The increasing global demand for freshwater necessitates exploring water conservation and wastewater reuse strategies. Graywater, comprising 50–80% of domestic sewage, presents a significant opportunity for reuse. However, its high concentration of pollutants, including nitrogen, pathogens, micropollutants, and linear alkylbenzene sulfonates (LAS), requires efficient and cost-effective treatment. On-site graywater treatment and reuse offer advantages in reducing transportation and treatment costs. While various treatment methods exist (physical, chemical, biological, and combined), limitations in efficiency, effluent quality, and high energy input hinder widespread adoption. Traditional aerobic biological treatments, although efficient in removing organics and nitrogen, suffer from drawbacks such as foaming caused by high LAS concentrations and low oxygen transfer rates, which increase treatment costs. Oxygen-based membrane biofilm reactors (O₂-MBfR) offer high oxygen transfer rates and efficient pollutant removal but are expensive to operate and maintain. Granular activated carbon (GAC), with its high porosity, large surface area, and adsorption properties, offers a potential solution. GAC-based biofilters combine adsorption, filtration, catalysis, biosorption, and biodegradation for efficient pollutant removal. This study aimed to develop a bio-enhanced granular-activated carbon dynamic biofilm reactor (BhGAC-DBfR) for graywater treatment, leveraging the advantages of GAC and biofilm formation to achieve efficient and low-energy treatment. The research investigated the biofilm characteristics, reactor performance, LAS adsorption and biodegradation, microbial community composition, and functional gene dynamics.

Existing literature highlights the challenges in efficient and cost-effective graywater treatment. Traditional aerobic biological treatment methods, while effective in removing organics and nitrogen, face challenges related to foaming from high LAS concentrations and low oxygen transfer rates, leading to high energy consumption. Oxygen-based membrane biofilm reactors (O2-MBFRs) have been explored to address these issues, offering high oxygen transfer rates, but their high operational and maintenance costs limit widespread applicability. Granular activated carbon (GAC) has been shown to be an effective filter media in wastewater treatment due to its high porosity, large specific surface area, and excellent adsorption capabilities. GAC biofilters can achieve efficient pollutant removal through various mechanisms including filtration, adsorption, catalysis, biosorption, and biodegradation. However, limited research exists on the formation, properties, and microbial ecology of GAC-biofilms in graywater treatment, particularly concerning the functional genes and enzymes involved in pollutant removal.

This study employed a laboratory-scale cylindrical BhGAC-DBfR with a controlled saturated/unsaturated ratio to investigate graywater treatment. Synthetic graywater, prepared according to NSF/ANSI Standard 350, was used. Acid-washed coconut husk GAC (2.0-3.2 mm particle size) was used as the filter media. The reactor was inoculated with diluted activated sludge to facilitate biofilm growth. The study was conducted in three phases, each with a different saturated/unsaturated ratio (1:2.3, 1:1.1, and 1:0.5). Hydraulic retention time (HRT) was maintained at 6.7 h. Batch experiments using fresh GAC, inactivated biofilm-GAC, and bio-enhanced GAC were conducted to differentiate LAS adsorption and biodegradation. Pseudo-first-order and pseudo-second-order models were used to simulate adsorption kinetics, while a two-step model (adsorption and biodegradation) was used for bio-enhanced GAC. Biofilm samples were collected from different reactor positions (top, stratification, bottom) for various analyses: SEM-EDX for morphology and elemental composition, FTIR and 2D-FTIR for functional group analysis, and DNA extraction for microbial community analysis using Illumina NextSeq 500 sequencing. PICRUSt 2.0 was used to predict functional genes involved in carbon and nitrogen metabolism. COD, LAS, TN, NH₄⁺-N, NO₂⁻-N, NO₃⁻-N, protein, carbohydrate, DNA, DO, and pH were measured using standard methods.

The BhGAC-DBfR demonstrated high removal efficiency. Increasing the saturated/unsaturated ratio promoted biofilm growth on the GAC surface, with the highest biomass concentration observed at the reactor top. The optimal saturated/unsaturated ratio (1:1.1) yielded maximum removal rates: 98.3% COD, 99.4% LAS, 99.8% ammonia nitrogen, and 83.5% total nitrogen. Batch experiments revealed that both adsorption and biodegradation were crucial for LAS removal. Microbial community analysis showed that α-Proteobacteria and γ-Proteobacteria dominated the biofilm in the top and stratification zones, playing roles in organic oxidation and nitrogen transformation. The bottom zone exhibited a distinct microbial community with anaerobic bacteria, indicating the influence of dissolved oxygen (DO) concentration on community structure. Functional gene analysis showed higher abundances of genes related to LAS mineralization, organic nitrogen metabolism, ammonium oxidation, and nitrate respiration in the top zone, consistent with the higher DO concentrations. The study characterized the biofilm's functional zones: oxygen flushing zone (aerobic), transition zone (anoxic), and anaerobic zone, each with distinct microbial communities and metabolic processes contributing to pollutant removal. LAS was efficiently removed, mostly in the unsaturated zone, with only a small percentage remaining in the effluent.

The results demonstrate that the BhGAC-DBfR effectively removes various pollutants from graywater through a combination of adsorption and biodegradation facilitated by a diverse and spatially structured microbial community. The optimal saturated/unsaturated ratio creates a dynamic environment promoting the development of a multifunctional biofilm capable of simultaneous organics and nitrogen removal. The findings highlight the importance of considering the interplay between oxygen availability, microbial community structure, and the diverse removal mechanisms involved in achieving efficient graywater treatment. The high removal efficiency, coupled with the relatively simple design and low energy input, indicate the BhGAC-DBfR's potential for practical application.

This study successfully developed an easy-to-maintain and energy-efficient BhGAC-DBfR for graywater treatment, achieving high removal rates of various pollutants. The optimal saturated/unsaturated ratio (1:1.1) maximized biofilm growth and pollutant removal. The integrated adsorption and biodegradation mechanisms, driven by a diverse microbial community, demonstrate the system's effectiveness. Future research should focus on optimizing reactor design, exploring its applicability to real graywater, and investigating the removal of antibiotic resistance genes (ARGs) to ensure effluent quality.

The study used synthetic graywater, which may differ from real graywater in terms of composition and pollutant concentrations. The long-term stability of the biofilm and the reactor's performance under variable influent conditions need further investigation. The potential for clogging due to particulate matter in real graywater should be addressed through pre-treatment strategies. Finally, while the study assessed the efficiency of removing various pollutants, a more comprehensive evaluation of potential pathogens and other contaminants is warranted.