| Highly toxic and bioaccumulable mercury compounds pose a serious risk to the environment and human health. As a possible source of mercury, municipal wastewater treatment plants(WWTPs) have not received enough attention regarding the occurrence,transport, and transformation of mercury species. In the present study, a WWTP located in Jiaoziuo, China was selected as the study site. Sewage and sludge samples were collected from each operation unit. Content, speciation, diurnal variation, annual variation, removal efficiency and mass balance of mercury were analyzed, respectively.Fractionation of Hg in the de-watering sludge was determined and the ecological risk induced by mercury were assessed. The main conclusions obatined were listed as follows.(1) Both total mercury(THg) and methylmercury(Me Hg) occurred in raw sewage for the four sampling. The THg concentrations in April(1212±598 ng·L-1) and December(803.5±100.6 ng·L-1) were higger than that in June(95.27±7.12 ng·L-1) and September(187.0±43.8 ng·L-1). The Me Hg concentrations were slightly higher in April(21.34±13.88 ng·L-1) in comparison with others seansons(2.62±0.93—3.62±0.18ng·L-1). Mercury in the raw sewage mainly occurred in inorganic forms, with Me Hg/ THg less than 3%. The concentration of dissolved total mercury(F-THg) ranged from 5.89±2.24 to 26.58±12.48 ng·L-1 and the ratios of F-THg/ THg were less than 8%,indicating that THg was mainly adsorbed on particulates. The concentration of dissolved methylmercury(F-Me Hg) was 0.41±0.02—2.55±2.36 ng·L-1 and the ratio of F-Me Hg/ Me Hg was less than 40%. Similar to THg, majority of Me Hg also occurred in the particulate phase. Concentrations of THg and Me Hg in raw sewage varied greatly during the sampling period, giving diurnal coefficients of variance 49.3﹪ and 65.06﹪(April), 23.44﹪ and 35.58﹪(September), respectively.(2) THg and Me Hg concentrations in the effluent of secondary sedimentation tank during the four sampling were 6.89±0.71—56.39±2.35 ng·L-1 and0.33±0.09—0.80±0.25 ng·L-1, respectively, much lower than that in in the influent.Inorganic mercury also accounted for the major fraction of mercury in the effluent(Me Hg/ THg was less than 6%). THg and Me Hg contents in the effluent could meet the requirement of "Urban sewage treatment plant pollutant discharge standard"(GB18918-2002). The concentration of F-THg in the effluent ranged from 1.70±0.27 to5.74±2.22 ng·L-1, giving ratios of F-THg/ THg higher than 40%. Diurnal coefficients of variance for THg and Me Hg were 29.61﹪ and 31.63﹪(April), 42.56﹪ and 26.49﹪(September), respectively.(3) Average removal efficiency by the current sewage treatment technique were higher than 88% and 85% for THg and Me Hg, respectively, with the removal mainly accomplished by the secondary treatment processes(oxidation ditch and the secondary sedimentation tank). Average removal efficiency for Me Hg was slightly higher in April(96.25﹪) in comparison with others seansons(85.34﹪—88.67﹪). In addition, more than 70% of F-THg(except September) and 56% of F-Me Hg in the raw sewage were also removed by the current treatment technique.(4) The THg and Me Hg concentrations in the sludge of grit chamber during the four sampling were 0.97±0.52—1.91±0.10 mg·kg-1and 3.14±0.27—7.65±4.35 ng·g-1, while that of pressure-filter was 2.82±0.43—4.95±0.10 mg·kg-1 and 7.21±1.30—9.30±0.91ng·g-1, respectively. THg contents could meet the requirement of "Urban sewage treatment plant pollutant discharge standard"(GB18918-2002). Mercury in the sludge of pressure-filter mainly occurred in inorganic forms(Me Hg/THg was less than 3﹪).Diurnal coefficients of variance for THg in the sludge of the secondary sedimentation tank, the thickener and the pressure-filter were less than 20 ﹪, while that of grit chamber were 87.80﹪ and 53.61﹪ in April and September. Diurnal coefficients of variance for Me Hg in the sludge were higher than 45﹪ in April, but less than 25﹪except that of the grit chamber(48.23﹪) in September.(5) It was found that mercury in the sludge of pressure-filter was mainly distributed in the humus-bound fraction(52.05 ﹪ —85.87 ﹪) and the residue fraction(13.05 ﹪—47.80﹪). Exchangeable fraction of Hg only accounted for 0.05﹪—0.10﹪ of the mercury contents, indicating that the bioavailability of Hg in the sludge was generally very low. Ecological risk induced by Hg in the de-watered sludge showed that the sludge in April and December were not suitable for agricultural production directly andthat in June and September can be used in dry farmland and fruit tree planting.(6) Mass balance calculations for THg in the whole plant reveal that 43.79﹪ and19.68﹪ of THg in the raw sewage was lost during April and December, on the other hand, 3.74 and 0.81 times of THg was added to the effluent and sludge of the plant in comparison with THg in the raw sewage during June and September, especially in the secondary treatment units. Mass balance calculations for Me Hg in the whole plant reveal that 92.21﹪, 47.59﹪, 48.05﹪ and 62.44﹪ of Me Hg in the raw sewage was lost for the four sampling, especially in the secondary treatment units. Me Hg lost in WWTPs might be attributed to degradation and sludge adsorption. |
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