###### Abstract  
A hybrid anaerobic reactor was operated under the complementary effluents concept to reduce the unbalanced/inhibitory capacity of the provided piggery effluent. Brewery wastewater was chosen to complement piggery effluent (60:40% \(v/v\), respectively). The HRT reduction from 6.7 to 3.0 days allowed the testing of an organic load increase from 4.5 to 10.0 g COP/L-d, which resulted in the almost doubling of biogas production. Biogas volumes (1.2 and 2.1 L/L-d, respectively) associated with its quality (>77% CH\({}_{4}\)) revealed that the hybrid anaerobic reactor responded positively to the operational changes and that piggery effluent can be advantageously digested using the brewery wastewater as the complementary effluent. The unit bottom and the packing bed were the main functional sections recognized in the hybrid. At the beginning of anaerobic digestion, bacterial populations belonged mostly to _Bacteroidales_ (33%) and _Clostridiales_ (35%). The process stability and the biogas quality at 3-d HRT were related to a change in the structure composition, since _Flawobacteriales_ (18%), _Bacillus_ (7%), _Pseudomonadales_ (11%) and members of the _Alcaligenaceae_ family (5%) also integrated the microbial communities. An evident change had also occurred in archaeal populations at this phase. _Methanosaeta_ became the dominant genus (95%), confirming that acetoclastic methanogenesis was the main way for methane production. aaerobic digestion; biogas/methane; unbalance/inhibiting organic waste streams; brewery wastewater; piggery effluent; microbial communities profiling; Next Generation Sequencing +
Footnote †: journal:  
taking advantage of the generated effluent through its treatment and recovery. Among the different ways of proceeding with the treatment of an effluent, anaerobic digestion stands out as an alternative and advantageous process [7]. Anaerobic digestion converts the substrate organic content into biogas (mainly CH\({}_{4}\) and CO\({}_{2}\)) under the absence of oxygen, preserving its nutrients in the digestate. Controlled anaerobic digestion and the consequent biogas application will release carbon dioxide back to the carbon cycle and displace fossil fuel use [8; 9]. Anaerobic digestion is a complex biochemical process of successive metabolic stages (hydrolysis, acidogenesis, acetogenesis and methanogenesis), that are coordinated by bacteria and archaea communities in close interaction and interdependence [10]. The performance and stability of the anaerobic digestion process are vulnerable issues that depend on inhibitory or recalcitrant factors intrinsic to the composition of the substrate [11]. Pre-treatments are a common approach to enhance the anaerobic process and provide higher methane production [12; 13]. Operational techniques based on the complementary effluents concept can alternatively contribute to solve the disadvantageous initial composition of an \"anti-substrate\". Effectively, the combination of effluents that complement each other has been successfully applied in anaerobic digestion of unbalanced, toxic and/or concentrated substrates to promote organic matter conversion, avoiding substrate chemical adjustments and/or pre-treatments [14; 15; 16; 17; 18; 19; 20]. In addition, the reduction in plant operational expenses can be expected. One of the relevant aspects in the scope of anaerobic digestion concerns the characteristics of the anaerobic reactor to be installed and its performance. Different types of anaerobic reactor have been used in the research of organic materials conversion [9], and the importance of reducing the reactor volume in the installation costs implies operating in conditions of stability, with a short hydraulic retention time that withstands high organic loads and allows its maximum conversion into methane. One way to achieve this performance is to concentrate the microbial biomass through its immobilization, as happens in the high-rate anaerobic reactors.