INTRODUCTIONHydroelectric energy is born from the action of water, from its movement. This can be understood as a form of solar energy, because the sun starts the Earth's hydrological cycle. The hydrological cycle of water originates from the atmosphere due to the earth's surface. Some of the water evaporates, but most of it seeps into the ground or becomes surface runoff. Rainwater and melted snow ultimately protect the pond, natural reservoirs and the ocean, where constant evaporation takes place. The hydrological cycle Water that enters the ground can be converted to groundwater, some of which flows into streams via springs or underground flows. In times of drought, groundwater can raise the land surface, increasing evaporation into the atmosphere. Water vapor enters the atmosphere through evaporation, then circulates, condenses into clouds, and some returns to earth as rain. Small hydroelectric plants account for the largest share of electricity production from renewable sources. It is estimated that the installed capacity worldwide is 47,000 MW, with a technical and economic potential of nearly 180,000 MW. Small hydroelectric power plants (SHPs) are generally powered by water flow, without requiring significant quantities of water, as well as by the construction of large dams and reservoirs, although it is useful if they are there to can be easily used. There is general international agreement on the definition of MHE, with the upper limit being between 2.5 and 25 MW depending on the country, but generally accepting the value of 10 MW. MHE can be divided into "mini-hydropower plants", generally described as a composition of up to 500 kW. Regardless of the applicable definition, MHE is...... middle of paper...... substrate characteristics;- Quantities of toxic elements; 8) Provide sufficient biomass resources by their own production or with the help of the supplier, but the maximum delivery distance cannot be more than 150 km;9) Ensure biomass storage;10) Constant monitoring of conditions biological, thermal and chemical in the tanks during the anaerobic digestion process which guarantees maximum production; 11) Provide technical services and administrative staff for different operations; b) Legal requirements: 1) Legal requirements vary by country; 2) Monitor local and regional legislation; purchase price of heat and electricity from renewable sources, the price is guaranteed for a certain period, financial support, etc...).