Article pubs.acs.org/est
Molecular Characteristics and Differences of Effluent Organic Matter from Parallel Activated Sludge and Integrated Fixed-Film Activated Sludge (IFAS) Processes Linda Y. Tseng,† Michael Gonsior,‡,* Philippe Schmitt-Kopplin,§,∥ William J. Cooper,⊥,# Paul Pitt,▽ and Diego Rosso⊥,# †
Department of Civil and Environmental Engineering, University of California, Los Angeles, California 90095-1593, United States University of Maryland Center for Environmental Science, Chesapeake Biological Laboratory, Solomons, Maryland20688, United States § Helmholtz Zentrum Munich, German Research Center for Environmental Health, D-85764 Neuherberg, Germany ∥ Chair of analytical Food Chemistry, Technische Universität München, D-85354 Freising-Weihenstephan, Germany ⊥ Department of Civil and Environmental Engineering, University of California, Irvine, California 92697-2175, United States # University of California, Urban Water Research Center, Irvine, California 92697-2175, United States ▽ Hazen and Sawyer, P.C., 498 Seventh Avenue, New York, New York 10018, United States ‡
ABSTRACT: A direct comparison between parallel activated sludge and integrated fixed-film activated sludge (IFAS) processes was performed in this study because both treatments received the same primary effluent, although differences may still remain due to different return flow rates. Modern ultrahigh resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry was applied to characterize the complexity of effluent organic matter (EfOM) and to evaluate both processes in their abilities to change the EfOM molecular composition. At different stages during the two processes a direct comparison of the performance and changes in molecular composition of the IFAS with those of the activated sludge was undertaken. Large differences in the molecular composition between both processes were only apparent in the early stage of the aeration cells and the first cell of the IFAS possibly due to the higher flow rate and a delay in aerobic bacterial degradation. Despite the double flow rate (0.263 m3 s−1) in the IFAS reactors compared to the activated sludge, by the end of the treatment the EfOM composition of both processes were undistinguishable from each other. However, a much more complex EfOM was generated in both processes, suggesting that bacteria are responsible for an increase in molecular diversity in the effluent.
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INTRODUCTION
The chemical composition of EfOM was thought to be similar to NOM.16 However, compounds such as synthetic organic compounds and soluble microbial products found in EfOM were reported to be chemically distinct from NOM.1,17,18 It has previously been shown that even bulk properties of EfOM such as spectrophotometric properties, molecular weight, dissolved organic carbon (DOC), and total dissolved nitrogen (TDN) concentrations are different from those of NOM.18−23 Although these bulk properties and selected chemical species have been studied extensively,24,25 there is a lack of information on the complex composition of EfOM. Detailed characterization of EfOM has the potential to elucidate the chemical diversity in EfOM, thus it can act as a
A detailed chemical analysis of wastewater effluent is intrinsic to the understanding of chemical dynamics and treatment efficiency.1 Since the late 1990s, the electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR MS) technique, though cost-prohibitive for many environmental applications,2 has made great strides in identifying thousands of molecular formulas in aquatic natural organic matter (NOM).3−7 However, most chemical characterization of complex organic matrices to date has focused on NOM in natural environments8−10 and chemical characterization studies related to engineered water systems also focused mainly on NOM and its effect during water treatment process and on the resulting drinking water quality.11−15 Recently a study demonstrated the versatility of FT-ICR MS as a technique that can provide a detailed chemical characterization of wastewater treatment effluent organic matter (EfOM) 1 © 2013 American Chemical Society
Received: Revised: Accepted: Published: 10277
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Figure 1. Process layout. Tank 11 is strict activated sludge (ASP, receives a flow of 0.131 m3 s−1) and Tank 12 is a hybrid of integrated fixed-film activated sludge in tandem with activated sludge (IFAS/ASP, receives a flow of 0.263 m3 s−1). Tank 11 and Tank 12 have separate secondary clarifier therefore separate return activated sludge (RAS).
measure to compare EfOM from different sources and to give indications of potential selective removal of chemical species and of the overall wastewater treatment performance. With the advancements of analytical technology, a more comprehensive chemical characterization of EfOM is now possible. The nontargeted ultrahigh resolution ESI FT-ICR MS analytical approach allowed mass spectrometric analysis of complex mixtures such as EfOM.1,26 This technique is capable of assigning unambiguously thousands of molecular formulas to ultrahigh resolution mass peaks and can therefore be used to evaluate the chemical makeup of complex mixtures of organic molecules. Samples from two parallel and independently operating fullscale processes treating the same wastewater were analyzed in this study. These side-by-side trains featured the activated sludge process (ASP) and the integrated fixed-film activated sludge (IFAS) system. The IFAS system was an evolution of the ASP where media were added to provide surface for biofilm growth.29 The resulting process operated with an increased biomass inventory thanks to the combination of suspended and attached biomass, and was able to sustain higher loading rates (gLOAD m−3 h−1, where load is both biodegradable chemical oxygen demand, or bCOD, and NH4+) and higher oxygen uptake rates (gDO m−3 h−1).27 There were several advantages to enhance ASP with the introduction of biofilm, such as smaller physical footprint,28 enhancement of nitrification in wastewater treatment, longer solids residence time29 typically associated with increased process stability,40 and increased removal of anthropogenic compounds from the water phase.41 Therefore these biofilm-based systems have gained increasing interest for domestic wastewater treatment.28−31 Because both the IFAS and ASP trains in the studied wastewater treatment plant received the same wastewater influent, this situation was advantageous for a comparison of the chemical composition of EfOM and the differences in process performance. Previous analyses of the chemical oxygen demand (COD) removal in the secondary treated effluent of both trains showed remarkably similar values suggesting a very similar performance of the two different treatment trains.27 Accordingly, the goal of this study was to characterize and directly compare the EfOM from the IFAS and the ASP treatment processes to evaluate whether the chemical diversity was similar during both processes and in their effluents. Since each train was composed of several individual well-mixed reactors in series (Figure 1), a side-by-side comparison of these reactors was evaluated
sequentially in each train for any potential change of the chemical diversity of EfOM.
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MATERIALS AND METHODS Process Conditions. The T.Z. Osborne Water Reclamation Plant is owned and operated by the City of Greensboro, NC. In this facility two tanks were selected for testing: Tank 11 was operated as strict activated sludge in Ludzack-Ettinger mode; Tank 12 was operated as hybrid IFAS/ASP, since the first half of its volume was converted to a large-scale pilot demonstration of the IFAS technology (IFAS media by AnoxKaldnes). The two tanks were fed the same primary effluent, and were parallel and independently operated with separate clarifiers thus separate return activated sludge (RAS) streams (Figure 1). The aeration system for the ASP cells in both processes used Sanitaire fine-pore disc diffusers, whereas the IFAS reactors were retrofitted with coarse-bubble nozzles. The surface of the IFAS reactors was continuously sprayed with secondary effluent to minimize the accumulation of foam. This large wastewater treatment plant treats an average flow of 1.75 m3 s−1 and at the time of sampling (beginning of the summer season) the process was operated in ordinary conditions (primary effluent total COD and NH4−N were 337 mg L−1 and 15.0 mg L−1, respectively) meeting effluent levels of total COD of 28.6 mg L−1 (Tank 11, ASP) and 28.2 mg L−1 (Tank 12, IFAS) and of NH4−N below 0.2 mg L−1 (Tank 11, ASP) and below 0.1 mg L−1 (Tank 12, IFAS). Sample Collection. A grab sample of mixed liquor was collected using a sampling pole approximately 1 m away from the wall of each reactor cell and 0.5 m below the surface. Reactor cells were well-mixed (the operating air flow rate far exceed the requirements for a well-mixed reactor, and the apparent superficial liquid velocity was of the order of 1 m/s), therefore each single grab sample was assumed to be representative of each reactor volume. At the time of sampling, the aggressive defoaming spraying at this facility has reduced to a negligible extent the foam accumulation in the IFAS reactors thereby reducing the possibility of chemicals differentially accumulating onto the floating foam. The grab samples from the IFAS cells were treated like the grab samples from other reactor cells, since only the liquid portion of each sample was used for analysis. Each sample was filtered on-site with a Fisherbrand 0.2 μm syringe filter containing mixed cellulose ester (MCE) membrane. The samples stored in Nalgene polypropylene copolymer (PPCO) bottles with polypropylene 10278
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Figure 2. Dissolved organic carbon (DOC) concentrations and solid-phase extraction (SPE) adsorption efficiencies in cells of Tank 11 (ASP) and Tank 12 (IFAS). Note: PE = primary effluent and SE = secondary effluent.
Visualization of FT-ICR MS data was undertaken using Van Krevelen diagrams33 and a previously developed modified Kendrick plots referred to as KMD-Z* plots.34 In this diagram the ratio of the Kendrick Mass Defect (KMD, eq 1) and the parameter Z* (eq 2) are plotted against the exact masses of all assigned molecular formulas to be able to directly show unambiguous homologous CH2-series.
cap were immediately shipped to the laboratory for extraction (sample storage time not exceeding 24 h). Sample Preparation. An aliquot of 200 mL of each 0.2 μm filtered sample was acidified to pH 2.0 with small amounts of concentrated HCl (puriss. p.a., ≥32%, Sigma Aldrich) and then gravity-fed through an Agilent Bond Elut solid-phase extraction (SPE) cartridge filled with 1 g of styrene-divinylbenzene polymer (PPL) resin. Prior to the extraction, the solid-phase resin was conditioned by passing 5 mL LC-MS grade methanol (Chromasolv, Merck) through the cartridge followed by washing off the methanol using acidified (pH 2) LC-MS grade water (Chromasolv, Merck). Following the extraction, the cartridge was washed with 5 mL acidified LC-MS grade water and then dried under pure N2 gas. The extract was then eluted from the cartridge with 2 × 5 mL LC-MS grade methanol and stored at −20 °C thereafter for subsequent analyses. To calculate the extraction efficiency, a 20 mL aliquot of the samples before and after passing through the cartridge was collected for dissolved organic carbon (DOC) analysis. Dissolved Organic Carbon (DOC) Analysis. DOC concentrations were measured using a high temperature combustion Shimadzu 5000A TOC Analyzer. Each standard (potassium hydrogen phthalate) were measured in triplicates, and all standards and samples were acidified to pH 2.0. FT-MS Analysis. All solid-phase extracted EfOM samples were diluted 1:100 with methanol and then analyzed at the Helmholtz Zentrum in Munich, Germany using a Bruker Apex QE 12 T FT-ICR mass spectrometer. Singly charged and unfragmented negative ions were generated at atmospheric pressure within an Apollo II Electrospray ionization (ESI) source at a flow rate of 3 μL min−1. Detailed information about the ESI-FT-ICR-MS method applied in this study is given elsewhere.32 The spray stability and ionization efficiencies were optimal using 100% methanol and did not improve when diluted with water. We purposely did not add any ammonium hydroxide because it did not help to improve the signal. Molecular formula assignments were based on the following elements: 1H, 12C, 16O, 14N, 32S. The isotopes 13C and 34S were also included to cross-validate assigned molecular formulas. The same calibration procedure used in a previous study of EfOM were used 1 and a mass accuracy of 0.2 ppm was achieved.
KMD = nominal mass(NM) − KM
(1)
where NM = nominal mass of the compound, that is, molecular weight strictly rounded to the next integer value [Da] KM = massmeasured ×(14.0000/14.01565)
(2)
Z* = modulus ×(NM/14) − 14
(3)
This approach also allows spreading the data points along the y-axes (KMD/Z*) and shows in more detail the molecular weight-dependent differences between samples. The ratio of KMD/Z* produced an almost unambiguous indicator for homologous series based on CH2. On rare occasions the same values can be produced but are located at different masses and therefore homologous series are practically unambiguous in the proposed diagram where this ratio is plotted against the exact mass of the assigned neutral molecules. This KMD/Z* diagram is much more informative when compared to traditional Kendrick diagrams.35 More details about why two independent parameters are needed to assign unambiguous homologous series and about the above-mentioned parameters were given in an earlier study.6 Only masses that had a minimum of 1% relative abundance and had a correspondent 13C isotope were considered in this study. The reproducibility of the FT-MS spectra acquired using the same instrument has been demonstrated in a previous study 34 and replicate samples showed only small differences that can be explained by very small intensity mass peaks that are just under or just above the signal-to-noise ratio. A direct comparison between samples was therefore possible.
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RESULTS AND DISCUSSION Electrospray ionization is very sensitive to salt and in most cases a desalination step is required prior to mass spectral analysis. Here we used solid phase extraction and a polymeric 10279
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Figure 3. Direct comparison of van Krevelen and modified Kendrick plots (KMD-Z* plots) of SPE-DOM samples collected in cell D and I, Tank 12, and Tank 11, respectively. The size of each symbol is proportional to the intensity of the peak from the FT-MS data.
would not be efficiently retained by the solid-phase resin employed for extraction. Additionally, the common practice of adding coagulants in preliminary operations and primary treatment effluent for odor control or to enhance COD removal in the primary settler may play a role, since the coagulant would preferentially remove organic matter that would be otherwise easily retained during SPE. The anoxic denitrification cells of Tank 11 (i.e., Cells A−C) had progressively increasing adsorption efficiencies of 62% in Cell A to 72% in Cell C, whereas the anoxic denitrification Cells AC of Tank 12 had more consistent adsorption efficiencies oscillating around 80%. Although the adsorption efficiencies of Tank 11 and Tank 12 seem to have different trends for the
resin (PPL) that has been shown to have the highest overall extraction efficiency for DOM over a broad polarity range. However, PPL still has its limitation, and varying DOM recoveries from PPL are possible based on the makeup of the initial DOM present in any given sample. In general, the adsorption efficiencies of the used SPE were high, with values in the range 75−85% of DOC retained for all activated sludge cells and secondary effluents (SE) in Tank 11 and Tank 12 (Figure 2). An exception was the primary effluent (PE) with an adsorption efficiency of 34%. The low adsorption efficiency of the PE may be explained by a possibly elevated abundance of highly hydrophilic, relatively short-chain organic acids (e.g., CH3CH2COOH) typically found in PE36−38 which 10280
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Figure 4. Direct comparison of van Krevelen and modified Kendrick plots (KMD-Z* plots) of SPE-EfOM in combined primary and the two separate secondary effluent samples present in Tank 11 and Tank 12, respectively.
in Tank 12), the molecular diversity (molecular formulas assigned) in the IFAS and the ASP was virtually identical (Figure 3) indicating that the residence time in the three IFAS cells was sufficient to yield similar organics treatment when compared to the ASP. It is important to recognize that the relative abundance of mass peaks and assigned formulas in Figure 3 is not a quantitative indicator. Yet, the intensity showed evidently that the grouping of organic compounds was more similar between Cell I of Tank 11 and Tank 12 than between Cell D of both tanks suggesting similar EfOM quality after both treatments. A hierarchical cluster analysis based on distance measures of dissimilarity using Pearson correlation (R2) and averaged linkages between groups also demonstrated the differences in relative abundances and in appearance of mass peaks and associated molecular formulas between Cell D of Tank 12 (start of IFAS) and the rest (Figure 5). The result of the hierarchical cluster analysis showed similarities between Tank 11 Cell C and Tank 12 Cell A (Figure 5). This again suggested that the doubled flow rate in Tank 12 might have
initial samples in the profile (PE and Cell A in Figure 2), the DOC measurements were single measurements, and thus these efficiency trends may be within the errors of the extraction and measurement. Replicates are needed in future studies to show whether there are in fact different adsorption and extraction efficiencies for such parallel process configurations, or if the apparent trends are an artifact driven by an erratic sample (i.e., Cell A in either tank, Figure 2). Nonetheless, the high adsorption efficiencies for activated sludge cells are evidence to conclude that the SPE procedure employed in this study is a satisfactory choice to extract activated sludge dissolved organic matter, but it is of limited applicability for the PE if high DOC recoveries are important. It should be noted here that the adsorption efficiencies given here are not representing true extraction efficiencies because a small percentage of very hydrophobic compounds may not completely elute from the resin using methanol and hence the actual recovery of EfOM is not known. However, the reproducibility of recovered EfOM is high as demonstrated in a previous study using diverse EfOM samples from biogas reactors.34 Despite the doubled process flow which halved the hydraulic retention time in the IFAS treatment, the effluent DOC from both the IFAS and ASP processes had very similar values of 8− 9 mg L−1 (Figure 2). Furthermore, the molecular complexity and chemical diversity of EfOM tracked using FT-MS showed that the effluents from the different treatments with different flow rates were very similar suggesting that both treatments (ASP and IFAS/ASP) have similar effects on the molecular diversity of EfOM (Figure 3). Moreover, Figure 4 shows consistent molecular diversity for Tank 11 SE and Tank 12 SE. However, significant differences in the chemical diversity were found in cell D between the IFAS (Tank 12) and the ASP (Tank 11) (Figure 3). These differences in absolute values (appearance of mass peaks) may be explained by reduced treatment efficiency of transforming organic matter in the early stage of aeration due to the doubled flow in the IFAS Tank 12 or the still inefficient release of higher molecular weight organic matter by aerobic bacteria. After the third IFAS reactor (Cell F
Figure 5. Hierarchical cluster analysis of EfOM molecular complexity of all treatment cells in both independent tanks. Note: Calculations were based on average value per group and using R-squared correlation. 10281
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high-intensity peaks likely arising from aliphatic sulfonates in the PE with H/C ratio >2 and O/C ratio in the low to medium range (Figure 4). These high-intensity peaks disappeared in Tank 11 and Tank 12, suggesting degradation and transformation. Furthermore, by tracking the intensities of a homologous series of aromatic sulfonates through the process, transformations of one class of sulfonates to another (i.e., dialkyl tetralin sulfonates, DATS, transform to dialkyl tetralin sulfonate intermediates, DATSI) were observed (Figure 6).
reduced the treatment efficiency in the beginning, but the similarity between Tank 11 SE and Tank 12 SE confirmed that the EfOM after both treatments was practically identical. An important finding was that the overall molecular diversity increased drastically within both treatment tanks strongly suggesting that the bacterial biomass released a high degree of complex dissolved organic matter with higher molecular weight even though the DOC and biochemical oxygen demand (BOD) were removed in an expected fashion. The number of assigned molecular formulas with associated mass peaks of relative abundances higher than 1% increased from 987 in Cell D of Tank 11 (ASP) to 1387 in Cell I of the same tank (Figure 3). In Tank 12 (IFAS), the number of formulas increased from 669 in Cell D to 1411 in Cell I indicating approximately a 50% increase in molecular diversity in Tank 11 and a 100% increase in Tank 12, respectively. Additionally, a significant increase in high molecular weight mass peaks and associated molecular formulas were observed between Cells D and I in both tanks (Figure 3). Even with the presence of already high molecular weight organic compounds in Tank 11 Cell D (Figure 3), the molecular diversity continued to increase in the following cells. The increase in molecular diversity of organic molecules also indicated that bacterial biomass produced/released complex and highly transformed organic compounds. Because of the relatively low molecular diversity of PE (Figure 4) and the low extraction efficiency of the intake, it can be concluded that the drastic increase in chemical diversity of hydrophobic high molecular weight organic compounds was directly associated with the bacterial biomass. Figure 4 demonstrates the marked transformation of PE organic matter by bacterial biomass to the organic matter in SE of both tanks. The possible release of refractory DOM by bacteria has fueled debates in the biogeochemical community in recent years after the proposed concept of the microbial carbon pump (MCP).42 The concept takes into consideration that refractory DOM is supplied to the deep ocean via bacterial transformation and it was partially based on studies were refractory DOM was formed after bacterial incubation.43 Our study showed that low BOD and high molecular weight DOM is produced throughout the ASP process supporting the foundation of the MCP concept and suggesting that refractory EfOM is formed throughout the treatment process. Another surprising observation was that a higher order of chemically similar formulas was found at later stages of the ASP and IFAS treatment which are analogous to higher orders and homologous series found in NOM.33,39 For example, the appearance of longer homologous series (i.e., series with the same KMD/Z* value) based on an exact difference of CH2groups throughout the entire molecular weight range (200−600 Da) was much more pronounced in the last cells (Figure 3). It seemed that the processes within the ASP and IFAS resemble a high degree of production and rearrangement of organic matter that resulted in the observed highly symmetric pattern (smooth extent of symmetric pattern around each nominal mass) across the molecular weight range and distinctly longer homologous series. In a previous study, a large diversity of linear alkyl benzene sulfonates (LAS), their co- and byproducts including all possible homologues were present in a secondary treated effluent.1 The FT-MS data presented in this study also showed the same high intensity homologous series suggesting that these specific sulfonates resembled an important component of EfOM throughout the ASP treatments. There were numerous
Figure 6. Trends of averaged intensities of all mass peaks associated with DATS and DATSI-type molecular formulas along the treatment trains from primary effluent (PE), across the ASP/IFAS-ASP (Cell D to Cell I) toward the secondary effluent (SE). Note: No DATSI-type associated mass peaks were found in PE.
This study focused on the characterization and molecular diversity of the DOC from the IFAS and ASP. The increase of molecular complexity and diversity in the EfOM of the IFAS and ASP suggested that bacterial biomass produced/released diverse and hydrophobic organic compounds with high molecular weight, and they were highly reworked and had higher orders and longer homologous series. In line with a previous study,1 the FT-ICR-MS data also showed LAS and their co- and byproducts as important components of EfOM during the treatment processes and were likely to be highly rearranged sulfonates via transformation and degradation. The results presented in this study suggest FT-ICR MS as a promising tool for comparing the EfOM from different treatment processes. Further studies are needed to investigate DOC extraction efficiency of replicates using SPE to address the extraction trend along wastewater treatment trains.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +1 410 326 7245; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully thank the T.Z. Osborne Water Reclamation Plant’s personnel coordinated by Donald Howard for their help during sampling and data collection. This research was supported by Hazen and Sawyer, P.C. and is contribution 4794 from the University of Maryland Center for Environmental Science, Chesapeake Biological Laboratory. 10282
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Environmental Science & Technology
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dx.doi.org/10.1021/es4002482 | Environ. Sci. Technol. 2013, 47, 10277−10284
