## 3 Results  
### Experiment on Inactivation of Methanogens in the Inoculum  
Pretreatment of anaerobic sludge with hydrochloric acid (pH 3.0, for 24 h) or high temperature (90 \({}^{\circ}\)C, 30 min) led to a change in the content of TS and VS in inoculum. After acid pretreatment of inoculum 1, VS increased by 5.3% and by 13.8%--after heat pretreatment. VS content after acid and heat pretreatment remained practically unchanged in inoculum 2. TS content in inoculum 1 increased by 53.2% after heat pretreatment and decreased by 32.2% in inoculum 2. Acid pretreatment increased the TS content by 25.49% in inoculum 1 and by 21.17% in inoculum 2. When using acid-pretreated inoculum 1 for DF of CW, on the first day of the batch experiment, the hydrogen content in the biogas was 33.3% and methane was absent (Figure 2). For acid-pretreated inoculum 2, a different picture was observed: the hydrogen content in biogas on the first day of the batch experiment was 0.8%, and methane--17%. Thus, inactivation of methanogens using acid pretreatment in inoculum 2 was not observed, since in control 2 (inoculum 2 without pretreatment) the methane content in biogas was 20.1%. On the second day of incubation, the amount of hydrogen content in inoculum 1 after acid pretreatment decreased to 11.0%. Acetic and butyric acids predominated among the metabolites formed (Figure 3). Particularly strong acidification on the 8 th day of the experiment was observed for inoculum 1 and inoculum 2 after acid pretreatment (up to pH = 3.94 and pH = 4.04, respectively), and for inoculum 2 after heat treatment (up to pH = 3.92). Acetate formation was observed in all vials, but not more than 63 mM (control 1). In vials with inoculum 1, after thermal inactivation, the formation of hydrogen was observed only on the second day of the experiment (7.91%). At the very beginning, the activity of methanogens was suppressed and only on the fifth day of the experiment, the appearance of methane (1.1%) began to be observed in the gas phase. On the second day, acetic acid prevailed in the liquid phase and its concentration was higher than in the samples with inoculum 1 after acid pretreatment. However, a sharp decrease in pH when using heat pretreated inoculum was observed, the pH of the medium on day 8 was 6.37. Inoculum 1 after acid pretreatment on day 8 had a pH of 3.94. Figure 2: Content of hydrogen (**a**) and methane (**b**) in biogas. In the reactor with untreated inoculum 1 (control 1), the pH also decreased on the 8 th day to a value of 6.67, despite the high content of acetic acid, only methane was observed in the biogas (16.5% on the 5 th day), hydrogen in the gas phase was not detected. In control  
Figure 3: Production of VFA in reactors on days 0 (**a**), 2 (**b**) and 8 (**c**) of incubation. 1—control 1 (inoculum 1 without treatment); 2—inoculum 1 after acid inactivation; 3—inoculum 1 after thermal inactivation; 4—control 2 (inoculum 2 without treatment); 5—inoculum 2 after acid inactivation; 6—inoculum 2 after thermal inactivation. 1, acetic acid predominated, the decrease in COD (71.4%) was the highest compared to the pretreated inoculum 1. After 8 days of the experiment, the greatest decrease in the COD value was observed in the control samples (71%). In the samples after acid inactivation, COD removal was 64-66%, and in the samples with thermal inactivation--44-46% (Table 2). In control 2, both hydrogen and methane (9.3% H\({}_{2}\) and 20.1% CH\({}_{4}\)) were recorded in the gas phase on the second day of the experiment. However, strong acidification (pH 4.65) was observed on the second day, after which the content of methane and hydrogen in the gas began to decrease sharply. Analysis of the VFA content in the liquid phase showed the predominance of butyric acid, COD removal was 71%. Thermal inactivation of inoculum 2 contributed to an insignificant increase in the hydrogen yield compared to control 2.