## 2 Materials and Methods  
### Hybrid Pump to Mix Fermenter Contents Using Biogas  
The research was conducted with the use of two devices: a hybrid pump for lifting a liquid with added solids, using a working medium such as gas or air, and a fermentation tank for disposing of liquid biomass with added solids by methane fermentation. The pump design was adapted to the shape of vertical fermenter. If a fermenter of a different shape is employed, the pump should be structurally modified. The fermentation tank prepared for research comprises an in-built hybrid pump that operates as a mammoth, geyser, or hybrid pump, depending on process requirements. The automatic control system can switch the operation into geyser mode or to a hybrid system. The times of each mode can be adjusted as required. In the pump-mixing system, biogas is directly collected above the surface of fermenting biomass and distributed into the fermenter contents. The pumped stream with a specific kinetic energy comprises biomass as fermenter contents and biogas as a working medium. The biogas supply line can be equipped with a line supplying hydrogen directly from the hydrolyser. The working medium that is part of the stream may have three forms: a substantial gas bubble, fine micro-bubbles, or both at the same time. The pumped stream saturates biomass with fresh biogas and homogenizes the composition and temperature of biomass throughout the fermenter volume. This is done by releasing a substantial gas bubble from the pump baffle assembly, which, flowing through the fermenting biomass bed, detaches the over-expanded microbial colonies and distributes them throughout the fermenter contents. This bubble, after passing through the biological bed, moves through the upper biomass fractions, mixes them, and finally rapidly agitates (shakes) the upper layer and the substrate surface, which results in a faster release of biogas. As it goes through the entire system, the bubble pumps substantial amounts of biomass upwards. The fermenting biomass is saturated with fresh biogas or biogas with added hydrogen primarily through the operation of a diffuser that generates a cloud of fine gas bubbles. The efficiency of saturation of the entire biomass volume with gas is affected by the size of bubbles that are generated by the diffuser and rise slowly upwards, and consequently saturate and slowly mix the fermenting biomass. The diffuser is mounted at the bottom of the upper pump baffle, hence the cloud of gas bubbles generated is also transported to farther biomass fractions by periodically releasing the gas bubble from the vertical baffle assembly. When doping hydrogen into biogas, both gases are thoroughly mixed in chambers formed by vertical baffles, even before their use for mixing. The gas-flow generation pump has a separate power supply for both functional parts, which means that the geyser section and the diffuser (or diffusers 1-2 and more) are separately powered, which enables the pump to operate in three variants: operation of geyser section, operation of diffuser-mammoth unit, and simultaneous operation of both sections. The choice of operation variant depends on the technological process requirements. In this study, a functional pump model was constructed, tests were carried out, technical documentation of the biogas fermenter with an in-built pump was produced, a prototype integrated into a standard layout of a biogas plant utilizing pig slurry on a farm was developed, and functional tests of the entire biogas plant were carried out. A schematic diagram of the pump in Figures 1 and 2 shows a view of the model used to test the functionality and validity of the previous solutions (left) and a view of the design of the pump integrated into the digester (right). Figure 3 shows a general view of the pilot plant used to test the effect of hybrid pump mixing on methane yield increase.