activées
Nous détaillons ici la méthodologie expérimentale et d’exploitation des résultats des essais en réacteur fermé. Nous nous appuyons sur les résultats obtenus lors de la campagne ACA1-P2 pour décrire le comportement de 3 micropolluants (IBP, ATE, DCF) choisis pour leurs propriétés physico-chimiques contrastées. Une analyse critique de cette méthodologie est également développée. Nous mettons ainsi en exergue la capacité de la méthodologie proposée à décrire les mécanismes d’élimination des micropolluants. Des pistes pour améliorer le protocole sont aussi proposées.
Pomiès, M., Choubert, J.M., Wisniewski, C., Miège, C., Budzinski H., Coquery M. A comprehensive strategy to calibrate sorption and biodegradation of micropollutant in a biokinetic model. Soumis à Bioresource Technology.
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A COMPREHENSIVE STRATEGY FOR CALIBRATING MICROPOLLUTANT SORPTION AND BIODEGRADATION IN A BIOKINETIC MODEL FOR
ACTIVATED SLUDGE
M. Pomiès1, J.M. Choubert1*, C. Wisniewski2,C. Miège1, H. Budzinski3, M. Coquery1
1 Irstea, UR MALY, 5 rue de la Doua, CS70077, 69626 Villeurbanne Cedex, France
(maxime.pomies@irstea.fr, jean-marc.choubert@irstea.fr, cecile.miege@irstea.fr, marina.coquery@irstea.fr)
2 UMR Qualisud, Univ. Montpellier 1, 15 Av. Ch. Flahault BP 14491, 34093 Montpellier Cedex 5, France (christelle.wisniewski@univ-montp1.fr)
3 EPOC/LPTC UMR5805, Université Bordeaux 1, 351 Cours de la libération, 33405 Talence Cedex, France (h.budzinski@epoc.u-bordeaux1.fr)
*corresponding author
Abstract
The activated sludge process removes several micropollutants from wastewater by sorption onto the sludge and/or biodegradation. The objective of this paper is to propose and evaluate a robust protocol determining the sorption coefficient and biodegradation kinetics rates in the dissolved and particulate phases of activated sludge. The protocol uses batch experiments integrating in-reactor conditions (oxidoreduction potential, with/without substrate). The protocol was tested for three pharmaceutical compounds (ibuprofen, atenolol and diclofenac) covering a wide hydrophobicity range (log Kow from 0.10 to 4.51). This work allowed determining the main removal mechanism plus the phase engaged in the biodegradation process.
We showed that ibuprofen was mainly biodegraded in aerobic conditions by cometabolism with biodegradable carbon substrate, whereas anoxic conditions suppressed biodegradation. For atenolol, biodegradation occurred under both redox conditions, but that biodegradation rate was higher in aerobic conditions. Finally, diclofenac was removed from the liquid phase by sorption, with zero biodegradation observed.
Keywords. Modelling, pharmaceuticals, experimental strategy, wastewater treatment, activated sludge.
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1. INTRODUCTION
Wastewater treatment plant (WWTP) effluent is a key vector of micropollutant discharge into the environment (Heberer, 2002; Ternes and Joss, 2006). On-going regulation in Europe regularly reinforces the requirements towards discharge of chemicals into the environment, with lists of priority substances regularly revisited (EC, 2008, 2012). A broad strand of research has highlighted how WWTPs remove many micropollutants from wastewater (Miege et al., 2009; Onesios et al., 2009), particularly by biological processes, even though they were not originally designed for this purpose. However, discharged WWTP effluent still contains several micropollutants that escape removal due to their high initial concentration in raw wastewater and/or their low affinity with suspended solids and unbiodegradable chemical structures.
Sorption onto the sludge and biodegradation are the two main mechanisms dictating the fate of micropollutants in biological wastewater treatment systems (Carballa et al., 2004). Both mechanisms have been widely modelled using parameters determined based on batch tests (Pomiès et al., 2013).
Sorption is generally described at equilibrium state with a linear relation between dissolved and particulate concentrations. Using a single partition coefficient Kd is thought to account for the two main sorption mechanisms, i.e. hydrophobic mechanisms and electrostatic interactions (Sipma et al., 2010). A common procedure for determining Kd value consists in spiking sludge with micropollutant at defined concentrations and then measuring dissolved concentrations when sorption equilibrium is reached. Particulate concentration at equilibrium is generally deduced from measurements in the dissolved compartment and initial concentration in the sludge (Hyland et al., 2012). However, although widely accepted, this procedure carries weaknesses (Stevens-Garmon et al., 2011; Pomiès et al., 2013). First, it is based on the general assumption that equilibrium state is reached after a few hours (Wang et al., 2003; Hyland et al., 2012). First, there is a need to describe sorption kinetics for many micropollutants and to clearly state for each of them the time required to meet full sorption equilibrium conditions. Second, in sorption studies, micropollutant biodegradation is generally suppressed (or inhibited) (Maurer et al., 2007; Wick et al., 2009), but the procedures are not standardized, making it difficult to compare Kd data in the literature.
Biodegradation inhibition may also induce potential bias in the Kd measurement, as it influences sorption performances by altering the physico-chemical properties of sludge
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(Stevens-Garmon et al., 2011). Finally, the colloidal compartment of sludge is seldom taken into account, yet recent research on anaerobic sludge suggests it should be considered a compartment where micropollutant can sorb (Delgadillo-Mirquez et al., 2011).
The biodegradation of a micropollutant is often described with a pseudo first-order kinetics in the dissolved compartment (Joss et al., 2006; Wick et al., 2009). The biodegradation process also suffers a lack of standardization. After a micropollutant spiking step, biodegradation is evaluated from the time-course evolution of micropollutant concentrations in the dissolved phase (Lindblom, 2009; Plosz et al., 2010) using either a synthetic substrate (Suarez et al., 2010) or raw wastewater (Plosz et al., 2010). Biodegradation kinetic constant, k, are determined. Many studies have assumed that micropollutant biodegradation can be either direct or cometabolic. Cometabolism defines the biodegradation of a nongrowth substrate (the micropollutant) by the biomass in the presence of a growth substrate which is general carbon substrate (Criddle, 1993). Moreover, the few studies investigating the effect of oxidoreduction potential (ORP) conditions (aerobic and anoxic) on biodegradation efficiencies have reported significant decreases under anoxic conditions for several pharmaceutical compounds, including ibuprofen, estrone, estriol (Joss et al., 2004; Plosz et al., 2010; Suarez et al., 2010).
So, a large variety of sorption and biodegradation parameter values have been reported (Pomiès et al., 2013). These discrepancies imply difficulties of use in modelling. It is now necessary to elaborate a unified protocol with defined conditions to describe micropollutant behaviour through WWTPs.
This study aimed to bring new insight for determining sorption (Kd) and biodegradation (k) parameter values for modelling micropollutant removal in activated sludge process. In this aim, we implemented a robust experimental strategy in batch experiments involving chemical analysis in both dissolved and particulate sludge phases. The protocol integrated different conditions of activated sludge process, i.e. redox conditions and biodegradable substrate (macropollutant) supply. Our experiments aimed to (i) determine the predominant removal mechanism (sorption or biodegradation) for a selection of micropollutants, (ii) determine the phase (dissolved or particulate) engaged in biodegradation, and (iii) quantify the values of the sorption (Kd) and biodegradation (k) parameters. We also tested the measured parameters to simulate the micropollutant removal in a full-scale activated sludge process. Our work targeted 3 pharmaceutical compounds (ibuprofen, atenolol, diclofenac) that share different physicochemical properties and different fates through WWTPs.
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156 2. THEORY ― CONCEPTS AND EQUATIONS 2.1. Sorption
We considered that sorption equilibrium was reached rapidly (30 minutes) and that equilibrium was in the range of linearity between particulate and dissolved micropollutant at working concentrations. The sorption coefficient (Kd) can be evaluated as follows (Eq. 1):
mp mp
S SS X Kd=
deg ,bio
FW
Each process in Table 1 can be added to calculate micropollutant flux biodegraded, . As shown in Table 1, this study defined pseudo first-order biodegradation rates separately for dissolved (with an S superscript) and particulate (with an X superscript) phases in aerobic (with an Ox superscript) and anoxic (with an Ax superscript) conditions. Moreover, we considered different biodegradation kinetic constants (k) according to the presence of different macropollutants as co-substrate:
2.2. Biodegradation
SS: suspended solids concentration (gSS.L-1).
Smp: dissolved micropollutant concentration (µg.L-1) ; Xmp: particulate micropollutant concentration (µg.L-1) ; Kd: sorption coefficient (L.gSS-1) ;
- absence of biodegradable substrate for aerobic (kS,endo,Ox, kX,endo,Ox) and anoxic (kS,endo,Ax, kX,endo,Ax) conditions.
- presence of nitrogen forms only for aerobic (kS,N,Ox, kX,N,Ox) and anoxic (kS,N,Ax, kX,N,Ax) conditions;
- presence of biodegradable COD (chemical oxygen demand) and nitrogen forms for aerobic (kS,C-N,Ox,kX,C-N,Ox) and anoxic (kS,C-N,Ax, kX,C-N,Ax) conditions;
(Eq. 1)
Biodegradation conditions Smp Xmp Biodegradation rate
Presence of biodegradable C and N substrates
Presence of biodegradable C and N substrates
Presence of nitrogen substrate only
Presence of biodegradable C and N substrates
Presence of biodegradable C and N substrates
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Table 1: Variables and equations involved in sorption and biodegradation processes
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where So: dissolved oxygen concentration
KO2,OHO: oxygen half-saturation coefficient for heterotrophic biomass SNHx: dissolved ammonia nitrogen concentration
KNHx,ANO: ammonia half-saturation coefficient for autotrophic biomass SNOx: dissolved nitrate nitrogen concentration
KNOx,OHO: nitrate half-saturation coefficient for heterotrophic biomass SB: dissolved biodegradable COD
K1, K2: half-saturation coefficients for heterotrophic biomass
Cometabolism is described including the influence of biodegradable substrate on micropollutant biodegradation in equations makes possible to describe cometabolism. This formalism dissociates cometabolism with biodegradable carbon and nitrogen (action of heterotrophic biomass), cometabolism with nitrogen only (action of autotrophic biomass) and the endogenous process. Biodegradation kinetic constants used in the biodegradation equations are easier to measure than the parameters of Criddle’s formalism (Criddle, 1993;
Delgadillo-Mirquez et al., 2011).
3. MATERIALS AND METHODS 3.1 Experimental strategy
3.1.1. WWTP sampling
The WWTP sampling procedure sampled (i) sludge for batch experiments and (ii) raw and treated wastewater for the WWTP mass balance evaluation. The WWTP studied (2,900 population equivalents) consisted in an activated sludge process (under extended aeration) treating carbon (C) and nitrogen (N) under a 50-days solid retention time working on around 7.3 gSS.L-1 at a temperature of around 23°C during sampling. Sludge for batch experiments was grab-sampled from the aeration tank.
To perform the WWTP mass balance evaluation, we sampled raw and treated wastewater to constitute 24 h flow-proportional composite samples. Samples were taken using a refrigerated automatic sampler with precleaned Teflon tubing and glass containers (Choubert et al., 2011).
Dissolved and particulate micropollutants were analysed in raw wastewater whereas only the dissolved phase was analysed in treated effluent. Sludge in the aeration tank was also sampled by grab-sampling.
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3.1.2 Batch experiments
We designed a combination of two batch experiments to simulate the conditions occurring in a full-scale biological process:
– two ORP conditions (aerobic tank A; anoxic tank B);
– three macropollutant supply conditions (no macropollutant addition; C and N addition; nitrogen addition).
Batch experiments were carried out during 4 days in two 200 L plastic tanks. Tank content was continuously stirred throughout the experiment. Samples were collected via a valve port fitted at the bottom of the tanks. The different phases of the experiment are presented in the following sections and illustrated in Figure 1 (using the example of batch A).
Preliminary phase (t = 0 to 18 h)
Tanks A and B were filled with 180 L of activated sludge (t = 0 h) collected in the WWTP aeration tank. Intermittent aeration was applied for 18 hours to eliminate residual substrate from the liquid sludge phase. Micropollutant concentration in the sludge was measured at t = 0 h.
Phase I “without any substrate” (t = 18 to 43 h)
At t = 18 h, each tank was spiked with a mixture of micropollutants (10 µg.L-1 each) in 10 mL of methanol solvent. These levels were in the usual concentration range of raw wastewater (Miege et al., 2009). Tank A was supplied with air via a diffuser located at the bottom of the reactor to ensure a dissolved oxygen concentration of 7-8 mg O2.L-1. In tank B, N2 gas was supplied to initiate anoxic conditions. No chemical agent (e.g. inhibitor) was used. ORP conditions were monitored in both tanks. In each reactor, sludge was sampled 3 times at t = 18.5 h (for dissolved and particulate analysis), t = 23 h (for dissolved analysis, tank B only) and t = 42 h (for dissolved analysis).