs to expand outward compared to those of untreated MPs. When two techniques were combined, the notable peeling effect caused by alkaline pretreatment diminished to a great extent, possibly due to the heat applied afterwards. The expanded MPs lost the previous changes occurred on their surfaces and reformed, but the cross-sectional layers appeared. As shown in Figure 2, ATHP pretreated reactors produced statistically higher amounts (22.0%) of methane than unpretreated ones regardless of the MPs dose. This can be attributed to that pretreatment process overwhelmed the impact of MPs on digesters. In contrast, the dose of MPs significantly affected the methane yield of unpretreated reactors. Among unpretreated reactor sets, R3 yielded the highest methane production (p<0.05), which was followed by R6, R1 and R0, in decreasing order. Both crystallinity and carbonyl index of MPs showed smaller changes by anaerobic digestion compared to more dramatic effect of sole ATHP. Figure 3(b) shows that there was an observable predominant impact of digestion, giving the MPs a squashed surface appearance and decreased surface thickness at some points. These images supported the current claim that MPs pretreated prior to anaerobic digestion becomes more prone to microbial attack. Acknowledgement: This study was funded by TUBITAK through the project grant 121Y156.