## 3 Results
### Changes in Activated Sludge Process  
ASP properties changed during the inhibition by acidic and alkaline pH. Visual observation revealed sludge color change (sludge from both reactors became darker). After 24 h of inhibition, constant unpleasant sludge odor from both reactors was detected. However, the odor intensity was stronger in the acidic reactor. The effluent from both reactors became more turbid and the color also changed.  
The sludge from the reactor at acidic pH had a dispersed or pin-point floc condition after 48 h. The formation of pin-pointed flocs refers to an environmental shock that keeps sludge microorganisms from agglomeration and causes a formation of a settleable floc. Respectively, pin-point floc condition strongly influences sludge settling abilities [25].  
The MLSS at the end of the experiments in acidic reactor was 1.50 \(\pm\) 0.30 g/L and in alkaline reactor 2.25 \(\pm\) 0.20 g/L. Additionally, sludge foaming was observed in the acidic reactor at 72 h of inhibition (Figure S3). White billowy foam usually occurs to young-aged sludge or sludge with low MLSS, sludge with high or too low organic loading, toxic conditions, excessive wasting, and a high abundance of _Zoogloea_ spp. [26]. As expected, a rapid decrease in MLSS in the acidic reactor (from 2.75 \(\pm\) 0.25 g/L at 0 h to 1.50 \(\pm\) 0.30 g/L at 72 h) led to an increase in the organic sludge load since the influent wastewater parameters were the same from 0 h to 72 h. Further, pH6.5 increased FNA toxicity risk for ASP, and there was a sludge washout during the experiment (excessive wasting). Besides, _Zoogloea ramigera_ and _Zoogloea uva_ had considerable augmentation in sludge of the acidic reactor (Figure 2a). Thus, foaming in a reactor with acidic pH denoted ASP troubleshooting due to FNA appearance.  
ASP process in the alkaline reactor had filamentous sludge bulking due to an increase in filamentous flocs in the sludge after 48 h (Figure 2b). Filamentous sludge bulking occurs mainly due to the generation of interfloc bridging by filamentous microorganisms, reduction of biomass density, and diffusion of flocs with poor precipitation properties [27]. Figure 2 represents sludge samples from the alkaline after 48 h and a previously reported [28] sample with interfloc bridging by filamentous microorganisms. At the same time, it was observed that total organic carbon (TOC) concentration in the alkaline reactor effluent stabilized and remained at 15.4 mg/L from 0 h to 72 h. TOC concentration in acidic  
Figure 2: _Zoogloea ramigera_ (**a**) in sludge of the acidic reactor and filamentous sludge in the alkaline reactor (**b**).  
reactor effluent increased gradually from 15.6 mg/L at 0 h to 21.0 mg/L at 72 h; so, in this reactor, ASP process organic carbon biodegradation was also affected by acidic pH 6.5.  
Sludge volume indices (SVI5 and SVI30) increased in both reactors after the start of inhibition by the acidic and alkaline pH. SVI5 increased from 222 mL/g to 256 mL/g after 48 h and decreased again to 230 mL/g in acidic reactor (Figure S4). SVI30 index in the alkaline reactor was not strongly affected by high pH (Figure S5). The SVI30 index had values up to 90 mL/g, which indicate proper sludge settleability despite the fact of increased filamentous sludge flocs in the reactor. However, sometimes, filamentous sludge bulking can occur at relatively low SVI values [25]. Hence, the SVI30 index is not the main indicator of the formation of high FA concentration in this case. Although, SVI5 changed more than SVI30 and can be specified as the first sign of the inhibition due to an increase from 225 mL/g (acclimatization cycle) to 238 mL/g at 0 h. The risk of the sludge bulking due to high SVI30 can be at SVI higher than 120 mL/g [27], which might occur after 72 h of inhibition (Figure S5) due to the gradual increase in SVI30 in acidic reactor.