High-Rate Degradation of Aromatic Sulfonates in a Biofilm Airlift

Jul 13, 2007 - FISIA Italimpianti, Via de Marini 16, 16149 GenoVa, Italy ... end, the reactor was inoculated with a mixed culture of biomass, adapted ...
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Ind. Eng. Chem. Res. 2007, 46, 6674-6680

High-Rate Degradation of Aromatic Sulfonates in a Biofilm Airlift Suspension Reactor Cristiano Nicolella* Dipartimento di Ingegneria Chimica, Chimica Industriale e Scienza dei Materiali, UniVersita` di Pisa, Via DiotisalVi 2, 56100 Pisa, Italy

Marcello Zolezzi FISIA Italimpianti, Via de Marini 16, 16149 GenoVa, Italy

Michela Furfaro, Claudia Cattaneo, and Mauro Rovatti Dipartimento di Ingegneria Chimica e di Processo,UniVersita` di GenoVa, Via Opera Pia 15, 16145 GenoVa, Italy

A wastewater containing aromatic sulfonates was treated in a laboratory-scale biofilm airlift suspension (BAS) reactor. The general objective of the experiments was to assess the performance of a particle-based biofilm reactor in the biodegradation of refractory organic compounds under varying operating conditions. To this end, the reactor was inoculated with a mixed culture of biomass, adapted to grow on the leachate as the sole source of carbon, which colonized carrier particles in the reactor and formed stable and uniform biofilms. Depending on their degradation kinetics (characterized in independent experiments) and the fraction of specific degraders in the biofilms, aromatic sulfonates attain different degradation efficiencies in the BAS reactor, with more compex molecules (e.g., trisusbtituted naphthalene sulfonates and some bisubstituted naphthalene sulfonates) showing the lowest degradation efficiencies. The BAS reactor achieved high biomass concentration (12 g L-1) and an overall degradation efficiency of 67% based on COD measurements for loading rates up to 0.45 kgCOD kgVS-1 day-1, corresponding to a specific degradation rate of 0.3 kgCOD kgVS-1 day-1. For higher loading rates, the hydraulic retention time of leachate in the reactor proved to be insufficient for complete degradation of aromatic sulfonate with slow degradation kinetics, resulting in a decrease of overall degradation efficiency. The fast degradation rate of most aromatic sulfonates is attributed to the long biomass retention time and reactor biomass concentration, resulting in high concentrations of degraders for specific compounds. Introduction Wastewater streams to point-source treatment works may contain organic sulfonates (e.g., benzene and naphthalene sulfonates) as the result of the manufacture of several chemical products (including dispersants, detergents, azo dyes, concrete plasticizers, and wetting agents) where they are used as intermediates or obtained as byproducts. Since these xeniobiotic compounds are in most cases scarcely biodegradable,1,2 their removal is usually performed by chemical or electrochemical oxidation,3-5 while they are reported to resist biodegradation in activated sludge processes for sewage treatment.6,7 Altenbach,7 while investigating the fate and behavior of several benzene and naphthalene sulfonates in municipal treatment works receiving wastewater from textile finishing industries, showed that municipal treatment works are able to develop a degradative capacity for some xenobiotic industrial sulfonates, e.g., monosubstituted naphthalene sulfonates and some bisubstituted naphthalene sulfonates. The biological oxidation of scarcely biodegradable compounds in wastewaters is accomplished by slow-growing microorganisms, requiring long retention time to be kept in a reactor. To obtain long biomass retention time, two conditions must be realized in a biological process for wastewater treatment: (i) low rate of biomass wash-out and (ii) high reactor * To whom correspondence should be addressed. Tel.: 39 50 511294. Fax: 39 50 511266. E-mail: [email protected].

biomass concentration. In activated sludge processes the biomass retention and, as a consequence, the treatment capacity are limited by the biomass concentration achievable in the oxidation tank. This limitation is particularly severe in the case of slowgrowing microorganisms, resulting in large volumes required to accomplish the degradation of target substrates and large areas to be occupied by the treatment works. To overcome these limitations, different types of particlebased biofilm reactors have been developed in the past two decades (e.g., the upflow sludge blanket, biofilm fluidized bed, expanded bed sludge blanket, internal circulation, and biofilm airlift suspension reactors).8,9 In these reactors biomass grows in the form of nearly spherical, compact biofilms, whose small size (typically around 1 mm) results in high specific biofilm surface area (up to 3000 m2 m-3) and high liquid-biofilm mass transfer rates. The good settling properties of the biofilms (with settling velocities up to 50 m h-1) allows biomass retention at high dilution rates, thus enabling high reactor biomass concentrations to be achieved without the need of biomass separation and recirculation as in the activated sludge process. With high biofilm specific surface area and high reactor biomass concentration, high volumetric conversion rates can be obtained even with slow-growing microorganisms, such as nitrifiers in the biofilm airlift suspension reactor10,11 and methanogens in the upflow sludge blanket,12 expanded bed sludge blanket,13 and internal circulation14 reactors. Wagner and Hempel15 investigated the biodegradation of naphthalene-2-sulfonate in an external loop

10.1021/ie0616601 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/13/2007

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biofilm airlift reactor inoculated with a pure culture of Pseudomonas sp. A3. Recently Nicolella et al.16 used an internal loop airlift reactor inoculated with industrial sewage sludge to develop particulate biofilms capable of degrading a mixture of aromatic sulfonates. Feasibility of biodegradation in particulate biofilm reactors was also demonstrated for other xenobiotic organics including chlorophenols,17 peroxide compounds,18 aromatic compounds,19 and other volatile organic compounds.20 In this work a laboratory-scale biofilm airlift suspension reactor is used to reduce the chemical oxygen demand (COD) of the infiltration water from an industrial site contaminated by aromatic sulfonates. The main aim of the experiments is to assess the performance of a particle-based biofilm reactor in the biodegradation of refractory organic compounds under varying operating conditions. Experimental Section Wastewater and Nutrient Medium. The experiments were performed using the infiltration water (hereafter called leachate) collected at an industrial site in the northwest of Italy contaminated by the wastes of a chemical factory. A waterproof, underground side wall is placed around the contaminated site, with drainage wells drilled to pump and treat contaminated infiltration water, and to maintain the aquifer surface below the lowest river level. The leachate extracted from the wells is treated in an activated sludge process, whose performance on scarcely biodegradable pollutants is enhanced by addition of powdered activated carbon (PAC) in the oxidation tank. Residual pollutants and color in the effluent are removed by wet oxidation and ion exchange. Leachate samples pumped from the drainage wells were collected once a week, transported to the laboratory, and cooled at 4 °C. Since the leachate does not contain enough macronutrients (ammonium nitrogen is less than 10 mgN-NH4 L-1 and phosphorus is less than 0.4 mgP-PO4 L-1), ammonium (NH4Cl) and phosphate (KH2PO4) salts were added in order to reach a C:N:P ratio of 100:5:1. Variable amounts of salts were dosed depending on the measured COD concentration in the leachate and assuming an average COD/total organic carbon (TOC) ratio of 3.3 as determined by the analytical laboratory at the factory site. Micronutrients (e.g., 0.1 mg L-1 MgSO4‚7H2O and 0.5 mL L-1 trace element solution)21 were also added to the leachate. Biofilm Airlift Suspension (BAS) Reactor. The experimental setup is schematically represented in Figure 1. The BAS reactor, whose main features and range of operating conditions are summarized in Table 2, consists of a concentric tube column with a three-phase separator on the top. Air introduced at the bottom of the inner column (riser) reduces the apparent density in this section, thus driving continuous liquid circulation from the outer column (downcomer), at velocity increasing with increasing gas superficial velocity. The BAS reactor is operated at liquid velocity higher than biofilm settling velocity and bubble rising velocity. Under these conditions circulation of liquid, biofilms, and gas bubbles is realized in the reactor, where the liquid mixing regime approaches perfect mixing.9 Liquid, biofilms, and gas bubbles are separated in the three-phase separator on the top of the column, constituted by an enlarged section where liquid velocity decreases below biofilm settling velocity and bubble rising velocity, allowing biofilms to drop back in the airlift column and bubbles to exit as offgas. The system was operated at room temperature and pressure, with pH controlled at 7, and dissolved oxygen always higher than 3 mg L-1. Air was sparged into the reactor through a

Figure 1. Experimental setup. Table 1. Experimental Setup Characteristics and Operating Conditions airlift column riser diameter downcomer diameter column height separator diameter separator height airlift volume biofilm carriers (sand) mean size density holdup concentration gas (air) superficial velocity holdup feed (leachate) flow rate COD concentration

35 mm 80 mm 1640 mm 150 mm 290 mm 8L 0.42-0.5 mm 2550 kg m-3 4% 100 kg m-3 9 mm s-1 1.3% 0.6-3.8 L h-1 300-1200 mg/L

sintered glass stone placed at the bottom of the riser section. The superficial air velocity in the reactor was controlled by means of a mass flow controller. To ensure control of the dilution rate for growth in suspension in the BAS reactor, a separation-recirculation loop was placed at the column outlet. The BAS effluent entered a hydrocyclone where biofilm particles washed out from the reactor were collected and, after shearing off excess biofilm, were reintroduced in the column. The liquid from the hydrocyclone passed into a settling tank where suspended solids settled and were periodically discharged. The excess sludge produced in the BAS process was estimated by weighing the sludge discharged from the settler. The clarified liquid was recirculated to the BAS reactor by means of a centrifugal pump at the desired recycle ratio. Particle-Based Biofilms. Particle-based biofilms used in the experiments were obtained as previously described.16 Active biomass growing on a mixture of aromatic sulfonates as the sole source of carbon was obtained by enriching a mixed microbial culture sampled from the treatment works at the factory site. Biomass enrichment was obtained by exposing the mixed culture to progressively increasing concentrations of aromatic sulfonates while maintaining microbial activity over a period of 4 months. The enriched mixed culture was then

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Figure 3. Adhered and suspended biomass concentration in the BAS reactor during batch and continuous phase of operation. Table 2. Operating Conditions and Effluent Concentrations during Different Phases of Operation

Figure 2. Particle-based biofilm formed in the BAS reactor.

inoculated in a BAS reactor where it colonized carrier particles and formed stable and uniform biofilms (see Figure 2). Assays. COD, ammonium, nitrate, nitrite, suspended solid, and adhered biomass concentrations were determined according to standard methods.22 COD concentration was measured according to the standard dichromate method. Ammonium, nitrite, and nitrate nitrogen and phosphorus concentrations were measured by colorimetric assays. Suspended solid concentration was determined by filtration of a known volume of liquid through a filter paper with pore size 0.45 µm. Adhered biomass was determined as dry weight of volatile solids per unit mass of carrier. Biofilm particles were sampled from the BAS reactor, washed with demineralized water to remove suspended biomass, dried at 105 °C for 2 h, cooled, and weighed at room temperature. The dried sample was calcinated at 600 °C for 2 h, and the residual weight was determined at room temperature. Biomass weight was estimated as the volatile solid weight, i.e., the difference between the dried sample and the calcinated sample. Each determination was based on the weighing of three samples. Aromatic (benzene and naphthalene) sulfonate concentrations were determined at the factory laboratory by gradient HPLC analysis (solvent A, H2O + NaH2PO4 + TBAHS; solvent B, CH3OH + H2O; column, ODS Hypersil 5 µm, 200 × 2.1 mm), with a detection limit lower than of 0.01 mg/L. Results and Discussion Operating Conditions. The BAS reactor was started up using particle-based biofilms (see Figure 2) developed as previously described.16 The reactor was operated at a dilution rate higher than the maximum specific growth rate of the mixed culture forming the biofilms.16 This condition is required to wash out the microorganisms growing in suspension and makes the organic substrate and nutrients available for biofilm growth.23 At any loading condition (i.e., inlet leachate flow rate), the dilution rate was controlled by introducing a hydrocyclone at the reactor outlet to separate the suspended biomass from the reactor effluent, which was partially recirculated to the column at the desired rate. This setup configuration (see Figure 1) allowed the achievement of high reactor biomass concentration (up to 12 g L-1). The concentration of adhered biomass in the BAS reactor is presented in Figure 3 as a function of operation time. Due to the high dilution rate applied to the column, the concentration of suspended biomass in the reactor, also plotted

inlet outlet inlet outlet hydraulic biomass starting residence concentration COD COD ASOD ASOD 3 (mg/L) (mg/L) (mg/L) (mg/L) phase day time (h) (kg/m ) 1 2 3 4 5 6 7 8

3 28 48 62 78 94 101 115

29 16 16 16 10 8 4 3

1.9 3.5 5.1 6.1 6.7 7.5 8.4 11.4

578 1008 913 845 1121 915 1033 1139

187 331 301 283 368 314 360 423

353 594 532 514 642 538 631 675

15 27 23 20 29 24 32 34

in Figure 3 as a function of time, was always much lower than the concentration of adhered biomass. It is worth noting that, because the dilution rate applied to the reactor was always much higher than the maximum specific growth rate of the microorganisms forming the biofilms, the volatile suspended solids detected in the reactor effluent are not to be attributed to growth in suspension but mainly to biofilm detachment. In addition, because the concentration of suspended biomass was up to 2 orders of magnitude lower than the adhered biomass concentration, the contribution of suspended biomass to the overall degradation process is assumed to be negligible. The BAS reactor was fed at increasing COD loading rates up to a maximum value of 9.0 kgCOD m-3 day-1. For each loading rate the reactor was operated until a condition of steady state in outlet concentration was achieved, characterized by a variation of less than 7% (corresponding to the accuracy of the COD assay) for at least four consecutive days. The operating conditions applied during different phases of operations and the steady-state COD and oxygen demand due to aromatic sulfonates (ASOD) in the reactor effluent are summarized in Table 2, where concentrations are averages of values recorded during the last 4 days of each phase of operation (i.e., pseudo-steadystate values). The organic content of the leachate collected at the factory site varied in the range 600-1100 mgCOD L-1, mainly because of varying weather conditions and precipitation levels at the factory site during the period of experiments. As can be observed in Table 2, oxygen demand due to aromatic sulfonates constitutes around 60% of the organic content of the leachate. COD loading rate varied during the experiments either because the leachate COD changed during the reactor operation or because the hydraulic retention time was changed on purpose to assess the effect on process performance. The variation of loading rate during the period of operation is presented in Figure 4, where eight phases of operation are depicted as characterized in Table 2. The increase in loading rate was initially due to the increase of COD concentration in the leachate (phase 1). The reactor was then operated at constant loading rate for a period

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Figure 4. Volumetric loading and degradation rate in the BAS reactor. Vertical lines identify different phases of operation, characterized by operating conditions as listed in Table 2.

Figure 5. Specific removal rate of aromatic sulfonates and COD in the BAS reactor as a function of the loading rate.

of 50 days, during which only reactor biomass concentration varied (from 3.5 to 6.1 kg/m3) because the hydraulic residence time was set to 16 h and the leachate COD was almost constant (phase 2-4). After this phase of operation, the loading rate to the reactor was progressively increased by increasing the volumetric flow rate of the leachate fed to the reactor (phase 5-8), up to values for which a drop in degradation efficiency was observed. General Quality Parameters. The specific degradation rate of COD increased with increasing specific loading rate as illustrated in Figure 5, which also reports the COD degradation efficiency as function of COD loading rate, both determined under conditions of steady outlet concentrations for all loading rates (see Table 2). For loading rates up to 0.45 kgCOD kgVS-1 day-1, data in Figure 5 show a linear trend whose slope represents the average degradation efficiency, confirming that COD degradation was stable and did not depend on loading rate or on biomass concentration in the range of conditions characterizing phases 1-5 of operation (see Table 2). The best linear fit of the experimental data corresponds to a degradation efficiency of 67%. This value is in line with the information on the degradable COD fraction in the leachate collected during the adaptation experiments previously reported,16 suggesting that the residual effluent COD for loading rates up to 0.45 kgCOD kgVS-1 day-1 is not due to kinetic and/or mass transfer limitations (i.e., insufficient hydraulic residence time in the reactor), but is due to the recalcitrant character of some components in the leachate. For higher loading rates the degradation efficiency starts to decrease and reaches a minimum of 62% corresponding to the highest loading rate applied in this work (0.8 kgCOD kgVS-1 day-1). Under these conditions, the degradation process is limited either by mass transfer (e.g., diffusion in the biofilms) or biochemical reaction kinetics, and the hydraulic residence time is not sufficient for the biodegradable fraction of COD to be completely biodegraded in the reactor.

Figure 6. Degradation kinetics for three aromatic sulfonates.

Data on degradation rate of aromatic sulfonates are also reported in Figure 5, where the specific degradation rate of oxygen demand due to aromatic sulfonates (ASOD) is plotted as a function of the specific COD loading rate. By comparing the degradation rate of aromatic sulfonates and COD, it can be observed that a fraction (ranging from 9 to 16%) of the COD degraded in the bioreactor is due to organic compounds other than aromatic sulfonates. Degradation of Aromatic Sulfonates. Kinetics of aromatic sulfonate degradation were investigated in flask experiments, performed by sampling the flask content every hour for determinations of COD, aromatic sulfonate concentration, and volatile suspended solid concentration. After inoculation with suspended biomass from the reactor effluent, the flasks were stirred, continuously sparged with air, and maintained at room temperature for 12 h. The estimation of kinetic parameters is based on three repetitions of the experiments. Typical results of kinetic experiments are presented in Figure 6, where measured concentrations are plotted as a function of batch time for three aromatic sulfonates (hydroxyl naphthalene mono-, bi-, and trisulfonate). Fitting of experimental data in Figure 6 using the Monod kinetic expression shows that degradation kinetics of aromatic sulfonates is zero order over a wide range of concentration. This is due to the low value of the Monod constant (a few milligrams per liter) typically reported for the biodegradation of aromatic sulfonates.15,16 Data on degradation kinetics of aromatic sulfonates are summarized in Table 3, where the estimated maximum specific degradation rates are apparent values referred to the whole biomass rather than to the active biomass (i.e., degraders of specific compounds and isomers), and including endogeneous decay. This may account for the lower maximum specific degradation rates of this work compared to that of Wagner and Hempel,15 which used a specific pure culture to degrade naphthalene-2-sulfonate, reported to be one of the most readily biodegradable among naphthalene sulfonates.2 The value of kmax obtained in this work is at the lower end of the range reported by Knightes and Peters25 for the biodegradation of 10 polycyclic aromatic hydrocarbons by a consortium of bacterial strains. The values reported in that work spanned a little less than 2 orders of magnitude depending on the compound considered. These differences in the biodegradation rate are attributed to physicalchemical processes that control bioavailability, which differ significantly for different compounds. Depending on their relative rate of degradation (see Table 3) and on the loading conditions (see Table 2), aromatic sulfonates attain different degradation efficiencies in the BAS reactor. Complex molecules with slow degradation kinetics are not completely degraded in the BAS reactor even at slow dilution rates (i.e., long hydraulic residence times), as detailed in Table 4, which reports the degradation efficiencies of refractory

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Table 3. Average Aromatic Sulfonate Concentrations in the Leachate and Their Maximum Specific Degradation Rates compound

abbreviation

average oxygen demand (mg/L)

maximum specific degradation rate (gCOD gVS-1 day-1)

1-aminobenzene-3-monosulfonic acid 1-aminobenzene-4-sulfonic acid naphthalene-1-monosulfonic acid naphthalene-2-monosulfonic acid 2-hydroxynaphthalene-5-sulfonic acid naphthalene-1,5-disulfonic acid naphthalene-1,6-disulfonic acid naphthalene-2,6-disulfonic acid naphthalene-2,7-disulfonic acid 2-hydroxynaphthalene-1,6-disulfonic acid 2-hydroxynaphthalene-3,6-disulfonic acid 2-hydroxynaphthalene-6,8-disulfonic acid 2-aminonaphthalene-5,7-disulfonic acid 2-hydroxynaphthalene-3,6,8-trisulfonic acid 2-hydroxy-3-naphthoic acid

1-NH2-3-BMS 1-NH2-4BMS 1-NMS 2-NMS 2-OH-5-NS 1,5-NDS 1,6-NDS 2,6-NDS 2,7-NDS 2-OH-1,6-NDS 2-OH-3,6-NDS 2-OH-6,8-NDS 2-NH2-5,7-NDS 2-OH-368-NTS BON

36 16 74 10 8 9 114 11 37 2 8 29 4 8 240

0.060 0.165 0.703 0.188 0.040 0.006 0.657 0.074 0.201 0.029 0.059 0.114 0.003 0.003 0.803

Table 4. Degradation Efficiency (%) for Refractory Aromatic Sulfonates in the BAS Reactor in Different Phases of Operationa compound

phase 1

phase 2

phase 3

phase 4

phase 5

phase 6

phase 7

phase 8

naphthalene-1,5-disulfonic acid 2-aminonaphthalene-5,7-disulfonic acid 2-hydroxynaphthalene-3,6,8-trisulfonic acid

7 24 8

7 24 8

11 36 12

13 43 15

9 30 10

8 26 9

4 15 5

5 15 5

a

Operating conditions in each phase are reported in Table 2.

aromatic sulfonates in all phases of operation. Compounds not listed in Table 4 attain complete degradation under all operating conditions. Benzene sulfonates and monosubstituted naphthalene sulfonates show the fastest degradation kinetics. These compounds reach complete degradation in the BAS reactor at all loading conditions investigated in this work. Most naphthalene disulfonates, hydroxy naphthalene monosulfonates, and hydroxy naphthalene disulfonates present fast kinetics as well, and they were completely removed in the BAS reactor. 1,5-Naphthalenedisulfonate represents an exception among these compounds, showing slow degradation kinetics and resisting microbial degradation, with degradation efficiency lower than 13%. This behavior reflects the results of Altenbach,7 who observed that microbial sludges from municipal treatment works degrade isomers of naphthalene sulfonates at different rates. In particular, naphthalene mono- and disulfonates can be arranged in the following order according to their biodegradability: 2-NMS > 1-NMS > 2,6-NDS > 2,7-NDS > 1,5-NDS. This reflects some of the features of the system investigated in this work (see Tables 3 and 4). Different degradation rates for isomers were attributed to the relative amounts of specific degrading microorganisms, which will depend on their growth kinetics and the relative isomer concentrations in the wastewater. In this work, the concentration of 1,6-NDS in the leachate was on average 1 order of magnitude higher than the concentration of 1,5-NDS (see Table 3). As a consequence, specific organisms degrading 1,5NDS might not have grown at a sufficient rate to sustain the dilution rate applied in the BAS reactor, thus resulting in low degradation efficiency for this compound. Trisubstituted naphthalenes show a marked recalcitrant character, with slow degradation kinetics (see Table 3). The degradation of 2-OH-3,6,8-NTS did not exceed 15% in the BAS reactor. As the presence of one amino group implies a low degradability of naphthalene disulfonates,26 the degradation efficiency of 2-aminonaphthalene-5,7-disulfonic acid was less than 45%. Performance of BAS Reactor and PAC AS. A comparison of the performance of the laboratory-scale BAS reactor and that of the powder activated carbon-activated sludge (PAC AS) process at the on-site treatment works operating at the factory

Table 5. Comparison of Degradation Performance in the Laboratory-Scale BAS Reactor Used in This Work and in the Treatment Works Operating at the Factory Site

parameter reactor volume (m3) biomass concentration (kg m-3) inlet flow rate (m3 day-1) hydraulic residence time (h) inlet COD (mg/L) outlet COD (mg/L) degradation efficiency (%) volumetric loading rate (kgCOD m-3 day-1) volumetric degradation rate (kgCOD m-3 day-1) specific loading rate (kgCOD kgVS-1 day-1) specific degradation rate (kgCOD kgVS-1 day-1) excess sludge production (kgVS mww-3) activated carbon dosage (kg/day)

lab-scale BAS reactor

treatment works (activated sludge + activated carbon)

8.00 × 10-3 7.5

6200 4.1

2.40 × 10-2 8 915 314 66 2.7

3720 40 1028 138 87 0.6

1.8

0.5

0.37

0.15

0.24

0.13

0.06

0.11 750

site on the same leachate is made in Table 5. The biological section of the treatment works is based on an activated sludge process operating with continuous addition of activated carbon in the oxidation tank (0.2 kg per m3 of leachate, resulting in an average of 750 kg/day of activated carbon dosed in the oxidation tank). The comparison is made based on the best performance offered by the two systems in terms of removal efficiency. As can be observed in Table 5, because of higher biomass concentration (resulting in higher concentration of specific sulfonatedegrading microorganisms), the volumetric degradation rate achieved in the BAS reactor before the performance drops (2.8 kgCOD m-3day-1) is almost 5 times higher than in the treatment works, even though in this process activated carbon is added to the leachate to assist removal of organic compounds and it may contribute to the overall removal efficiency, which on average is higher in the PAC AS process than in the BAS reactor. Data on the specific loading rate (food-to-microorganism ratio, i.e., kilogram of degradable COD fed per kilogram of biomass in the reactor in the unit of time) are also reported in

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θc )

XR V PX

(3)

where V is the reactor volume. The biomass retention time in the reactor varies in the range 21-50 days depending on the inlet flow rate and reactor biomass concentration. Long biomass retention time and low food-to-microorganism ratio are key factors determining the high degradation rate achieved in the BAS reactor for organic compounds with slow degradation kinetics. Figure 7. Comparison of excess sludge production measured in a BAS reactor (2, ]) and calculated for an activated sludge process (solid line).

Table 5. Due to the high reactor biomass concentration (see Figure 3), this parameter is lower than the typical values reported for activated sludge process, where it can exceed 1 kgCOD kgVSS day-1 for high organic loading rates.24 One of the advantages claimed for particle-based biofilm reactors is the low production of excess sludge compared to that of activated sludge processes.8 This is mainly due to the higher mean biomass residence time in biofilm reactors than in activated sludge processes. In an activated sludge process, the excess sludge production (PX) can be calculated from the observed biomass yield (Yobs) as24

PX ) YobsQ(CS0 - CS)

(1)

where Q is the volumetric flow rate and CS is the COD concentration in the leachate. The observed biomass yield (Yobs) is a function of the biomass retention time (θc):

Yobs )

Y 1 + kdθc

(2)

where Y is the biomass yield and kd is the biomass decay coefficient. The excess sludge produced in the experiments of this work can be compared to the excess sludge estimated for an activated sludge process using eq 1 and the yield coefficient of the biomass in the BAS reactor, measured in independent experiments as 0.23 gVSS/gCOD.16 This comparison is made in Figure 7, where the excess sludge collected during the experiments (per unit volume of wastewater fed to the reactor) is plotted as a function of the volumetric degradation rate. The continuous line represents the estimated behavior of an activated sludge process (eq 1) operating with the same biomass and substrate (i.e., same yield coefficient), the same loading rate and degradation efficiency, and with a biomass retention time of 5 days and a cell decay coefficient of 0.06 day-1.24 The excess sludge produced in the PAC AC process at the factory site is also reported in Figure 7, where it can be observed that the sludge produced in the BAS reactor is generally lower than that estimated and measured for the activated sludge process, with the exception of a few peaks which may be attributed to random biofilm detachment. This behavior is typical of particle-based biofilm reactors, as reported by Heijnen et al.,27 who observed no excess sludge production in a full-scale biofilm airlift suspension reactor. This was attributed to the high reactor biomass concentration, resulting in a slow food-to-biomass ratio. The biomass retention time in the BAS reactor (θc) may be calculated using experimental data on excess sludge production (PX) and reactor biomass concentration (XR) as

Conclusions Depending on their degradation kinetics, characterized in independent experiments, and the fraction of specific degraders in the biofilms, aromatic sulfonates attain different degradation efficiencies in the BAS reactor, with more compex molecules (e.g., trisusbtituted naphthalene sulfonates and some bisubstituted naphthalene sulfonates) showing the slowest degradation efficiencies. The BAS reactor achieved high biomass concentration (12 g L-1) and an overall degradation efficiency of 67% based on COD measurements for loading rates up to 0.45 kgCOD kgVS-1 day-1, corresponding to a specific degradation rate of 0.3 kgCOD kgVS-1 day-1. For higher loading rates, the hydraulic retention time of leachate in the reactor proved to be insufficient for complete degradation of aromatic sulfonates with slow degradation kinetics, resulting in a decrease of overall degradation efficiency. The degradation rate achieved solely by biological oxidation in the BAS reactor is higher than that in the activated sludge process installed at the industrial site producing the leachate, where a fraction of the organic load is adsorbed on activated carbon dosed continuously in the oxidation tank. The high capacity for degradation of aromatic sulfonates observed in the BAS reactor may be attributed to high biomass concentration (resulting in high concentration of specific degraders for different compounds) and long biomass retention time for the growth of slow-growing microorganisms able to degrade refractory organic substrates such as the aromatic sulfonates considered in this work. Literature Cited (1) Cook, A. M.; Laue, H.; Junker, F. Microbial desulfonation. FEMS Microbiol. Lett. 1999, 22, 399. (2) Nortermann, B.; Kuhm, A. E.; Knackmuss, H. J.; Stolz, A. Conversion of substituted naphthalenesulfonates by Pseudomonas sp. BN6. Arch. Microbiol. 1994, 161, 320. (3) MacKay, A. A.; Pignatello, J. J Application of Fenton-based reactions for treating dye wastewaters: Stability of sulfonated azo dyes in the presence of iron(III). HelV. Chim. Acta 2001, 84, 2589. (4) Panizza, M.; Zolezzi, M.; Nicolella, C. Biological and electrochemical oxidation of naphthalene sulfonates in a contaminated site leachate. J. Chem. Technol. Biotechnol. 2006, 81, 225. (5) Shiyun, Z.; Xuesong, Z.; Daotang, L. Ozonation of naphthalene sulfonic acids in aqueous solutions. Part I: elimination of COD, TOC and increase of their biodegradability. Water Res. 2002, 36, 1237. (6) Kolbener, P.; Baumann, U.; Cook, A. M.; Leisinger, T. 3-nitrobenzenesulfonic acid and 3-aminobenzesulfonic acid in a laboratory trickling filter: biodegradability with different activated sludges. Water Res. 1994, 28, 1855. (7) Altenbach, B. Determination of substituted benzene- and naphthalene sulfonates in waste water and their behaviour in sewage treatment. Ph.D. Thesis No. 11437, ETH, Zurich, 1996. (8) Nicolella, C.; van Loosdrecht, M. C. M.; Heijnen, J. J. Particlebased biofilm reactor technology. Trends Biotechnol. 2000, 18, 312. (9) Nicolella, C.; van Loosdrecht, M. C. M.; Heijnen, J. J. Wastewater treatment with particulate biofilm reactors. J. Biotechnol. 2000, 80, 1.

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ReceiVed for reView December 22, 2006 ReVised manuscript receiVed April 23, 2007 Accepted April 24, 2007 IE0616601