Biological drying (biodrying) is one of the methods of biological processing of waste, used mainly as part of the mechanical biological treatment of mixed municipal waste. Biological drying uses the heat released during the decomposition of organic matter to reduce the amount of water in dried waste. The aim of the analyses was to provide a microbiological and energetic (fuel) assessment of the process of biodrying of alternative fuel (RDF) obtained by mechanical sorting of mixed municipal waste. The resulting alternative fuel (obtained with just sorting) is characterised by varied moisture content and the presence of diverse groups of microorganisms. The analyses were intended to assess 3 alternative methods of biodrying of alternative fuel in order to produce a stable end product for utility power generation and the cement industry. The analyses were performed using special bioreactors equipped with custom (innovative) fluidised bed, aeration system (air flow rate 500 m3 · h−1), effluents drain systems, post-process air offtake and 4 temperature sensors. The assessment of the impact of the employed bed aeration methods on the quality of the alternative fuel was performed in 3 repetitions with the same external parameters. The obtained results show that after 8 days of biodrying, in the most favourable option, the moisture content in the fuel was reduced to the level of 18.7%, i.e. by 39%, the resulting fuel was microbiologically stable and the calorific value of the fuel was increased on average by 3.2 MJ · kg−1.