A process for recovering reusable materials, in particular phosphate, from sewage sludge products, namely sewage sludge, sewage sludge ash or sewage sludge slag, by extraction, in which a suspension of the sewage sludge product is produced in water, alcohol, water-alcohol mixture or an aqueous solution, gaseous carbon dioxide (CO2) or supercritical carbon dioxide (scCO2) is introduced as an extraction agent into the suspension of the sewage sludge product, undissolved solids are separated from the liquid suspension agent, carbon dioxide is removed from the suspension agent, and reusable materials dissolved in the suspension agent are precipitated and separated from the suspension agent.