## 1 Introduction  
Currently, there is an increase in milk production worldwide, and dairy processing is now considered the largest source of food industrial wastewater, estimated at 192.5 \(\times\) 106 m\({}^{3}\) in the EU [1; 2; 3]. According to the literature, anaerobic systems are suitable and cost-effective methods for the treatment of high-strength dairy wastewater [4; 5; 6; 7]. Nowadays, in full-scale applications, many different types of anaerobic reactors are used, such as upflow anaerobic sludge blanket (UASB) reactors, anaerobic filters (AFs), and anaerobic fluidized bed reactors (AFBRs) [2; 4; 7; 8]. However, the major limitation of their use in the treatment of dairy wastewater is the need for wastewater pre-treating due to a high content of suspended solids as well as fat, oil, and grease (FOG), which inhibit the development of anaerobic microflora [4; 9; 10]. Reactors with suspended biomass are more recommended for dairy wastewater treatment, but they can be operated only at low loading rates [7; 11; 12]. Moreover, all of the typical reactors have poor biomass retention resulting from high concentrations of FOG in dairy wastewater [2]. Therefore, in recent years, searching for new designs of hybrid anaerobic reactors that combine the positive features of individual anaerobic reactors for the treatment of dairy wastewater has been a challenge [7]. The new constructions of anaerobic hybrid digesters must both enhance biogas production and overall treatment efficiency and be easy to scale up to full-scale applications [1]. According to the newest studies, higher biogas production efficiency can be reached by increasing free electron flow via mediated/direct interspecies electron transfer (MIET/DIET) [13; 14; 15; 16; 17; 18]. Some authors have also suggested DIET is the essential mechanism for interspecies electron transfer in UASB reactors [19; 20; 21]. However, UASB reactors are typically operated at high OLRs to enhance the biogas production rate, but a high concentration of organic load increases the rate of acidogenesis, resulting in volatile fatty acid (VFA) accumulation and a sharp drop in pH leading to reactor failure [7; 22; 23]. In novel hybrid reactors, the acidogenic and methanogenic phases are separated to isolate each metabolic pathway that requires specific conditions for smooth and efficient operation [7; 24; 25; 26]. In addition, the optimization of operating conditions has the primary effect on the successful performance of hybrid reactors, among which OLR is the most important in forming the appropriate groups of anaerobic microorganisms [7; 27; 28]. The phyla Firmicutes, Bacteroidetes, and Synergistetes have been identified as major consortia that cooperate in maintaining the appropriate environmental niche for efficient and stable methane production [27]. In addition, other phyla such as Chloroflexi, Actinobacteria, and Proteobacteria are commonly recognized in anaerobic digesters [29; 30]. Recently, the performance of a structured-bed hybrid baffled reactor (SBHBR) with anaerobic and oxic/anoxic chambers has been designed to enhance simultaneous organic matter and nitrogen removal in dairy wastewater [31]. Hybrid anaerobic reactors with different inert media were successfully used to increase the efficiency of dairy wastewater treatment [32; 33]. A hybrid anaerobic baffled reactor also ensured efficient digestion of dairy wastewater and promoted the growth of acetoclastic and hydrogenotrophic methanogenesis in the reactor, represented mainly by Methanosaeta and Methanobacterium species, as well as other communities such as Syntrophomonadaceae and Syntrophaceae microorganisms [34]. The potential of granular activated carbon supplementation to increase the anaerobic degradation of dairy wastewater was investigated by [PERSON] et al. [35]. They found stable and efficient methane production and high activity of electroactive microorganisms such as Synergistes and Geobacter, as well as the methanogens Methanoalinea and Methanosaeta.