Heavy Metals Precipitation in Sewage Sludge

There is a great need for heavy metal removal from strongly metal-polluted sewage sludges. One of the advantages of heavy metal removal from this type of sludge is the possibility of the sludge disposal to landfill with reduced risk of metals being leached to the surface and groundwater. Another advantage is the application of the sludge as soil improver. The use of chemical precipitation to remove dissolved heavy metals from sewage sludge implies a high cost for chemicals. This work shows, for real sewage sludge for the first time that the addition of NaOH as first precipitating agent considerably saves the addition of Na2S, that is one of the most effective metal precipitating agents and also expensive. After solubilization of heavy metals by chemical leaching with previous aeration, the next step was the separation of the sludge solids from the metalrich acidic liquid (leachate) by centrifugation and filtration. Afterwards, the filtered leachate was submitted to the application of NaOH and Na2S, separately and in combination, followed by filtration. The results showed that when iron and aluminium are present in the leachate, adsorption and/or coprecipitation of Cr, Pb, and Zn with Fe(OH)3 and Al(OH)3 might occur at increasing pH conditions. The combination of hydroxide and sulfide precipitation was able to promote an effective removal of heavy metals from leachate. Applying NaOH at a pH of 4-5 as a first precipitation step, followed by filtration and further addition of Na2S to the filtered liquid at pH of 7-8 as a second precipitation step, decreased considerably the dosage of the second precipitant (almost 200 times), compared to when it was solely applied. This has practical applications, as the claimed costs drawbacks of H2S addition is considerably reduced by the addition of the less expensive NaOH. The best removal efficiencies obtained were: Pb: ~ 100%, Cr: 99.9%, Cu: 99.7%, and Zn: 99.9%.