## 1 Introduction  
Since the 1990s, the gross inland energy consumption of biogas has increased 25-fold, exceeding 163 MWh in 2019 in the European Union [1]. This increase has been supported by technological development and policy guidelines and strategies promoting this type of energy production. Germany, one of the leading biogas producers in Europe, is already generating 1 MWh per capita, and similar or even higher levels are considered possible in many countries [2]. The biogas plants in Croatia generated 316.5 GWh in 2018 [3], which roughly equals 0.08 MWh per capita, indicating significant improvements in this sector are desirable and necessary. The European Union demands such improvements, outlined in the new regulative, REPowerEU [4], and sets new targets for the member states to produce 20% more biogas and biomethane by 2030. In the EU, the technically and economically most important substrate for biogas production is maize silage, which is considered the benchmark in terms of the biomass volume per hecare and biogas produced [5]. However, corn silage is also used as livestock feed, so demands are increasing, leading to a sharp rise in silage prices in the EU market. Therefore, gradually replacing silage with alternative biomass sources is both a long and short-term goal. Lignocellulose waste, such as crop residues, energy crops, municipal solid waste, or forest waste, are abundant and renewable resources [6]. They are rich in sugars and other energy-valuable structural components and are considered one of the alternative biomass sources for biogas production. In Europe, especially in Mediterranean countries, one of the sectors producing large amounts of lignocellulosic waste is the olive (_Olea europaea_) industry. As olive is mainly cultivated for olive oil extraction, this sector is a major producer of olive cake, as the primary residue and pruning residues. Disposal of these waste, in many cases, causes environmental problems such as odor production and change in the soil microbiome. On the other hand, these wastes have considerable potential for further utilization as an energy source [7]. Carbohydrate components of lignocellulosic biomass (cellulose and hemicellulose) are fermentable, making it a suitable substrate for bioenergy and biofuel production. However, lignocellulosic biomass is highly resistant to microorganism enzymatic biodegradation. Therefore, optimized (pre)treatments that make the carbohydrates available for further processing are often performed [6; 8], increasing the time and money cost of the processes. Another option for boosting biogas production during anaerobic digestion is bioaugmentation, a process that implies the addition of targeted and desired microorganisms in bioreactors. In the case of biogas production, these microorganisms, mainly bacteria and methanogenic archaea, can enhance the primary degradation of substrate, fermentation, or methane production. These can be added as pure cultures, i.e., _Acetobacteroides hydrogenigenes_[9], a hydrogen-producing acetate-type fermenter, or _Methanoculleus bourgensis_[10], methanogenic archaea. Another option is to add a microbial consortium that combines methanogens, hydrolytic, acidogenic, or acetogenic bacteria [11; 12; 13]. Finally, the added microorganisms can be purchased from banks of cell cultures [9; 10] or previously isolated from a suitable environment and propagated [12; 14]. In the presented study, the idea was to isolate the targeted consortium of microorganisms from the inoculum to be used for anaerobic digestion. The isolated bacteria are intended to be specifically optimized to use the olive waste (olive cake and pruned biomass) as a substrate for biodegradation. These microorganisms augmented the biogas reactors. To enhance the process, isolated microorganisms are to be immobilized onto a suitable bioccarrier. Microbial cell immobilization can propagate the process stability, intensify biochemical transformations, protect the microbial cells from unfavorable influences, and optimize the cost-effectiveness [14; 15].