## 3 Results and Discussion  
### Experimental Studies in Laboratory bioreactors  
Experimental studies were performed in batch and semi-continuous modes of operations in laboratory bioreactors and in a two-phase pilot biogas plant with a computer control system. The \"bottleneck\" was the hydrogen production because of the hydrolysis/acidification as a hybrid first stage. Using mixed microbial consortia seemed to be the easiest way to realize fermentative hydrogen production. However, there are a lot of factors that must be specified such as temperature, pH, organic load, and hydraulic retention time. These consortia must be enriched in hydrogen-producing bacteria and hydrogen-consuming microorganisms must be properly inhibited. Its necessity to strengthen the competitiveness of hydrogen-producing bacteria over hydrolyzing ones to improve volatile fatty acids production remains an open issue [4]. Such kinds of experiments were carried out in our laboratory bioreactors with different substrates. * Glucose--batch and continuous mode of operation with organic loads of 2, 3, 4, and 6 \(\mathrm{g/L}\));
* Corn steep liquor. All experimental data obtained on day 3 for biogas yield and the maximal value of the \(\mathrm{H_{2}}\) content in the biogas are represented in Table 2.  
As can be seen in Table 2, even 2 g of pure glucose led to high hydrogen production. Even though at an organic load of 6 g a higher hydrogen content was obtained, the biogas quantity is less, and the overall hydrogen yield is smaller. In this case, the other substrate (CSL) was used, the 15 mL load led to a longer process duration (5 days) and higher hydrogen content along with bigger biogas volume. The semi-continuous process of daily addition of substrate (Figure 5) showed that a higher hydrogen content (up to about 50%)might be obtained, but the process maintained without automation control of pH is quite unstable for a long period of bioreactor operation (longer than about 30 days). The experiments were transposed to slightly bigger laboratory bioreactors with a working volume of 2 dm\({}^{3}\). The biogas and the hydrogen yield increased with the substrate addition. However, the H\({}_{2}\)S content also increased when CSL was used as a substrate. It might be due to the sulfur content in this substrate based on the technology of corn seeds treatment. The Draeger X-am 7000 device was used for biogas analysis. It has a limitation on H\({}_{2}\)S measurement above 1000 ppm. Even in a small concentration (18 mL) of CSL, about 705 ppm of H\({}_{2}\)S was reached. The subsequent increase in CSL addition led to an H\({}_{2}\)S content greater than the maximum limitation of the device. The results concerning the batch and continuous experiments are demonstrated in Table 3.  
The preliminary experiment showed that in the two-stage process, pH monitoring and continuous control have a great impact (Table 4). The biogas yield decrease at the pH means lower than pH 4.5 which might be reached on the 7 th day of cultivation. Although the graphics in Figure 6 show that the daily hydrogen yield is greater during the first 5-6 days of the process, the overall methane yield is greater for the whole process (2.46 dm\({}^{3}\) biogas/L working volume) in comparison with the hydrogen yield (1.74 dm\({}^{3}\)/L working volume). \begin{table}
\begin{tabular}{c c c c} \hline
**Substrate and** & **Experiment** & **Cumulative Biogas** & **Maximal Value of** \\
**Organic Loading** & **Duration (day)** & **Yield from 1 L of** & **the H\({}_{2}\) Content in** \\
**(g/L)** & **Working Volume (dm\({}^{3}\))** & **the Biogas (\%)** \\ \hline Glucose (2 g/L) & 3 & 1.42 \(\pm\) 0.10 & 42.3 \(\pm\) 0.02 \\ Glucose (6 g/L) & 3 & 1.05 \(\pm\) 0.14 & 46.7 \(\pm\) 0.02 \\ CSL (10 mL/L) & 3 & 0.39 \(\pm\) 0.12 & 4.7 \(\pm\) 0.21 \\ CSL (15 mL/L) & 5 & 0.80 \(\pm\) 0.18 & 21.0 \(\pm\) 0.05 \\ \hline \end{tabular}
\end{table} 
Table 2: Influence of the organic load and the process duration on the hydrogen content and the biogas yield.