## 3 Results and Discussion  
### Chemical Composition of Substrates  
BWW holds the lowest concentration of organic material (7 g/L COD, 4 g/L TS and 1 g/L VS), having an acidic pH and low nitrogen content (Table 2) as its main characteristics. These characteristics are related directly to the primary treatment carried out on site before the wastewater was collected for this work. On the contrary, PE contains remarkably high concentrations of organic compounds (around 90 g/L COD, Table 2). Other features measured in PE, such as neutral pH, and contents of nitrogen and of VFA, favor the attainment of a more balanced substrate that promotes the start-up and the working stability of the anaerobic process. BWW presents a lower total VFA concentration than PE (3.1 versus 5.6 g/L HAc), acetic acid being the main component (about 72%, Table 2). In turn, the longer chain acids (isobutyric and butyric) are mainly found in greater quantities in PE. It shows some degradation of the collected flow, probably due to the effluent handling process during the passage of the original piggery effluent through a solid-liquid separator. Additionally, the absence of total phenols in BWW (Table 2) can be a beneficial factor when it is digested \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Parameters**} & \multirow{2}{*}{**Units**} & \multicolumn{3}{c}{**BWW**} & \multicolumn{3}{c}{**PE**} & \multirow{2}{*}{**[43]**} & \multirow{2}{*}{**[44]**} \\  & & & & & & & & \\  & & & & & & & & \\ \hline pH & & 5.1 & 3-12 & 4.5 & 3.2-3.9 & 7.3 & & \\ \hline COD & (g/L) & 7 \(\pm\) 0 & 2–6 & 8.2-20 & 22.5–32.5 & 93 \(\pm\) 5 & 0.82 \(\pm\) 0.23 & 26–51 & 20–91 \\ TS & (g/L) & 4 \(\pm\) 0 & 5.1–8.7 & 5.1–8.75 & & 47 \(\pm\) 1 & & 28–54 & 23–61 \\ VS & (g/L) & 1 \(\pm\) 0 & & & 32 \(\pm\) 1 & & & 16–51 \\ TKN & (mg/L) & 25 \(\pm\) 0 & 25–80 & 0.02–0.03 & 320–450 & 5000 & & 1500–6100 \\ NH4\({}^{+}\)-N & (g/L) & 0.01 \(\pm\) 0 & & & 3 \(\pm\) 0 & 0.51 \(\pm\) 0.14 & 1.15–4.8 \\ \hline Total VFA & (g/L HAc) & 3.13 & 1–2.5 & & & 5.64 & & \\ \hline Acetic acid & (g/L) & 2.27 & & & 1.37 & & \\ Propionic acid & (g/L HAc) & 0.55 & & & 0.56 & & \\ Isobutyric acid & (g/L HAc) & 0.07 & & & 2.16 & & \\ Butyric acid & (g/L HAc) & 0.24 & & & 1.54 & & \\ \hline Total phenols & (g/L caffe acid) & 0.00 & & 0.9 \(\pm\) 0.0 & & \\ \hline \hline \end{tabular}  
* BWW—Brewyer wastewater; PE—Piggery effluent; COD—chemical oxygen demand; TS and VS—total and volatile solids; TKN—total Kjeldahl nitrogen; HAc—Acetic acid.  
\end{table} 
Table 2: Chemical composition of effluents. together with PE, as it can minimize the potential inhibition capacity of this effluent with a total phenol content of 0.9 g/L caffe acid. Untreated BWW is generally described in the literature as a typical pollutant with high organic concentration and low nutrient content, characterized by wide variations in these parameters [41; 45; 46], yet the values found in the literature were much lower comparatively with PE (Table 2). Moreover, the lower values of COD and TS found for BWW in this work (Table 2) are justified by the primary treatment to which the effluent was subjected. Effectively, the provided PE is an unusual effluent with several inhibiting characteristics. It is known that the piggery effluent composition is highly variable depending on several factors, such as the farm management practices, animal feed, and storage conditions at the piggery and treatment plant; nevertheless, the PE under study comparatively presents higher concentrations than those stated by other authors (Table 2). This can be explained by the breeding conditions of the animals and the effluent management carried out on this farm in which, as mentioned, the substrate obtained is devoid of most of the solids. In addition to the toxic potential associated mainly with the total of acids and phenolic compounds found in the PE, high concentrations of nitrogen must be considered. Nitrogen performs contrasting functions in the digestion process.