# Structure of Microbial Communities When Complementary
###### Abstract  
Olive oil and pig productions are important industries in Portugal that generate large volumes of wastewater with high organic load and toxicity, raising environmental concerns. The principal objective of this study is to energetically valorize these organic effluents--piggery effluent and olive mill wastewater--through the anaerobic digestion to the biogas/methane production, by means of the effluent complementarity concept. Several mixtures of piggery effluent were tested, with an increasing percentage of olive mill wastewater. The best performance was obtained for samples of piggery effluent alone and in admixture with 30% of OMW, which provided the same volume of biogas (0.8 L, 70% CH\({}_{4}\)), 63/75% COD removal, and 434/489 L CH\({}_{4}\)/kg SV\({}_{\text{in}}\), respectively. The validation of the process was assessed by molecular evaluation through Next Generation Sequencing (NGS) of the 16S rRNA gene. The structure of the microbial communities for both samples, throughout the anaerobic process, was characterized by the predominance of bacterial populations belonging to the phylum Firmicutes, mainly _Clostratiales_, with Bacteroidetes being the subdominant populations. Archaea populations belonging to the genus _Methanosarcina_ became predominant throughout anaerobic digestion, confirming the formation of methane mainly from acetate, in line with the greatest removal of volatile fatty acids (VFAs) in these samples. anaerobic digestion; biogas production; unbalance/inhibiting organic waste streams; monitoring; microbial community; Next Generation Sequencing 202111129331112931129311293112931129311293112931129311293112931129311293112931129311293112931129311293112931129312931129311293112931293112931293129311293129312931  
With regard to pig farming, the large and intensive animal production results in large amounts of manure, part of which can end up in the sewage system. Piggery effluent (PE) is responsible for gas emissions, notably greenhouse gases and nitrogen-based gas (ammonia and nitrous oxide) [12; 13]. So, to safely dispose of PE, it is always necessary to provide a treatment process for the manure. PE comprises two fractions: The solid, with animal excrement and food leftovers, and the liquid fraction, containing urine and water from washing and disinfections. It is rich in organic matter, solid residues (usually above 40 g/L), and nitrogen (mostly ammonia) [14; 15]. Moreover, it contains high amounts of phosphorus and potassium and other compounds that create bad odors (e.g., volatile fatty acids and phenolic compounds). Additionally, proteins, lipids, and cellulosic components are also present [14]. Anaerobic digestion is a complex biochemical process based on close interactions between various microorganisms, bacteria, and archaeal species, that includes several steps, as hydrolysis, acidogenesis, acetogenesis, and methanogenesis, resulting in the production of methane (CH\({}_{4}\)), carbon dioxide (CO\({}_{2}\)), and water (H\({}_{2}\)O). During the process, a coordinated metabolism is carried out by different groups of microorganisms, which can be affected by various environmental factors, such as temperature, pH, volatile fatty acids, ammonia, nutrient balance, or the presence of toxins and inhibitors [16; 17; 18]. The harmful characteristics of OMW for the anaerobic process have been overcome with previous procedures providing composition substrate change. Dilution with water, chemical correction, and physical, chemical, and biological treatments, individually or in combination, are pre-treatment actions that present the disadvantages regarding the water consumption and the consequent increase in the flow volume to be treated and the possibility of arising new compounds more toxic/ recalcitrant than the originals [19]. Other economic factors concern the loss of part of the organic load contained in the substrate and the inherent decrease in the available energy potential, as well as the cost of the pre-treatment equipment itself and its operation.