REVIEW ARTICLE On the performance of electrocoagulation-assisted biological treatment processes: a review on the state of the art Zakaria Al-Qodah 1 & Yahiya Al-Qudah 2 & Waid Omar 1 Received: 25 April 2019 /Accepted: 23 July 2019 # Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract The combined treatment systems have become a potential alternative to treat highly polluted industrial wastewater to achieve high-quality treated effluents. The current review focuses on the treatment systems compromising electrocoagulation (EC) as a pretreatment step followed by a biological treatment step. The reasons for applying EC as a pretreatment process were mainly to (1) detoxify the wastewater by removing inhibitors of the biotreatment step or (2) to remove the major part of the COD or (3) the dissolved materials that could cause fouling to membrane bioreactors or (4) to increase the activity of the microorganisms. This combination represents a new and promising application characterized by higher performance and removal efficiency. The main published findings related to this application are presented and analyzed. Besides, the statistical models used to optimize the process variables and the kinetics of microorganism growth rate are discussed herein. Most of the previous investigations were conducted in a laboratory-scale level with biologically treated water as a feed to the EC process. Only a few works applied a hybrid system consisting of the biological step and the EC step. In all studies, improved performance and higher removal efficiencies of the combined process were achieved particularly when applying aluminum electrodes, providing more than 95% removal efficiency. Many researchers have reported that they had faced a significant problem in the operation of the electrocoagulation process associated with the reduction of electrodes’ efficiency caused by deposits of the coagulation complex. This problem needs to be effectively resolved. Keywords Electrocoagulation . Combined treatment processes . Biological treatment . Pretreatment processes . Membrane bioreactors . Wastewater treatment integrated processes Introduction Biological treatment processes usually present a high potential of a relatively simple, effective and economically feasible ap- proach for the treatment of different types of biodegradable wastes (Foresti 2002; Sato et al. 2006). Biological methods are generally inexpensive, simple, and easy to apply to remove different types of organic compounds found in industrial wastewater. However, most of the industrial wastewater contains some nonbiodegradable and toxic pollutants in addi- tion to considerable quantities of colloidal matter and suspended solids (SS) (Khoufi et al. 2007; Al-Qodah et al. 2018; Al-Qodah et al. 2019). The existence of these pollutants at high loads could alter the biological treatment process or at least significantly reduces its efficiency. Also, the quality of treated wastewater by single conventional biological process- es may not respond to new and strict international and national environmental standards. These facts have promoted a contin- uous and increasing desire to develop innovative, high-perfor- mance, cost-effective, and sustainable combined techniques for wastewater treatment (Al-Malack 2007; Bani-Melhem and Elektorowicz 2011). During the last two decades, several physicochemical pre- treatment processes have been applied as a pretreatment step to treat highly polluted industrial wastewater before the tradi- tional biological treatment process (Chiavola et al. 2010; Salem et al. 2008). These pretreatment processes include flocculation/settling (Zhou et al. 2019), adsorption (Al- Responsible editor: Bingcai Pan * Zakaria Al-Qodah zak@bau.edu.jo; z_alqodah@hotmail.com 1 Chemical Engineering Department, Faculty of Engineering Technology, Al-Balqa Applied University, Amman 11134, Jordan 2 Chemistry Department, Faculty of Science, Al-Balqa Applied University, Salt, Jordan Environmental Science and Pollution Research https://doi.org/10.1007/s11356-019-06053-6