Water scarcity is increasingly recognized as a global crisis with significant impacts on water resources, food security, and environmental sustainability. In response to this issue, the reuse of greywater has emerged as one of the key strategies.This study investigates the effectiveness of various media, including sugarcane bagasse biochar and steel slag, in adsorption columns for reducing the Chemical Oxygen Demand (COD) of greywater. The chemical and physical properties of sugarcane bagasse biochar and steel slag were analyzed using X-Ray Fluorescence (XRF), Fourier Transform Infrared Spectroscopy (FTIR), and Brunauer-Emmett-Teller (BET) tests. Adsorption columns made of polyethylene pipes were filled with biochar and slag, respectively, and the effects of influent flow rate and contact time on the performance of the columns in COD reduction were evaluated. The results indicated that the specific surface area of biochar decreased from 88.664 m²/g to 22.698 m²/g, while that of slag changed from 0.640 m²/g to 0.355 m²/g. This reduction was attributed to the adsorption of pollutants and the decrease in the surface capacity of the media. The results also showed that the COD removal efficiency decreased with increasing flow rate, with the highest efficiency achieved at a flow rate of 15 mL/min. Additionally, longer contact times improved the performance of the adsorption columns in reducing COD. The study utilized Shannon Entropy method to optimize the design and performance of the adsorption columns, and the findings indicate that contact time is the most influential parameter in the design of adsorption columns.
Introduction

Water scarcity is a major challenge with serious social, economic, and environmental impacts. Population growth, climate change, and mismanagement worsen the crisis. Reusing greywater for non-potable purposes like irrigation and car washing offers a promising solution. Proper treatment of greywater is crucial, and various methods exist, including physical, chemical, and biological. Utilizing industrial waste as adsorbents in greywater treatment reduces industrial waste volume, conserves resources, and represents an economical and sustainable approach.
In Khuzestan province, large industries produce significant waste, including steel slag and sugarcane bagasse. If not managed properly, these wastes can lead to increased costs for waste storage, transportation, material accumulation space, fire hazards, and environmental pollution. One effective solution is to reuse or recycle these industrial wastes to purify polluted water. This study investigated using industrial wastes as adsorbents in specialized columns and aimed to optimize the system's performance using the Shannon entropy method.

Materials and methods
Gray water was artificially prepared using lactic acid, bentonite, sodium decyl sulfate, glycerol, sodium bicarbonate, and sodium sulfate to maintain its quality during the experiment.
A double-walled PVC adsorption column (15 cm diameter, 80 cm height) was used. The system included a metal frame, polyethene column, storage tanks, and valves. Greywater was stored in a 250-liter tank, and a 20-liter tank with a submersible pump maintained a constant inflow rate. Artificial greywater was made using lactic acid, bentonite, sodium dodecyl sulfate, glycerol, sodium bicarbonate, and sodium sulfate. Two types of beds were used: steel slag (as a mineral bed) and sugarcane bagasse biochar (as an organic bed). Steel slag was sieved, cleaned, and graded, while biochar was produced from sugarcane bagasse at 300°C. Three-bed combinations were tested: Sand and gravel (10 cm) + three 20 cm layers of steel slag, Sand and gravel (10 cm) + 60 cm layer of biochar, Sand and gravel (10 cm) + three 10 cm layers of steel slag + 30 cm layer of biochar. The Shannon entropy method was used to determine the weight of each feature affecting the adsorption process, including inflow rate, organic and mineral bed composition, and residence time.

Results and discussion
The surface area, pore size, and chemical composition of adsorbent materials affect their adsorption capacity and selectivity. XRF analysis showed that both biochar and slag contain compounds such as Fe, CaO, SiO₂, and Al₂O₃. The presence of these minerals indicates chemical adsorption of organic materials. BET analysis revealed that biochar’s surface area increased from 88.664 m²/g to 22.698 m²/g after use, while slag’s decreased from 0.640 m²/g to 0.355 m²/g. This reduction in surface area is attributed to pollutant adsorption.
The highest COD removal efficiencies were achieved with columns containing more biochar. Biochar’s high surface area and porosity enhance its adsorption capacity for organic pollutants like COD. Conversely, the absorbent column containing more slag compared to the other columns showed a lower ability to remove COD. This is because of the presence of magnesium and calcium oxides in steel industries. When these oxides come into contact with water, hydroxide ions are formed. This increases the volume of slags and reduces their contact surface and pores. As a result, the gray water has less contact with the absorbent column, leading to a low COD removal efficiency by the slag absorbent.
Higher inflow rates decreased COD removal efficiency, with the best results at 15 ml/min and the lowest at 30 ml/min. longer contact times improved COD removal, with optimal performance observed at 312 minutes. Increased contact time allows more interaction between greywater and adsorbent, enhancing pollutant removal.
According to the Shannon entropy results, the contact time of gray water with the absorbent column bed was found to be the most influential parameter, while the flow rate of gray water entering the absorbent column was considered the least influential. It's important to note that the importance of organic and mineral substrates is equal. The results indicate that contact time is more significant than flow rate and bed type in the adsorbent column, as it determines the duration of interaction between the adsorbent and the pollutant, allowing for sufficient adsorption to take place (George et al, 2022; Prasad et al., 2022). Therefore, understanding and optimizing contact time is crucial for efficient adsorption in fixed bed columns.
Conclusion
The results of this study indicate that using biochar and slag as low-cost adsorbents significantly reduces COD in greywater. Among the studied parameters, contact time had the greatest effect, while inflow rate had the least impact on the column's efficiency. Increasing contact time improved COD removal efficiency, while higher inflow rates decreased it. Additionally, using organic beds like biochar enhances COD reduction more effectively than mineral beds such as slag.