Spatial and Temporal Variation of Mercury in Municipal Sewage Sludge in China

Abstract

In order to determine the occurrence of mercury (Hg) in the dewatered sewage sludge (SS) from municipal wastewater treatment plants (MWTPs) in China, 315 SS samples were collected from 40 MWTPs. The total Hg (THg) contents of the sludge samples were analyzed using a DMA-80 Hg analyzer. It was found that THg in the samples ranged from 0.45-15.42 mg ·kg-1. The THg data followed a log-normal distribution with a geometric mean of (2.19±3.16) mg ·kg-1. THg contents in all the sludge samples meet the criteria set for disposal (by co-landfilling) of MWTPs sludge (GB/T 23485-2009). In terms of THg contents, 97.8% of the sludge was suitable for land application in neutral and alkaline soils, while 86.7% of the sludge was suitable for land application in acidic soil. THg in SS varied greatly, not only among cities (variation coefficient of 105%), but also in the same MWTP (variation coefficient of 0.6%-53.6% over seven days). Mercury contents in SS of China showed a descending trend of North > Northeast > Northwest > Southwest > East > Central > South China. Using the corresponding urban soil background values of THg as references, Hg pollution levels of SS were evaluated using the geological cumulative index method. It was found that SS from more than 60% of the cities sampled were heavily polluted by Hg. Mercury contents in the SS of China showed a temporal trend of increasing then decreasing, with 2000-2009 being the peak period. Results of the present study provide significant data support for the prevention and control of mercury pollution in sewage and SS in China.