Pyrolysis of 15 Swedish Sewage Sludges on a Pilot Scale: An Evaluation of Sludge Biochar Contents

Sammanfattning

With limited phosphorus supplies there is a growing need of recovering phosphorus from sewage sludge. Potentially reduced risks of heavy metal and organic micropollutant contamination makes sludge biochar, a product of pyrolysed sewage sludge, a promising alternative as an organic fertilizer. The effect of pyrolysis temperature, treatment methods and sludge characteristics on sludge biochar contents were examined by pyrolysing sewage sludges from fifteen different WWTPs at pyrolysis temperatures between 600-800 °C in a pilot-scale, continuous screw, slow pyrolysis unit. Mass flows of nutrients and heavy metals were determined. Sludge biochar contents of mercury could be reduced to trace amounts at the lowest pyrolysis temperature while cadmium and nitrogen concentrations decreased with increasing pyrolysis temperature. A pyrolysis temperature of 650-750 °C could be optimal depending on if nitrogen retention or cadmium removal is prioritised. The concentrations of other heavy metals increased during pyrolysis due to a high retention and loss of mainly organic matter, causing more of the biochars to exceed limit values compared to the sludges. The likelihood of a biochar meeting legal requirements regarding heavy metal content increased if the sludge was undigested and if the initial heavy metal content of the sludge was low, highlighting the importance of upstream work. Considering the removal of cadmium and mercury, as well as the immobilisation of other heavy metals and the removal of organic micropollutants, pyrolysis of sewage sludge can on the whole provide environmental advantages to sludge spreading. An accumulation of chromium and nickel in the biochar could be caused by erosion of the machinery. The water scrubber had a low potential of removing cadmium and mercury from the flue gas.