| In order to understand the transport and transformation of mercury in a sewage treatment plant, sewage and sludge samples were collected from all the operation units of the Second Sewage Treatment Plant(STP) of Jiaozuo for 10 months.Content, speciation, temporal variation and mass balance of mercury in every operation units were analyzed, and potential ecological risk of mercury in sewage was evaluated. The main results were listed as follows:1. Removal effect of COD, TP, NH3-N, and TSS were efficient in the STP, and contents of these pollutant in the effluent could meet class A requirement of "Urban sewage treatment plant pollutant discharge standard"(GB18918-2002).2. It was found that the contents of THg((0.23±0.02~9.73±0.82)×103 ng/L)and dissolved THg(2.32±0.28 ~ 124±20.2ng/L) in influent were highly variable,giving average concentrations of 2.00×103ng/L and 16.2ng/L. THg was positively correlated to TSS(P=0.003). THg in influent mainly exists with particulate phase,and dissolved THg only represented 0.04%~5.5% of THg in the raw sewage. The contents of Me Hg(1.46±0.00~22.6±2.86ng/L) and dissolved Me Hg(0.11±0.01~2.17±0.13ng/L) in influent were highly variable. Dissolved Me Hg increased with temperature. The concentrations of dissolved Me Hg in influent were higher in the warmer months(May, June, July, and August) than in the colder months(September,October, November, and December).3. The rate of removal efficiency of total mercury, dissolved mercury, methyl mercury, dissolved methylmercury were not reduced through primary treatment..Compared with primary treatment, secondary treatment on total mercury, dissolved mercury,methyl mercury, dissolved methylmercury average removal efficiency were improved, sewage treatment plant for mercury removal effect occurs mainly in the secondary processing4. The rate of total mercury, dissolved mercury, methyl mercury, dissolved methylmercury removal efficiency were 77.1%,13.7%,45.1%,7.7%. total mercury(methyl mercury) removed greater than dissolved mercury(methyl mercury)in the filter. mercury removal efficiency: primary treatment> filter>secondary treatment, mercury removal effect occurs mainly in the secondary processing.5. The contents of THg(1.78±0.22~43.3±0.67ng/L) in final effluent were highly variable, giving an average concentration of 10.6ng/L. The contents of Me Hg were 0.13±0.01~1.11±0.26ng/L, giving an average concentration of 0.33ng/L.Compared with the filter effluent, various forms of mercury have no obvious change,in chlorine disinfection process6. The contents of THg and Me Hg in pressure-filter sludge were(2.02±0.29~6.88±1.42)×103ng/g and 3.76±0.613~10.4±2.13ng/g, giving average concentrations of(3.94±0.53) ×103ng/g and 6.35±0.84ng/g, respectively. The ratio of Me Hg/THg was less than 1%. Using the Index of geoaccumulation and potential ecological risk index method, ecological risk induced by THg in the pressure-filter sludge was evaluated. The result indicated that THg in pressure-filter sludge have high potential ecological risk. Thus the sludge might not be applied to farm land.7. Contents of dissolved THg had no obvious change in oxidation ditch.Dissolved Me Hg were negative correlated to ORP(P<0.01). There were no correlation between dissolved Me Hg and DO, conductivity, PH, or temperature.8. Me Hg and THg were positively correlated to TSS(P<0.01) in secondary sedimentation tanks. The variation of Me Hg and THg contents have no obvious regulation in the depth or radius direction. There were no correlation between the four forms of mercury and ORP, DO, PH, conductivity, or temperature. Mercury deposited on the bottom of secondary sedimentation tank, together with TSS. There was not significant methylation or demethylation in secondary sedimentation tank.9. A mass balance calculation for the whole plant shows that 164.6g/day of THg entered the plant and 122g/day of THg left, so 43g/day of THg was lost.657.5mg/day of Me Hg entered and 316.4g/day of Me Hg left the plant, indicating a loss of 341mg/day of Me Hg, probably caused by sedimentation/accumulation or volatilization of THg and Me Hg in the oxidation ditch and the secondary sedimentation tank. Demethylation of methylmercury might also have occurred in oxidation ditches. |
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