The Migration And Transformation Behaviors Of Heavy Metals During The Microwave Pyrolysis Of Sewage Sludge

In order to improve the product quality obtained from pyrolysis of sewage sludge, and solve the secondary pollution problems such as heavy metals, the migration and transformation characteristics of heavy metals during microwave-induced pyrolysis of sewage sludge were explored in this study. The results may shred a light on the treatment of sewage sludge for harmless, minimization and recycling utilization..Municipal sewage sludge from Wuhan, China was selected in this study. The microwave-induced pyrolysis and electrical furnace pyrolysis of sewage sludge were carried out. The Simplified and Improved BCR Sequential Extraction Method were used to investigate the general character and characteristic of migration and transformation of heavy metals in sewage sludge pyrolysis product at different temperatures and with different heating methods. In addition, the heavy metal leaching characteristics of the bio-chars and bio-oils were studied. Finally, three different types of risk assessment system were applied to the heavy metal risk assessment in sewage sludge pyrolysis products, to obtain a comprehensive migration and transformation characteristics of heavy metals during the microwave-induced pyrolysis of sewage sludge.The results showed that the majority of heavy metals in sewage sludge were enriched with different degrees in bio-chars after the pyrolysis treatment. On the whole compared with electrical furnace pyrolysis, microwave-induced pyrolysis is more conducive to increase the residual fraction(F4) of heavy metals in bio-chars to enhance the content of heavy metal stability. Bio-chars derived from microwave-induced pyrolysis can get a better pore structure characteristics and more stable heavy metals forms distribution at lower temperatures. Avery small amount of heavy metals were relaesed into the bio-oils with the volatile. The increase of pyrolysis temperature increase the content of acid exchangeable(F1) fraction in bio-oil, which is harmful to the utilization of bio-oil. The temperature of 600? may be more appropriate for microwave-induced pyrolysis of sewage sludge, which could balance the stablization of the total amount of heavy metals and form orientated transformation.Leaching characteristics of pyrolysis products are mainly affect by p H, pyrolysis temperature, heating method and acid exchangeable(F1) fraction. The leaching concentrations of most hevy metals in sewage sludge were significantly reduced for an order of magnitude after pyrolysis. Sewage sludge pyrolysis process contributed significantly to improve the effectiveness of heavy metal fixation, and reduce the heavy metals leaching toxicity. The principle which pyrolysis temperature and pyrolysis mode influence the leaching characteristics depending on fluctuating the acid exchangeable(F1) fraction and sewage sludge chars pore structure characteristics, on the other hand, p H value is one of the main factors affecting the physical adsorption and chemisorption of heavy metals in sewage sludge.