# Effect of Bioaugmentation on Biogas Yields and Kinetics in Anaerobic Digestion of Sewage Sludge
## 3 Results and Discussion
### Process Stability  
To evaluate the stability of anaerobic digestion, pH, alkalinity, concentration of VFA and the VFA/alkalinity ratio in digest supernatant should be analyzed. The average values of these parameters in the feedstock and digest are given in Table 1.  
The results indicate that the pH of feedstock was at a comparable level in the bioaugmented and non-bioaugmented reactors. Conversely, alkalinity decreased in the bioaugmented reactors (R2, R3, and R5) by 5.5%, 8.4%, and 16.7%, respectively, but the differences were not statistically significant. An increase in pH was observed in the digest. The values for all runs were in a range favorable for methanogens, i.e., from pH 7.59 to pH 7.99. Similarly, the digest alkalinity increased more than 4-fold. In the bioaugmented reactors, pH and alkalinity were lower compared with the sewage sludge, and these differences were statistically significant. Bioaugmentation caused a decrease in the VFA of the mixture fed to the reactor, but the difference was not statistically significant. Higher VFA removal was reported for anaerobic digestion of sewage sludge: 71.9%--in R1, and 90.2%--in R4. For bioaugmented reactors, these values were 68.8% (R2), 55.6% (R3), and 76.2% (R5). The digest was characterized by low concentrations of VFA--they did not exceed the value of 290 g m\({}^{-3}\) in any reactor. In the presence of the bioaugmenting mixture, the VFA/alkalinity ratio increased to 0.071 and 0.087 (for 9% and 13  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Reactor**} & \multirow{2}{*}{**Value**} & \multicolumn{2}{c}{**VFA (g m\({}^{-3}\))**} & \multicolumn{2}{c}{**Alkalinity (g CaCO\({}_{3}\) m\({}^{-3}\))**} & \multicolumn{2}{c}{pH} \\ \cline{3-8}  & & **Feed** & **Digest** & **Feed** & **Digest** & **Feed** & **Digest** \\ \hline \multirow{2}{*}{R1} & Average & 838 & 235 & 812 & 3722 & 6.81 & 7.99 \\  & lower/upper 95\% mean & 719/957 & 66/404 & 764/860 & 3614/3830 & 6.68/6.94 & 7.95/8.03 \\ \hline \multirow{2}{*}{R2} & Average & 771 & 241 & 769 & 3391 & 6.83 & 7.86 \\  & lower/upper 95\% mean & 678/864 & 112/370 & 732/806 & 3268/3514 & 6.70/6.96 & 7.80/7.92 \\ \hline \multirow{2}{*}{R3} & Average & 660 & 291 & 749 & 3336 & 6.82 & 7.72 \\  & lower/upper 95\% mean & 542/778 & 203/379 & 712/786 & 3229/3443 & 6.68/6.96 & 7.66/7.78 \\ \hline \multirow{2}{*}{R4} & Average & 1520 & 149 & 880 & 3670 & 6.68 & 7.71 \\  & lower/upper 95\% mean & 147/2893 & 115/183 & 610/1150 & 3524/3816 & 6.44/6.92 & 7.44/7.98 \\ \hline \multirow{2}{*}{R5} & Average & 1062 & 253 & 754 & 3195 & 6.75 & 7.59 \\  & lower/upper 95\% mean & 163/1961 & 167/339 & 578/930 & 3078/3312 & 6.53/6.97 & 7.37/7.81 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Volatile fatty acids (VFA) concentration, alkalinity and pH values in feed and digest for specified reactors.  
\(\
icefrac{{v}}{{v}}\) dose, respectively) and 0.079 (17% \(\
icefrac{{v}}{{v}}\)). For the sludge, the ratio equaled 0.063 in R1 and 0.041 in R4. The process was carried out under stable conditions in both phases.