Efficiency of an integrated process of electrooxidation and anaerobic digestion of waste activated sludge

Abstract

Background Most of the organic content of waste activated sludge (WAS) comprises microbial cells hard to degrade, which must be pre-treated for the energy recovery by anaerobic digestion (AD). Electrooxidation pre-treatment (EOP) with a boron-doped diamond (BDD) electrode have been considered a promising novel technology that increase hydrolysis rate, by the disintegrating cell walls from WAS. Although electrochemical oxidation could efficiently solubilise organic substances of macromolecules, limited reports are available on EOP of WAS for improving AD. In this endeavour, the mathematical optimization study and the energy analysis of the effect of current density (CD) during EOP and the initial total solids concentration [TS] from WAS on methane (CH4) production by AD was investigated. Results In the present work, biogas production from WAS conversion are comprehensively affected by CD and [TS]. The highest COD and VS removal by 60 and 39% respectively, were achieved with WAS at 3% of [TS] pre-treated at CD of 24.1 mA/cm2, and with a maximum CH4 production of 305 N-L/kg VS and a positive energy balance of 0.83 kWh/kg VS. Therefore, the low current densities used in boron-doped diamond (BDD) electrode are adequate to produce the strong oxidant ●OH radical on the electrode surface, allow the oxidation of organic compounds that favours the solubilization of COD from WAS. Conclusions The improvement of VS removal indicates that EOP help to disintegrating cell walls from WAS. This allows a decomposition reaction that leads to biodegrade more compounds during AD. The energy balance was positive, suggesting that even without any optimization the energy used as electricity could be approximately recovered as energy from the increased methane production. However, this kind of analysis have not been sufficiently studied so far, therefore, is important to understand how critical parameters can influence the pre-treatment and AD performances. The current study highlights that the mathematical optimization and energy analysis can get the whole process more convenient and feasible.

Electrochemical valorization of waste activated sludge for short-chain fatty acids production

“…Electrochemical treatment is a robust, scalable, and environmentally friendly approach that has been widely studied for wastewater treatment processes. For instance, to improve sludge dewaterability (She et al, 2016;Masihi and Badalians Gholikandi, 2018;Cai et al, 2019;Lu et al, 2022), to enhance biogas recovery from anaerobic digestion (Barrios et al, 2021;Hao Huang et al, 2021), for sludge conversion and pathogen removal (Zeng et al, 2019;Jafari and Botte, 2021;Martínez-Huitle and Brillas, 2021), for pollutant removal (Qiao and Xiong, 2021), and for nutrient recovery such as nitrogen and phosphorous (Arslan et al, 2017;Yang et al, 2020;Xu et al, 2021). Our previous research has shown that electrochemical treatment of WAS under alkaline conditions could improve sludge defragmentation and solubilization (Jafari and Botte, 2021), electrochemically enhanced digestors.…”