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Article

Toxic Effects of Liquors Generated During Kraft Pulp Production Process on Aerobic Biomass and Growth of Selenastrum capricornutum

by
Constanza Hidd
1,2,
Gabriela Morales
1,2,
Naomi Monsalves
1,2 and
Gladys Vidal
1,2,*
1
Engineering and Biotechnology Environmental Group, Environmental Science Faculty and EULA–Chile Center, Universidad de Concepcion, Concepcion 4030000, Chile
2
Water Research Center for Agriculture and Mining (CRHIAM), ANID FONDAP Center, Concepcion 4030000, Chile
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(10), 4494; https://doi.org/10.3390/su17104494
Submission received: 28 March 2025 / Revised: 26 April 2025 / Accepted: 13 May 2025 / Published: 15 May 2025
(This article belongs to the Topic Wastewater Treatment by Physical, Chemical, Photochemical, and Biological Processes, and Their Combinations)
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Abstract

:
The kraft pulp process generates liquors with different physicochemical characteristics at each treatment stage. These liquors can accidentally spill into the biological treatment, hindering it and harming ecosystems where the effluents are discharged. Due to the lack of studies on the effects these liquors can have on the aerobic biomass of activated sludges and ecosystems, this investigation aims to assess the toxicity of each liquor spill to the aerobic biomass of an activated sludge, using Selenastrum capricornutum as a bioindicator of water quality. This evaluation used a laboratory-scale activated sludge, which was fed with an effluent with pH 6.62–6.67 and chemical organic demand (COD) of 611–638.5 mg/L. The liquors used had the following parameters: pH = 13 and COD = 1911 mg/L (white); pH = 13 and COD = 141,350 mg/L (black); pH = 13 and 2755 mg/L (green); and pH = 7.5 and COD = 358 mg/L (condensate). White liquor produced the greatest toxicity (EC20 of 17.8 mgCOD/L) and lowest oxygen uptake rate (8.42 mgO2/L·h with 287.7 mgCOD/L) in the aerobic biomass compared to the other liquors. White liquor presented the greatest inhibition of Selenastrum capricornutum, with 81.7% (48 h) and 98.0% (96 h). Meanwhile, black liquor presented an inhibition of 94.7% (48 h), but a 13% increase in microalga growth at 96 h of culture. The information from this study makes it possible to calculate how much liquor can be fed to an activated sludge system, keeping it optimized to eliminate liquor discharges generated within the kraft mill’s processing units.

1. Introduction

The kraft pulp industry is among the most important pulp production industries in the world (90% of pulp mills) and is fundamental for the transformation of wood into stronger cellulose fibers, and most of the chemical products used in the process can be recovered [1,2]. This process consists of four stages: wood preparation, pulping, bleaching, and chemical recovery [3,4,5]. The process begins with the treatment of wood chips with a solution composed of NaOH and Na2S, known as white liquor, which is aimed at disintegrating lignin. This stage yields black liquor, which is composed of white liquor plus the removed lignin [6]. Black liquor, due to its high energy value, is concentrated by evaporation, generating condensates containing volatile organic compounds such as methanol, terpenes, and aldehydes, in addition to the concentrated black liquor. This liquor is composed of an organic fraction that is burned to produce energy and an inorganic fraction that is dissolved in weak white liquor, producing green liquor, which, after a causticization process, allows white liquor and NaOH to be obtained [7,8].
The kraft pulp process uses large quantities of chemical substances, in particular NaOH, solvents, and chlorinated compounds. Therefore, the effluents produced are characterized by the presence of a wide variety of both organic and inorganic compounds, some of which have been identified and classified as carcinogenic, mutagenic, or endocrine-disrupting. In addition, the high organic matter content in these effluents can give rise to eutrophication of the receiving water bodies, negatively affecting biodiversity and the abundance of numerous organisms [9,10]. Given that this process generates considerable liquid waste (60 and 90 m3 per ton of pulp produced), the need for an effective treatment prior to its discharge into receiving water bodies to remove mainly organic matter, suspended solids, color, and thus some of its toxicity is apparent [11,12] (Figure 1).
The most commonly applied secondary treatment is activated sludge (AS) after a primary treatment. AS is a biological treatment which, due to microorganism activity, can remove and decompose organic matter (lignin, proteins, hemicellulose, cellulose, and lipids) [1,13]. However, in a kraft pulp mill, there may be accidental spills of liquor from the pulp digestion (white, green, black, and condensate), which in this biological system could be toxic to the microorganisms due to the resin acid content [7,9]. It has been reported that black liquor spills onto the aerobic biomass of AS cause a 50% decrease in COD removal, the disappearance of indicator organisms (ciliates and rotifers), and a reduction in the heterotrophic activity of the biomass [14,15,16]. White and green liquors have been less studied; however, from the literature, it is known that the presence of sulfides (contained in both liquors) has weakening and disintegrating effects on floccules in the aerobic biomass of activated sludge systems, causing poor sludge settling [17], with reported half maximal effective concentrations (EC50) of sulfur compounds of 0.276 mM, 0.141 mM, and 0.0085 mM in Vibrio fischeri, Scenedesmus vacuolatus, and Daphnia magna, respectively [18].
However, there is no complete information on the effects of each of the liquors generated during the kraft pulp process on the aerobic biomass of AS. This measurement is important, as spills in the kraft mill process will ultimately be discharged into the treatment system for a final discharge into the ecosystem. Therefore, this research aims to assess the possible toxicity of each type of spill—white liquor, green liquor, black liquor, and condensate—to the aerobic biomass and Selenastrum capricornutum, a freshwater microalga used as a bioindicator of water quality.

2. Materials and Methods

2.1. Collection of Kraft Pulp Effluent and Accidental Toxic Spill Flows

The assessed effluent comes from a kraft pulp mill in the Biobío Region that uses Eucalyptus globulus as a raw material and has an elemental chlorine-free (ECF) bleaching system. The effluent collection took place after the mill’s primary treatment, which consists of the removal of suspended solids via settling tanks. It was stored in 20 L drums and refrigerated at 4 °C in the dark. The influent was supplemented with nitrogen in the form of urea (CO(NH2)2) in order to achieve an optimal BOD5:N:P proportion (100:5:1). The pH was kept neutral. In addition, four streams from a kraft pulp mill that can appear as accidental toxic spills were analyzed: white liquor, green liquor, black liquor, and condensate. They were stored in 5 L drums at 4 °C in the dark.

2.2. Physicochemical Characterization of a Kraft Pulp Effluent and Liquors Formed as By-Products of the Kraft Pulp Process

To determine the physicochemical parameters of the effluent samples and each of the flows of the kraft pulp process, three different samples were filtered with a Whatman membrane with a pore size of 0.7 µm and analyzed based on the protocol described in the standard method [17]. In situ pH and electrical conductivity (EC) parameters were determined using an OAKTON-PC650 multi-parameter meter (Eutech Instruments; Singapore). The organic matter present in the samples was determined in the form of COD (colorimetric method, 5220-D), total organic carbon (TOC) (catalytic combustion oxidation method and NDIR detection, TOC LCPH analyzer, Shimadzu, Kyoto, Japan), and biological oxygen demand (BOD5) (azide-modified Winkler method, 5210-B). Solids were measured based on total suspended solids (TSS) and volatile suspended solids (VSS). TSS was determined using gravimetric methods, in which the sample was filtered (1.5 µm) and dried at 105 °C. To determine VSS, the samples were dried for 30 min at 550 °C and subtracted from the TSS value (gravimetric method, 2540-D). The analyzed nutrients were measured in the form of NO3−-N and NO2−-N (spectrophotometer UV–Vis Shimadzu UV 1800, Kyoto Japan), total nitrogen (TN) (Spectroquant-Nova 60, kits Merck, Darmstadt, Germany), and total phosphorus (TP) (Spectroquant-Nova, Merck kits, Darmstadt, Germany). The total concentration of phenolic compounds (UV phenol) was measured via UV absorbance in a 1 × 1 cm quartz cuvette at 215 nm and pH 8.0 (0.2 M KH2PO4 buffer). The samples underwent membrane filtration (0.45 µm). Color was measured at wavelengths of 440 nm, lignosulfonic acids at 346 nm, aromatic compounds at 254 nm, and lignin derivatives at 280 nm in a 1 × 1 cm quartz cuvette (Spectronic Unicam UV-Visible Series GenesysTM 10, Waltham, MA, USA.

2.3. Measurement via Respirometry of Aerobic Biomass Activity of an Activated Sludge System Exposed to Different COD Concentrations of White, Black, and Green Liquors and Condensate

Measurement of aerobic biomass activity via respirometry was conducted based on the methodology recommended by the OECD [19]. The oxygen consumption of the aerobic biomass was measured in the presence of different COD concentrations of each liquor. The aerobic biomass was obtained from an aeration tank of a laboratory-scale AS system fed with primary pulp effluent. The general procedure for the assays consisted of a first exposure stage in which the biomass, with a concentration of 1.5 gTSS/L, was put into contact with 3.2 mL of feed solution and a defined quantity of liquor spill to be assayed, diluted with distilled water until a total volume of 100 mL was reached. The mixture was maintained under aeration and forced stirring conditions for 3 h. Subsequently, oxygen consumption in 10 mL of the mixture was measured in a closed cell of an oxygen monitoring system, and the value was recorded every 15 s for a period of 15 min. The respiration rate (OD) was calculated in accordance with Equation (1). This procedure was repeated at different COD concentrations of each liquor.
Respiration rate (OD) = (Q1 − Q2)/(∆t·60),
where Q1 is the initial oxygen concentration and Q2 the final oxygen concentration (mgO2/L), ∆t is the time interval between measurements (h), and OD is the respiration rate (mgO2/L·h).
The inhibitory effect of the assayed potential liquor spill at each concentration was calculated in accordance with Equation (2):
Ir = (1 − ((OD − ODa)/ODb))·100%,
where Ir is respiration inhibition (%), OD is the O2 respiration rate in assay solutions (mgO2/L·h), ODa is the respiration rate of the abiotic control (mgO2/L·h), and ODb is the respiration rate of the blank (mgO2/L·h).

2.4. Determination of Selenastrum capricornutum Growth and Inhibition Rates upon Exposure to Different COD Concentrations of White, Black, and Green Liquors and Condensate

The method used in the bioassays was extracted from the OECD [19]. The freshwater microalga species Selenastrum capricornutum was used in its exponential growth phase. The culture medium used for algal growth was proposed by the EPA [20], in which nitrogen and phosphorous concentrations are increased five times to obtain cultures with high cellular densities. The toxicity assays were performed under static conditions at 20 ± 1 °C. The initial cellular density of each concentration of liquor spill to be assayed and the control was approximately 104 cells·mL−1. The assays were performed in test tubes with 10 mL of assay solution, which contained culture medium, a microalga population, and different dilutions of each of the liquor spills to be assayed (% v/v): 3.12%, 6.25%, 12.5%, 25%, 33%, 41%, 50%, 64%, and 100% for the white, green, and black liquor and 3.12%, 6.25%, 12.5%, 25%, 50%, and 100% for the condensate. The exposure time was 96 h. The test tubes were installed at a constant photon flux density of 90 ± 10 µmol·m−2·s−1 using cold-white fluorescent lights. The Selenastrum capricornutum growth rate was determined using Equation (3), while the inhibition of the growth rate caused by liquors at different COD concentrations was determined using Equation (4).
K = (Ln(Nt/N0))/∆t,
where K is the growth rate, Nt is the cell density at the end of the assay (cells/mL), N0 is the initial cell density (cells/mL), and ∆t is the time interval between measurements (h).
Ic = ((Kc − Ki)/Kc)·100%,
where Ic is the growth inhibition (%), Kc is the average growth rate for the control (div/day), and Ki is the average growth rate for concentration i (div/day).

3. Results and Discussion

3.1. Physicochemical Characterization of a Kraft Pulp Process Effluent Used as Feed for a Laboratory-Scale Activated Sludge System and White, Black, and Green Liquors and Condensate

Table 1 shows the physicochemical characteristics of the effluent from the kraft pulp process used to feed a laboratory-scale activated sludge system. This effluent presented a pH of 6.6, with organic matter measured as COD and BOD5 having average values of 624.8 and 342.0 mg/L, respectively. These data are similar to those reported by Villamar et al. [21], who observed a neutral pH in effluents that use Eucalyptus globulus and COD concentrations of 467.9 mg/L. With respect to the TSS and VSS concentrations of 0.05 mg/L and 0.03 g/L, they can be explained by the fact that the effluent was obtained after the primary treatment.
Biodegradability assessed via the BOD5/COD ratio presents a value of 0.55. By comparison, Morales et al. [7] reported a ratio of 0.46 in Eucalyptus globulus effluents, indicating slightly lower biodegradability. This difference could be related to the fact that this study found lower concentrations of recalcitrant high-molecular-weight compounds (>1 kDa) such as lignins, sulfonic acids, and aromatic and phenolic compounds [1], which are also responsible for the color of the analyzed effluent. Regarding nutrients, average concentrations of 2.7 and 0.7 mg/L were detected for TP and TN, respectively.
Table 2 shows the physicochemical characteristics of the by-products formed during the kraft pulp process. The white, black, and green liquors present a pH of 13, which is expected, as the compounds that make up these liquors are NaOH, Na2S, and Na2CO3 from the lignin digestion, bleaching, and chemical recovery processes [8]. This make-up also explains the EC values obtained (32.9, 89.9, and 37.1 mS/cm, respectively). Furthermore, an alkaline pH favors dissolution and maintenance of a greater concentration of ions in solution.
In the comparison of the organic matter contents of the different analyzed spill liquors, black liquor presents the greatest COD and TOC concentrations: 141,350 mg/L and 3072 mg/L, respectively. This is because black liquor is composed of two-thirds organic chemical products extracted from wood and one-third inorganic chemical products [2]. The condensate presents lower COD and TOC concentrations—358.0 and 71.7 mg/L, respectively—which indicates that it is condensate A, the first condensate from the multi-evaporator stage, which is the least contaminating [22].
Color was not detected in the green and white liquor flows, as it occurs mainly due to recalcitrant compounds such as lignin, phenols, and aromatic compounds [23]. The black liquor and condensate, meanwhile, do contain these compounds, and thus the presence of color was detected.

3.2. Assessment of the Toxicity of Liquors Generated During the Kraft Pulp Production Process to the Aerobic Biomass of an Activated Sludge Treatment System

Table 3 shows the toxicity to the aerobic biomass from an activated sludge system as a result of exposure to flows generated by the kraft pulp process (white liquor, green liquor, black liquor, condensate), expressed in terms of EC20 and EC50. These figures represent the contaminant concentrations that cause 20% and 50% decreases in biomass activity, respectively.
Analysis of the obtained EC20 indicates that the white and green liquors and condensate require a similar concentration (117.8–167.1 mgCOD/L) to decrease the biomass activity by 20%, while black liquor concentrations of 1400 mgCOD/L are required to obtain this same result. With respect to EC50, white liquor requires a concentration of 168.9 mgCOD/L to reduce activity in the activated sludge by 50%, while green and black liquor concentrations of 300.3 and 2854.4 mgCOD/L, respectively, are needed to achieve the same effect. The toxicity of the liquors to the aerobic biomass could be related to pH, as its feed changes from pH 6.62 (Table 1) to pH = 13, corresponding to acute exposure (Table 2). As Li et al. [24] indicate, NaOH is a solution that causes sludge disintegration, as it alters the floccules and cells present. Therefore, the compositions of white and green liquors (mixture of hot water, Na2S, and NaOH) are the main reason for the toxicity to the aerobic biomass [8]. Meanwhile, exposure of the aerobic biomass to the black liquor produced toxicity 10 times lower than that of the other analyzed liquors, even though its physicochemical characteristics presented considerably higher values (Table 2). This result is consistent with the study of Kelley et al. [15], who reported EC50 results above 1000 mg/L in an assessment of the acute toxicity of black liquor to Daphnia magna. Meanwhile, Chamorro et al. [9] investigated the effects of different concentrations of black liquor on an activated sludge system and found that it causes a decrease in activity after each instance of exposure. However, this activity subsequently recovers.
The results obtained from the EC50 and EC20 from the exposure of the aerobic biomass from an activated sludge system to the different liquors do not have a direct correlation with the characterization of COD, TOC, lignosulfonic acids, lignin, or aromatic compounds discussed in Table 2.
Figure 2 shows the respiration rate (OD) (mgO2/L) of the aerobic biomass from an activated sludge system upon exposure to different concentrations of liquors over time. White liquor concentrations (Figure 2a) of 114.7 and 668.9 mgCOD/L stand out, as they produce the greatest variations in OD: 4.24–2.38 mgO2/L and 5.65–4.43 mgO2/L, respectively.
For green liquor (Figure 2b), at concentrations of 137.8 mgCOD/L, the oxygen consumption rate varies from 5.16 to 3.03 mgO2/L, while at a concentration of 413.3 mgCOD/L, it varies from 5.13 to 4.57 mgO2/L. Regarding black liquor (Figure 2c), at 2000 mgCOD/L, the oxygen consumption rate varies from 4.32 to 0.29 mgO2/L, while at 4000 mgCOD/L, it varies from 4.48 to 2.76 mgO2/L. Finally, for the condensate (Figure 2d), a concentration of 89.5 mgCOD/L causes the oxygen consumption rate to go from 4.24 to 0.71 mgO2/L, while at higher concentrations, the rate goes from 4.56 to 2.86 mgO2/L.
These results indicate a general trend: biomass exposure to greater liquor concentrations produces less OD variation. This suggests that the microbiological consortium present in the sludge is not using the available oxygen; thus, its activity is inhibited. Figure 2 presents a visualization of this information, showing that white and green liquors have the greatest influence on the biomass, while exposure to black liquor appears to have a smaller effect.
Figure 3 shows the inhibition percentage (%) and oxygen uptake rate (OUR) (mgO2/L·h) of the biomass upon exposure to different liquor concentrations.
White liquor, at a concentration of 287.7 mgCOD/L, presents a biomass inhibition percentage of 59.5% and an OUR of 8.42 mgO2/L·h. Green liquor presents an inhibition percentage of 60.9% at a concentration of 275.5 mgCOD/L, with an OUR of 4.29 mgO2/L·h. Meanwhile, black liquor requires a considerably greater concentration, reaching 3000 mgCOD/L to achieve a similar inhibition. In addition, the OUR caused by exposure to black liquor is greater even with an inhibition percentage of 59.4%, indicating that it is less toxic to the biomass.

3.3. Evaluation of the Toxicity of Liquors Produced in the Kraft Pulp Process to Selenastrum capricornutum, a Bioindicator of Acute Toxicity

Figure 4 shows the inhibition percentage (%) of Selenastrum capricornutum growth at different concentrations of white liquor, green liquor, black liquor, and condensate after an incubation of 48 h and 96 h. The microalga exposed to white liquor presents growth only up to a concentration of 0.24 mgCOD/L of white liquor. On the other hand, it was determined that concentrations greater than 0.50 mgCOD/L cause 18.5% inhibition in its growth. There is greater inhibition at white liquor concentrations of 1.21 mgCOD/L, at which inhibition reaches 81.7% at 48 h and 98.0% at 96 h. A similar pattern is observed for green liquor. Regarding Selenastrum capricornutum exposure to black liquor, different behaviors are observed at 48 h and 96 h. At 48 h of exposure, microalga growth inhibition ranges from 3.1% at 0.04 mgCOD/L to 94.7% at 1.41 mgCOD/L of black liquor, while at 96 h, there is a 13% increase in microalga growth when the black liquor concentration reaches 0.40 mgCOD/L. This growth begins to be inhibited by 2.8% at 0.47 mgCOD/L of black liquor, with inhibition reaching 55.7% at 1.4 mgCOD/L. This behavior suggests a hormetic response, which refers to a dose–response phenomenon characterized by stimulation at low concentrations and inhibition at higher concentrations [25]. This could be explained by the use of black liquor as a carbon source, considering its high organic matter content (Table 2), which comes from sugar and lignin fragments [26]. This phenomenon has been described by Duan et al. [27] in microalgae exposed to complex organic contaminants. In their study, Chlorella pyrenoidosa exhibited growth stimulation at 0.1 mg/L of bisphenol A, associated with enhanced metabolic and nucleotide pathways that increase energy production. Meanwhile, at 10 mg/L, the expression of genes related to the Krebs cycle, glycolysis, fatty acid metabolism, and mitochondrial electron transport was suppressed. Moreover, other studies have demonstrated the ability of certain microorganisms to directly use black liquor as a substrate. Rasooly-Garmaroody et al. [28] reported the production of bacterial cellulose using black liquor as the sole carbon source. Similarly, Kumar and Verma [23] compiled evidence showing how various microorganisms can valorize black liquor compounds through specific metabolic pathways. Brown et al. [29] also demonstrated the growth of Paenibacillus glucanolyticus in media containing black liquor, producing lactic acid without prior pretreatment. These studies highlight the potential of black liquor as a carbon source for microbial growth. The stimulation of microalgal growth at low concentrations has been reported by Chauhan et al. [30], who found that the addition of black liquor from a kraft pulp process increases the growth rate of Spirulina by 38%. Chamorro et al. [9], meanwhile, analyzing the effects of different concentrations of black liquor on Daphnia magna (bioindicator), reported that it was not feasible to determine the EC50 over 24 and 48 h of study, although exposure caused morphological alterations. They also mention that the pulp effluents present compounds such as triterpenes, ketones, and phytosterols that could have a beneficial effect once their acute toxicity to the exposed organisms abates. Finally, with respect to the condensate, it allows Selenastrum capricornutum growth up to a concentration of 0.17 mgCOD/L at 48 h and 96 h. However, at a concentration of 0.35 mgCOD/L, inhibitions of 27% at 48 h and 3% at 96 h are observed. This behavior of Selenastrum capricornutum upon exposure to the condensate could be due to its physicochemical characteristics (Table 2), as it has a neutral pH, near the pH of the feed solution, reducing the toxic impact on the microalga.

4. Conclusions

Black liquor presented the highest levels of COD, TOC, EC, and color compared to the rest of the assessed liquors, with 14,1350 mg/L, 3072 mg/L, 89.9 mS/cm, and 5500 abs, respectively. It was followed by green and white liquors, with COD, TOC, and EC values of 2755 mg/L and 1911 mg/L, 1948 mg/L and 1341 mg/L, and 37.13 mS/cm and 32.95 mS/cm, respectively. Finally, the lowest values were obtained for the condensate, with 358 mg/L, 71.7 mg/L, and 0.03 mS/cm, respectively.
Regarding the effects of the liquors on the biomass from an activated sludge system, it was demonstrated that white liquor is the stream that produces the greatest toxicity, followed by green liquor, with an EC20 of 117.8 and 129.9 mgCOD/L, respectively.
The evaluation of the effects of exposure to liquors from the kraft pulp process on Selenastrum capricornutum growth at 48 and 96 h showed that white and green liquors produced growth inhibition near 100% at both 48 and 96 h, while black liquor caused 94.7% inhibition at 48 h of incubation, which fell to 55.7% at 96 h, producing a 13% growth increase. With respect to the condensate, this stream stimulated Selenastrum capricornutum growth in both incubation periods.
The conclusions of this publication are highly relevant, as there is very little information about the physicochemical characteristics and toxicity of black, white, and green liquors and condensate coming from kraft processes. The information from this study makes it possible to calculate how much liquor can be fed to an activated sludge system, keeping it optimized to eliminate liquor discharges generated within the kraft mill’s processing units.

Author Contributions

Conceptualization, G.V.; methodology, C.H. and G.M.; software, C.H.; validation, N.M., G.M. and G.V.; formal analysis, C.H.; investigation, C.H. and G.M.; resources, G.V.; data curation, N.M.; writing—original draft preparation, N.M.; writing—review and editing, G.M., N.M. and G.V.; visualization, C.H.; supervision, G.V.; project administration, G.V.; funding acquisition, G.V. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the ANID/FONDAP/1523A0001 grant.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data have been included in the present paper.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. A diagram of the kraft mill process.
Figure 1. A diagram of the kraft mill process.
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Figure 2. Respiration rate (OD) (mgO2/L) of the aerobic biomass from a laboratory-scale activated sludge system upon exposure to different concentrations of liquors from the kraft pulp process over nine minutes. (a) White liquor at concentrations of 114.7, 191.1, 477.8, and 668.9 mgCOD/L. (b) Green liquor at concentrations of 137.8, 192.9, 275.5, and 413.3 mgCOD/L. (c) Black liquor at concentrations of 2000, 3000, 3400, and 4000 mgCOD/L. (d) Condensate at concentrations of 89.5, 125.3, 143.2, and 214.8 mgCOD/L.
Figure 2. Respiration rate (OD) (mgO2/L) of the aerobic biomass from a laboratory-scale activated sludge system upon exposure to different concentrations of liquors from the kraft pulp process over nine minutes. (a) White liquor at concentrations of 114.7, 191.1, 477.8, and 668.9 mgCOD/L. (b) Green liquor at concentrations of 137.8, 192.9, 275.5, and 413.3 mgCOD/L. (c) Black liquor at concentrations of 2000, 3000, 3400, and 4000 mgCOD/L. (d) Condensate at concentrations of 89.5, 125.3, 143.2, and 214.8 mgCOD/L.
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Figure 3. Biomass inhibition percentage (%) and oxygen uptake rate (OUR) (mgO2/L·h) of the aerobic biomass from a kraft pulp treatment system in contact with different concentrations of white liquor, black liquor, green liquor, and condensate (mgCOD/L).
Figure 3. Biomass inhibition percentage (%) and oxygen uptake rate (OUR) (mgO2/L·h) of the aerobic biomass from a kraft pulp treatment system in contact with different concentrations of white liquor, black liquor, green liquor, and condensate (mgCOD/L).
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Figure 4. Selenastrum capriconutum inhibition percentage (%) at different liquor concentrations (log mgCOD/L) with incubation times of 48 h (■) and 96 h (□). (a) White liquor; (b) green liquor; (c) black liquor; (d) condensate.
Figure 4. Selenastrum capriconutum inhibition percentage (%) at different liquor concentrations (log mgCOD/L) with incubation times of 48 h (■) and 96 h (□). (a) White liquor; (b) green liquor; (c) black liquor; (d) condensate.
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Table 1. Physicochemical characterization of the kraft pulp effluent after primary treatment.
Table 1. Physicochemical characterization of the kraft pulp effluent after primary treatment.
ParameterUnitRangeMean ± SD *
pH-6.62–6.676.65 ± 0.04
EC *mS/cm2.80–2.812.81 ± 0.01
CODmg/L611.0–638.5624.8 ± 19.5
BOD5mg/L324.0–360.0342.0 ± 25.5
TSSmg/L0.03–0.060.05 ± 0.02
VSSg/L0.02–0.050.03 ± 0.02
ColorAbs0.09–0.100.10 ± 0.00
Lignosulfonic acidsAbs0.05–0.050.05 ± 0.00
LigninAbs2.39–2.412.40 ± 0.01
Aromatic compoundsAbs0.49–0.530.52 ± 0.03
Total phenolsmg/L159.7–161.4160.6 ± 1.2
TPmg/L2.7–2.72.7 ± 0.0
TNmg/L0.5–0.80.7 ± 0.2
NO3−mg/L<0.1 –
NO2−mg/L0.03–0.050.04 ± 0.01
* EC: electric conductivity; COD: chemical oxygen demand; BOD5: biological oxygen demand; TSS: total suspended solids; VSS: volatile suspended solids; Abs: absorption units 1×, 1 cm; TP: total phosphorous; TN: total nitrogen; NO2−: nitrite; NO3−: nitrate; SD: standard deviation; n = 3.
Table 2. Physicochemical characterization of by-products of the kraft pulp process: white liquor, black liquor, green liquor, and condensate.
Table 2. Physicochemical characterization of by-products of the kraft pulp process: white liquor, black liquor, green liquor, and condensate.
ParameterUnitLiquor
WhiteBlackGreenCondensate
pH-13.0 ± 0.113.0 ± 0.113.0 ± 0.17.6 ± 0.1
EC *mS/cm32.9589.9037.130.03
COD *mg/L1911 ± 209141,350 ± 22752755 ± 95358 ± 7
TOC *mg/L1341.0 ± 2.53072.0 ± 3.11948.0 ± 3.571.7 ± 0.5
ColorAbs-5.500-0.014
Lignosulfonic acidsAbs0.50021.500-0.058
LigninAbs2.051.52.00.258
Aromatic compoundsAbs3.462.56.00.6
Total phenolsmg/L203405640637
* EC: electric conductivity; COD: chemical oxygen demand; TOC: total organic carbon; Abs: absorption units 1×, 1 cm; n = 3.
Table 3. EC50 and EC20 caused by exposure of an aerobic biomass from an activated sludge system to white liquor, black liquor, green liquor, and condensate.
Table 3. EC50 and EC20 caused by exposure of an aerobic biomass from an activated sludge system to white liquor, black liquor, green liquor, and condensate.
LiquorEC20 *EC50 *
(mgCOD/L)
White 117.8168.9
Black 1400.02854.5
Green 129.9300.3
Condensate167.1ND
* EC20: effective concentration 20%; EC50: effective concentration 50%; ND: not determined.
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Hidd, C.; Morales, G.; Monsalves, N.; Vidal, G. Toxic Effects of Liquors Generated During Kraft Pulp Production Process on Aerobic Biomass and Growth of Selenastrum capricornutum. Sustainability 2025, 17, 4494. https://doi.org/10.3390/su17104494

AMA Style

Hidd C, Morales G, Monsalves N, Vidal G. Toxic Effects of Liquors Generated During Kraft Pulp Production Process on Aerobic Biomass and Growth of Selenastrum capricornutum. Sustainability. 2025; 17(10):4494. https://doi.org/10.3390/su17104494

Chicago/Turabian Style

Hidd, Constanza, Gabriela Morales, Naomi Monsalves, and Gladys Vidal. 2025. "Toxic Effects of Liquors Generated During Kraft Pulp Production Process on Aerobic Biomass and Growth of Selenastrum capricornutum" Sustainability 17, no. 10: 4494. https://doi.org/10.3390/su17104494

APA Style

Hidd, C., Morales, G., Monsalves, N., & Vidal, G. (2025). Toxic Effects of Liquors Generated During Kraft Pulp Production Process on Aerobic Biomass and Growth of Selenastrum capricornutum. Sustainability, 17(10), 4494. https://doi.org/10.3390/su17104494

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