Free nitrous acid pre-treatment enhances anaerobic digestion of waste activated sludge and rheological properties of digested sludge: A pilot- scale study Jia Meng a, b , Haoran Duan a , Huijuan Li a , Shane Watts a , Peng Liu a , Sohan Shrestha a , Min Zheng a , Wenbo Yu a , Zhongwei Chen c , Yarong Song a , Jason Dwyer d , Shihu Hu a, * , Zhiguo Yuan a, ** a Advanced Water Management Centre, The University of Queensland, St. Lucia, QLD, 4072, Australia b State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, 73 Huanghe Road, Harbin, 50090, China c School of Mechanical and Mining Engineering, The University of Queensland, St. Lucia, QLD, 4072, Australia d Queensland Urban Utilities, Brisbane, QLD, 4000, Australia article info Article history: Received 22 July 2019 Received in revised form 28 December 2019 Accepted 15 January 2020 Available online 20 January 2020 Keywords: Anaerobic digestion Free nitrous acid Pilot-scale Viscosity Rheological properties abstract In this study, the effects of free nitrous acid (FNA) pre-treatment on the rheological properties of digested sludge were investigated at a pilot-scale, along with the improvement in volatile solids (VS) destruction and biogas production. Two pilot-scale anaerobic sludge digesters were operated for one year, one receiving thickened waste activated sludge (TWAS) without pre-treatment (control) and one receiving TWAS pre-treated for 24 h at an FNA concentration of 4.9e6.1 mgN/L (nitrite ¼ 250 mgN/L, pH ¼ 5.0, T ¼ 22e30  C). The results confirmed the enhancing effect of FNA pre-treatment on methane production (37 ± 1%), consistent with previous laboratory studies. Equally importantly, FNA pre-treatment sub- stantially reduced the shear viscosity of TWAS by 51 ± 8% at 100 s 1 and 49 ± 7% at 250 s 1 , likely due to the solubilization of the TWAS (11.1 ± 0.8%). Similarly, FNA pre-treatment also reduced these viscosity parameters of the digested sludge by 80 ± 4% and 78 ± 4%, respectively, caused by both enhanced VS destruction and disintegration of the digested sludge. The dewaterability of digested sludge, assessed by dewatered solids content, capillary suction time and specific resistance to filtration, was not improved by FNA pre-treatment. The polymer requirement for dewatering was reduced by 24 ± 0.6% due to the lower solids concentration in the digested sludge achieved with FNA pre-treatment. The changes to sludge rheological properties revealed in this study further enhances the business case for the FNA pre- treatment technology. © 2020 Elsevier Ltd. All rights reserved. 1. Introduction Anaerobic digestion (AD) of waste activated sludge (WAS) is an important step in contemporary wastewater treatment plants (WWTPs). The main aim of anaerobic sludge digestion is to reduce sludge production, and to recover renewable energy from the sludge (Abelleira-Pereira et al., 2015; Zuo et al., 2019). However, the application of anaerobic sludge digestion in WWTPs still faces challenges. One challenge is the low biodegradability of WAS with the methane production of 110e145 mL/gVSS added in general (Gossett and Belser, 1981; Bougrier et al., 2006). Furthermore, the low hydrolysis rate of WAS, known to be the rate-limiting step of the anaerobic sludge digestion process, requires a long hydraulic retention time (HRT), l5 days or longer, for the digester (Miron et al., 2000). Thermal hydrolysis pre-treatment (THP) has found wide applications due to its significant improvement to biogas produc- tion by both enhancing the sludge biodegradability and hydrolysis rate (Wilson and Novak, 2009). Also, THP can improve the rheo- logical properties and dewaterability of the digested sludge (Zhang et al., 2018). However, the THP process requires high capital in- vestment and also requires significant energy input. Recently, sludge pre-treatment using free nitrous acid (FNA/ * Corresponding author. ** Corresponding author. E-mail addresses: s.hu@awmc.uq.edu.au (S. Hu), z.yuan@awmc.uq.edu.au (Z. Yuan). Contents lists available at ScienceDirect Water Research journal homepage: www.elsevier.com/locate/watres https://doi.org/10.1016/j.watres.2020.115515 0043-1354/© 2020 Elsevier Ltd. All rights reserved. Water Research 172 (2020) 115515