INTRODUCTIONHydroelectric energy arises from the action of water, from its movement. It can be understood as a form of solar energy, as the sun initiates the earth's hydrological cycle. The hydrological cycle of water comes from the atmosphere due to the Earth's surface. Some of the water evaporates, but most seeps into the ground or turns into surface runoff. Rainwater and slush ultimately protect the pond, natural reservoirs, and ocean, where constant evaporation occurs. The hydrological cycleWater that enters the soil can be converted into groundwater, some of which flows into streams through springs or groundwater flows. Groundwater in times of drought can raise the surface of the soil, which increases evaporation into the atmosphere. Water vapor enters the atmosphere by evaporation, then circulates, condenses into clouds and some returns to the earth in the form of rain. Small hydroelectric plants hold the largest share of electricity production from renewable sources. Globally installed capacity is estimated to be 47,000 MW, with a technical and economic potential of nearly 180,000 MW. Small hydroelectric plants (SHPs) are usually driven by the flow of water, without needing significant quantities of water, as are the construction of large dams and reservoirs, although it is helpful if they are present so they can be easily used. There is a general international agreement on the definition of MHE, the upper limit varies between 2.5 and 25 MW in different countries, but in most cases accept the value of 10 MW. MHE can be further divided into "mini hydroelectric power station", which is usually described as having a composition of up to 500 kW. Regardless of the definition applied, MHE is... half of the paper... substrate characteristics;- Amount of toxic elements;8) Provide sufficient biomass resources through own production or with the help of the supplier, but the maximum distance for delivery cannot be more than 150 km;9) Provide storage for biomass;10) Constant monitoring of biological, thermal and chemical conditions in the tanks during the anaerobic digestion process which ensures maximum production;11) Provide specialized technicians and personnel administrative for different operations;b) Legal requirements:1) Legal requirements vary in different countries;2) Monitor local and regional legislation;3) National law instruments that support renewable energy projects are different ( for example, purchase prices for heat and electricity from renewable sources, guaranteed price for a certain period, financial support, etc.).