# Effect of Bioaugmentation on Biogas Yields and Kinetics in Anaerobic Digestion of Sewage Sludge
## 3 Results and Discussion
### Microbial Structure of Digesters
decreased; this was a strong association, with an R\({}^{2}\) value of 0.91, indicating that 91% of the variation in _Cytoplasa_ sp. abundance was associated with the dose of bioaugmenting mixture. Similarly, an increased dose of this mixture was associated with decreases in the percent abundance of the order Verrucomicrobiales (R\({}^{2}\) = 1.00), the phyla Cloacimonetes (R\({}^{2}\) = 0.83), and Choroflexi (R\({}^{2}\) = 1.00), including order Anaerolineales (R\({}^{2}\) = 0.99). \begin{table}
\begin{tabular}{l c c c c} \hline \hline  & **Kingdom**; **Phylum**; **Class**; **Order**; **Family**; **Genus** & **R1** & **R2** & **R3** \\ \hline Bacteria; Proteobacteria; Gammaproteobacteria; Pseudomonadales; Pseudomonadaceae; _Pseudomonas_ & 0.8 & 3.9 & 3.6 \\ \hline Bacteria; Proteobacteria; Deltaproteobacteria; Syntrophobacterales; Unclassified; Unclassified & 0.8 & 1.2 & 0.8 \\ \hline Bacteria; Proteobacteria; Deltaproteobacteria; Syntrophobacterales; Syntrophocaeae; _Syntrophus_ & 1.4 & 1.8 & 1.7 \\ \hline Bacteria; Proteobacteria; Deltaproteobacteria; Surrophobacterales; Syntrophaceae; _Syntithella_ & 0.9 & 0.8 & 0.7 \\ \hline Bacteria; Proteobacteria; Deltaproteobacteria; Desulfobacterales; DesulfMembers of the genus _Cytoplaspa_ are important for anaerobic decomposition of biopolymers, such as xylan or cellulose. _Cytoplaspa xylanolytica_ is a mesophilic anaerbo that grows by fermentation of mono-, di-, and polysaccharides (but not cellulose) to acetate, propionate, succinate, CO\({}_{2}\), and H\({}_{2}\); xylan-grown cells of this species have xylanase and various glycosidase activities. _Cytoplaspa hutchinsonii_ can rapidly digest crystalline cellulose without free cellulases or cellulosomes [42]. As extracellular polymeric substances may comprise up to 30% of activated sludge, these activities of _Cytoplaspa_ sp. may be crucial for efficient degradation of organics in fermented sludge. The abundance of _Pseudomonas_ sp. increased from 0.4% in R1 to nearly 4% in bioaugmented R2 and R3 and it was the only identified genus whose abundance increased by such a large amount. Such an increase can be advantageous for anaerobic digestion because [PERSON] et al. [31] observed that the presence of selected strains of genera _Pseudomonas_, _Bacillus_, and _Actinomycetes_ improved the anaerobic digestion of biosolids, increasing net CH\({}_{4}\) production by 29% and diminished odor formation. In addition, [PERSON] et al. [43] reported that an increase in the proportion of some functional organisms, including _Pseudomonas_ sp., led to an increase in the efficiency of anaerobic digestion when the proportion of more biodegradable, low molecular weight fractions (<20 kDa) was increased 10 times because of solubilization of some of the proteins, polysaccharides, nucleic acids, and humic-like substances. In our study, the concentration of low molecular weight SCOD in the digest was negatively correlated (R\({}^{2}\) = 0.97) with the abundance of _Pseudomonas_ sp. in the biomass in the bioaugmented reactors, indicating that this genus played an important role in SCOD degradation. Syntrophic bacteria comprised a significant part of the biomass. From 3.6 to 7.1% of the identified sequences belonged to syntrophic prokaryotes from phylum Cloacimonetes. The analysis of the proteome of _Candidatus Cloacimonas acidaminovovans_ indicated that this bacterium derives carbon and energy from the fermentation of amino acids and that it is a syntroph producing H\({}_{2}\) and CO\({}_{2}\) from formate [44]. On the other hand, the abundance of phylum Cloacimonetes was linked with lowered methane production in reactors fed with protein-rich substrates [7]. In this study, the abundance of other syntrophic microorganisms belonging to _Syntrophus_ sp. and _Smithella_ sp. was relatively stable in the reactors (1.4-1.8% and 0.7-0.9%, respectively). _Smithella_ sp. are syntrophic acetogens involved in propionate degradation in anaerobic digesters, while _Syntrophus_ sp. oxidates fatty acids and benzoate [41; 45].