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Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lesa20

Effects of black liquor shocks on activated sludge treatment of bleached kraft pulp mill wastewater a

a

Gabriela Morales , Silvana Pesante & Gladys Vidal

a

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Engineering and Environmental Biotechnology Group, Environmental Science Faculty & Center EULA-Chile, University of Concepción, Concepción, Chile Published online: 02 Apr 2015.

Click for updates To cite this article: Gabriela Morales, Silvana Pesante & Gladys Vidal (2015) Effects of black liquor shocks on activated sludge treatment of bleached kraft pulp mill wastewater, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 50:6, 639-645 To link to this article: http://dx.doi.org/10.1080/10934529.2015.994974

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Journal of Environmental Science and Health, Part A (2015) 50, 639–645 Copyright © Taylor & Francis Group, LLC ISSN: 1093-4529 (Print); 1532-4117 (Online) DOI: 10.1080/10934529.2015.994974

Effects of black liquor shocks on activated sludge treatment of bleached kraft pulp mill wastewater GABRIELA MORALES, SILVANA PESANTE and GLADYS VIDAL

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Engineering and Environmental Biotechnology Group, Environmental Science Faculty & Center EULA-Chile, University of Concepcion, Concepci on, Chile

Kraft pulp mills use activated sludge systems to remove organic matter from effluents. Process streams may appear as toxic spills in treatment plant effluents, such as black liquor, which is toxic to microorganisms of the activated sludge. The present study evaluates the effects of black liquor shocks in activated sludge systems. Four black liquor shocks from 883 to 3,225 mg chemical oxygen demand¡COD L¡1 were applied during 24 hours in a continuously operating lab¡scale activated sludge system. Removal efficiencies of COD, color and specific compounds were determined. Moreover, specific oxygen uptake rate (SOUR), sludge volumetric index (SVI) and indicator microorganisms were evaluated. Results show that the addition of black liquor caused an increase in COD removal (76¡67%) immediately post shock; followed two days later by a decrease (¡19¡50%). On the other hand, SOUR ranged between 0.152 and 0.336 mgO2 g¡1 volatile suspended solids¡VSS min¡1 during shocks, but the initial value was reestablished at hour 24. When the COD concentration of the shock was higher than 1,014 mg/L, the abundance of stalked ciliates and rotifers dropped. Finally, no changes in SVI were observed, with values remaining in the range 65.8¡40.2 mL g¡1 total suspended solids¡TSS during the entire operating process. Based on the results, the principal conclusion is that the activated sludge system with the biomass adapted to the kraft pulp effluent could resist a black liquor shock with 3,225 mgCOD L¡1 of concentration during 24 h, under this study’s conditions. Keywords: Activated sludge, black liquor, kraft pulp mill, indicator microorganisms, specific oxygen uptake rate, sludge volumetric index.

Introduction Bleached kraft pulp industry generates between 60 and 90 m3 of wastewater per ton of pulp produced.[1] These effluents are characterized by high concentrations of dissolved organic compounds, suspended solids, toxicity and color.[1–3] Therefore, they need to be treated to reduce the impact on the aquatic ecosystem into which they will be discharged.[4] Kraft pulp industry uses activated sludge systems to remove organic matter from effluents.[2–6] These systems remove between 65 and 99% of biological oxygen demand (BOD5) and from 25 to 85% of chemical oxygen demand (COD) when the reactor is operated under a hydraulic retention time (HRT) from 12 to 42 h, with an organic loading rate (OLR) between 0.6 and 4.9 gBOD5(L¡1¢d)1 [2, 4-5] . Under these conditions, the specific oxygen uptake

Address correspondence to Gladys Vidal, Engineering and Environmental Biotechnology Group, Environmental Science Faculty & Center EULA-Chile, University of Concepci on, P. O. Box 160¡C, Concepci on 4070386, Chile; E-mail: [email protected] Received August 15, 2014.

rate (SOUR) of the biomass usually varies between 0.07 and 0.42 gO2/gVSS¡1¢d;[7] whereas, the sludge volumetric index (SVI), used to characterize settling properties of the sludge, has a range of 150 to 35 mL gTSS¡1.[8] Further, microorganisms observed in biomass (e.g. stalked ciliates and rotifers) indicate good system performance.[9–11] The activated sludge system is based in a group of microorganisms, mainly bacteria, which degrade organic matter in presence of oxygen.[9,10] Due to its biological nature, the system is sensitive to unfavorable conditions, especially the input of toxics that affect performance.[12–13] Specifically in the pulp industry, several studies found that wood extractives are toxic to aquatic organisms and microorganisms in wastewater treatment systems.[14] In general terms, resin acids are considered as one of the mainly contributors to the acute toxicity of effluents.[15] The acute toxicity associated to hardwood pulp is principally due to extractives like resin acids (9.8% dry matter), phenols and fats, although other active extractives are also present into hardwood pulp. For example, Eucalyptus globulus has high contents of aromatic, polar and semipolar compounds as well as resin acids (0.26% dry matter).[16,17] Furthermore, it has been determined that compounds such as biocides, monochloric acetic acid, resins,

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640 soft soap and turpentine have an acute toxic effect on activated sludge, inhibiting oxygen uptake rate (OUR) between 6 and 72% and dissolved organic carbon (DOC) removal between 3 and 58%.[6] Additionally, there are process streams that may appear as acid spills, alkali spills from the bleaching process or toxic spills as black liquor.[18] Black liquor is one of the most toxic liquors; it is characterized by high pH (> 12), high COD concentrations (1,000¡33,600 mg L¡1), extractives and sulfide[19] and has been reported in spills into the wastewater treatment system of kraft pulp industry, generating adverse effects. Studies have evaluated effects of black liquor shocks in a treatment system with multiple bioreactors (steps). In the first step (carrier bioreactor) for black liquor concentrations > 3,000 mgCOD L¡1, the protozoa disappeared. In the fourth step (activated sludge system), 50% reduction in COD removal was noted.[13,18] Our objective was to evaluate the effects of black liquor shocks in activated sludge, studying the SOUR, SVI and indicator microorganisms.

Materials and methods Raw wastewater Effluent samples (later used as influent for Activated Sludge (AS) reactors) were obtained from a kraft pulp mill (located in southern Chile) that processes Eucalyptus globulus and Eucalyptus nitens with an elemental chlorine free (ECF) bleaching system. Samples were obtained after primary treatment, which consisted of a settling tank to reduce fiber and suspended solids. Effluent was stored in 30 L PVC tanks, which were refrigerated in a dark room at 4 C.[20]

Morales et al.

Fig. 1. Schematic of activated sludge system: (1) influent, (2) pump, (3) aeration tank, (4) biomass, (5) biomass recirculation, (6) sedimentation, (7) aeration, (8) effluent.

was not necessary to add phosphorus.[21] When needed, the pH was adjusted to approximately 7 using HCl 0.5 N. The AS systems were operated at a temperature of 25.2 § 2.1 C. The dissolved oxygen (DO) concentration was maintained between 3.9 and 8.0 mg L¡1, using a diffuser aeration system. The sludge was periodically recycled from the settling units to the aerobic reactors to maintain approximately 3.0 gVSS L¡1. Figure 1 shows a scheme of the activated sludge system and its components. Black liquor trials Black liquor from a local kraft pulp mill was added to the feed of AS1 in four trials, during 24 h. Trials were denominated: S1, S2, S3 and S4 and the concentrations of black liquor added were 2, 4, 10 and 30 mL¡black liquor/L¡of influent, respectively. As indicated, the pH was adjusted to approximately 7 using HCl. After the 24 h of each test, AS1 was fed with raw wastewater without black liquor.

Inoculum The inoculum consisted in 5 g L¡1 of volatile suspended solid (VSS) of sludge obtained from an aerobic reactor treating effluent in the same kraft pulp mill where effluent samples were obtained. Activated sludge systems Two laboratory-scale systems of activated sludge (AS) were implemented. These AS systems included an aerobic reactor (0.820 L) and a settling unit (0.450 L), both constructed of glass. One AS was used as control (AS0) and the other was used for the trials (AS1). In the normal operation period (no trials), raw wastewater was fed to both systems by a peristaltic pump with the flow rate adjusted to the desired hydraulic retention time (HRT) based on the net liquid volume of the reactor. To achieve the proportion of BOD5:N:P of 100:5:1 in the raw wastewater, nitrogen was supplemented with urea but it

Operational conditions Both AS systems were continuously operated for 103 days. The operation considered 20 days for start-up with a HRT of 2 days. During the next 83 days, HRT was maintained at 1 day, corresponding to an organic loading rate (OLR) around 0.31 gCOD(L¡1¢d)¡1. During the entire operational period (normal and trials), the efficiencies of AS0 and AS1 were estimated for COD, BOD5, total phenolic compounds, color, lignin and derivatives, lignosulfonic acids and aromatics compounds, according to Equation 1. Eð%Þ D

Qi  Ci ¡ Qo  Co Qi  Ci  100

(1)

where E (%) is the removal percentage; Q the flow rate (L d¡1); C the parameter concentration (mg L¡1); subindex

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Black liquor shocks on activated sludge treatment

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“i” and “o” correspond to the inflow and outflow, respectively. Temperature, pH, DO and electrical conductivity (EC) were also monitored daily during normal operation and each hour during the trials. To evaluate biomass and microorganisms behavior, three indicators were measured weekly during normal operation and at hours 0, 2, 6 and 24 during the trials. Biomass activity was analyzed by the oxygen uptake rate (OUR) and specific oxygen uptake rate (SOUR). Microorganisms were studied using a microscope, estimating the number of indicator microorganisms (stalked ciliates and rotifers). Finally, the sludge volumetric index (SVI) was used to characterize the settling properties of the sludge.

Analytical methods Chemical oxygen demand (COD), biological oxygen demand (BOD5), total suspended solids (TSS) and volatile suspended solids (VSS) were measured according to APHA–AWWA–WPCF.[22] Color was measured in liquid samples at pH 9, adjusted with NaOH 0.5 N, by spectrophotometry at a wavelength of 440 nm in a 1 £ 1 cm glass cell.[21] Total phenolic compounds concentration was measured in liquid samples by UV absorbance in a 1-cm quartz cell at 215 nm with 0.2 M KH2PO4 buffer and transformed into concentration using a calibration curve with phenol as standard solution.[21] Spectrophotometric measurements of filtered samples (0.45 mm) were performed at wavelengths of 346 nm (lignosulfonic acids), 254 nm (aromatic compounds), 280 nm and 272 nm (lignin¡derived compounds) in a 1x1 cm quartz cell and were determined according to the Chamorro et al.[23] procedure. All spectrophotometric measurements were performed using a model Thermo Scientific Genesys TM10 UV-Visible Spectrophotometer (Thermo Fisher, Waltham, MA, USA). Temperature and DO were measured using a HQ¡10 oxygen meter with an LDO sensor. Total nitrogen (TN) and total phosphorous (PO4¡2¡P) were measured by means specific kits using thermoreactor TR-320 and photometer NOVA-60 of SpectrocuantÒ system from Merck (Darmstadt, Germany). OUR was evaluated by respirometry using a biological oxygen monitor (BOM) YSI 5300 System (YSI Incorporated Life Sciences, Yellow Springs, OH, USA). The system consisted in an air¡tight respiration vessel fitted with a DO probe YSI 5231; the vessel was continuously stirred and thermally controlled; and the biomass was washed twice previously with phosphate buffer according to the Mosquera et al.[24] procedure, modified at 25 C. OUR was determined by lineal regression from the slope obtained by plotting dissolved oxygen concentration versus time and SOUR with the value of VSS used in the assay.

Microscopic examination was performed within 1 h of collection, using a microscope Leica Microsystems (Wetzlar, Germany), model DM500. Stalked ciliates and rotifers abundance were determined with a sub-sampling technique: 25 mL volume of the mixed liquor was taken with an automatic micropipette, and three replicates of this volume were counted in the microscope.[11] SVI was measured as the volume occupied (mL) by 1 g of VSS, after 30 min in a 100 mL graduated cylinder.[2]

Results and discussion Table 1 shows the physicochemical characteristics of kraft mill influent. The pH was maintained near neutral (7.3 § 0.3). The average organic matter concentration was 227 § 61 mg L¡1 and 449 § 134 mg L¡1 for BOD5and COD, respectively, where BOD5 corresponds to compounds that are easily biodegradable, such as carbohydrates and organic acids.[2] Similar values were obtained in studies on the primary effluent from the kraft mill industry, with ranges of 150¡168 mg L¡1 for BOD5and 445¡491 mg L¡1 for COD.[20,22] Biodegradability was determined by the relation BOD5/ COD, which indicated an average value of 0.46, showing that the effluent is not totally biodegradable due to the presence of compounds with molecular weights greater than 1,000 Da, such as lignin and its derivatives.[21,25-27] This recalcitrant COD fraction produces the color found in the effluent, and after aerobic treatment has average absorbance values in the range of 0.32 § 0.18.[26] On the

Table 1. Physicochemical characterization of kraft pulp effluent used. Parameter

Unit

pH EC mS cm¡1 mg L¡1 BOD5 COD mg L¡1 TSS g L¡1 VSS g L¡1 Abs Color (pH 9, VIS 440 nm) Abs Lignosulfonic acids (UV 346 nm) Total phenolic mg L¡1 compounds (UV 215 nm) TP mg L¡1 TN mg L¡1 Sulfate mg L¡1

Range

Averagea § SD

6.7¡7.9 2.7¡2.9 143¡285 283¡677 0.17¡0.96 0.12¡0.62 0.11¡0.21 0.43¡0.60

7.3§0.3 2.8§0.1 227§61 449§134 0.48§0.26 0.32§0.18 0.14§0.03 0.51§0.07

76¡149

108§24

1.3¡2.1 < 0.5 212¡240

1.7§0.3 < 0.5 226§19

EC: electrical conductivity, BOD5: biological oxygen demand, COD: chemical oxygen demand, TSS: total suspended solids, VSS: volatile suspended solids, TP: total phosphorus, TN: total nitrogen. a Average values correspond to 11 different samples except for sulfate and BOD5 which were measured in 4 samples.

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Morales et al.

other hand, the total nitrogen concentrations (TN) were below detection levels (< 0.5 mg L¡1) and total phosphorous (TP) was 1.7 mg L¡1. Table 2 presents the perturbation conditions for system AS1. The feed pH was always maintained near neutrality (7.1¡7.5); while EC increased in direct relation with the concentration of black liquor fed in the reactor, from an initial condition of 2.8 mS cm¡1 to 7.5 mS cm¡1 in S4. This increase expresses the addition of dissolved ions that come from the black liquor containing sodium hydroxide, sodium sulfate and other sulfur salts that are toxic for the microorganisms.[19] Moreover, the organic matter concentration of COD increased from 883 to 3,225 mg L¡1 between S1 and S4, respectively. Figures 2 and 3 present reactor behavior (during 103 days of operation of AS0 and AS1) with respect to removal efficiency for COD, color and specific compounds (e.g., lignin, lignosulfonic acids, aromatic compounds and total phenolic compounds) from the effluent. Until day 61 of the normal operating period, both systems had an average COD removal of 56% for an OLR of 0.38 § 0.12 kgCOD m¡3¢d¡1. The average removal efficiency of BOD5 was 85 § 8% with a maximum of 94%, values that are considered typical for activated sludge systems.[1] In general, these results are similar to those obtained by Xavier et al.[28] with COD elimination between 57¡67% for OLR from 0.26 § 0.05 to 0.66 § 0.04 kgCOD m¡3¢d¡1. Color elimination varied between ¡24 and 50% in both reactors. On day 61, an increase in color was observed and could be associated to the low oxygen concentration in the settling unit (< 1 mgO2 L¡1). Milestone et al.[29] reported this situation for biological systems with increases between 10¡40% in some periods. This situation is associated to low aeration zones where anaerobic bacteria produce the de-methoxylation of the aromatic unit of lignin, liberating intact hydroxylated aromatic structures. These units generate the catechols, which are transformed in quinones under slightly oxidizing conditions, augmenting effluent color.[28] Thus, the kraft pulp effluent color is closely related with lignin and its derivatives as well as their elimination, which is confirmed by the correlation found between the variation in color elimination and the mentioned compounds. The removal efficiencies obtained were ¡21¡26% for lignin and its derivatives; ¡21¡71% for aromatic Table 2. Characterization of applied toxic shocks. Shock S1 S2 S3 S4

Operation EC Day pH (mS cm¡1) 61 83 91 97

7.5 7.1 7.3 7.5

3.7 3.7 4.2 7.5

COD (mg L¡1)

OLR Increase (%)

883 § 2 1,015 § 19 1,774 § 81 3,225 § 116

200 210 410 740

EC: electrical conductivity, COD: chemical oxygen demand, OLR: organic loading rate, S1: shock 1, S2: shock 2, S3: shock 3, S4: shock 4.

Fig. 2. Evolution of COD removal efficiency in AS0 (Blank) (O) and AS1 (&) and OLR in AS0 (r) and AS1(~). S1: shock 1, S2: shock 2, S3: shock 3, S4: shock 4.

compounds; ¡26¡11% for lignosulfonic acids; and ¡27¡41% for total phenolic compounds. After day 62, four OLR peaks were observed (Fig. 2 upper), and each one represented a toxic shock. The analyses performed after S1 and S2 showed COD elimination from 77 and 73%, respectively. Then, once the reactor is again fed with normal influent, efficiency falls and then stabilizes, obtaining results similar to AS0 with a value of 50% in S1 and 37% in S2. Sandberg and Holby[18] mention that in moderate spills, the main part of the degradable COD will be removed with good efficiency. This result was found in S1 and S2, resulting in increases of 200 and 230% in the OLR, respectively. In the cases of S3 and S4, the OLR was 410 and 740% higher than average and efficiency increased in 66 and 60%, respectively. However, two days after S3, elimination dropped to 4%. On day 99, only 2 days after the application of S4, the lowest efficiency value (¡19%) was observed. This negative value was produced because of black liquor retention in the system that was diluted in the effluent, contributing to the concentration of organic matter. In this sense, removal efficiency returned to normal 150 h after black liquor entry in a multiple reactors treatment system.[13] Moreover, a progressive diminishment in the removal efficiency of compounds and color was observed. Since the application of S2, at day 82, the removal efficiency of aromatic compounds, lignin and its derivatives diminishes until reaching a minimum of ¡50% at 2 days after S4. The persistence in the elimination inefficiency was produced by the high content of lignin and black liquor

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Black liquor shocks on activated sludge treatment compounds that remain in the system and that are diluted with the effluent as in the case of COD. Table 3 presents the operating parameters and the stability indicators for the different operating conditions. Under normal conditions, SOUR varies in a range of 0.078¡0.316 in AS0 and in the range of 0.127¡0.336 mgO2 gVSS¡1¢min¡1 in AS1. These values are similar to those obtained by Pozo[21] with an average of 0.083 mgO2 gVSS¡1 ¢min¡1. During toxic shocks, from hour 0 to 2, SOUR remained constant in S1, S2 and S3 but was 21% lower in S4. This diminishment could be an indication of black liquor toxicity, also indicated by the high COD (3,225 mg L¡1) and EC (7.5 mS cm¡1) levels in this influent. Following the variations presented at hour 6, the initial SOUR values were reestablished in S1, S2 and S4 at hour 24. This situation coincides with a higher concentration of organic matter, which is associated to an increase in biomass activity in the system. Under the reference conditions, bacteria are limited by the access to biodegradable organic matter and nutrients, while low black liquor concentrations provide extra COD to degrade. Thus, an increase in efficiency (up to 77%) is observed after the shocks are applied. In particular, a diminishment of 37% in SOUR and a lower increase in post¡shock efficiency (67%) were observed in S3. Studies show that SOUR was inhibited at a concentration of 3,500 mgCOD L¡1 with an increase in COD produced by the entry of black liquor.[13] In the present study, the maximum COD concentration applied was 3,225 mg L¡1 and total inhibition of activity

was not observed, indicating that the system’s robustness was able to reestablish SOUR. A second influential factor was the neutralization of the influent pH. The majority of the bacteria cannot proliferate at pH levels below 4.0 or higher than 9.5 and the optimum value is between 6.5 and 7.5.[18] Further, the toxicity of resin acids and other black liquor extractives increases at pH 8 due to their greater bio-availability.[19,30] In this way, when the feed pH is adjusted to approximately 7, the bacteria can resist higher black liquor concentrations before being inhibited.[18] During the normal feeding period in AS1, the indicator microorganisms identified were stalked ciliates, such as Vorticella sp., with an abundance of 344 § 105 organisms mL¡1. These species are frequent when the treatment is working correctly. Rotifers were also observed with an abundance of 229 § 91 organisms mL¡1, indicating mature sludge.[9] During the application of toxic shocks, the following tendency was observed in S2, S3 and S4: Stalked ciliate abundance dropped between 22 and 52% from hour 0 to 6 and later increased at hour 24. In a similar context, a significant drop in stalked ciliates, flagellates, and free swimming bacteria was observed 3 h after black liquor entry; these microorganisms reappeared 3 hours after the toxic shock application.[13] On the other hand, the rotifers diminished 16% in S2 and up to 80% in S4 from hour 0 to 24. However, even when stalked ciliated and rotifers diminish and the activated sludge system is inhibited for a period, the bacteria survive and the elimination of organic matter restarts once the feed returns to normal.[18]

Table 3. Evolution of reactor operating parameters and stability indicators for different operating conditions. Microorganisms (organisms mL¡1) Operating Condition Normal S1

S2

S3

S4

Time (h) 0 2 6 24 0 2 6 24 0 2 6 24 0 2 6 24

COD Influent (mg L¡1) 428 § 125 883 § 2

1,014 § 18

1,773 § 81

3,225 § 116

pH

EC (mS cm¡1)

DO (mg L¡1)

SVI (mL g TSS¡1)

SOUR (mgO2/gVSS¢min)

Stalked Ciliates

Rotifers

8.9 9.1 9.0 8.4 8.7 9.2 8.5 8.7 8.9 8.7 8.6 8.5 8.5 8.7 8.2 8.6 8.4

4.0 3.8 3.9 3.9 4.2 3.5 3.6 3.6 3.8 3.3 3.9 3.9 4.2 3.0 3.7 4.1 5.5

5.9 7.8 8.3 3.8 4.7 6.0 4.3 6.2 6.9 7.6 6.8 5.6 5.6 7.1 4.0 5.8 4.4

42.4 41.8 41.8 41.8 41.8 40.2 46.0 42.1 46.0 51.5 65.8 60.1 55.5 40.9 43.6 43.6 46.3

0.210 0.289 0.271 0.184 0.261 0.289 0.289 0.336 0.255 0.243 0.255 0.254 0.152 0.212 0.167 0.201 0.275

344 446

229 320

280 240 133 200 266 333 133 187 120 60 93 66

80 53 80 67 93 80 106 53 133 173 53 27

COD: chemical oxygen demand, EC: electrical conductivity, DO: dissolved oxygen, SVI: sludge volumetric index, TSS: total suspended solids, VSS: volatile suspended solids, SOUR: specific oxygen uptake rate.

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Morales et al. decreased from ¡19 to 50% as well as removal of color and specific compounds. During the each shock, SOUR was unstable (0.152¡0.336 mgO2 gVSS¡1 ¢min¡1 but by hour 24, the respective initial values were reestablished except for S3. From S2 (1,014 mgCOD L¡1) abundance of stalked ciliates and rotifers dropped until 60 and 27 organisms mL¡1, respectively in S4 (3,225 mgCOD L¡1). No changes in sludge sedimentation were observed with values remaining in the range 65.8¡40.2 mL gTSS¡1. This study shows that good stabilization of the activated sludge biomass could resist a black liquor shock with a COD concentration of 3,225 mg L¡1, during 24 h, with a HRT of 1 d and a relation BOD5:N:P of 100:5:1.

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Acknowledgments Fig. 3. Evolution of color and specific compounds removal efficiency in AS1. Color(&), lignin UV 280 nm (r), lignin UV 272 nm (~), lignosulfonic acids 346 UV nm (O), aromatic compounds UV 254 nm (~), total phenolic compounds UV 215 nm (O).

The abundance of rotifers and stalked ciliates was observed to diminish when the shock contained more black liquor. Further, with higher black liquor concentrations, the mobility of indicator microorganisms dropped and a larger quantity of flagellates, suctorian ciliates and free swimmers were observed. These organisms indicate an inadequate removal efficiency due, for example, to an excessive organic load, a situation present in S3 and S4[11] and one which agrees with the diminished efficiency of organic matter elimination days after shock application. Additionally, abundant foam was formed in the aeration tank, probably due to the presence in the black liquor of fatty acids that form foam in basic pH (9¡10) environments.[18] The toxic shocks did not produce significant changes in the SVI, whose values remained below 65 mL gTSS¡1 for the entire process. Similarly, Xavier et al. [28] observed an average SVI of 50 mL gTSS¡1 in an activated sludge system, while Sandberg and Holby[18] observed values in the range of 70¡100 mL gTSS¡1. SVI is influenced by factors such as floc size, solid concentration, and the presence of filamentous organisms.[31] During the study, the medium floc size (150¡500 mm) remained constant and no filamentous microorganisms were observed.

Conclusions Black liquor shocks ranging from 883 to 3,225 mgCOD L¡1 in an activated sludge system showed an immediate increase in COD removal efficiency from 67 to 76%. However, after 2 days of operation the removal efficiency

The authors thank Dr. S. Chamorro for her support with microorganism evaluation.

Funding This work was supported by FONDECYT Grant No. 1120664, INNOVA BIOBIO 13.386-EM.TES, CONICYT/FONDAP/15130015 and Red Doctoral REDOC. CTA, MINEDUC Grant UCO1202 to the University of Concepcion.

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Effects of black liquor shocks on activated sludge treatment of bleached kraft pulp mill wastewater.

Kraft pulp mills use activated sludge systems to remove organic matter from effluents. Process streams may appear as toxic spills in treatment plant e...
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