Fate of N-nitrosodimethylamine, trihalomethane and haloacetic acid precursors in tertiary treatment including biofiltration
Graphical abstract
Highlights
► The removal of NDMA, THM and 5HAA precursors by BAC is investigated at full and pilot scale. ► The fate of 4 pharmaceuticals containing a dimethylamino moiety are individually investigated. ► Adsorption is an important factor for the observed removal NDMA precursors during BAC filtration. ► Both adsorption and biodegradation play a role on the removal of THM and HAA precursors.
Introduction
Disinfection by-products in water are of great concern to public health since bladder and colorectal cancers have been associated with exposure to them in drinking water, and experimental evidence suggests that exposure also occurs through inhalation and dermal absorption (Villanueva et al., 2007). The US Environmental Protection Agency (USEPA) allows a maximum of 80 μg/L of total THMs (TTHMs) and 60 μg/L of five HAAs in drinking water based on its current regulation guidelines (Richardson et al., 2007). For NDMA, even if it is not yet included in the drinking water regulation, the USEPA classifies it in the group B2, which includes compounds that are probably carcinogenic to humans (EPA, 2008). In Australia, NDMA has been included in the draft Australian Drinking Water Guidelines at a maximum concentration of 100 ng/L (ADWG, 2010). Moreover, NDMA was recently identified as one of the DBPs with the greatestpotential impact on public health (Hebert et al., 2010). While THMs and HAAs are mainly formed when water is disinfected with chlorine (Richardson et al., 2007), NDMA has been related to the presence of chloramines, specifically dichloramines generated during the disinfection process (Schreiber and Mitch, 2006).
Population increases, particularly in cities, and scarce water resources have increased the demand for use of highly treated municipal wastewater as a source of potable water (Shannon et al., 2008). Studying the fate of DBP precursors during secondary effluent treatment is of importance as an increasing number of municipal water treatment plants are engaged in the practice of potable reuse of treated wastewaters. A common method to measure the precursors of DBP in water is by means of formation potential tests. In these tests, chlorine or chloramines are added to a buffered sample at relatively high concentrations and kept reacting for at least seven days to achieve the maximum formation of the specific DBPs (Greenberg et al., 1992, Mitch et al., 2003).
While THM and HAA precursors are difficult to characterize because different fractions of NOM may generate these DBPs upon disinfection (Xie, 2004), some particular NDMA precursors may be easier to monitor because the specific dimethylamino moiety is required to generate the nitrosamine upon chloramination. Traditionally, dimethylamine (DMA) was the first NDMA precursor considered during water chloramination since direct reaction between this molecule and chloramine produces the carcinogen. Dimethylamine is not only the typical catabolic product of proteins in animals and plants (implying a typical concentration in urine of 40 μg/L) (Tricker et al., 1994) but also amongst the most frequently produced amines by the chemical industry. Recently, it has also been shown that other pharmaceuticals and personal care products with substituted amino groups can serve as NDMA precursors during chloramine disinfection (Lee et al., 2007, Kemper et al., 2010, Shen and Andrews, 2010). In addition, natural organic matter is another typical source of NDMA (Westerhoff and Mash, 2002, Chen and Valentine, 2006, Chen and Valentine, 2007, Chen and Valentine, 2008). Wastewater treatment plants (WWTPs) remove 90% of dissolved organic nitrogen (DON) and the typical concentration of DON remaining after treatment at WWTPs is in the range from 1 to 3 mgN/L (Pehlivanoglu-Mantas and Sedlak, 2006). This remaining percentage consists of either difficult-to-remove DON species, or DON produced during biological treatment.
Finding technologies and processes to reduce DBP precursors in water is thus of great interest. Adsorption on activated carbon (AC) is one of the proven methods for removing natural organic matter (NOM). However, the capacity for adsorption is limited and replacement and disposal is costly. AC with active biomass established on its surface is called biological activated carbon (BAC). A BAC filter consists of a fixed bed of granular AC supporting the growth of bacteria attached on the surface. This technology has been used for many years in drinking water treatment, usually after ozonation, and has proven to significantly remove NOM, pharmaceuticals and personal care products and ozonation by-products as well as odour and taste compounds (e.g. geosmin and 2-methylisoborneol) (Simpson, 2008, Reungoat et al., 2011).
Although some work has already been published showing the efficiency of BAC to remove DBP precursors in drinking water (Simpson, 2008), to our knowledge, there are no reports regarding the degradation of THM, HAA and N-nitrosamine precursors in secondary treated effluent using ozone/BAC. In the present study, we investigated the removal of NDMA, HAA and THM precursors in full and pilot-scale biofilters treating the effluent from a municipal WWTP. Additionally, the fate along the treatment train of specific pharmaceuticals containing tertiary amines (hence suspected to be NDMA precursors) is presented.
Section snippets
Chemicals
All chemicals used for chemical analysis were of analytical grade and commercially available. NDMA (5000 μg/mL in methanol) had a purity of >99.9% and was obtained from Supelco. Deuterated d6-NDMA and d14-NDPA (N-nitrosodipropylamine) were used as surrogate and internal standard, respectively (1000 μg/mL in dichloromethane, >98.9%, supplied by Accustandard and Ultra Scientific, respectively). For the NDMA formation potential test, ammonium chloride (TraceSELECT®, ≥99.9% purity), sodium
Nitrosamines
NDMA, NDEA, NMOR, N-Pip and NDBA were analysed in all the samples taken from South Caboolture Water Reclamation Plant. Sampling points along the treatment train are indicated in Fig. 1. The selected N-nitrosamines were also analysed in the effluent of the three pilot scale columns, which are BAC filter placed after pre-ozonation and dissolve air flotation and filtration (BAC 1), sand filtration after main ozonation (SAND) and a BAC filter after main ozonation (BAC 2). NDMA and NMOR were
Conclusions
Both, the bulk of NDMA precursors and individual pharmaceuticals that may form NDMA are dramatically reduced by the BAC filters even without preceding main ozonation. DOC removed by the BAC contains a higher concentration of NDMA precursors than the bulk DOC, which means that the NDMA precursors are preferentially adsorbed or degraded in the BAC. The percentage of removal in the BACs is unaffected by a main ozonation step when considering long filtration times (EBCT = 60 min). On the contrary,
Acknowledgements
The authors want to specifically acknowledge Urban Water Security Research Alliance for funding the “NDMA formation potential project” and “The Enhanced Treatment Project” and Unity Water and their staff for giving access to the plant. Thanks to Miss Hollie King for correcting the English.
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