ZVI Addition in Continuous Anaerobic Digestion Systems Dramatically Decreases P Recovery Potential: Dynamic Modelling

The objective of this study is to show the preliminary results of a (dynamic) mathematical model describing the effects of zero valent iron (ZVI) addition during the anaerobic digestion of waste activated sludge from wastewater treatment systems. A modified version of the Anaerobic Digestion Model No. 1 (ADM1) upgraded with an improved physico-chemical description, ZVI corrosion, propionate uptake enhancement and multiple mineral precipitation is used as a modelling platform. The proposed approach is tested against two case studies which correspond to two lab scale anaerobic digesters (AD2, AD1), with and without adding ZVI, respectively, and running in parallel for a period of 87 days. Experimental results show that ZVI enhances methane production. However, the P recovery potential is dramatically reduced as soluble P decreased by one order of magnitude in AD2 with respect to AD1. Simulations demonstrate that the model is capable to satisfactorily reproduce the dynamics of hydrolysis, acetogenesis, acidogenesis, nutrient release, pH and methanogenesis in the control anaerobic digester (AD1). This study also evidences the enhancement of methane production by the influence of ZVI on the acidogenesis and methanogenesis processes in AD2. In addition, it also identifies saturation conditions for siderite (FeCO3) and vivianite (Fe3(PO4)2), which causes changes in the biogas composition (% CH4 versus % CO2) and P release (lower values). This is the first study analysing the decrease of P recovery potential due to the addition of ZVI into AD systems.