Tannery Wastewater Treatment by Sequencing ... - ACS Publications


plant treating the wastewater of a large tannery district in Northern Italy. ... production (∼2·107 m2leather/y), and water consumption (∼2.5...
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Environ. Sci. Technol. 2003, 37, 3199-3205

Tannery Wastewater Treatment by Sequencing Batch Biofilm Reactor C . D I I A C O N I , * ,† A . L O P E Z , † R. RAMADORI,‡ AND R. PASSINO‡ Istituto di Ricerca Sulle Acque C.N.R., Via F. De Blasio 5, 70123 Bari, Italy, and Istituto di Ricerca Sulle Acque C.N.R., Via Reno 1, 00198 Roma, Italy

The paper reports the results of an investigation aimed to evaluate the performances of an innovative process for treating tannery wastewater. In such a process biological degradation, carried out in a sequencing batch biofilm reactor (SBBR), is combined with chemical oxidation by ozone. The treatment was carried out at laboratory scale on a real primary effluent coming from a centralized plant treating the wastewater of a large tannery district in Northern Italy. SBBR performances without and with ozonation were compared with very satisfactory results particularly in the latter instance when the recorded COD, TKN, and TSS average removals, (96%), (92%), and (98%), respectively, permitted to achieve the fixed limits enforced by Italian regulation without needing any additional polishing step. With or without ozonation, the process that resulted was characterized by a specific sludge production (0.1 kgVSS/kgCODremoved) significantly lower than the values featuring conventional biological systems (i.e., 0.3-0.5 VSS/kgCODremoved). Moreover, as in the reactor the biomass density results were very high, i.e., 98 gVSS/Lsludge, it was possible to achieve and maintain biomass concentration as high as 20 gVSS/L.

Introduction The tannery industry is one of the most polluting industries. It is characterized by considerable water consumption associated with the large use of different chemicals. Because of this it is considered as one of the industries with a major environmental impact (1, 2). In the tannery industry pollution problems due to chromium and sulfide have been faced and solved, i.e., sulfides are removed by precipitation or transformed by chemical oxidation and chromium is precipitated or recovered and recycled to tanning process (3, 4). Presently, however, high contents of both organic substances and ammonium still are problems that need effective and feasible technological solutions. In particular, referring to organic substances, the major problems are due to chemicals (e.g., fungicides, dyes, surfactants, tannins, etc.) particularly refractory to biological degradation and/or potentially inhibitory toward nitrification process (5). The progressively more stringent discharge limits fixed for COD and NH4-N by many countries make the need for technological solutions very pressing (6-8). The situation is particularly tricky in Italy where the tannery industry is one of the leaders in the world in terms * Corresponding author phone: +39805820511; fax: +39805313365; e-mail: [email protected] † Istituto di Ricerca Sulle Acque C.N.R., Bari. ‡ Istituto di Ricerca Sulle Acque C.N.R., Roma. 10.1021/es030002u CCC: $25.00 Published on Web 06/14/2003

 2003 American Chemical Society

of turnover (5.5 billions of euro), number of firms (∼2400), production (∼2‚107 m2leather/y), and water consumption (∼2.5‚ 107 m3/y). Conventionally, tannery wastewater treatments are based on the following scheme: (physicochemical pretreatment) f (activated sludge stage) f (physicochemical polishing step). Usually, in the case of discharge into public sewerage, such a scheme does not include the last step. Therefore, in such instances, a clariflocculation step precedes a biological treatment commonly carried out at municipal wastewater treatment plants (9, 10). The efficiency of such a clariflocculation step is rather high, in fact it is possible to remove up to 60-70% of both COD and suspended solids (3, 7, 11-13). In case of discharge into superficial water bodies, a tertiary polishing step is necessary because the residual COD after biological treatment is still too high (∼500 mg O2/L). The most common polishing processes are the Fenton one (14) and clariflocculation with lime and/or iron salts at high pHs (4, 6, 8). Although through such polishing processes it is possible to obtain removal efficiencies sufficiently high to meet the limits imposed by locally different regulations, it must be pointed out that such good performances are associated with a high production of chemical sludge (∼2 kgsolids/m3treated-effluent), large consumption of chemicals, salinity increase, and high operation costs. It is then evident that the present interest toward innovative technologies compared with conventional treatments gives more advantageous results in terms of plant compactness, operational flexibility, sludge production, and operational costs. The present paper deals with the results obtained by treating real tannery wastewater through a treatment scheme in which the conventional steps, activated sludge and Fenton or clariflocculation, have been replaced by a Sequencing Batch Biofilm Reactor (SBBR) and an ozonation step, respectively. SBBR is an attached biomass system working in fill and drawn mode; its main advantages are the following: (1) it is possible to carry out in a single operative unit all the phases of an integrated oxidative treatment (biological f chemical f biological); (2) the selection of biomass particularly effective for degrading toxic and/or recalcitrant compounds is favored (15, 16); (3) it is possible to maintain a uniform biomass concentration along the whole height of the bed and, then, performance stability is assured even under overloading conditions (17-21); (4) it is possible to insert a chemical oxidation step (i.e., ozonation) inside the biological treatment. As for ozonation, its specific aim was to increase the biodegradability of refractory compounds occurring in tannery wastewater through their partial oxidation. The technological effectiveness and the economic convenience of such an approach are well documented for tannery wastewater (22-25) as well as for other types of effluents (26-31).

Materials and Methods Biological Reactor. In Figure 1 is shown a scheme of the Sequencing Batch Biofilm Reactor (SBBR) whose features are reported in Table 1. The SBBR essentially consisted of a plexiglass closed cylindrical vessel filled with biomass support material (KMT elements from Kaldnes-Norway) kept between two sieves and aerated by air injection through porous stones placed close to both sieves. The reactor was equipped with an external loop allowing wastewater recirculation by a VOL. 37, NO. 14, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Scheme of the sequencing batch biofilm reactor used during the investigation.

TABLE 1. SBBR Features diameter [cm] height [cm] geometric volume [L] fixed bed volume [L] packing media initial bed porosity average working volume [L] recirculation upflow speed [m/h]

TABLE 2. Operating Parameters of the Ozonation Unit 23 73 30 16 KMTa 0.75 12 2.5-3.0

liquid-phase volume [Lwastewater] ozonated air flow rate [Lair/h] ozone concentration in air [mgO3/Lair] ozonation time [h] transferred ozone [mgO3/ Lwastewater]

5 100 5.2 1 60

a Height: 7 mm; diameter: 11 mm; specific surface: 690 m2/m3; density: 0.95 g/cm3.

peristaltic pump. Such a recirculation was aimed to obtain a homogeneous distribution, along the reactor height, of substrate, oxygen, and microorganisms and to maintain a constant temperature of 20 °C by means of a thermostatic bath. A pressure meter (manostat) measured online biofilter head losses. When a pressure gradient of 0.9 was reached, a washing step was carried out by means of compressed (1.5 bar) air. Removed biomass was collected and measured as TSS and VSS. The system was fully automatic. The operative schedule (i.e., filling, recirculation, aeration, drawing, etc. phases) could be set by switching on or off, manually or automatically, the air compressor and the various valves. Parameters such as DO, temperature, and pH were measured online. Ozonation. An ozonation step was integrated in the SBBR treatment. In practice, after being treated biologically, a fixed fraction of the working volume was ozonated under semibatch conditions and recycled to the SBBR for a further biological treatment. The system is considered semibatch as in the ozonation unit, ozonated air was continuously purged, for a scheduled time, through a fixed volume of wastewater. At the end of the ozonation, residual ozone was purged out from the liquid phase by bubbling compressed air for about 5 min. As the aim of the ozonation step was to enhance the biodegradability of recalcitrant compounds minimizing their 3200

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FIGURE 2. Typical profiles of O3 concentration in the gas phase and COD and BOD5 concentrations in the liquid phase during an ozonation test (experimental conditions in the text). mineralization, preliminary tests were carried out to assess the experimental conditions to fix for achieving such a goal. Such conditions are reported in Table 2. Figure 2, instead, shows ozone, COD, and BOD concentration profiles recorded during an ozonation test. Ozonated air was produced by means of a Fischer ozone generator (Model 502, Germany). Analytical Instruments, Methods, and Procedures. A Total Carbon Analyzer (Model 5050 Shimadzu Co Japan) was used for TOC analyses. COD, BOD5, TKN, NH4-N, NO3-N, PO4-P, TSS, and VSS were measured according to standard methods (32). Biomass density was evaluated measuring, as VSS, the amount of biomass and dividing the resulting value by the

TABLE 3. Composition Range of SBBR Influent (Tannery Wastewater) during the Investigation parameter COD [mg/L] COD [mg/L] TOC [mg/L] TKN [mg/L] NH4-N [mg/L] NO3-N [mg/L] pH conducibility [µS/cm] chlorides [mg/L] PO4-P [mg/L] TSS [mg/L] VSS [mg/L] Cr [mg/L] a Filtered sample. growth.

b

3300-3600 2700-3000a 800-900a 250-300 220-260a absent 8.0-8.2 34 000-38 000 6000-7000 10b 800-1000 500-700 0.2-0.3

Added to wastewater to support biomass

its specific volume. This latter was measured following the Beun method (33) properly modified, i.e., 100 mL of a dextran blue solution (1 g/L) was added to 100 mL of representative biomass (sample); the mixture was gently mixed and the sludge was allowed to settle in order to take a portion of supernatant solution; the whole procedure was, then, repeated replacing the biomass sample with distilled water (blank); and the two liquid portions were analyzed by a spectrophotometer at 620 nm. Since dextran blue is not sorbed by biomass but can only dissolve in aqueous phase, the biomass volume can be easily calculated by the following equation:

volume of biomass (mL) ) 620 sample absorption 100 ‚ 100 (1) 620 blank absorption Bed porosity of the aerated biofilter was evaluated, at the beginning of the investigation and under steady-state conditions, by measuring the volumetric water contents of the filter. Wastewater Composition. The SBBR was fed with real primary effluent provided by one of the largest centralized tannery wastewater treatment plants located in Santa CrocePisa (Northern Italy). The plant treats the wastewater produced by about 450 tannery firms. In the investigated period, the composition of the SBBR influent wastewater was that reported in Table 3. SBBR Operative Schedule. The operative schedule of the SBBR is reported in Table 4. In practice, the reactor was operated for 9 months: 6 months without ozonation (period A) and 3 months including an ozonation step in the treatment cycle (period B). In both periods, three cycles per day, each lasting 8 h, were carried out. In the two periods, however, the phases/cycle were different, i.e., filling, aerobic biological oxidation, and drawing (period A); filling, aerobic biological oxidation (I), intermediate drawing, ozonation, intermediate filling, aerobic biological oxidation (II), and final drawing (period B). As shown in Figure 3, the ozonation step was carried out in batch, outside the SBBR. After ozonation the fraction of the working volume drawn during the intermediate drawing phase was recycled to SBBR during the intermediate filling phase. Period A was split in six subperiods each one lasting 30 days. As shown in Table 4, the first four subperiods (1-4) were characterized by progressively increasing organic loads obtained by increasing the hydraulic load. During the last subperiod 6, instead, filling time was increased. As for period B, it was split in two subperiods (7 and 8), lasting respectively 40 and 50 days, during which the fractions of working volume ozonated were respectively 17% and 50%.

FIGURE 3. Phases sequencing of a typical SBBR cycle during period B: (1) filling; (2) aerobic oxidation I; (3) intermediate drawing; (4) intermediate filling; (5) aerobic oxidation II; (6) final drawing. During the first four subperiods of period A, the aim of the experimentation was to assess the maximum value of the organic load compatible with good COD and TKN removal efficiencies. Once such a value was assessed, during subperiod 6, the influence of the filling time on SBBR performances was investigated. Of course, in period B, the scope of the investigation was to study the effect of ozonization on SBBR performances

Results and Discussion Period A (SBBR Treatment without Ozonation). Removal Efficiencies. The SBBR performances recorded during period A are summarized in Table 5. The data in this table indicate that throughout the whole period, despite the rather high COD removal efficiencies (averagely 84%), the limit of 160 mgO2/L, presently in force in Italy, was never achieved. Such a failure was due to the presence in the feed of a relevant fraction of recalcitrant organics. This is proved by the fact that, regardless of the organic load values, always the same value of residual COD (∼550 mgO2/L) was measured in treated effluents. The recalcitrance of such a fraction is additionally confirmed by the COD trend shown in Figure 4 which clearly points out that COD removal essentially stops after 5 h. During the successive 3 h, in fact, the value of the residual COD does not decrease anymore. A significant negative difference between the COD values actually measured at the end of the filling phases (data nonreported) and the corresponding values calculated on the basis of a simple mass balance was always recorded. Such a result, a feature of SBBRs, is due to storage phenomena particularly favored by short filling times such as those adopted in the first five subperiods. In Table 5, the data referring to TKN and NH4-N indicate that SBBR removal efficiencies toward these two parameters resulted independent from the organic load, at least up to a value of 2.6 kgCOD/m3‚d (subperiods 1-3). This result clearly demonstrates one of the claimed advantages of periodic over continuous biofilters, i.e., the possibility to achieve significant NH4-N removals through nitrification even at relatively high organic loads. This is due to the fact that in continuous biofilters, because of the competition for oxygen between heterotrophic and nitrifying biomass, nitrification occurs simultaneously to COD removal, and, accordingly, it can take place only when organic load is rather VOL. 37, NO. 14, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 4. Operative Conditions of SBBR throughout the Experimentation period a

period b

parameter

1

2

3

4

5

6

7

8

duration [d] organic load [kgCOD/(m3‚d)] nitrogen load [kgN/(m3‚d)] hydraulic load [L/d] hydraulic retention time [d] time cycle [min] phase: filling aerobic oxidation (I) drawing/filling (intermediate) aerobic oxidation (II) drawing (final) ozonated working volume per cycle [L]

30 0.65 0.05 3 4 480

30 1.3 0.09 6 2 480

30 2.6 0.19 12 1 480

30 3.3 0.23 15 0.8 480

30 2.6 0.19 12 1 480

30 2.6 0.19 12 1 480

40 2.6 0.19 12 1 480

50 2.6 0.19 12 1 480

10 460

10 460

10 460

10 460

10 460

160a 310

10

10

10

10

10

10

160a 200 5/5 100 10 2

160a 200 5/5 100 10 6

a

Under aeration.

TABLE 5. SBBR Performances during Period A (Average Values) subperiod parameter organic load

1

[kgCOD/(m3‚d)]

COD influent [mg/L] end feeda [mg/L] effluent [mg/L] removal efficiency [%] TKN influent [mg/L] end feeda [mg/L] effluent [mg/L] removal efficiency [%] NH4-N influent [mg/L] end feeda [mg/L] effluent [mg/L] removal efficiency [%] NO3-Nc influent [mg/L] end feeda [mg/L] effluent [mg/L] removal efficiencyd [%] SS influent [mg/L] end feeda [mg/L] effluent [mg/L] removal efficiency [%]

2

3

4

5

6

0.65

1.3

2.6

3.3

2.6

2.6

3480 794 550 84.2

3520 1045 550 84.4

3500 1533 550 84.3

3540 1797 552 84.4

3500 1533 550 84.3

3390 b 550 83.8

289 42.4 20 93

293 66.3 21 93

290 112 23 92

295 208 146 50.5

290 112 23 92

288 b 22 92.3

248 20 6 97.6

250 37 6 97.6

260 72 7 97.3

255 141 128 49.8

258 70 6 97.7

250 b 3 98.8

0 4.8 5.3 97.7

0 5.4 6.5 97.6

0 4 6 97.7

0 0.6 1.1 99

0 3 5 97.7

0 b 9.8

910 85 10 98.9

900 155.8 7 99.2

854 300 23 97.3

860 371.7 23 97.3

860 301 22 97.4

880 b 28 96.8

a Theoretical value calculated at the end of the filling phase assuming that no reactions take place during such a short time. period 160 min). c Nitrite formation was never detected. d Calculated by

(TKNend feed - TKNeffluent) - (NO3 - Neffluent - NO3 - Nend feed) (TKNend feed - TKNeffluent)

.

low, i.e., less than 1.5 kgCOD/m3‚d (34). Conversely, in periodic systems such as the SBBR used in the present study, because COD removal and nitrification occur in series, it is possible to obtain relevant nitrification even at higher organic load values. In the present study, in fact, 2.6 kgCOD/m3‚d represents the actual maximum organic load value compatible with a significant nitrification efficiency. As shown in Table 5, a further increase of the organic load (subperiod 4) causes a relevant reduction of both TKN and NH4-N removal efficiencies. This is why during the successive subperiod 5, the organic load value was again fixed at 2.6 kgCOD/m3‚d, and the performances recorded during subperiod 3 were confirmed. Referring to nitrate nitrogen (NO3-N), as shown in Table 5, its concentration in treated effluents was always 3202

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b

Meaningless (fill

‚ 100

very low, even lower than the Italian limit of 20 mg/L. Such a result indicates that denitrification occurs at a great extent in the SBBR. In fact, from Table 5 it is possible to argue that the amount of total nitrogen removed during each subperiod (TKNend-feed - TKNeffluent) result is always greater than the amount of formed oxidized nitrogen (N-NO3effluent - N-NO3end-feed). Such a discrepancy cannot be ascribed to the amount of nitrogen necessary for biomass growth because, as reported later on, such an amount yields very low results. Instead, it can be justified assuming that some denitrification occurs inside the more internal layers of the biofilm where oxygen cannot penetrate and denitrifying bacteria easily find carbon sources occurring either as internal storage products due to the high transient conditions and as

FIGURE 4. Typical COD profile recorded during a cycle in subperiod 3.

FIGURE 6. Biomass concentration (as VSS/L) profiles through bed height before and after washing step during subperiod 6.

TABLE 6. Selected SBBR Parameters Recorded in Period A during Interval between Two Washing Steps frequency of washing step [d] total COD removed [kgCOD] biomass wash out [kgVSS] by washing by effluent specific biomass production [kgVSS/kgCODremoved] sludge density [gVSS/Lsludge] bed porosity at the beginning of the investigation before washing after washing pressure gradient terminal pressure (i.e. before washing) pressure after washing

FIGURE 5. Biomass concentration (as VSS/L) profiles through bed height before and after washing step during subperiod 3. hydrolysis products of particulate organic matter present in the feed. SBBR also showed interesting filtration performances. In fact (see Table 5), the suspended solids average removal efficiency was about 98%, and SS concentration in treated effluent was always lower than 30 mg/L. Biomass Distribution in the SBBR. Biomass distribution inside the SBBR was assessed during the subperiod 3 measuring biomass concentration (as VSS/L) along the height of the biofilter before and after washing step. The results, shown in Figure 5, point out both the high biomass concentration inside the biofilter, about 18-20 gVSS/Lfilter, and its homogeneous distribution along the height of the bed. Such features are different from those of continuous biofilm systems in which biomass concentration results much lower (averagely 3-4 gVSS/Lfilter) and most of the solids accumulate in the first part of the filter. Because of such uneven biomass distribution, several problems arise when a sudden increase of influent concentration occurs. In particular, in such instances, substrate transported toward the higher part of continuous biofilter finds less biomass, and this causes a consequent reduction of removal efficiency (17). The results reported in Figure 5 prove that such a drawback is overcome by the SBBR, and good substrate removal performances can be obtained even in case of significant load variation. Figure 5 also demonstrates that the washing step removes only a small amount of biomass (i.e., about 2 gVSS/ Lfilter). In continuous biofilters, instead, as washing steps are much more energetic, biomass removal is almost complete (35). As shown in Figure 6, when, as in subperiod 6, the duration of the filling phase was increased (from 10 to 160 min) biomass

20 0.53 0.049 0.047 0.002 0.092 98 0.75 0.25 0.35 0.9 0.3

distribution became less homogeneous. This is due to the fact that an increased length of the filling phase is obtained through a lower feed flow rate, and, accordingly, a minor portion of substrate reaches the higher part of the biofilter, being mostly utilized by the biomass occurring in the lower layers of the SBBR. In Figure 6 it is interesting to observe that during the washing step, biomass is removed only from the first 20 cm of the biofilter without perturbing the higher layers where bacteria with low growing rate (e.g., nitrifiers) accumulate. This contributes to explain the great stability of the nitrification process and nitrogen removal performances in SBBR. Specific Sludge Production. The specific sludge production (SSP) has been calculated assuming that the fixed SBBR operating conditions, in particular the very high sludge age, about 160 days, guarantee the complete metabolization of all particulate organic matter occurring in the feed. Accordingly, the specific sludge production (kgVSS/kgCODremoved) has been calculated dividing the biomass production (VSS discharged with the effluent + VSS removed during the washing step) by the amount of COD removed. It must be pointed out that the above assumption is very conservative. In fact, it basically considers that the VSS leaving the reactor do not come from the influent. The sludge specific production value and the other parameters characterizing in period A the SBBR during the interval between two washing steps are reported in Table 6. The SSP value reported in this table (i.e., 0.1 kg VSS/ kgCODremoved) is much lower than those commonly reported for continuous biofilm reactors usually falling in the range 0.3-0.5 kgVSS/kgCODremoved (35-38). Such a rather low production of sludge resulted substantially the same throughout the whole experimentation and was confirmed by the measured very low phosphorus consumption (1.7 gPO4-P/ kgCODremoved). VOL. 37, NO. 14, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 7. SBBR Performances during Period B (Average Values) subperiod parameter organic load

[kgCOD/(m3‚d)]

COD influent [mg/L] intermediate effluent [mg/L] ozonated intermediate effluent [mg/L] final effluent [mg/L] removal efficiency [%] TOC influent [mg/L] intermediate effluent [mg/L] ozonated intermediate effluent [mg/L] final effluent [mg/L] removal efficiency [%] TKN influent [mg/L] intermediate effluent [mg/L] final effluent [mg/L] removal efficiency [%] NH4-N influent [mg/L] intermediate effluent [mg/L] final effluent [mg/L] removal efficiency [%] NO3-N influent [mg/L] intermediate effluent [mg/L] final effluent [mg/L] SS influent [mg/L] final effluent [mg/L] removal efficiency [%]

7

8

2.6

2.6

3530 420 363 390 88.7

3580 300 240 150 95.8

854 131 121 125 85.4

860 108 100 58 93.2

288 22 21 92.7

285 21 21 92.6

248 4 3 98.8

250 3 3 98.8

0 9 8

0 9 2

860 21 97.5

860 21 97.5

Bed Porosity. At the beginning of the experimentation (i.e., when no biomass was attached on the biofilter), bed porosity was 0.75 (see Table 6). After biofilm growth (see Table 6) it was reduced up to about 0.35-0.25. More precisely, the porosity was 0.35 at the end of washing step and continuously decreased during SBBR operation up to a value of about 0.25. Within this porosity range, an increase of pressure gradient, from 0.3 to 0.9, was recorded indicating that progressive biomass stratification was taking place. Such an increase, however, was uneven. In fact, after a period of regular increase, the value of 0.9 was achieved rather suddenly indicating the need of a washing step. According to the above results concerning bed porosity, the maximum volume of biomass that could stay in the SBBR without causing clogging problems is about 50% of the bed volume [(i.e., initial bed porosity (0.75) - bed porosity before washing (0.25)]. Referring to the biomass amount contained in the biofilter, of course, it will depend on its density which, in the SBBR, was particularly high, i.e., 98 gVSS/Lsludge. This value result is higher not only than those usually measured in continuous biofilters, i.e., 25-30 gVSS/Lsludge (39), but also those (50-60 gVSS/Lsludge) obtained in aerobic systems in which sludge granulation occurs (33). It follows that in the used SBBR, a biomass concentration 2-3 times greater than that achievable in other high-rate systems can be maintained. Period B (SBBR Treatment Integrated with Ozonation). The data reported in Table 7 indicate that during subperiod 8, the SBBR showed better COD removal efficiency (95.6%) than in subperiod 7 (88.7%), achieving for the first time a residual COD concentration (150 mgO2/L) lower than the Italian limit of 160 mgO2/L. The different results during the two subperiods were due to the different fraction, 17% and 50%, of working volume that was ozonated. The COD profiles of intermediate and final effluents recorded during both subperiods (see Figure 7) indicate that 3204

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FIGURE 7. COD concentration profiles of intermediate and final effluents during subperiods 7 and 8. in the SBBR, stationary conditions were achieved rather slowly. This can be ascribed to the need of the biomass to become acclimatized toward new biodegradable substrates produced during ozonation (29). The COD and TOC data reported in Table 7 demonstrate that the very low COD value in the final effluent produced during subperiod 8 is due to the removal during the second aerobic biological oxidation of the biodegradable organic compounds produced in the ozonation phase. In addition, it is possible to observe the existence of the well-known processes usually occurring during ozonation: mineralization of some organic substrates and partial oxidation, with corresponding biodegradability enhancement, of some others (40) even if the contribution of the latter is major. In fact, as for mineralization, it is proved by the difference between TOC values of intermediate effluents before (108 mgC/L) and after (100 mgC/L) ozonation. Conversely, referring to the partial oxidation, its occurrence is demonstrated by the increase of the average oxidation number calculated {nox ) 4 × [(TOC-COD)/TOC]} (29) for intermediate effluent before and after ozonation, i.e., -0.16 and +0.48, respectively. The effect of ozonation was beneficial even toward the denitrification process. In fact, the difference of NO3-N concentrations between intermediate and final effluents, i.e., from 9 to 3 mg/L, has to be ascribed to the denitrification process favored by the formation of biodegradable organic byproducts during the ozonation step. No difference in sludge production was recorded during the two periods A and B. This result is rather interesting. Presently, in fact, to achieve the COD limit of 160 mgO2/L, at most Italian tannery wastewater treatment plants, a tertiary polishing step based on Fenton’s process is carried out. The sludge produced during such a step averages amounts of 4 kgsolids/kgCODremoved, i.e., a value by far greater than that recorded during the present study.

Acknowledgments The authors thank Miss G. Dell’Olio for their valuable assistance in carrying out most of the analytical part of the work.

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Received for review January 8, 2003. Revised manuscript received April 29, 2003. Accepted May 1, 2003. ES030002U

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