**1. Introduction**

The transition from oil-based energy to bioenergy can strive for technological efforts in this direction. Anaerobic digesters are widely used to treat organic waste to produce high-value products (biogas and biofertilizers) with the help of microorganisms [1, 2]. Currently, bioreactors occur in many different types. The sizes of these reactors can vary over several orders of magnitude, from mini-bioreactors (1–10 mL) to plant-scale reactors (2–500 m3 ) [3–6]. The need for automated multi-parallel mini-bioreactor systems is becoming more prominent. It occurs that no device is capable yet of meeting all the challenges of miniaturizing large-scale processes while keeping the functionality of conventional bioreactors [7–9]. As the bench-scale bioreactors are expensive, 3D printing can be considered as an alternative solution for the fabrication of miniaturized bioreactor systems [10–15]. The smaller the reactor, the more efficient it can become in terms of experimental

throughput [16]. This highlights the need for automated multi-parallel minibioreactor systems [17–19]. The pilot-scale reactors are often considered impractical since they require more feedstock, space, and energy than the mini-scale reactors. This makes the commercial bioreactors expensive, unrealistic, and inefficient as a process screening method. Additionally, the current state-of-the-art minibioreactors do not apply to complex microbial systems (e.g., the biogas production through anaerobic digestion) [20, 21].

A downsized approach of anaerobic digestion using mini-digesters is presented. In this study, 40 mL bioreactors were designed, fabricated, and operated to evaluate the anaerobic digestion performance and stability at a small scale. The start-up and operation of the mini-bioreactors were investigated. The results demonstrated that AD in low working volumes was feasible and efficient in terms of biogas quantity and quality. The results also established links between scale-down and process stability.
