A novel method to improve methane generation from waste sludge using iron nanoparticles coated with magnesium hydroxide

In response to the low efficiency of the anaerobic digestion (AD) process in generating methane gas, we apply for the first time the use of coated/Fe⁰ with Mg(OH)₂ to enhance the production rate of methane gas from the degradation of waste sludge. A series of batch tests investigated several operations factors followed by a semi-continuous operation system examined the long-term production of methane gas in the presence of the coated/Fe⁰ were performed. The coating ratio of Mg(OH)₂/Fe⁰ and the dosage of coated/Fe⁰ were optimized to acquire the highest production rate of methane as 0.5% and 25mg/gVS, respectively. Under these optimum conditions, the methane production increased by 46.6% in the batch tests and 120% in the semi-continuous operation system compared to the control reactor. The results revealed that both Fe⁰ and Mg(OH)₂ did not significantly improve the production of methane when each one was used alone at different dosages, and the improved methane production originated from the synergetic effect of combining these two materials. The crucial role of Mg(OH)₂ coating layer was associated with the controlled reactivity release of Fe⁰, which was indicated by the slow release of Fe²⁺ and Fe³⁺ in the bioreactors. Furthermore, the addition of coated/Fe⁰ stimulated bacterial growth, increased methane content, and maintained the pH within the optimum range in the bioreactors. The dosing time of coated/Fe⁰ was investigated during the four stages of AD process, and the best dosing time was found in the methanogenic stage (on Day 4). Overall, based on the experimental and predicted methane production, the coated/Fe⁰ has a great potential for the practical applications of AD.