## 3 Results  
Microbial respiration in coffee samples with different Arabica/Robusta proportions was determined (Figure 2). The respiration in the first ratio sample (Arabica/Robusta: 50%/50%) slightly differed, but was not statistically significant (_p_ > 0.05), from other investigated samples (Arabica 80% and Arabica 100%). There was a 10% reduction in respiration in the sample containing 50% Robusta. Oppositely, statistically significant (_p_ < 0.05) respiration differences were observed for all investigated samples in comparison with control samples. After 50 h, the respiration in the SCG samples (1, 2, and 3) was 215%, 206%, and 198%, respectively; after 100 min: 219%, 236%, and 235%; at the end: 202%, 220%, and 219% compared to the control samples (without SCG). The microbial respiration rates were calculated and are presented in Figure 2 and Table 1. The main kinetic parameters (\(V_{0}\), \(V_{average}\), and \(V_{max}\)) indicated significant differences between the control and other investigated samples. Figure 2: Microbial respiration and its rate in activated sludge depending on Arabica and Robusta content: (**a**) Respiration; (**b**) rate, \(V_{max}\) is maximal rate. SCG represent a source of nutrient for community of microorganisms, due to content of polysaccharides, proteins, lipids, and phenolic compounds, which serves as substrate for various species of microorganisms. It is still speculated if the phenolic compounds can stimulate growth another way than as a substrate, although inhibitory effect on some microorganisms is known. Our results imply the possibility of carbon source, since the content of phenolic compounds significantly decreased, and this decrease could not happen without activity of microorganisms. Another hypothetical effect, although not verified, is the sorption ability of SCG particles that could protect microorganisms in consortia against harmful substances such as heavy metals. These are positive results for WWTP, as the communities of microorganisms are protected from inhibitory effects contrary to pure cultures alone and even degrading SCG, which arrive to the WWTP. Thus, the biggest differences were observed in samples with a higher portion (50%) of Robust coffee cultivar. The results shown in Table 1 correspond to the obtained data in Figure 2.  
The differences were confirmed by PCA (Figure 3); factorial analysis distinguishes three samples with spent coffee as one component and the control (composed without spent coffee) as another component. PCA is a powerful method to reduce the complexity of a dataset and highlight relations between the variables. SCG samples are one cluster and control samples are another cluster. Based on eigenvalue (>1), two components were formed since eigenvalue distinguishes clusters of variances [32]. The first component consisted of three samples with spent coffee and the second component was only the control sample without spent coffee. Consequently, PCA results clearly and unambiguously indicate the influence of spent coffee addition on microbial respiration. Figure 3 represents an individual plot. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{4}{c}{**Kinetic Parameters**} \\ \cline{2-5}  & **Initial Rate** & **Average Rate** & **Maximal Rate** & \\  & \(V_{0}\) & \(V_{average}\) & \(V_{max}\) & \(t\) **(hours)** \\  & **(mg O\({}_{2}\)/L/h)** & **(mg O\({}_{2}\)/L/h)** & **(mg O\({}_{2}\)/L/h)** & **(mg O\({}_{2}\)/L/h)** \\ \hline Control (without coffee ground) & \(15.93\pm 1.45\) & \(14.73\pm 1.34\) & \(25.54\pm 2.32\) & 2 \\ Arabica (50\%)/Robusta (50\%) & \(15.94\pm 1.44\) & \(35.95\pm 3.27\) & \(49.99\pm 4.54\) & 38 \\ Arabica (80\%)/Robusta (20\%) & \(26.54\pm 2.41\) & \(36.47\pm 3.32\) & \(47.51\pm 4.32\) & 21 \\ Arabica (100\%) & \(26.55\pm 2.43\) & \(35.46\pm 3.22\) & \(45.56\pm 4.14\) & 32 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Kinetic parameters of microbial respiration of spent coffee grounds (SCG) samples (n = 3, mean \(\pm\) SD).