## 4 Conclusions  
The inhibiting capacity of the unusual and concentrated piggery effluent can be advantageously overcome by applying the complementary effluents concept. The brewery wastewater used as the complement provided the decrease in the piggery effluent concentrations, such as nitrogen, VFA and TP contents, making the anaerobic digestion process viable without any previous change in its composition. The tested hybrid anaerobic reactor  
Figure 7: Relative abundance and dynamics of archaeal communities and taxonomic classification of archaeal reads at genus level in samples collected at two operating conditions and effluents. BWW—Brewery wastewater, PE—Piggery effluent, HRT—Hydraulic retention time, OUT—Operational taxonomical unit.  
withstood large substrate streams (HRT of 6.7 d and 3 d) whose high organic loading rates of 4.5 and 10.0 g COD/L-d provided productions in biogas of 1.2 and 2.1 L/L-d (77.8-79.5% CH\({}_{4}\)), respectively.  
Different functional sections of the hybrid anaerobic reactor could be recognized: (a) the unit bottom (0-14 cm) identified as the sludge blanket section, presents the highest matter concentration (COD, TS and VS) and carries out the main consumption of VFA and the basification of pH; (b) the unit central section (14-37 cm) where a decrease in previous concentrations was observed and VFA is maintained in low content and pH in the same basic value; (c) the unit upper section (37-48 cm) shows the greatest reduction in the matter concentrations that is justified by the functional action of the packing bed in retaining the material in digestion. VFA and pH were maintained at the preceding values.  
The metagenomic analysis presented a considerable shift of the microbial populations during the experiments towards a specialization to the operational conditions in the anaerobic processes, with the loss of the initial population diversity. The high relative abundance of _Bacillus_, _Clostridiales_ and _Pseudomonas_ could be indicative of the important contribution of these bacteria to the efficient bioconversion at HRT 3 d. Likewise, _Methanosaeta_ was the predominant methanogenic archaeal genus, strongly suggesting that acetoclastic methanogenesis could be the main pathway in the process.  
Long-term studies will provide more insights on the hybrid reactor behavior digesting unbalance/inhibiting effluents concerning its efficiency under extreme operating conditions of organic load. Microbial biomass stratification and function and the packing bed effect on the unit removal capacity will be the predictable goal of future research in the continuation of this work.  
Conceptualization, A.E. and I.P.M.; data curation, A.E., A.N. and I.P.M.; formal analysis, A.E. and I.P.M.; investigation, A.E., A.N. and I.P.M.; methodology, A.E., A.N. and I.P.M.; resources, A.E. and I.P.M.; supervision, A.E. and I.P.M.; validation, A.E. and I.P.M.; writing--original draft, A.N.; writing--review and editing, A.E. and I.P.M. All authors have read and agreed to the published version of the manuscript.  
This research was funded by ERANETLAC/0001/2014--GREENBIOREFINERY project.  
Not applicable.  
**Data Availability Statement:** Publicly available datasets were analyzed in this study. These data can be found at the NCBI Sequence Read Archive (SRA) database with accession numbers of SRR13579864, and SRR13697433-SRR13697436 (bioproject PRJNA701622).  
Authors would like to thank SOCIEDADE CENTRAL DE CERVEJAS E BE-BIDAS (SCC, Vialonga, Portugal) and VALORGADO (Salvaterra de Magos, Portugal) for supplying the brewery wastewaters and piggery effluents. Special thanks to [PERSON] for laboratory assistance.  
The authors declare no conflict of interest.