## 3 Results and Discussion
### Box-Type Digester Assays
The inundation time was also increased along time, starting from 20 min per day, until reaching 2 h by the end of the experiment and with the feedstock showing depletion. The experiment was called to an end after 49 days of this second run. The substrate was showing signs of exhaustion and, while still producing biogas, daily yield decreased no matter how aggressive the reinoculation system adopted was. Accumulated methane production along time in this recovery run in both experiments can be seen in Figure 2.  
The final specific methane productions in E1 and E2 were 119.14 and 125.50 I\({}_{\rm N}\) CH\({}_{4}\)kgVS\({}^{-1}\), respectively. That amounts to 41.6% and 43.8% of the results attained in the BMP tests. It has to be said that, as the experiments went, a lot of the biomethanogenic feed substances in the substrate might have been lost in the first runs wasted percolate and dissipated or aerobically decomposed during the time the second batch of inoculums was in the adaptation process. Both experiments running in parallel showed very similar behavior, which allowed us to perform the same actions throughout the experimentation process. While the experiment with the conductive material had slightly better results in terms of VFA's COD reduction on the first stages and in final biogas production, the differences in performance in both experiments were negligible. Two more experiments were performed. In this second run, it was decided to go on the cautious side. Thus, the box reactors were filled with just 8 kg of substrate to keep a lesser substrate/inoculum ratio. The substrates assayed were BSGs in E3 and a mixture consisting of 6.4 kg of BSGs and 1.6 kg of DGSBs (thus, 80% BSGs and 20% DGSBs) in E4. The percolate tanks were filled with 10 L each of the percolate used in the previous experiment and which was now supposed to be perfectly adapted to the substrates after the previous runs. After the first inoculation of the feedstocks with a 3 L sprinkling of inoculum from the percolate tank and leaving the experiments to rest and reach mesophilic conditions for the first day, the adopted recirculation rate was of a total of 0.75 L distributed in 3 runs throughout the day. From then on, recirculation was adjusted according to biogas production and the evolution of VFA's COD in the percolate, which was carefully controlled throughout the experiments. In fact, after the 7 th experimentation day, while recirculation was increased in E3 to 1 L distributed in 4 runs, it had to be temporarily stopped in E4 for 2 days and, after that, reduced to a single 0.25 L run per day as VFA's COD showed a steady increasing trend reaching levels around 16 g/L, which we deemed as dangerous. On the 13 th day, when conditions were apparently stabilized in E4 and recirculation was increased to 0.5 L distributed in 2 runs, the sudden increase in VFA's COD (which reached 17.64 g/L) forced to stop temporarily recirculations in the experiment and let it rest again for two days and start again with a single recirculation of 0.25 L. At the same time, conditions in E3 had allowed to gradually increase the recirculation rate up to 2.4 L distributed in 6 runs. Figure 2: Methane production through time in \(\mathrm{l_{N}}\) CH\({}_{4}\) in experiments E1 (DGSBs as substrate, subfigure (**a**) and E2 (DGSBs as substrate with GAC used in the percolate tank, subfigure (**b**)) and the second phase of the experiment. After those first two weeks, conditions got permanently stabilized in E4, which allowed a gradual increase in the recirculation rate, performing the same operational way as in E3 but only with a 14-day lag. Thus, at the beginning of the 4 th week, while inundation cycles were starting to be performed in E3, the recirculation rate in E4 was of 9 L spared in 6 daily runs. The experiments were called to an end after 49 days. Accumulated methane production along time in both experiments are shown in Figure 3.  
The final specific methane productions in E3 and E4 were 295.06 and 217.57I\({}_{\text{N}}\) CH\({}_{4}\)kgVS\({}^{-1}\) respectively.