Long-term performance of subsurface-flow constructed wetlands treating Cd wastewater.

This study was conducted to investigate the long-term performance of subsurface-flow constructed wetland (SFCW) units treating a wastewater containing cadmium (Cd). The hydraulic retention time (HRT) was found to have significant effects on the SFCW performance, especially on Cd removal. During the 320 days of Cd feeding, the HRTs of 1, 3, 5 and 8 days resulted in the Cd removal efficiencies of 50, 90, 99 and 99%, respectively. The actual Cd breakthrough times in the SFCW effluent were found to be longer than the theoretical values calculated from the maximum adsorption capacities only, especially at the HRTs longer than 1 day, and were dependent on the operating HRT and Cd loading. Other mechanisms such as filtration, sedimentation and plant uptake were also responsible for Cd removal in the SFCW beds. The extents of Cd plant uptake were 21 and 6% of the Cd inputs for the SFCW units operating at the HRTs of 3 and 1 days, respectively. Based on Cd mass balance and fractionation analysis, the SFCW media were found to be most effective in Cd removal through adsorption of the residual and Fe/Mn oxide bound fractions. The results of this long-term study re-affirmed the necessity to determine actual breakthrough times of Cd or other heavy metals in the SFCW effluent which are dependent on HRT and Cd loading and are usually longer than the theoretical values calculated from the maximum adsorption capacity only.

Comparison of biological specimens for manganese determination among highly exposed welders.

This research aimed to determine if less invasive biological specimens (other than blood), such as feces and clipped toenails could be used to determine manganese concentrations among occupationally exposed human subjects. In addition to blood samples, which have routinely been used in determining manganese concentration, specimens were collected from welders working at the Electricity Generating Authority of Thailand, Mae Moh Thermal Power Plant, Lampang Province. Manganese concentrations in these three biological samples were determined by atomic absorption spectrophotometer. Correlations of manganese concentrations among these three biological samples were measured, and found to be rather poor (Pearson's r <+/-0.2, p > 0.1 for any pair-wise comparisons). Blood remains the recommended material for biomonitoring manganese concentrations in occupationally exposed subjects.