## 3 Results and Discussion
### Semi-Continuous Experiments  
In the initial part of the regime phase (Figure 3b), methane production seems to be linked only to the applied organic loading. In fact, reactors 1 and 3, and 2 and 4, respectively, behave similarly. This behaviour was confirmed for reactors 2 and 4 (low loading) until the end of the experiment, while reactors 1 and 3 present a different production pattern. Reactor 1 and 3's productions slowed gradually since about day 60 for the latter; this tendency was more pronounced, but a sudden recovery was also evident from about day 75 of the total operation. For the first 20 days of regime phase (1-HRT) the average yield (Figure 3b) was similar for reactors 1 and 3, and 2 and 4, respectively; it was equal to about 0.2 NL-gVS\({}_{\text{added}}\)\({}^{-1}\) for reactors 1 and 3 and to about 0.25 NL-gVS\({}_{\text{added}}\)\({}^{-1}\) for reactors 2 and 4, respectively. Then a continuous decrease was evident for reactor 1 that reached a value of about 0.17 NL-gVS\({}_{\text{added}}\)\({}^{-1}\) in the last days of the experiment. On the contrary, after a sharper decrease until day 75, first stabilization and then a slight increase was registered; in the last days of operation the yield of reactor 3 surpassed that of reactor 1.  
pH (Figure 3c) in all reactors during the regime phase was close to 7, seldom needing NaHCO\({}_{3}\) addition to increase the buffering capacity. A marked tendency toward a reduction is evident in reactor 3 after the beginning of the feeding of the new sludge. TS (data not displayed) and VS concentrations (Figure 3d) were stable during the regime phase; TS were slightly higher for reactors 1 and 3 (loading 1 gVS-L\({}^{-1}\).day\({}^{-1}\)). The COD concentration (Figure 3e), except for a few spikes (e.g., week 9--reactor 2), is quite low when the process is stable and tends to increase when the process is inhibited (reactors 1 and 4, since weeks 9-10). The ammonium ion concentration (Figure 3f) displays a tendency to decrease, and reaches very low values (it is practically absent) in reactor 1 since week 4 of operation and in reactor 2 since week 6. The VFA concentration and FOS/TAC (Figure 3g,h) are two important process stability indicators, they are conveniently low for reactors 2 and 4, while they display a tendency to increase since week 10 for reactors 1 and 3. In reactor 3, after the beginning of GAC addition, VFA suddenly decreases and the FOS/TAC stabilizes. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Market Waste** & **Anchovy Sludge** &
\begin{tabular}{c} **Mix (Market w. +** \\ **Anch. Sludge)** \\ \end{tabular} \\ \hline pH & 7.6 \(\pm\) 0.00 & 7.6 \(\pm\) 0.06 & 7.5 \(\pm\) 0.01 \\ COD [mg/L] & 7008 \(\pm\) 398 & 11470 \(\pm\) 130 & 7073 \(\pm\) 385 \\ ammonium ion [mg/L] & 1435 \(\pm\) 60 & 1861 \(\pm\) 61 & 1411 \(\pm\) 39 \\ chloride [mg/L] & 1280 \(\pm\) 93 & 1563 \(\pm\) 105 & 1363 \(\pm\) 274 \\ VFA [mg/L] & 550 \(\pm\) 156 & 3662 \(\pm\) 69 & 651 \(\pm\) 129 \\ FOS/TAC & 0.11 \(\pm\) 0.03 & 0.4 \(\pm\) 0.02 & 0.12 \(\pm\) 0.02 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Analyses of digestate from BMP experiments. Figure 3: Semi-continuous experiments results: (**a**) methane cumulative production, (**b**) methane yield, (**c**) pH, (**d**) volatile solids, (**e**) COD, (**f**) ammonium ion, (**g**) VFA, and (**h**) FOS/TAC. It is worth pointing out that the reactors during the start-up phase, as mentioned above, suffered from a severe overloading that, however, did not seem to influence significantly the regime phase, indicating a good recovery.