N-Nitrosodimethylamine Reduction by Slow Sand Filtration during Drinking Water Treatment
Date Issued
2010
Date
2010
Author(s)
Chang, Ching-Yu
Abstract
N-Nitrosodimethylamine (NDMA) was recently found as one of the emerging disinfection by-products (DBPs) in the wastewater and drinking water treatments. The toxicity of NDMA, which has been classified by USEPA as Group B2 of probable human carcinogen, was much higher than that of conventional DBPs such as THMs and HAAs. Thus, the control of NDMA in drinking water is an important issue for public health protection. Previous studies showed that NDMA can be destructed effectively by UV irradiation; however, it requires high UV dosage and NDMA precursors are likely to be formed again after UV treatment. Recently, microbial degradation of NDMA was studied extensively in laboratory and field investigations. However, isolation of NDMA-degrading bacteria from the field was not successful. Hence, the information regarding to the NDMA biotransformation mechanisms in the field was limited.
The objectives of this study were to identify the organisms responsible for the bio-reduction of NDMA, to provide information concerning environmental factors affecting the efficiency of NDMA biotransformation, and to investigate the distribution of NDMA-reducing bacteria in the field.
First, the candidates of NDMA-reducing bacteria were isolated from the biofilm attached on the slow sand filter in a water treatment plant by traditional plating methods. Followed by NDMA bio-reduction capability tests, kinetic experiments were conducted to demonstrate the roles of initial NDMA concentrations and environmental factors on NDMA removal. Third, a laboratory simulated sand column system was installed to establish a natural distribution of microbes along different depths of the column through continuous acclimation. Finally, the distribution of NDMA-reducing bacteria in the slow sand filter obtained in different seasons and which from simulated sand column was analyzed by terminal restriction fragment length polymorphism (T-RFLP).
The Methylobacterium sp. was isolated from the slow sand filter, and the ability of NDMA bio-reduction was observed after growth on a low nutrient medium, R2A. The bacterial isolate was unable to grow in the presence of NDMA, but it performed NDMA biotransformation through possible cometabolism when incubated with R2A medium. In the kinetic experiments, first-order and second-order reaction equations were used to fit the kinetic data from the reduction curve. The results demonstrated that 1st and 2nd order kinetic reaction rate constants (hour-1) and percent removal after 10 hours of contact time increased as the initial concentration decreased at 25℃ and 15℃, but no significant difference was found at 35℃. For the same NDMA initial concentration, the rate constant and extent of NDMA removal were higher at 25℃ than 15℃, however, biotransformation of NDMA seemed unfavorable to the bacterial isolate for a higher temperature at 35℃. Also, the enzyme’s maximum rate, Vmax, was the highest at 25℃. It was suggested that the optimal temperature for NDMA biotransformation was approximately at room temperature for the isolated Methylobacterium sp.. Different proportions of sterile influents from slow sand filter were spiked into the reaction matrix, and the percentage of NDMA removal in 100% of field scale influent water was only 5.4%, revealing that the compositions of organic or inorganic compounds in the filter influent might affect the efficiency of NDMA bio-reduction in the slow sand filter. In addition, mixed bacteria might have no direct influence on NDMA bio-reduction, but it could affect the growth of the Methylobacterium sp..
T-RFLP showed that NDMA-reducing bacteria like Methylobacterium sp. was detected in the slow sand filter, and its relative proportion as total community were two-fold increased in the winter than in the summer.
The laboratory simulated sand column fed with influents was spiked with 100
Subjects
N-Nitrosodimethylamine (NDMA)
disinfection by-product
biodegradation
slow sand filtration (SSF)
Terminal Restriction Fragment Length Polymorphism (T-RFLP)
Methylobacterium
cometabolism
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