Combination of anodic oxidation and biological treatment for the removal of phenanthrene and Tween 80 from soil washing solution

Surfactant enhanced soil washing (SW) and soil flushing are efficient processes for polycyclic aromatic hydrocarbons removal from contaminated soils. However, management of soil washing solutions is a complex challenge for environmental engineering. This study investigated the implementation of a combined process coupling anodic oxidation (AO) – an electrochemical advanced oxidation process – and aerobic biological treatment (BT) for proper handling of a synthetic SW solution containing phenanthrene, Tween 80 and humic acids. As a sole process, AO required a treatment duration of 5 h and a current intensity of 1 A to remove 95% of phenanthrene and Tween 80 as well as 71% of COD. Such operating conditions resulted in high electrical energy consumption (67 kWh (kg COD)1). In contrast, BT alone allowed a total degradation of both phenanthrene and Tween 80, but an insufficient removal of COD (44%) was observed due to the production of non-biodegradable by-products. In order to promote synergistic effects, AO and BT were combined and two different treatment strategies were investigated depending on whether AO was used as pre- or post-treatment. The production of more biodegradable by-products was observed after short AO pre-treatment (3 h, 500 mA), resulting in 80% COD removal after BT. The opposite combination (with AO post-treatment) led to 76% and 93% COD removal after the AO post-treatment at 500 mA for 3 h and 5 h, respectively. Therefore, such combined process appears as a sustainable way to enhance both quality of treated effluent and process cost-effectiveness.