They are based in the selective solubilisation of fertilizing elements (N, P and K) to be kept in sewage sludge, and heavy metals to be released and chelated by fly ash.

<p style="text-align: justify;"> Different pH values (acidification), extractive agents and physical conditions were assayed (nitric acid, ammonium acetate, water, high temperature, etc.). &nbsp;</p> <p style="text-align: justify;"> &nbsp;</p> <p style="text-align: justify;"> With nitric acid and low pH values, a choosy release of heavy metals was intended. With ammonium acetate and water, the same discriminating release was achieved for N, P and K from the sludge. Both trials were also performed at high temperature to increase effectivity, but no significative improvements were recorded regarding ambient temperature.</p> <p style="text-align: justify;"> &nbsp;</p> <p style="text-align: justify;"> The chemical analyses of separated fractions from all the trials did not showed a good selective separation of the two interesting fractions; a loss of each type of elements (fertilizers and heavy metals) is assessed in the filtrates.</p> <div> &nbsp;</div>