Toxic compounds biodegradation and toxicity of high strength wastewater treated under elevated nitrogen concentration in the activated sludge and membrane bioreactor systems Jarungwit Boonnorat a, ⁎, Nimaradee Boonapatcharoen b , Pradthana Prachanurak c , Ryo Honda d , Supaporn Phanwilai e a Division of Biology, Faculty of Science and Technology, Rajamangala University of Technology Thanyaburi (RMUTT), Klong 6, Pathum Thani 12110, Thailand b Excellent Center of Waste Utilization and Management (ECoWaste), King Mongkut's University of Technology Thonburi (KMUTT), Bangkhuntien, Bangkok 10150, Thailand c Department of Civil and Environmental Engineering, Faculty of Engineering, Srinakharinwirot University, Ongkharak, Nakhon Nayok 26120, Thailand d Research Center for Sustainable Energy and Technology (RSET), Institute of Science and Engineering, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan e Department of Environmental Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand HIGHLIGHTS • The AS achieves the enhanced toxic compounds degradation under elevated N condition. • The degradation enhancement is attrib- utable to the abundance of heterotro- phic nitrifying bacteria and nitrifying bacteria. • The AS system under the elevated N condition is less toxic to the aquatic en- vironment. • The MBR is more effective in toxic com- pounds degradation and high-strength wastewater treatment. GRAPHICAL ABSTRACT abstract article info Article history: Received 22 December 2016 Received in revised form 21 February 2017 Accepted 8 March 2017 Available online xxxx Editor: D. Barcelo This research has assessed the removal efficiencies of toxic compounds in the high strength wastewater (the leachate and agriculture wastewater mixture) using the activated sludge (AS) and membrane bioreactor (MBR) technologies under two carbon to nitrogen (C/N) ratios (C/N 14 and 6) and two toxic compounds concen- trations (8–396 μg/L and 1000 μg/L). In addition, the toxicity evaluations of the AS and MBR effluents to the aquatic environment were undertaken at five effluent dilution ratios (10, 20, 30, 50 and 70% v/v). The findings indicate that the AS treatment performance could be enhanced by the elevation of the nitrogen concentration. Specifically, the C/N 6 environment helps promote the bacterial growth, particularly heterotrophic nitrifying bac- teria (HNB) and nitrifying bacteria (NB), which produce the enzymes crucial to the toxic compounds degrada- tion. The improved biodegradation makes the effluents less toxic to the aquatic environment, as evidenced by the lower mortality rates of both experimental fish species raised in the nitrogen-elevated diluted AS effluents. On the other hand, the elevated nitrogen concentration minimally enhances the MBR treatment performance, given the fact that the MBR technology is in itself a biological treatment scheme with very high compounds re- moval capability. Despite its lower toxic compounds removal efficiency, the AS technology is simple, inexpensive Keywords: Toxic compounds C/N ratio Activated sludge (AS) Membrane bioreactor (MBR) Wastewater treatment Science of the Total Environment 592 (2017) 252–261 ⁎ Corresponding author. E-mail address: jarungwit_b@rmutt.ac.th (J. Boonnorat). http://dx.doi.org/10.1016/j.scitotenv.2017.03.078 0048-9697/© 2017 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv