## 1 Introduction  
With the rapid economic development and urbanization, the quantity of wastewater discharge increases and the demands for freshwater resources are increasing worldwide [1; 2]. Wastewater treatment plants (WWTPs) suffer from many difficulties to meet the rigorous discharge standards nowadays, e.g., high pollutant concentration, complicated composition, poor biodegradability, and unstable process [3; 4]. It is important for WWTPs to maintain high microbial activities of the activated sludge, which are critical in removing pollutants, particularly persistent organic pollutants (POPs), emerging contaminants or potential toxic elements (PTEs). For instance, biological treatment in WWTPs was reported to effectively degrade phenol [5], polycyclic aromatic hydrocarbons (PAHs) [6; 7], polychlorinated biphenyl (PCBs) [8], imidazole [9], 1-methyl-imidazole [9], \(N\),\(N^{\prime}\)-alkyl-imidazoliumchlorides [9], and SS-ethylenediamine-\(N\),\(N^{\prime}\)-disuccin acid [10,11], and the activated sludge was also capable of adsorbing and removing PTEs [12,13]. However, the biochemical treatment units in WWTPs are sensitive to high-level of toxic emerging substances or PTEs in influents and the paralysis often occurs in case of a shock load. It is reported that the spill of cyanide and other illegal chemical dumping can cause the shutdown of WWTPs and consequently release contaminants to the surrounding environment [14]. Toxicity assessment methods require development for monitoring the change of influent toxicity and guiding the rapid response strategies to protect WWTPs from the shock load [15]. One important source of the shock load in WWTPs is industrial wastewater, which is characterized by its extreme toxicity, inhibitive ability and high concentrations of complex pollutants [16, 17, 18]. Industrial wastewater harms the biochemical treatment units of WWTPs and seriously disturbs their operation by inhibiting the activated sludge activities in WWTPs [19]. The ratio of 5-day biological oxygen demand (BOD\({}_{5}\)) to chemical oxygen demand (COD), designated as B/C, can be significantly reduced by industrial wastewater to <0.3, indicating a poor biodegradability and causing the paralysis of the biochemical treatment units [20,21]. Due to the shock load and intermittent impacts of PTEs, cyanide and other toxic substances in industrial wastewater, WWTPs might lose efficiencies in removing pollutants to some extent and the effluent quality is affected [21,22], consequently increasing the operation cost [23]. It is therefore necessary to monitor the industrial shock load and improve the resistance of WWTPs to maintain the removal efficiency of refractory pollutants. However, due to the uncertainty and complexity of pollutants in industrial wastewater discharge, most of which are not conventional contaminants to be regularly measured in WWTPs, systems monitoring a broad spectrum of pollutants and integrally evaluating the influent toxicity are of great urgency in WWTP management. Biological monitoring is an alternative approach to evaluate the influent quality by assessing the toxicity, instead of the accurate composition of wastewater [24]. Many studies on biological early-warning systems use various indicators to monitor the influent toxicity in WWTPs, e.g., animals and plants [25,26], special bacterial strains [27,28], microbial current [29, 30, 31], and respiratory rate [32]. Among them, luminescent bacteria are widely studied [33, 34, 35] but suffer from the difficulties in online cultivation and monitoring [36, 37, 38, 39]. Additionally, most of these approaches are still in laboratory test, leaving a huge gap between the state-of-art concept and industrial practices. Besides, these biological measurements are indirect methods as the indicator organisms are exogenous to the activated sludge and unable to represent the real domestication and adaptability of the activated sludge during wastewater treatment process [14].