## 3 Results and Discussion
### Removal Efficiencies of Solids and COD
#### 3.3.3 Nitrogen and Phosphorus  
Table 4 shows the concentrations of TKN, N-NH4+, and o-N in the influent and effluent of the bioreactor, for each experimental assay. Figure 6: Average concentrations (\(n=3\) samples per HRT) of total COD in the influent (tCOD influent) and effluent (tCOD effluent) of the bioreactor, and respective removal efficiencies (error bars: standard deviation) (concentrations: on the left \(y\) axis; removal efficiencies: on the right \(y\) axis). Figure 7: Average concentrations (\(n=3\) samples per HRT) of soluble COD in the influent (sCOD influent) and effluent (sCOD effluent) of the bioreactor, and respective removal efficiencies (error bars: standard deviation) (concentrations: on the left \(y\) axis; removal efficiencies: on the right \(y\) axis). The used influent showed high concentrations of TKN with an average concentration of 2998 mg N.L\({}^{-1}\). Compared with the literature for similar organic substrates [29; 31; 32], this can be considered an organic substrate containing a high concentration of TKN. High levels of TKN were also found in the effluent of all assays, revealing the intensive hydrolysis of proteins during the DF [32]. To support this observation, the organic nitrogen (o-N) showed a significant degradation in all assays, contributing to the increase in N-NH\({}_{4}\)\({}^{+}\) from the influent to the effluent of the bioreactor. [PERSON] et al. [33] investigated the mechanisms of DF inhibition by N-NH\({}_{4}\)\({}^{+}\), using glucose as the organic substrate, under continuous and batch conditions. In the batch tests, the bioH\({}_{2}\) production rate was highly dependent on the medium pH and the presence of N-NH\({}_{4}\)\({}^{+}\) higher than 2000 mg N.L\({}^{-1}\). At pH 6.2 and with an initial N-NH\({}_{4}\)\({}^{+}\) concentration of 2000 mg N.L\({}^{-1}\), the bioH\({}_{2}\) production rate decreased from 56 to 16 mL.h\({}^{-1}\) (71.4% decrease rate) and the N-NH\({}_{4}\)\({}^{+}\) registered an increase to a concentration of 10000 mg N.L\({}^{-1}\) during the DF process. In turn, at pH 5.2, the bioH\({}_{2}\) production rate decreased from 49 to 7 mL.h\({}^{-1}\) (85.7% decrease rate) with a variation of N-NH\({}_{4}\)\({}^{+}\) from 2000 to 16000 mg N.L\({}^{-1}\). Therefore, the concentration of N-NH\({}_{4}\)\({}^{+}\) of 2000 mg N.L\({}^{-1}\) can adversely affect the performance of DF, especially under batch conditions and in an acidic medium. Similar conclusions were registered by these authors [33] in the study developed with a continuously operated bioreactor. The bioreactor (V = 2 L) was initially acclimated to 800 mg N.L\({}^{-1}\) of N-NH\({}_{4}\)\({}^{+}\). BioH\({}_{2}\) production and yield were 170 mL.h\({}^{-1}\) and 1.9 mol H\({}_{2}\).mol\({}^{-1}\) glucose, respectively, for this N-NH\({}_{4}\)\({}^{+}\) concentration, having decreased to 105 mL.h\({}^{-1}\) and 1.1 mol H\({}_{2}\).mol\({}^{-1}\) glucose (decrease rates of 38.2% and 42.1% for bioH\({}_{2}\) production and yield, respectively) with the increase in N-NH\({}_{4}\)\({}^{+}\) to 7800 mg N.L\({}^{-1}\). Even if the DF consortium became less sensitive to N-NH\({}_{4}\)\({}^{+}\) under continuous mode than in batch mode, the authors concluded that the production of bioH\({}_{2}\) was affected by high concentrations of this nitrogen species. In any case, it is important to notice that [PERSON] et al. [33] registered bioH\({}_{2}\) production for a concentration of N-NH\({}_{4}\)\({}^{+}\) as high as 7800 mg N.L\({}^{-1}\) which was 4.4-times higher than the average concentration determined in the influent of the present study (N-NH\({}_{4}\)\({}^{+}\): 1765 mg N.L\({}^{-1}\)) and 3.5-times higher than the average concentration determined in the effluent of the bioreactor for the three experimental assays (N-NH\({}_{4}\)\({}^{+}\): 2231 mg N.L\({}^{-1}\)). Thus, it is supposed that N-NH\({}_{4}\)\({}^{+}\) did not work as an inhibitor during the DF experiments for all the tested HRTs in the present work.