基于重金属安全性的污泥水热处理温度选择

Choice of Hydrothermal Treatment Temperature for Sewage Sludge Based on Heavy Metals Safety Assessment

摘要:

采用水热反应处理污水污泥，研究了170~270 ℃范围内水热反应温度对污泥水热反应过程中重金属的分布和浸出的影响，分析了重金属质量平衡，并研究了氧气参与的影响.结果表明，水热反应后重金属主要保留在水热炭中，少量迁移到水热液中.对水热炭采用2种浸出标准评价，结果显示水热炭中的重金属稳定性较好，其卫生填埋比干污泥更为安全；而以回收利用为目的则需要选择合适的水热温度，某些温度如210 ℃以下的水热炭反而不如污泥本身安全.总体上重金属的浸出量随水热温度而降低；以重金属的稳定化为目标，260 ℃时水热液中重金属质量分数最少，同时水热炭中的重金属浸出量也非常低，是合适的温度.采用了2种消解方法检测水热炭中的重金属质量分数，发现重金属在210 ℃以上的水热炭中主要以残渣态存在，实现了对重金属的稳定化；同时也显示水热处理难以实现将Zn，As，Pb，Cu等从固相中迁移到液相以降低固相中的重金属质量分数.另外氧气的参与促进了少量重金属向液相转移，但是仍保证了炭中重金属的稳定性.

Abstract:

Sewage sludge was treated in a hydrothermal reactor within temperature of 170~270 ℃ to investigate the influence of hydrothermal temperature on heavy metals’ distribution and their leaching from solid products. The balance of heavy metals was calculated; and the influence of oxygen presence in the hydrothermal process was checked. Experimental results show that after hydrothermal treatment, heavy metals remain mainly in the solid products (hydrothermal chars) with small part moving into liquids. Leaching tests of hydrothermal chars were carried out according to two Chinese standards and the results showed that heavy metals in the hydrothermal chars were relatively stable, and that for sanitary landfilling hydrothermal chars were safer than the dried sludge; but for land application purpose, proper reaction temperature should be chosen to produce chars to replace sludge itself; as chars obtained at temperatures lower than 210 ℃ were inferior to sludge itself in respect to heavy metal leaching. Generally leaching of heavy metals from hydrothermal chars decreased as the hydrothermal temperature increased. From point view of heavy metals’ stabilization, temperature of 260 ℃ corresponded to the char with the least heavy metal leaching and to the liquid with the lowest heavy metal concentrations. Therefore, 260 ℃ was the best choice of hydrothermal temperature for the tested sewage sludge. Two digestion methods were adopted to digest hydrothermal chars for their heavy metals measurement. It was found that heavy metals in the chars obtained at temperatures over 210 ℃ existed mainly in residual species, which were quite stable. Also it was difficult for the heavy metals such as Zn, As, Pb and Cu to move from solid phase into liquid phase in the hydrothermal process. The appearance of O2 in the hydrothermal process slightly enhanced the mobility of heavy metals to the liquid phase, while heavy metals left in the chars were still stable.