Study On Remaining Property Of Heavy Metals In Liquefaction Residue Of Sewage Sludge

The sewage sludge of urban sewage treatment plant is an inevitable secondary pollutant in wastewater treatment process. The improvement of environmental quality standard results in a drastic increase of sewage sludge production. Thus, how to properly handle the sewage sludge attracts more and more attention. Recently, the technology of converting sewage sludge into bio-oil by direct thermochemical liquefaction has been extensively studied. However, there were few studies reported focusing on remaining properties of heavy metals in liquefaction residue of sewage sludge. Thus, this paper intends to carry out the following two points of researches:(1) Studying the influence of liquefaction parameters on residual rules of heavy metals in liquefaction residue of sewage sludge, including reaction temperature, solid-liquid ration and solvent filling ratio.(2) By using the method of toxicity leaching procedure (TCLP), investigating the leaching characteristics of heavy metals in sewage sludge versus liquefaction residue.The main results and conclusions were as follows:(1) After liquefaction, most heavy metals still remained in the liquefaction residues, while almost organic components in the sewage sludge were converted into other products (bio-oil and gas products). Thus the content of heavy metal in liquefaction residues increased.(2) The liquefaction temperature had no obvious impact on the residual characteristics of heavy metals in liquefaction residues. The main reason may be that the heavy metals had higher boiling point or melting point. Therefore, they could exist stably in the liquefaction residues during the liquefaction process. It was uneasy for them to convert and transfer, now that the reaction temperatures were only set in the range of553K-633K.(3) With the increased solid-liquid ratio, the content of most heavy metals in liquefaction residue increased.(4) The content of Cu, Cd and Ni increased with the increase of solvent filling ratio, while the concentration of Pb, Zn, Cr and Mn increased firstly and then decreased.(5) The leaching content of heavy metals in the liquefaction residues decreased significantly compared with that in sewage sludge. The leaching content of heavy metals could directly reflect their environmental risk values. Compared with raw sewage sludge, the environmental risk of heavy metals in the liquefaction residues declined significantly. Now that the leaching content of Zn in liquefaction residues still exceeded standard value, the liquefaction residue still had some risk to the environments. It was also shown that preliminary treatment was necessary before recycling and final treatment of the liquefaction residues of sewage sludge.(6) In the liquefaction process, NaOH was used as the catalyst to enhance the conversion of sewage sludge to target product (bio-oil). At the same time, the addition of NaOH could also facilitate heavy metals to transform from unstable fractions to stable ones, which was confirmed by the significant decrease in leaching content of heavy metals.