Coupling Granular Activated Carbon with Waste Iron Scraps Enhances Anaerobic Digestion of PBAT Wastewater: Performance Improvement and Mechanistic Insights

Abstract

Poly(butylene adipate-co-terephthalate) (PBAT) wastewater, characterized by high chemical oxygen demand (COD) and acidity, poses significant challenges to anaerobic digestion (AD) due to toxicity and volatile fatty acids (VFAs) accumulation. This study coupled granular activated carbon (GAC) and waste iron scraps (WISs) to synergistically enhance AD performance. Batch experiments demonstrated that, compared with the control, the GAC/WISs group achieved a COD removal efficiency of 53.18% and a methane production of 207.53 ± 5.80 mL/g COD, which were 5.48- and 12.14-fold increases, respectively, while reducing the accumulation of total VFAs by 98.48% (to 15.09 mg/L). Mechanistic analysis revealed that GAC adsorbed inhibitors and enriched methanogens, while WISs buffered pH and promoted direct interspecies electron transfer (DIET) through hydrogenotrophic methanogenesis. Metagenomic sequencing showed shifts in microbial communities, with enrichment of syntrophic bacteria (Syntrophobacter) and functional genes (pta, bcd, and pccA), indicating metabolic reprogramming. This study provided a theoretical foundation and engineering strategy for the anaerobic treatment of PBAT wastewater.

1. Introduction

Poly(butylene adipate-co-terephthalate) (PBAT) is a prominent biodegradable polymer with widespread applications in packaging and other sectors. The global demand for PBAT is growing at a rapid annual rate of 15.3%, driven by the excellent material properties and environmental benefits [1,2]. However, the production of PBAT poses a significant environmental challenge. The process generates highly concentrated and acidic organic wastewater, characterized by a chemical oxygen demand (COD) of 10–20 g/L and a pH ranging from 3.5 to 5.5 [3,4]. This effluent contains high concentrations of unreacted monomers, oligomers, and other biotoxic substances, which present a substantial challenge to conventional biological treatment systems [5].

Anaerobic digestion (AD), which converts organic matter into methane, is a sustainable technology for treating such high-concentration organic wastewater [6]. However, the anaerobic treatment for PBAT wastewater is limited by two primary factors. First, toxic components in the wastewater directly inhibit methanogens, potentially by disrupting the cell membranes [7,8]. Second, the highly acidic condition of the wastewater (pH < 6.3) severely impairs the initial stages of digestion, namely hydrolysis and acidogenesis. This leads to the accumulation of volatile fatty acids (VFAs) and reduces the VFA-to-methane conversion efficiency by 40–60% [9,10]. While conventional chemical neutralization can mitigate pH inhibition, this approach incurs substantial operational costs and poses a risk of secondary inhibition from alkali overdosing, which compromises microbial activity [11,12]. Therefore, the development of more economical, efficient, and ecologically sound strategies to enhance the AD process is imperative.

The addition of functional materials to anaerobic reactors has been reported as a promising strategy for regulating microbial communities and improving treatment efficiency [13,14]. Granular activated carbon (GAC), owing to its high surface area and porosity, can physically adsorb organic inhibitors and thus mitigate toxicity [11,15]. In addition, GAC can act as both a microbial carrier and an electron mediator, enriching key methanogenic archaea—such as Methanosarcina, Methanobacterium, and Methanothrix—and potentially accelerating direct interspecies electron transfer (DIET) [16,17,18,19]. Similarly, waste iron scraps (WISs), which are inexpensive and widely available, also exhibit unique performance in enhancing effects. Under anaerobic conditions, WISs can consume protons through corrosion, thereby regulating pH and buffering acidic shocks to maintain stable reactor performance [4,20]. Moreover, WISs can stimulate overall microbial metabolic activity, enhance electron transfer rates, and enrich functional microbial populations, including acidogenic bacteria (Corynebacterium), syntrophic bacteria (Syntrophomonas), and electroactive microorganisms (Synergistaceae). By providing Fe²⁺ or H₂, WISs facilitate hydrogenotrophic methanogenesis and may even promote DIET, for example, between Syntrophomonas and Methanothrix, thereby collectively enhancing the degradation of recalcitrant organic matter [4,21].

GAC offers advantages in adsorption, bio-enrichment, and promoting DIET. WISs provide complementary roles in pH buffering, metabolic stimulation, enhancing specific methanogenic pathways, and mediating DIET. It is hypothesized that coupling GAC and WISs could generate synergistic effects, which would address the dual challenges of toxicity and acidity inhibition in PBAT wastewater treatment more comprehensively and efficiently. Therefore, this study aimed to systematically evaluate the effects of GAC and WISs on the anaerobic digestion of PBAT wastewater. The specific objectives were: (1) To check the synergistic effects of GAC and WISs on enhancing methane production and COD removal efficiency. (2) To investigate the potential effects of GAC and WISs on mitigating the accumulation of VFAs and promoting the degradation of recalcitrant organic matter. (3) To investigate the effects of GAC and WISs on the methanogenic activity and electron transfer activity. (4) To analyze the shifts in microbial community structure and functional gene expression that underpin the enhanced performance of the AD process.

2. Materials and Methods

2.1. Inoculum Sludge and PBAT Wastewater

The inoculum sludge was obtained from the activated sludge tank at the Zhuzhuanjing Wastewater Treatment Plant (Hefei, China). After collection, it was purged with nitrogen gas (N₂) and acclimated under anaerobic conditions for one week. The mixed liquor suspended solids (MLSSs) concentration of the inoculum was 21.70 ± 0.77 g/L, while the mixed liquor volatile suspended solids (MLVSSs) concentration was 10.78 ± 0.33 g/L. The acetoclastic methanogenic activity was 53.30 ± 5.31 mL CH₄/g MLVSS/d, and the hydrogenotrophic methanogenic activity was 134.12 ± 3.97 mL CH₄/g MLVSS/d. The PBAT wastewater was prepared artificially, according to previous studies [4], with the detailed composition and water quality parameters provided in

Table 1. Water quality parameters for PBAT simulated wastewater.

Parameters	pH	COD	BOD5	NH3-N	TP
Concentration (mg/L)	3.86 ± 0.13	9456.10 ± 219.96	3850.00 ± 50.00	135.11 ± 5.22	1.25 ± 0.15

and

Table 2. Main components in artificial water distribution.

No.	Chemical Name	Molecular Formula	Concentration (mg/L)
1	Cyclopentanone	C5H8O	2491
2	1,4-Butanediol	C4H10O2	1605
3	n-Butanol	C4H10O	996
4	Tetrahydrofuran	C4H8O	249
5	Acetic Acid	C2H4O2	125
6	Propionic Acid	C3H6O2	249
7	Butyric Acid	C4H8O2	125
8	Ammonium Chloride	NH4Cl	135
9	Sodium Dihydrogen Phosphate	NaH2PO4	1.25

.

2.2. Batch Experiment Design

Batch experiments were conducted to assess the impact of GAC and WISs on the anaerobic treatment of PBAT wastewater. The assays were conducted in 250 mL serum bottles with a working volume of 150 mL. PBAT wastewater and anaerobic sludge were added to the bottles at an MLVSS concentration of 4.49 ± 0.12 g MLVSS/L and an MLVSS–COD ratio of 1:2. After mixing, the pH was 5.47 ± 0.15. All bottles were purged with N₂ to remove dissolved oxygen, sealed, and incubated in an air-bath shaker at 35 ± 1 °C and 140 rpm under dark conditions. GAC (0.5–1.0 mm) and WISs (0.3–0.7 cm) were used, which were purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China) and Gongyi Longxin Water Purification Materials Co., Ltd. (Gongyi, China), respectively. Four experimental groups were established: Control, GAC, WISs, and GAC/WISs. Each group contained three replicates, and the experiment was conducted over four successive cycles, each lasting 14 days. The detailed experimental design is presented in

Table 3. Experimental design for each batch.

Group	GAC (g/L)	WISs (g/L)	Wastewater Volume (mL)	Sludge Volume (mL)
Control	0	0	100	50
GAC	20	0	100	50
WISs	0	20	100	50
GAC/WISs	10	10	100	50

. The control group, without any functional materials, served to reflect the intrinsic anaerobic digestion performance of PBAT wastewater and provided a baseline for evaluating the enhancement effects of the additive groups. The GAC group (with 20 g/L GAC) and WISs group (with 20 g/L WISs) were designed to investigate the individual effects of each material, while the GAC/WISs group (with 10 g/L GAC + 10 g/L WISs) aimed to validate the coupling effect under equivalent total dosage (20 g/L), thereby effectively eliminating interference from differences in material dosage. This design allowed systematic examination of whether GAC and WISs could produce a synergistic effect.

2.3. Sampling and Analytical Methods

The biogas compositions in each serum bottle were measured every two days during each cycle. Biogas production was measured volumetrically, using calibrated gas-tight glass syringes (100 mL, Returnable, Shanghai, China). Methane production was quantified using a gas chromatograph (SP-6890, Shandong Ruihong Co., Ltd., Tengzhou, China), and the COD concentration of the supernatant was determined by standard methods [22]. At the end of each of the four batch cycles, supernatant samples were collected to determine organic matter and VFAs concentrations using a gas chromatograph (7890A, Agilent Technologies, Santa Clara, CA, USA). In addition, 50 mL of anaerobic sludge was withdrawn from each bottle; one portion was used to assess acetoclastic methanogenic activity, and the other to determine hydrogenotrophic methanogenic activity. Sodium acetate was used as the sole carbon source in the determination of acetoclastic methanogenic activity, while H₂/CO₂ (80%/20%) was used as the sole carbon source in the determination of hydrotropic methanogenic activity, following procedures detailed in a previous study [23]. Sludge samples from each group were also analyzed for electron transport system (ETS) activity and extracellular polymeric substances (EPSs). ETS activity was determined according to Wang et al. (2016) [24], while protein and polysaccharide contents in EPSs were measured following Wu et al. (2021) [25]. Mixed liquor suspended solids (MLSSs) and mixed liquor volatile suspended solids (MLVSSs) were determined using standard methods [22].

Metagenomic sequencing of sludge samples was performed by Shanghai Majorbio Bio-Pharm Technology Co., Ltd. (Shanghai, China), and the detailed protocols are provided in Supplementary Material S1. The pretreated sludge samples were selected for metagenomic sequencing to analyze the microbial community structure and functional potential. The specific procedures were as follows: (1) DNA Extraction and Quality Control: Total DNA was extracted from the samples using the FastDNA^® Spin Kit for Soil (MP Biomedicals, Santa Ana, CA, USA). The integrity and concentration of the DNA were assessed by using agarose gel electrophoresis and a Qubit fluorometer (Thermo Fisher Scientific, Waltham, MA, USA). (2) Library Construction: Qualified DNA was randomly fragmented into approximately 350 bp fragments using a Covaris M220 focused-ultrasonicator (Covaris, Woburn, MA, USA). Subsequently, a sequencing library was constructed, following the instructions of the manufacturer, with the NEXTFLEX Rapid DNA-Seq Kit (Bioo Scientific, Austin, TX, USA). (3) Sequencing: High-throughput sequencing was performed on the Illumina NovaSeq 6000 platform (Illumina, San Diego, CA, USA) using a PE150 strategy to generate raw data. (4) Bioinformatics Analysis: High-quality sequences were taxonomically annotated using Kraken2 and its standard database to resolve the microbial community composition of the samples. Meanwhile, functional annotation of the assembled genes was conducted, with pathway annotation performed using the KEGG database and functional classification carried out with the COG database. To evaluate whether functional genes were enriched in each treatment group compared with the Control, the odds ratio (OR) was calculated using Equation (1), as described by Liang et al. (2019) [26].

$$\mathrm{OR}={\displaystyle \frac{{\mathrm{\phi }}_{\mathrm{R}}/\left(1-{\mathrm{\phi }}_{\mathrm{R}}\right)}{{\mathrm{\phi }}_0/\left(1-{\mathrm{\phi }}_0\right)}}$$

(1)

${\mathrm{\phi }}_{\mathrm{R}}$ is the absolute abundance of a certain gene in the sludge from each experimental group, and ${\mathrm{\phi }}_0$ is the absolute abundance of a certain gene in the sludge from the control group.

3. Results

3.1. COD Removal Efficiency and Methane Production Performance

The temporal variation in COD removal efficiency across the experimental groups is presented in Figure 1a. At the end of the fourth stage, the control group achieved a COD removal efficiency of 9.69%, while the GAC, WISs, and GAC/WISs groups reached 39.07%, 45.09%, and 53.18%, respectively. The GAC/WISs treatment showed 5.48-fold higher efficiency compared to the control group. The GAC group exhibited the highest early-stage COD removal (79.00%), followed by a subsequent decline, whereas the GAC/WISs group maintained consistently high performance throughout the study period.

Methane production (Figure 1b) showed that the control group maintained a low methane yield of 17.09 ± 2.64 mL/g COD over four cycles. In contrast, the GAC, WISs, and GAC/WISs groups produced 116.85 ± 4.49, 174.22 ± 5.43, and 207.53 ± 5.80 mL/g COD, respectively, representing 6.83-, 10.19-, and 12.14-fold increases over the Control.

3.2. Removal of Typical Refractory Organic Compounds

PBAT wastewater contains typical refractory organics, such as cyclopentanone, tetrahydrofuran, 1,4-butanediol, and n-butanol. These compounds serve as primary COD sources and exhibit biotoxicity, which inhibits the efficiency of anaerobic digestion. Analysis of refractory organic removal (Figure 2) revealed that all treatment groups achieved high removal efficiencies (91%) for 1,4-butanediol and n-butanol. For cyclopentanone and tetrahydrofuran, the GAC group achieved removal efficiencies of 59.08% and 57.17%, respectively, while the WISs group showed lower removal efficiencies of 20.47% and 11.72%. The GAC/WISs group yielded intermediate removal efficiencies of 41.65% and 37.80% for these compounds.

3.3. VFAs Accumulation

Volatile fatty acids (VFAs) are key intermediates in anaerobic digestion, and excessive accumulation often leads to a decline in pH, methanogenic inhibition, and process instability [10,27]. VFAs analysis (Figure 3) showed that the control group exhibited severe accumulation, with total VFAs reaching 1004.58 mg/L, dominated by butyric acid (858.40 mg/L) and propionic acid (136.03 mg/L). The GAC group reduced total VFAs to 707.82 mg/L (29.54% reduction), with butyric and propionic acids decreased by 30.80% and 24.39%, respectively. The WISs group achieved total VFAs of 212.8 mg/L (78.77% reduction), with butyric acid reduced to 19.17 mg/L (97.82% removal). The GAC/WISs combination showed the lowest VFAs accumulation at 15.09 mg/L (98.48% reduction), with butyric acid at 2.03 mg/L and propionic acid at 13.06 mg/L.

3.4. Methanogenic Activity and Electron Transfer Activity

Methanogenic activity measurements (Figure 4a) revealed that the control group exhibited low acetoclastic methanogenic (AM) activity of 3.14 ± 0.44 mL CH₄/g MLVSS/d and hydrogenotrophic methanogenic (HM) activity of 5.08 ± 0.55 mL CH₄/g MLVSS/d. Compared to the control group, the GAC group showed 6.96- and 4.72-fold increases in AM and HM activities, respectively. The WISs group displayed elevated HM activity (31.30 ± 4.74 mL CH₄/g MLVSS/d). The GAC/WISs group achieved the highest AM activity (28.72 ± 3.91 mL CH₄/g MLVSS/d) and high HM activity (30.87 ± 2.83 mL CH₄/g MLVSS/d).

INT-ETS activity reflects microbial respiratory efficiency in anaerobic systems [28]. INT-ETS activity analysis (Figure 4b) showed that the GAC/WISs group displayed the highest value of 36.05 ± 2.87 μg/ (mg SS·d), which was 3.16-, 1.26-, and 1.61-fold higher than the Control, GAC, and WISs groups, respectively. Extracellular polymeric substances (EPSs) maintain sludge structure, protect microbes, and facilitate interspecies interactions [28]. EPS analysis (Figure 4c,d) revealed that the control group had the highest polysaccharide content, while GAC/WISs showed the lowest. For proteins, GAC/WISs exhibited the highest total EPS content—1.38-, 1.09-, and 1.06-fold greater than the Control, GAC, and WISs groups, respectively, with notable elevation in the S-EPS layer.

3.5. Microbial Community Structure

(1)

Archaeal community

Genus-level analysis revealed that methanogenic pathways varied across different groups (Figure 5a). In the control group, Methanothrix (32.63%) and Methanoregula (20.05%) were the dominant methanogens. Among them, Methanothrix had been reported as an acetoclastic methanogen [29], while Methanoregula was known as a hydrogenotrophic methanogen [30]. The relative abundance indicated that the acetoclastic methanogenic pathway was dominant, with acetoclastic methanogens accounting for 46.57% and hydrogenotrophic methanogens for 34.88%. In the GAC group, Methanoregula (25.00%) and Methanothrix (19.52%) remained the dominant methanogen genera. The proportion of acetoclastic methanogens was 46.06%, while that of hydrogenotrophic methanogens increased to 43.53%. In contrast, in the WISs group, Methanobacterium (reported as hydrogenotrophic methanogens, 36.33%) and unclassified_o__Methanobacteriales (28.54%) became the dominant methanogen genera [31]. The relative abundance of hydrogenotrophic methanogens increased to 74.60%, whereas the proportion of acetoclastic methanogens decreased to 17.33%. In the GAC/WISs group, Methanothrix (24.73%) and Methanoregula (20.77%) were the dominant methanogen genera. The proportion of acetoclastic methanogens was 46.06%, and that of hydrogenotrophic methanogens was 41.78%.

(2)

Bacterial community

The bacterial community at the genus level is shown in Figure 5b. In the control group, bacterial diversity was low, with Rummeliibacillus overwhelmingly dominant (82.48%). In the GAC group, Rhodococcus became the sole dominant genus (85.54%). The WISs group exhibited the highest bacterial diversity, characterized by the co-dominance of Lysinibacillus, Romboutsia, and Petrimonas. In the GAC/WISs group, Rhodococcus remained abundant at 65.58%, while Lysinibacillus remained abundant at 11.37%, which was reported to mediate electron transfer via membrane proteins [32]. Additionally, Sedimentibacter (2.10%) was detected, a known syntrophic acetate oxidizer that produces H₂ and CO₂ for hydrogenotrophic methanogens [33].

3.6. Metagenomic Analysis of Functional Genes and Pathways

(1)

Syntrophic bacteria and DIET-related genera

As shown in Figure 6a, profiling of the key functional genera revealed that syntrophic bacteria—namely Syntrophobacter (3.08- and 1.31-fold) and Syntrophomonas (1.78- and 1.18-fold)—were enriched in the WISs and GAC/WISs groups, respectively. The electroactive bacterium Geobacter showed pronounced enrichment in the GAC and GAC/WISs groups (3.81- and 18.68-fold). Pelotomaculum, another syntrophic oxidizer, remained at 1.45-fold relative to the Control, underscoring the pivotal role of these interlinked syntrophic metabolic pathways.

(2)

Metabolic enzyme profiles

As shown in Figure 6b, in the GAC/WISs group, genes encoding for the classical coenzyme A pathway (e.g., buk, acd, crt) were suppressed, while genes for the syntrophic oxidation pathway, such as butyrate metabolism (bcd, etfA) and propionate metabolism (sdhA, pccA), were upregulated [34,35]. The upregulation of genes for acetate conversion (acs) and the rate-limiting enzymes for acetoclastic methanogenesis (pta, ackA) directly explains the superior acetoclastic activity in the GAC/WISs group [36,37]. Additionally, the terminal step of methanogenesis was accelerated, as evidenced by the increased abundance of the methyl–coenzyme M reductase gene (mcrA).

4. Discussion

The superior performance of the GAC/WISs combination, evidenced by a 5.48-fold increase in COD removal and a 12.14-fold increase in methane production compared to the control group, highlighted a clear synergistic effect that addressed multiple limiting factors in PBAT wastewater anaerobic digestion. Interestingly, the GAC group showed the highest COD removal (79.0%) at the beginning (end of stage 1), but this was followed by a significant decline, likely due to the gradual saturation of the adsorption sites after initial efficient uptake. Despite the initial limitation of GAC when used alone, the enhanced and sustained performance of the combined system originated from the strategic coupling of GAC and WISs. Here, GAC provided a high adsorption capacity and a conductive surface for biofilm formation [11,15], while WISs contributed electron-donating properties and hydrogen generation through corrosion [4,20]. Together, this coupling of GAC and WISs reinforced refractory organic degradation, mitigated volatile fatty acids (VFAs) accumulation, and promoted direct interspecies electron transfer (DIET), ultimately driving shifts in methanogenic pathways and boosting overall process efficiency [16,17,18,21].

The reinforcement of refractory organic degradation was demonstrated by the selective removal patterns observed, with GAC playing a dominant role in adsorbing and degrading stable compounds, such as cyclopentanone (59.08% removal) and tetrahydrofuran (57.17%), likely through adsorption and biofilm formation [38]. The coupling with WISs enhanced the process by introducing reductive mechanisms, such as Fe²⁺ release [39], which complemented the GAC’s action and maintained robust removal efficiencies even with a halved GAC dosage. This synergy not only accelerated the breakdown of hard-to-degrade organics but also contributed significantly to COD reduction. Furthermore, the high removal efficiencies (91%) for more biodegradable compounds like 1,4-butanediol and n-butanol across all groups underscored the complementary roles of GAC adsorption and WIS-associated reduction. By integrating physical adsorption with biochemical enhancement, the coupled GAC/WISs system effectively eliminated the biotoxicity of refractory organics, improved the anaerobic digestion environment, and consequently optimized substrate utilization efficiency and methane production.

Simultaneously, the dramatic reduction in VFAs accumulation (98.48% in GAC/WISs) represented a critical breakthrough in overcoming acidification challenges in anaerobic digestion of acidic wastewater [10,27]. The coupling of GAC and WISs amplified this effect, as the corrosion-driven H₂ production of WISs alleviated VFAs inhibition, while conductive surfaces of GAC facilitated microbial attachment and electron transfer, targeting specific VFAs like butyric acid (97.82% removal). This integrated approach not only prevented process instability but was also directly linked to improved COD removal and methane production by fostering a balanced microbial environment.

These performance gains were fundamentally driven by transformations in methanogenic pathways and DIET facilitation, both of which were enhanced by the GAC/WISs coupling. The combination achieved peak acetoclastic methanogenesis (AM) activity (6.96-fold increase over the Control) and maintained high hydrogenotrophic methanogenesis (HM) activity (31.30 mL CH₄/g MLVSS/d), indicating a shift to a balanced AM-HM system [40,41,42]. The conductive nature of GAC, coupled with the electron-donating capacity of WISs, created an optimized electron transfer network that promoted DIET, as evidenced by the highest INT-ETS activity in the GAC/WISs group (3.16-fold over the Control) [43]. The EPS analysis highlighted positive regulation of microbial microenvironments in the GAC/WISs group. The control group had the highest polysaccharide content, while GAC/WISs showed the lowest, suggesting reduced environmental stress and cell lysis under combined treatment. For proteins, GAC/WISs exhibited the highest total EPS content. Protein levels increased across the S-EPS, L-EPS, and T-EPS layers, with a notable elevation in the S-EPS layer. The high protein content in the outer layer formed a barrier against PBAT wastewater toxins, preserving internal microbial activity, and supported DIET due to protein conductivity. Microbial community shifts further illustrated this: enrichment of Rhodococcus for refractory organic degradation [44,45] and Lysinibacillus for electron mediation [32], alongside archaeal (Methanothrix and Methanosarcina) dominance that supported DIET [29,46].

At the metabolic level, metagenomic analysis revealed a profound microbial metabolic reprogramming enabled by the GAC coupling with WISs, characterized by a distinct shift from classical coenzyme A pathways to DIET-mediated syntrophic metabolism. This transition was evidenced by the significant upregulation of syntrophic oxidation genes (including butyrate metabolism genes bcd and etfA, and propionate metabolism genes sdhA and pccA) coupled with marked suppression of conventional VFAs degradation pathways (such as buk, acd, and crt) [34,35,47]. The substantial enrichment of electroactive Geobacter alongside key syntrophic bacteria (Syntrophobacter and Syntrophomonas) confirmed robust DIET establishment, with the conductive materials facilitating direct interspecies electron exchange [3,48,49,50]. The presence of Pelotomaculum, another syntrophic oxidizer, further reinforced these interconnected metabolic pathways. Concurrently, the markedly increased expression of acetate conversion genes (acs) and critical methanogenesis genes (pta, ackA, mcrA) synergistically accelerated VFAs degradation and enhanced methane production efficiency [36,37]. The operation of the syntrophic oxidation pathway depended on the efficiency of interspecies electron transfer, and DIET provided a guarantee for electron transfer between syntrophic bacteria. Ultimately, the GAC coupling with WISs not only reinforced refractory organic degradation and VFAs mitigation but also underpinned the observed COD and methane yield enhancements by fostering an integrated electron transfer and metabolic network, demonstrating the holistic benefits in PBAT wastewater treatment.

5. Conclusions

This study verified the synergistic effects of GAC and WISs in enhancing the anaerobic digestion of PBAT wastewater. Compared to individual additions, the coupling of GAC and WISs significantly improved methane production and COD removal efficiency. The underlying mechanisms were that GAC effectively reduced the inhibitory toxicity of refractory organics through adsorption and microbial enrichment, while WISs buffered the system pH by the release of Fe²⁺ and H₂. Coupling GAC and WISs effectively mitigated the accumulation of VFAs and promoted the transformation of complex organic matter. Furthermore, the favorable microenvironment synergistically enhanced the activity of both hydrogenotrophic and acetoclastic methanogenic pathways and significantly improved the DIET efficiency among microbial communities. Finally, analysis of metagenomic sequencing indicated that GAC enriched electro-active fermentative bacteria and methanogens, whereas WISs stimulated the growth of hydrogenotrophic methanogens. Coupling GAC and WISs synergistically drove the functional succession of the microbial community and the upregulation of key metabolic genes. This study provided a theoretical foundation and engineering strategy for the treatment of PBAT wastewater.