## 3 Results and Discussion  
### Analysis of Biodegradation During Composting  
Two composite samples incorporating alum sludge fractions of 63 \(\upmu\)m and 250 \(\upmu\)m were compared with the neat PBAT for their biodegradation performance. The tensile strength, % elongation at break, Young's modulus, and weight loss of the composite sam ples at different compositing stages (0 day, 15 days, 30 days, and 60 days) are presented in Figure 2. Over the 60-day compositing period, a progressive decline in tensile strength and % elongation at break was observed for all samples (Figure 2a,b). The composite with the 250 um particle fraction exhibited the greatest reduction at the end of 60 days, with decreases of 55.16% and 63.59% in tensile strength and % elongation at break, respectively, compared to the undegraded samples. In contrast, the composite with the 63 um particle fraction experienced a sharp reduction in tensile strength and % elongation at break by 15 days (\(-32.66\%\) and \(-33.66\%\)) but demonstrated greater resistance to degradation thereafter. Values recorded at 30 and 45 days showed pronounced variability, as indicated by the larger error bars in the graph. Regarding [PERSON]'s modulus (Figure 2c), a slight increasing trend in stiffness was observed for all samples up to 45 days, after which the rate of change became less significant. This trend can be attributed to the preferential degradation of the amorphous regions in the polymer during compositing, which increases crystallinity and, consequently, stiffness. A similar observation was reported by [PERSON] et al., where the crystallinity of the PBAT polymer was increased after enzymatic degradation [27]. Once the amorphous regions are largely degraded, the rate of degradation slows, requiring a longer period for further breakdown. This explains the plateau observed in the Young's modulus vs. compositing days curve. Figure 2d indicates the weight loss of all samples throughout the compositing period. The neat PBAT sample exhibited the highest weight loss of 1.6% at 60 days, while the 63 um sample showed the least weight loss, indicating its higher resistance to degradation. This observation aligns with the result in the tensile strength analysis and accelerated UV degradation tests (data are provided in the Supplementary document). The 63 um sample exhibited a sharp reduction in weight within the first 15 days, followed by a slower rate of degradation. This trend suggests a two-stage degradation mechanism: an initial rapid degradation phase followed by a slower degradation phase. The initial phase (the rapid  
Figure 2: Composting analysis of the alum sludge/PBAT composites and neat PBAT, including (**a**) tensile strength, (**b**) % elongation at break, (**c**) Young’s modulus, and (**d**) weight loss over a 60-day compositing period. The insert in (**d**) shows sample preparation for compositing using vegetable composits. degradation stage) was driven by microorganisms breaking down the more accessible regions of the sample, such as the surface or the amorphous regions of the polymer. In contrast, the subsequent phase (the slow degradation stage) was limited by reduced microbial penetration and diffusion into the less accessible regions, such as the inner parts of the sample or the crystalline regions of the polymer. The improved resistance to degradation observed in the 63 \(\upmu\)m sample, as reflected in its better-maintained mechanical properties and reduced weight loss, could be attributed to the higher surface area of the smaller particles, which facilitates stronger interfacial bonding between the album sludge and the PBAT matrix. This finding suggests that such waste particles can be used to prepare composites for applications requiring extended composability, allowing for the fine-tuning of degradation rates to match specific timeframes. Figure 3a illustrates the stress-strain curves for the samples after 60 days of compositing.