## 3 Results and Discussion
### Box-Type Digester Assays  
Two sets of two experiments in parallel were performed. In the first roll of experiments, which we'll call E1 and E2, only DGSBs were used as substrate. In experiment E1, the box tank for solid substrate was filled with 9.97 kg of feedstock, while 10 L of inoculum were put in the percolate tank. The amounts of feedstock and inoculum for experiment E2 were 10.006 kg and 10 L, respectively. In this experiment, GAC (20 \(\times\) 40) was added to the percolate tank with the purpose to assess the possible influence of the presence of a conductive material. To avoid the possible interference with the pumping recirculation system and the clogging of pipes and distribution devices, the charcoal was put in a cage-like container, which allowed the flow of inoculum and percolate to pass through and get in contact with the conductive material. On the first day, the substrates were inoculated by sprinkling 3 L of inoculum (in one run) in each digester and letting them rest, allowing them to reach mesophilic conditions. The same operation was performed on the following two days. On the third day, recirculation was increased to a total of 4.5 L in three runs (of 1.5 L each). And on the fourth day, it was observed that biogas production, which had been increasing both in the box digesters and in the percolate tanks in the previous days, had plummeted. It was detected that Volatile Fatty Acids (VFA's) COD in the percolate tank had experienced a sudden increase (reaching 13.1 and 12.77 g/L respectively in E1 and E2) so inoculum recirculation was stopped. On the following days, the efforts were focused on the recovery of the inoculums from the failure by acidification. All actions performed (addition of a total of 15% fresh inoculums by volumeand of calcium bicarbonate on several occasions) proved worthless. VFA's COD kept increasing reaching 40.98 and 34.11 g/L in each experiment. Biogas production disappeared in percolate tanks and was residual in box digesters, being observed (but not measured) the presence of H\({}_{2}\) in the latter. Finally, after 15 days from the beginning of the experiments, it was decided to stop and reevaluate decisions and the course of actions. On the following days, two new batches of inoculum were acclimatized by progressively adding 2 L of inactivated inoculum from the previous run of experiment E1 and with a VFA's COD of around 41 g/L to 9 L of fresh inoculum each. Biogas production in the new batches was observed and after 21 days the adaptation seemed satisfactory, so these new batches of 11 L of inoculums each were used in a new run to end the experiments with the partially digested substrate. The digesters were again inoculated by sprinkling another 3 L of the new inoculums and allowed to rest and reach the mesophilic range of temperatures for the rest of the day. After that, a 1 L recirculation was performed. Biogas production started, but on the following day it was highly reduced, so recirculation was stopped for 10 days while biogas production and VFA's COD in the percolate tanks were controlled. After that and since VFA's COD had been highly reduced, recirculations started again, starting with 0.5 L in two runs (of 0.25 L each) per day. After a week and as the process stabilized, the volume of percolate recirculation was progressively increased, first with another run of 0.25 L (totalling 0.75 L) at which point, the box digesters (which had been inactive) started showing activity and biogas production. Percolate recirculation was increased in the following days as VFA's COD was kept under control and its amount was below 7 g/L. Thus, after 12 days the recirculation volume was 12 L spared into 4 runs along the day. After that, and as the reactor setting allowed the closing of the return line and the accumulation of percolate in the box reactor (acting as a percolating reactor system), recirculation was substituted by daily \"substrate floodings\".