# Effect of Bioaugmentation on Biogas Yields and Kinetics in Anaerobic Digestion of Sewage Sludge
## 3 Results and Discussion  
### Impact of Bioaugmentation on the Efficiency of Anaerobic Digestion of Sewage Sludge  
The characteristics of both the mixtures feeding the reactors and the digests are presented in Figure 3. As indicated in the figure, the characteristics of SS varied in the control reactors (R1 and R4), which probably resulted from the changes in the chemical composition of sewage (Figure 1, Table 1).  
Bioaugmentation by a mixture of Bacteria and Archaea decreased all parameters in the feedstock (due to dilution) as compared to sewage sludge. Decreases in COD concentration from 45.8 g dm\({}^{-3}\) to 39.8 g dm\({}^{-3}\) and 38.6 g dm\({}^{-3}\), VS from 31.0 g kg\({}^{-1}\) to 28.1 and 26.4 g kg\({}^{-1}\), and TS from 40.1 g kg\({}^{-1}\) to 36.4 g kg\({}^{-1}\) and 34.2 g kg\({}^{-1}\) were observed for R1, R2, and R3, respectively. The concentration of SCOD in the bioaugmented reactors was 1.25 and 1.20 g dm\({}^{-3}\) and was lower than that in the control (1.40 g dm\({}^{-3}\)). Similarly, in R4 and R5, the concentration of VS and TS decreased by 15% (from 26.1 to 22.1 g kg\({}^{-1}\) and from 33.9 to 28.8 g kg\({}^{-1}\), respectively), COD by 13.5% (from 38.3 to 33.1 g dm\({}^{-3}\)),and SCOD by 9% (from 2.5 to 2.3 g dm\({}^{-3}\)). The higher the dose of the bioaugmenting mixture, the greater the decrease in the aforementioned characteristics as compared to the control reactors.  
The degrees of removal pertaining to VS and other parameters were used to evaluate the process efficiency. On average, the removal efficiencies of VS in R2, R3, and R5 (the bioaugmented reactors) amounted to 47.3%, 49.2%, and 46.4%, respectively. These values were greater than the ones obtained  
Figure 3: Concentration of organic compounds (expressed as chemical oxygen demand (COD), total solids (TS), volatile solids (VS), and soluble chemical oxygen demand (SCOD)) in feed and digest, as well as related removal efficiencies (average values are given, error bars represent 95% confidence limits for means): (**a**) COD changes in phase 1, (**b**) COD changes in phase 2, (**c**) TS changes in phase 1, (**d**) TS changes in phase 2, (**e**) VS changes in phase 1, (**f**) VS changes in phase 2, (**g**) SCOD changes in phase 1, (**h**) SCOD changes in phase 2.  
for sewage sludge (Figure 3); however, the difference was not statistically significant. The highest removal of VS was observed in the presence of 13% \(v/v\) dose of the bioaugmenting mixture (R3), despite the shortening of HRT from 20 to 17.4 d. On the other hand, the removal efficiency of TS reached 40.5%, 41.7%, and 37.9% in R2, R3, and R5, respectively; for the control reactors, it was slightly lower, equaling 40.0% and 35.6% for R1 and R4 control reactors, respectively. As far as the removal efficiency of COD is concerned, comparable values were observed for all reactors. However, the maximum COD removal efficiency, reaching 47.6%, was obtained with 13% \(v/v\) dose of the bioaugmenting mixture (R3). This observation is in line with the findings of [PERSON] et al. (2019), who reported that the application of bioaugmentation during the mature landfill leachate treatment slightly improved the COD removal efficiency. Interestingly, visible differences were observed for the efficiency of SCOD removal (Figure 3g,h). In comparison with the non-bioaugmented reactors, the SCOD removal efficiency increased by 24% and 27%, in the reactors (R2, R3, R5) supplemented with 9% and 13% of bioaugmenting mixture. In contrast, in the presence of 17% \(v/v\) of bioaugmenting mixture, the increase was only by 2.5%. To sum up, the removal efficiency of organic compounds increased, although the hydraulic retention time was shortened from 20 to 16.7 d. [PERSON] et al. (2018) also reported that the addition of mixed microorganisms could shorten the sludge acclimation time, as well as improve the treatment efficiency.