Potential use of zero-valent iron in enhancing performance and resource recovery during the anaerobic digestion of domestic sewage

Abstract

Incorporating metallic iron (Fe 0 ) into anaerobic digesters can improve organics (chemical oxygen demand (COD)), phosphorus, and nitrogen from contaminated water. However, no study has systematically assessed Fe0 -supported anaerobic digestion (AD) systems for removing organic compounds and nutrients from domestic sewage (DS), limiting our understanding of their potential to replace tertiary treatment units. Besides, existing studies often focus on single contaminants at high concentrations, which may not reflect real-world effluents with multiple pollutants. Variations in experimental conditions and the type of wastewater effluent treated complicate comparisons across studies. Additionally, there is a lack of comprehensive evaluations of predictive models for methane (CH₄) yields in Fe0 -supported AD systems, hindering the identification of the most effective model and affecting future research and applications. Moreover, there is little information on sludge characteristics from Fe0 -aided AD systems and their potential applications. This research focused on three primary objectives: (i) assessing the impact of Fe0 type and dosage in AD systems for the simultaneous removal of COD and nutrients (orthophosphate (PO4 3- ), ammonium (NH4 + ), nitrate (NO3 - )), and (ii) characterizing the solids and biogas in Fe0 -supported AD of DS, and (iii) evaluating the Gompertz, Logistic, and Richard models for methane yield prediction. Two distinct experiments were conducted at various scales. In the first experiment, lab-scale reactors containing DS were subjected to varying dosages of Fe0 (0 to 30 g/L) over 32 experimental runs conducted for 76 days at a constant temperature of 37 ± 0.5℃. In the second experiment, bench-scale reactors with DS were fed with Fe0 and operated over 15 experimental runs for 53 days at 24 ± 3℃ temperature. Iron scraps (SI) and steel wool (SW) were used as the Fe0 sources. A control experiment was also conducted. It was found out that: (a) the optimal Fe0 dosage for organic and nutrient removal was 10 g/L SI, (b) NH4 + and NO3 - removal showed the lowest removal efficiency, and (c) maximum removal efficiencies for COD, PO4 3- , and NH4 + + NO3 - were 88.0%, 98.0%, and 40.0% for 10 g/L SI; 88.2%, 99.9%, and 25.1% for 10 g/L SW; and 68.9%, 7.3%, and 0.7% for the control system. Fe0 significantly enriched nutrients in the sludge, improved settling characteristics, and increased the percentage of methane content in biogas by over 12%. All tested methane prediction models showed good accuracy (error < 10%), with the Richard model demonstrating the highest level of fit (error < 1.6%). These findings confirm the effectiveness of Fe0 -supported AD in removing organics and nutrients from DS, producing agriculturally suitable sludge, and enhancing biogas methane content for potential energy recovery