## 3 Results and Discussion
### Digestate Main Characteristics  
The main physicochemical characteristics of the digestates resulting from the BMP tests carried out in this study are shown in Table 3. The pH ranged from 7.77 to 8.22 across the board, except for the P.o.:Ur. test which showed a higher value (9.2). pH values between 6.5 and 8.5 suggest a stable AD performance, although at pH under 6.3 or over 7.8 the methanogenesis stage may suffer failures and be partially hindered [38]. A higher pH may be related to the formation of ammonia during the hydrolysis stage, which is inhibitory to methanogens. Aqueous ammonia (NH\({}_{4}\)\({}^{+}\)) is in equilibrium with free ammonia (NH\({}_{3}\)); however, this equilibrium shifts to free ammonia formation as the pH increases. Concentrations of aqueous ammonia between 1.7 and 14 g NH\({}_{3}\) L\({}^{-1}\) have been reported as inhibitory to methanogens, especially to acetoclastic species [38]. However, the total ammonium nitrogen (TAN) limits, from which inhibitory effects are observed, are highly related to other factors, such as substrate, inoculum, environmental conditions, and microbial acclimation [39]. In any case, it is generally considered that TAN values below 2500 mg N-NH\({}_{4}\)\({}^{+}\) L\({}^{-1}\) do not indicate significant inhibition [26]. Another parameter directly linked to the stability of the AD process is the buffer capacity of the system, measured as the TA. It is generally accepted that TA, as a measure of carbonate buffer capacity, should be above 3000 mg CaCO\({}_{3}\) kg\({}^{-1}\)[26]. However, there is very little information about a TA upper limit, which may indicate an inhibition process. In fact, in this study, the TA value of the digestate derived from the inhibited P.o.:Ur. test was as high as 15,100 mg CaCO\({}_{3}\) kg\({}^{-1}\)Based on the physicochemical characteristics, the assayed digestates could be roughly separated into three main groups: (i) P.o. and P.o.:R.s. ; (ii) P.o. :Cas., P.o. :Pep., and P.o.:Syn.Cas. ; (iii) P.o.:Ur. The first group is characterized by showing the lowest pH, TA, and TAN values, although within acceptable values for the adequate performance of the AD process. The experimental methane yield was significantly lower than those achieved from the second group, despite the higher theoretical value (Table S2). This could be related to the lignocellulosic character of sun-dried _P. oceanica_. Moreover, the relatively low COD value suggested that the main limiting step was the hydrolytic stage, although the differences between both P.o. and P.o.:R.s. tests might also indicate that the methanogenesis was more limited in the P.o. test, based on the higher COD value and the lower methane yield. These differences might be related to the more balanced C/N ratio of the P.o.:R.s. test, which allowed for more consistent and stable bacterial and archaea growth. Similar results have been observed recently when other lignocellulosic biomasses, such as thermo-chemical pretreated rice husk, were co-digested with cow manure at different C/N ratios (13.8-75.7) [40]. The results showed that the highest methane yield was achieved at a C/N ratio of 24.7, meaning a yield improvement of 76% against the control [40]. In the present study, an improvement of 146% in methane yield of P.o.:R.s. compared to the mono-digestion of the sole _P. oceanica_ was observed (Figure 2). The evaluation of _P. oceanica_ as feedstock for an AD process is scarce in the literature. Only two studies reported specific methane yields using _P. oceanica_ as a substrate. [PERSON] and [PERSON] [4] reported values from 29 to 62 L CH\({}_{4}\) kg\({}^{-1}\) VS, depending on the operating conditions. These results were slightly lower than that obtained in this study from the sole digestion of _P. oceanica_ (79 \(\pm\) 3 NL CH\({}_{4}\) kg\({}^{-1}\) VS). Additionally, [PERSON] and [PERSON] [4] also evaluated the effect of some pretreatments, such as washing, thermal, or acidification with HCl, on methane production.