Heavy metal removal from sewage sludge by pyrolysis treatment

Abstract: Sewage sludge is the product from wastewater treatment that mostly is considered as a waste material. However, it contains several nutrients, especially phosphorus, potassium and nitrogen which are excellent fertilizers. The downside is the harmful content it also carries with pathogens, heavy metals and a variety of organic pollutants that in many cases have unknown effects on the ecosystem. A possible solution to this problem could be to pyrolyse the sewage sludge and by that decrease the levels of heavy metals and also render both pathogens and organic pollutants harmless. In this thesis project pyrolysis of dried sewage sludge has been evaluated at temperatures 650 750, 850 and 950 °C with addition of chlorine in the form of PVC and straw. An energy balance for pyrolysis and drying of dewatered sewage sludge has also been suggested. The results of the pyrolysis evaluation indicate that cadmium concentration can be reduced significantly with increasing temperature in the product char. But also other heavy metals like lead and zinc are affected at the higher temperatures evaluated. Mercury is completely removed from the char residue. The more latent volatile metals copper, chromium and nickel cannot be reduced to lower concentrations at these temperatures. They are instead enriched under these conditions. Chlorine addition to the sludge enhances the evaporation of all heavy metals but copper, nickel and chromium. The energy balance over the system indicates that the drying process requires more energy than is released from the sludge into the pyrolysis gases. The energy carried by the pyrolysis gases is however larger than what is required to drive the pyrolysis process.