Letter pubs.acs.org/journal/estlcu
Superior Removal of Disinfection Byproduct Precursors and Pharmaceuticals from Wastewater in a Staged Anaerobic Fluidized Membrane Bioreactor Compared to Activated Sludge Daniel L. McCurry,†,∥ Samantha E. Bear,∥,‡ Jaeho Bae,§ David L. Sedlak,∥,‡ Perry L. McCarty,† and William A. Mitch*,†,∥ †
Department of Civil and Environmental Engineering, Stanford University, Jerry Yang and Akiko Yamazaki Environment and Energy Building, 473 Via Ortega, Stanford, California 94305, United States ‡ Department of Civil and Environmental Engineering, University of California, Berkeley, California 94720, United States § Department of Environmental Engineering, Inha University, Incheon, Republic of Korea ∥ Engineering Research Center for Re-Inventing the Nation’s Urban Water Infrastructure (ReNUWIt), National Science Foundation, Washington, DC, United States S Supporting Information *
ABSTRACT: A recently developed, staged anaerobic fluidized membrane bioreactor (SAF-MBR) system has demonstrated performance comparable to that of conventional activated sludge systems at similar hydraulic residence times. The low biosolid generation rate, small footprint, and energy-positive potential (from captured methane) suggest the possible advantages of using the SAF-MBR upstream of advanced treatment for decentralized water reuse. Previous research conducted on laboratory-scale systems indicated that the SAFMBR system is capable of removing pharmaceutical compounds. The purpose of this study was to compare the removals of disinfection byproduct precursors as well as pharmaceuticals by parallel pilot-scale SAF-MBR and full-scale aerobic systems treating municipal wastewater. Significantly better removals of antivirals (acyclovir and lamivudine), β-blockers (atenolol, metoprolol, and propranolol), a β-blocker aerobic biotransformation product (metoprolol acid), an anticonvulsant (carbamazepine), antibiotics (sulfamethoxazole and trimethoprim), N-nitrosomorpholine, and the precursors of Nnitrosodimethylamine were obtained by the SAF-MBR system. Concentrations of trihalomethane precursors were lower in the SAF-MBR effluent in two of the three samplings.
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would meet or exceed typical U.S. permit limits of ≤30 mg/L for BOD5 and ≤30 mg/L for TSS. Population growth in arid regions has motivated consideration of municipal wastewater as a secure, local water supply,7 a trend heightened by recent droughts. Trace organic contaminants, such as pharmaceuticals and disinfection byproducts (DBPs), are a concern for potable reuse systems. The National Research Council highlighted the importance of DBPs, particularly N-nitrosodimethylamine (NDMA),7 because their concentrations in recycled water are closer to levels of health concern than those of pharmaceuticals.7,8 Additionally, neutral, low-molecular weight DBPs (and certain odorous compounds present in wastewater9) are poorly removed during
INTRODUCTION
Up to 3% of U.S. electrical generation capacity is expended on municipal wastewater treatment,1 largely for aeration during activated sludge treatment. By avoiding aeration and generating methane that can be harvested for energy, anaerobic treatment offers the possibility of net energy production2 but has generally been avoided for dilute wastewaters because of concerns of inadequate treatment efficiency, especially at lower temperatures. Recent progress with anaerobic membrane bioreactors has surmounted many of these concerns.3−5 Shin et al. evaluated a staged anaerobic fluidized membrane bioreactor (SAF-MBR) pilot plant used to treat primary municipal wastewater effluent.6 At temperatures ranging from 8 to 30 °C over 485 days, the SAF-MBR achieved a 3−9 mg/L effluent five day biological oxygen demand (BOD5) and nondetectable total suspended solids (TSS) at total hydraulic residence times (HRT) of 4.6−6.8 h, similar to the HRTs of conventional aerobic treatment systems. This effluent quality © 2014 American Chemical Society
Received: Revised: Accepted: Published: 459
September 10, 2014 October 13, 2014 October 14, 2014 October 14, 2014 dx.doi.org/10.1021/ez500279a | Environ. Sci. Technol. Lett. 2014, 1, 459−464
Environmental Science & Technology Letters
Letter
Figure 1. Mean concentrations of pharmaceuticals and related compounds in wastewater effluents, plotted on two y-axis ranges. Error bars represent the standard deviation of values from the three sampling events (January, March, and April). NQ indicates not quantifiable because of matrix interference.
recycle flow (27 m/h upflow velocity). The second 0.77 m3 anaerobic fluidized membrane bioreactor contained 264 kg of GAC and was fluidized to 100% bed expansion using a 0.53 m3/min recycle flow (75 m/h upflow velocity). This reactor contained 1.85 m long polyvinylidene fluoride membranes (pore size of 30 nm; total surface area of 39.5 m2). The pilot system was operated with a HRT of 6.8 h and a SRT of 36 days. After the SAF-MBR had been operated for one year, 24 h composite samples of primary effluent, aerobic secondary clarifier effluent, and SAF-MBR membrane permeate were collected upstream of the addition of disinfectant on three occasions in 1 L polyethylene jugs. Samples were immediately placed on ice and shipped by express courier to Stanford University (for water quality and DBP analyses) and the University of California at Berkeley (for pharmaceutical analyses). Upon receipt, samples were filtered (0.7 μm nominal pore size glass fiber filters previously baked at 400 °C for 4 h) and stored at 4 °C until they were analyzed; samples were processed within 1 week of sample collection. Water quality parameters are listed in Table SI-1 of the Supporting Information. Analyses. For pharmaceutical analyses, samples were split into two 400 mL aliquots for solid phase extraction (SPE) and analysis by liquid chromatography/tandem mass spectrometry, as described previously14,15 and summarized in the Supporting Information. For DBPs, samples were split into duplicate 500 mL aliquots, adjusted to pH 8 with 4 mM borate buffer, treated with 5 mg/L as Cl2 preformed monochloramine, and held in the dark at 20 ± 1 °C. On the basis of a Uniform Formation Condition (UFC) protocol mimicking typical chloramination conditions in drinking water treatment plants,16 the chloramine dose was increased from 2.5 mg/L as Cl2 to achieve a total chlorine residual of >2 mg/L as Cl2 after the 3 day reaction time in the wastewater effluents. Samples were chloraminated rather than chlorinated, because nitrosamine formation is associated with chloramination,17,18 because chlorination of non-nitrified secondary wastewater effluents usually results in de facto chloramination, and because chloramines are frequently applied upstream of microfiltration to inhibit biofouling in wastewater reuse plants.19 As described in detail previously, the Supporting Information summarizes preformed monochloramine stock preparation,20 chlorine residual measurement,21 and the measurement by gas chromatography/mass spectrometry of halogenated DBPs22 and nitrosamines.23,24
reverse-osmosis (RO) treatment, with rejections as low as 20%.10 Biological removal of trace organics upstream of an RO/ advanced oxidation process (AOP) train would reduce the needed removal efficiency of these systems. Although previous research suggests that SAF-MBR treatment may be a promising alternative to aerobic treatment for BOD5 removal, direct comparisons of DBP precursor and pharmaceutical removal efficiencies are needed for the treatment of authentic municipal wastewater. A recent labscale SAF-MBR study demonstrated that treatment of primary municipal wastewater effluent reduced the concentrations of 20 pharmaceuticals (median removal of 97%).11 Another study evaluated the removal efficiencies (median removal of 30%) within a lab-scale upflow anaerobic membrane bioreactor of 38 pharmaceuticals, personal care products, or pesticides spiked into a synthetic wastewater.12 A third study found higher concentrations of alkylphenols in a pilot anaerobic membrane bioreactor effluent than in a parallel full-scale activated sludge plant, which was attributed to their generation from efficient biotransformation of alkylphenol polyethoxylates under anaerobic conditions.13 Unfortunately, no study to date has evaluated removal of DBPs or their precursors by SAF-MBR treatment, nor have removal efficiencies of pharmaceuticals been compared against those of conventional aerobic systems. The purpose of this study was to compare the abilities of a SAF-MBR pilot unit and a full-scale activated sludge system to remove pharmaceuticals and DBP precursors from the same municipal primary wastewater effluent source.
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MATERIALS AND METHODS Treatment Systems. The pilot SAF-MBR and the full-scale aerobic treatment systems were located at the Bucheon municipal treatment plant in South Korea. The aerobic train treated ∼270,000 m3/day of municipal wastewater and included primary clarification and four-stage biological nutrient removal (anoxic, anaerobic, anoxic, and aerobic sections); the HRT and solid retention times (SRT) were 9.9−11.4 h and 24.2−27.7 days, respectively. Full nitrification and partial denitrification were achieved. The ∼5.5 m3/d (1 gpm) pilot SAF-MBR system consisted of two reactors operated sequentially that received 2 mm microscreened primary effluent.6 The first 0.99 m3 anaerobic fluidized bed reactor contained 139 kg of ∼0.9 mm diameter Calgon F300 granular activated carbon (GAC), which was fluidized to 40% bed expansion using a 0.15 m3/min 460
dx.doi.org/10.1021/ez500279a | Environ. Sci. Technol. Lett. 2014, 1, 459−464
Environmental Science & Technology Letters
Letter
Figure 2. Concentrations of (A) N-nitrosodimethylamine, (B) N-nitrosomorpholine, and (C) dimethylnitramine, and (D) the sum of chloroform, bromodichloromethane, dibromochloromethane, and bromoform (THM4) after application of 5 mg/L preformed monochloramine as Cl2 at pH 8 and 20 °C after 3 days. Error bars represent standard deviations of experimental replicate values (n = 3−4).
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RESULTS AND DISCUSSION Pharmaceuticals. Antivirals (abacavir, acyclovir, emtricitabine, and lamivudine), an anticonvulsant (carbamazepine), antibiotics (sulfamethoxazole and trimethoprim), β-blockers (atenolol, metoprolol, and propranolol), and a β-blocker transformation product (metoprolol acid) were quantified in the wastewater samples (Figure 1 and Table SI-3 of the Supporting Information). Target compounds were chosen on the basis of their expected occurrence and expected differences in their removal in conventional wastewater treatment plants. Concentrations generally were lower in both secondary effluents than in the primary effluents. However, the concentration of metoprolol acid was higher in the aerobic secondary effluent than in the primary effluent, in accordance with its production as an aerobic biological transformation product of metoprolol and atenolol;25,26 metoprolol acid was either present at a low concentration or not detected in the SAF-MBR effluent. For several other compounds, concentrations in secondary effluents occasionally were greater than those detected in the primary effluent. Although increases could result from release of parent pharmaceuticals from metabolites (e.g., conjugates), such increases were not consistent across the different sampling periods, and because they were often associated with concentrations near the detection limits (ranging from 0.3 to 25 ng/L), the increases were probably not statistically significant. For compounds that could be measured in primary effluent, percentage removals between primary and secondary effluents were determined (Table SI-3 of the Supporting Information). For aerobic treatment, the poor removal of carbamazepine (90%) observed across pharmaceuticals. The question of whether compound concentrations were lower in the SAF-MBR than in aerobic effluents was evaluated. Because of the fluctuations in the influent concentrations, the relative difference between the aerobic and anaerobic effluent concentrations (eq 1) was evaluated for each sampling event. relative difference = 461
Caerobic effluent − Canaerobic effluent Caerobic effluent
(1)
dx.doi.org/10.1021/ez500279a | Environ. Sci. Technol. Lett. 2014, 1, 459−464
Environmental Science & Technology Letters
Letter
observed, a reduced level of formation of trihalomethanes was potentially attributable to lower DOC and specific UV absorbance at 254 nm (SUVA254) in the SAF-MBR effluent than in aerobic effluent (Table SI-1 of the Supporting Information); both measures have been previously correlated with THM formation potential.37,38 The question of whether concentrations of NDMA, NMOR, DMNA, or THM4 were lower in SAF-MBR effluent than in aerobic effluent was evaluated in the same fashion as described for the pharmaceuticals (eq 1); only levels of NDMA and NMOR were significantly lower (p < 0.05) in SAF-MBR effluent (Table SI-6 of the Supporting Information). Implications. Previous lab-scale anaerobic membrane bioreactor research demonstrated that anaerobic treatment,12 including SAF-MBR,11 could remove a range of pharmaceuticals from wastewater effluents. Our results demonstrate that the efficiency of removal of pharmaceuticals and DBP precursors by a pilot-scale SAF-MBR treatment equaled or exceeded that of a parallel, full-scale conventional aerated system at comparable hydraulic residence times. If the SAFMBR were used as a secondary biological treatment system upstream of advanced treatment trains for potable wastewater reuse systems, its high trace organic removal efficiencies would decrease the burden on RO membrane systems to reduce trace organic concentrations. Research is needed to better understand removal efficiencies that can be achieved by SAF-MBR treatment across a wider range of compounds, and the mechanisms responsible. In the one previous comparison between a pilot-scale anaerobic membrane bioreactor and a full-scale activated sludge system, higher concentrations of alkylphenols were observed in the effluent of the anaerobic system, ironically because of enhanced biotransformation of the parent alkylphenol polyethoxylates under anaerobic conditions.13 Aerobic membrane bioreactor research has suggested that improved pharmaceutical removal there may be obtained with a higher SRT that results in a more diverse microbial community.27,29 Sorption to biofilms in aerobic membrane bioreactors was considered to be unimportant except for the most hydrophobic pharmaceuticals (e.g., propranolol).27,29 In the SAF-MBR system, similar considerations likely apply, although here sorption to an activated carbon carrier and subsequent bioregeneration of activated carbon may also play a role. SAF-MBR research has indicated that scouring of the membranes by the GAC particles controlled fouling,39 avoiding the need for chemical cleaning or energy-intensive gas scouring commonly used in other anaerobic membrane bioreactor systems.40 The significantly lower level of biosolid production by anaerobic compared with aerobic treatment6 and the lack of secondary clarifiers would reduce plant size. Together with its energy-positive potential, the small footprint of the SAF-MBR system suggests its suitability in treatment trains for decentralized direct potable reuse (DPR) applications. Coupling SAF-MBR treatment to advanced treatment (e.g., RO and AOPs) might allow the direct introduction of recycled water to distribution systems, avoiding the need to install separate recycled water distribution systems, and the high pumping costs needed to move water to users from centralized potable reuse plants. Energy produced by SAF-MBR treatment could be used to partially offset energy requirements for a reuse system. Demonstration of trace organic removal will be critical for the future development of such applications.
One half of the analytical detection limit was used for nondetect values; method detection limits were adjusted to account for matrix interference. Using the three sampling events as experimental replicates, a two-sided t test evaluated whether the relative difference was different from zero for each pharmaceutical (Prism 6, GraphPad Software, La Jolla, CA). Of the 11 compounds, the levels of nine were significantly lower (p < 0.05) in SAF-MBR effluent than in aerobic effluent (Table SI3 of the Supporting Information). The mean concentration of one of the two remaining compounds (emtricitabine) was lower, but the difference was not significant. Concentrations of abacavir were statistically indistinguishable because measured concentrations were at or near the detection limit of 1 ng/L. DBPs and Precursors. N-Nitrosodimethylamine (NDMA) was initially present in primary effluent (3.5−19 ng/L), in aerobic effluent (9−20 ng/L), and to a lesser degree in SAFMBR effluent (
