Environ. Sci. Technol. 2007, 41, 884-890
Calculation Methods to Perform Mass Balances of Micropollutants in Sewage Treatment Plants. Application to Pharmaceutical and Personal Care Products (PPCPs) MARTA CARBALLA,* FRANCISCO OMIL, AND JUAN M. LEMA Department of Chemical Engineering, School of Engineering, University of Santiago de Compostela, E-15782 Santiago de Compostela, Spain
Two different methods are proposed to perform the mass balance calculations of micropollutants in sewage treatment plants (STPs). The first method uses the measured data in both liquid and sludge phase and the second one uses the solid-water distribution coefficient (Kd) to calculate the concentrations in the sludge from those measured in the liquid phase. The proposed methodologies facilitate the identification of the main mechanisms involved in the elimination of micropollutants. Both methods are applied for determining mass balances of selected pharmaceutical and personal care products (PPCPs) and their results are discussed. In that way, the fate of 2 musks (galaxolide and tonalide), 3 pharmaceuticals (ibuprofen, naproxen, and sulfamethoxazole), and 2 natural estrogens (estrone and 17β-estradiol) has been investigated along the different water and sludge treatment units of a STP. Ibuprofen, naproxen, and sulfamethoxazole are biologically degraded in the aeration tank (50-70%), while musks are equally sorbed to the sludge and degraded. In contrast, estrogens are not removed in the STP studied. About 40% of the initial load of pharmaceuticals passes through the plant unaltered, with the fraction associated to sludge lower than 0.5%. In contrast, between 20 and 40% of the initial load of musks leaves the plant associated to solids, with less than 10% present in the final effluent. The results obtained show that the conclusions concerning the efficiency of micropollutants removal in a particular STP may be seriously affected by the calculation method used.
Introduction The dramatic increase in the production and emission of synthetic organic chemicals for industrial and domestic use has led to their presence in a wide range of environmental samples, both liquid (wastewater, drinking water, and groundwater) and solid (sludge and sediments). Since the sources of these contaminants cannot be eliminated, controlling the release of these compounds will require optimization of specific treatment processes in sewage treatment plants (STPs). To date, it is known that municipal STPs are able to partially remove some micropollutants, including pharma* Corresponding author e-mail:
[email protected]; phone: +34 981 59 44 88, ext. 16016; fax: +34 981 54 71 68. 884
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ceutical and personal care products (PPCPs). Ozonation, UVradiation, membrane filtration, and activated carbon are potential treatments that might improve the effectiveness of PPCPs removal in a STP (1, 2). However, implementation of these techniques would increase the cost of wastewater treatment. Alternatively, understanding the fate of these substances within the STPs might yield removal methods based on better management or minor modifications of existing STPs. To evaluate the efficiency of STPs, several studies have been carried out in which the difference between the concentration of the chemical in the influent and in the final effluent has been compared (3-6). But only a few studies dealt with the fate of PPCPs along the different units of the STP treatment, with most of them based on concentrations measured in the liquid phase (7-9). Other authors modeled the fate of micropollutants in STPs by quantities of use and fugacity calculations (10). However, to understand the fate of micropollutants along STPs, it is essential to obtain information on the distribution of these substances between the aqueous and the solid phases to get insight into the contribution of each process (sorption, volatilization, or biodegradation) on the overall removal. To perform mass balances in STPs, only a few studies (11-13) included sludge measurements in their calculations and many researchers only measure dissolved concentrations and calculated the amount associated to solids by using estimated or experimentally determined solid-water distribution coefficients (Kd). This fact can be explained by the complexity of sludge analysis as well as the few analytical techniques available. In some cases, when sorption does not play a significant role, calculations based on dissolved concentrations are adequate, but it can lead to errors when sorption is important. In this way, the existing discrepancy between results from different studies could be more related to the calculation methods rather than to the accuracy of the analysis. The main objective of this work is to study the suitability of different methodologies to perform mass balance calculations of micropollutants in STPs according to the data available in each case. For that purpose, two different methods for the solid phase are proposed and compared, one based on measured data and the other based on calculated data. The proper application of a mass balance would facilitate identification of the mechanism responsible for micropollutants elimination in STPs.
Materials and Methods Sewage Treatment Plant. The sewage treatment plant considered was previously described in Carballa et al. (9). A basic flow-scheme of the plant with the location of the liquid and sludge sampling points is shown in Figure 1. The STP corresponds to a population of approximately 100,000 and includes three main sections: pretreatment, primary treatment, and secondary treatment. The excess secondary sludge and the solids obtained from the primary sedimentation are treated in a specific unit from which a solid waste and a liquid stream, recycled to the inlet of the plant, are obtained. Three analytical campaigns, during 1 year, were carried out. Integrated (24-h) liquid samples were obtained at each sampling point, and for each sample pH was adjusted to 2 to avoid biological degradation. On the other hand, grab sludge samples were taken only from the inlet of the plant, outlet of the aeration tank, outlet of thickener (primary sludge), and outlet of the flotator (excess sludge). 10.1021/es061581g CCC: $37.00
2007 American Chemical Society Published on Web 01/03/2007
FIGURE 1. Diagram of the sewage treatment plant and location of the sampling points. PPCPs. The substances considered in this work were those detected by Carballa et al. (9) in the STP studied, which are galaxolide (HHCB), tonalide (AHTN), ibuprofen (IBP), naproxen (NPX), sulfamethoxazole (SMX), estrone (E1) and 17β-estradiol (E2). HHCB, AHTN, IBP, and NPX were determined in the liquid phase during all sampling campaigns, while estrogens and SMX were only analyzed in the last campaign (April 2002). The concentrations in the sludge were measured during one sampling period (April 2002) and only for some substances (HHCB, AHTN, IBP, and estrogens). Analytical Methods. Wastewater and sludge characterization (solids, organic matter, etc.) was performed according to Standard Methods (14). The soluble content of PPCPs was determined as described in Carballa et al. (9). The PPCPs concentrations in the sludge were analyzed by Dr. Ternes in Germany. The dried sludge was extracted successively at room temperature in an ultrasonic bath with methanol and acetone and the solvent fractions were combined. For estrogens (15), a cleanup step was carried out with gel permeation chromatography (GPC) and silica gel. The extracts were then derivatized and analyzed by GC-ion-trap-MS. In the case of musks and anti-inflammatories (16), a solid-phase extraction using RP-C18 and Oasis MCX cartridges, respectively, was carried out with the aqueous extracts. Finally, the detection was done by LC APCI tandem MS for anti-inflammatories and by GC/MS in SIM mode for musks.
Mass Balance Calculations Two approaches have been used to carry out the mass balance of each compound along the different units of the STP (eq 1). The difference lies in the way how the fraction sorbed to sludge is obtained.
m ) Q(S + XTSS)
(1)
where m is the mass flux of PPCP (µg PPCP/d), Q is the flow (m3/d), S is the PPCP concentration in the liquid phase (µg PPCP/m3), X is the PPCP concentration in the sludge phase
FIGURE 2. Scheme of mass balance calculations by Method I (a) and Method II (b). (µg PPCP/kg TSS), and TSS is the suspended solids concentration (kg/m3). Method I. The first method (Figure 2a) uses measured data, thus it is only possible for those PPCPs determined in both liquid and sludge phase, i.e., musks (HHCB, AHTN), ibuprofen, and the natural estrogens (E1 and E2). Method II. The second method (Figure 2b) uses the solidwater distribution coefficient (Kd) to calculate the concentrations in the sludge from those measured in the liquid phase, thus being possible for all substances considered. The Kd values used were those reported by Ternes et al. (17) and Andersen et al. (18). The removal efficiency of a substance in a specific unit was calculated as the difference between total mass flux entering (mi) and that leaving (mout) the unit, divided by the total mass flux of the substance at the inlet of the STP (Mi), as indicated in eq 2. This means that the total mass flux at VOL. 41, NO. 3, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Input Data and PPCPs Concentrations in the Aqueous (µg/L) and Sludge (µg/g) Phasesa wastewater characterization
inlet (point 1)
pretreat. effluent (point 2)
primary effluent (point 3)
final effluent (point 5)
pret. solids
primary sludge
excess sludge
treated sludge
Q (m3/d) TSS (kg/m3)
53,000 0.25
56,000 0.25
55,000 0.12
53,000 0.02
0.5 750
900 6
2,300 2
20 450
S (HHCB) S (AHTN) S (IBP) S (NPX) S (SMX) S (E1) S (E2b) S (E1 + E2)
2.8 (2.1-3.4) 1.2 (0.7-1.7) 4.2 (2.6-5.7) 3.2 (1.8-4.6) 0.6 0.0024 0.0016 0.0040
2.9 (2.3-3.4) 1.4 (1.1-1.6) 4.3 (2.8-5.8) 3.0 (1.8-4.1) 0.5 0.0024 0.0030 0.0054
1.6 (1.3-1.8) 0.8 (0.6-1.0) 4.3 (2.8-5.8) 3.2 (1.6-4.8) 0.6 0.0034 0.0024 0.0058
1.0 0.5 0.4 (0.2-0.6) 1.1 (0.1-2.1) 0.3 0.0025
