The aim of this work was to determine the influence of various variants of bioleaching on effectivity of releasing chosen critical metals: rhodium, cadmium, indium, niobium and chromium from ashes which are a byproduct of municipal waste and sewage sludge thermal processing. The research was conducted in 3 variants that considered different process factors such as temperature (24ºC and 37ºC), mixing intensity and aeration. After 5 days of the process the analyses were made of metals content, sulfate concentration, pH, general number of bacteria number, index of sulfur oxidizing bacteria. The best results of bioleaching were achieved by running the process at the temperature of 24ºC with aeration. The efficiency of rhodium and cadmium release from the byproduct of municipal waste thermal processing was above 90%. The efficiency of indium and chromium release reached 50–60%. Only niobium leached better in mixing conditions. The byproduct of sewage sludge thermal processing was far less susceptible to bioleaching. The highest effectivity (on a level of 50%) was reached for indium in temperature of 24°C with aeration. The efficiency of bioleaching depended on waste’s physiochemical properties and type of metal which will be released. Aeration with compressed air had a positive influence on the increase of sulfur oxidizing bacteria what corresponded with almost double increase of sulfate concentration in leaching culture. Such conditions had a positive influence on the increase of the efficiency of bioleaching process. Heightening the temperature to 37°C and slowly mixing did not impact bioleaching in a positive way.

Bioleaching research considers both the bio- and anthroposphere in the search for novel ways to recover resources and elements, which is important to the concept of sustainable development. Since the efficient, cost-effective and simple recovery of resources is of increasing importance in the circular economy model, the bioleaching of metals is a method currently gaining interest. The process is also of importance considering the need for the neutralization of waste materials/resources that allow for their safe storage and use. In this study, Acidithiobacillus thiooxidans bacteria, which is commonly found and widely utilized in the bioleaching process due to its high tolerance to heavy metals, was used in a twenty-eight-day experiment. The manner in which bacteria inhabit incineration residues was observed using fluorescence optical microscopy and scanning electron microscopy. The concentration of elements in incineration residues and in the post-reaction solutions was measured using inductively coupled plasma mass spectrometry and the efficiency of element recovery was calculated based on the results. Municipal waste incineration bottom ash and sewage sludge incineration fly ash were considered in the experiment. The extraction rates were far from satisfactory, with the average 20 and 50% for bottom ash and sewage sludge ash, respectively. The obtained results were consistent with microscopic observations where the relative number of bacteria increased only slightly over time in the sewage-sludge fly ash and was barely observed in the bottom ash of municipal- -waste incineration.