汞在城市污水处理厂的赋存特征及质量平衡——甲基汞

Occurrence and mass balance of mercury at a sewage treatment plant. Part Ⅱ: methylmercury

以焦作市第一污水处理厂各工艺单元的进出物料为研究对象,通过样品中甲基汞和溶解态甲基汞含量的测定,初步探讨了甲基汞在污水处理厂物料中的赋存特征和迁移过程.结果发现,污水处理厂进水中甲基汞和溶解态甲基汞的含量分别在6.73—49.53 ng·L-1和0.55—8.14 ng·L-1之间,日均值分别为21.34±13.88 ng·L-1和2.55±2.36 ng·L-1.污水厂外排水中甲基汞含量在0.42—1.15 ng·L-1之间,满足《城镇污水处理厂污染物排放标准》(GBl8918—2002)中规定的烷基汞含量限值(10 ng·L-1)要求.污水厂对原污水中甲基汞和溶解态甲基汞的去除率分别达到了96.3%和85.1%,甲基汞的去除主要发生在二级处理工艺单元(氧化沟+二沉池).与污水厂进水相比,出水中溶解态甲基汞所占甲基汞的比率显著升高(15.4%±11.4%升至48.3%±17.9%)(P<0.001),甲基汞占总汞的比率也有显著升高(1.9%±1.2%升至3.9%±1.2%)(P<0.001).沉砂池、二沉池、浓缩池和压滤机房污泥中甲基汞含量分别为7.65±4.35、13.53±6.54、10.48±8.17、8.80±6.48 ng·g-1,占总汞的比率分别为0.9%±0.5%、0.3%±0.1%、0.2%±0.2%、0.2%±0.1%.各处理单元污泥中甲基汞含量均不稳定,日变异系数均达到了45%以上.污水处理厂进出物料中甲基汞的质量平衡计算表明,每天离开污水处理厂的甲基汞比进入污水处理厂的甲基汞减少了1889 mg,占进水中甲基汞质量的88.5%.甲基汞的损失主要发生在二级处理工艺单元,可能有两个方面的原因,一是甲基汞在氧化沟和二沉池中的吸附和沉积,二是甲基汞在污水处理过程中发生了去甲基化.

Sewage and sludge samples were collected from each operation unit of Jiaozuo Sewage Treatment Plant. Occurrence and transport processes of methylmercury （ MeHg） were investigated on the basis of MeHg and dissolved MeHg analysis. It was found that the contents of MeHg and dissolved MeHg in the influent of the plant varied in the range of 6.73-49.53 ng·L-1 and 0.55-8.14 ng·L-1 （average 21.34±13.88 ng·L-1 and 2.55±2.36 ng·L-1, respectively）. MeHg content in the effluent （0.42-1.15 ng·L-1） met the requirement of “Urban Sewage Treatment Plant Pollutant Discharge Standard” （GB18918-2002）. On average, 96.3% MeHg and 85.1% dissolved MeHg were removed from the raw sewage by the current sewage treatment processes, and in particular by the secondary treatment units （ oxidation ditch and the secondary sedimentation tank ） . Dissolved MeHg represented （ 15. 4 ± 11. 4 ）% of MeHg in the raw sewage, while this ratio （ dissolved MeHg/MeHg） increased to （48.3±17.9）% in the effluent of the secondary sedimentation tank. The ratio of MeHg/THg reached 3.9%±1.2% in the effluent of the secondary sedimentation tank, while this ratio in the raw sewage was 1.9%±1.2%. MeHg in the sludge of the grit chamber, the secondary sedimentation tank, the thickener and the pressure-filter were 7.65±4.35,13.53±6.54,10.48±8.17 and 8.80±6.48 ng·g-1 respectively, accounting for 0.9%±0.5%,0.3%±0？1%,0.2%±0.2% and 0？2%±0.1% of the total mercury contents in the corresponding sludge. In the later three sludges, the content of MeHg was highly variable, with coefficients of variation over 45%. A mass balance calculation for MeHg in the whole plant shows that 1889 mg·d-1 MeHg was lost during the treatment processes （ accounting for 88.5%of MeHg in the raw sewage） , especially in the secondary treatment units. The possible reasons may include sedimentation/accumulation and de-methylation of MeHg in the oxidation ditch and the secondary sedimentation tank.