Bleached Effluent Research Articles

Mg(OH)2-BASED HYDROGEN PEROXIDE BLEACHING OF CMP PULPS AT HIGH CONSISTENCY

The objective of this study was to investigate the bleaching performance of a Mg(OH)2-based hydrogen peroxide process at a high consistency. In this work, an industrially produced chemimechanical pulp (CMP) was bleached via Mg(OH)2- or NaOH-based hydrogen peroxide processes at 10% and 25% consistencies. The results showed that the pulp bleached under the conditions of 1.5% Mg(OH)2 and 3% H2O2 at 25% consistency had a similar brightness to, a lower yellowness index, and a higher opacity than the pulp produced under the conditions of 2.1% NaOH, 3% Na2SiO3, and 3% H2O2 at the same consistency. The temperature (70 oC) and time (150 min) of the bleaching were the same for both processes. Under the conditions stated above, the Mg(OH)2-based process had a higher yield than the NaOH-based process did. The bleaching effluent of the Mg(OH)2-based process had a higher residual H2O2, but a lower Chemical Oxygen Demand (COD) load and turbidity, compared with that of the NaOH-based process. However, the strength properties and water retention value (WRV) of the pulp bleached via the Mg(OH)2-based process were lower, while its bulk was higher than those of the pulp bleached via the NaOH-based process.

TCF BLEACHING CHARACTER OF SODA-ANTHRAQUINONE PULP FROM OIL PALM FROND

The OQP, OQPO, and OLQP bleaching sequences have been applied to oil palm frond soda-anthraquinone pulp. Oxygen delignification resulted in a delignification of approximately half of the kappa number. In comparison between OQP and OQPO sequences, the OQPO sequence was able to improve brightness from 75.2% ISO to 82.3% ISO. A TCF bleaching sequence involving laccase was also suitable for the frond pulp bleaching. Results indicated that laccase has a good bleaching capability. Short beating in a PFI refiner (about PFI 2000 revolutions) was sufficient to attain good pulp strength of TCF bleached pulp. There was no total organically bound chlorine in the TCF bleaching effluent. The results of this study show that soda-anthraquinone pulping and TCF bleaching is a promising alternative to produce high-quality pulp from oil palm frond for writing and printing paper.

Anaerobic treatment of cellulose bleach plant wastewater: chlorinated organics and genotoxicity removal

This study assessed the removal efficiency of organic matter and how it relates to the decrease of toxic and mutagenic effects when an anaerobic reactor is used to treat the bleaching effluent from two kraft pulp mills. Parameters such as COD (chemical oxygen demand), DOC (dissolved organic carbon), AOX (adsorbable organic halogen), ASL (acid soluble lignin), color, chlorides, total phenols and absorbance values in the UV-VIS spectral region were measured. The acute and chronic toxicity and genetic toxicity assessments were performed with Daphnia similis, Ceriodaphnia sp. and Allium cepa L, respectively. The removal efficiency of organic matter measured as COD, ranged from 45% to 55%, while AOX removal ranged from 40% to 45%. The acute toxic and chronic effects, as well as the cytotoxic, genotoxic and mutagenic effects, decrease as the biodegradable fraction of the organics is removed. These results, together with the organic load measurement of the effluents of the anaerobic treatment, indicate that these effluents are recalcitrant but not toxic. As expected, color increased when the anaerobic treatment was applied. However, the colored compounds are of microbial origin and do not cause an increase in genotoxic effects. To discharge the wastewater, it is necessary to apply a physico-chemical or aerobic biological post-treatment to the effluents of the anaerobic reactor.

TCF bleaching character of soda-anthraquinone pulp from oil palm frond

The bleaching sequences have been applied to oil palm frond soda-anthraquinone pulp. Oxygen delignification resulted in a delignification of approximately half of the kappa number. In comparison between OQP and OQPO sequences, the OQPO sequence was able to improve brightness from 75.2% ISO to 82.3% ISO. A TCF bleaching sequence involving laccase was also suitable for the frond pulp bleaching. Results indicated that laccase has a good bleaching capability. Short beating in a PFI refiner (about PFI 2000 revolutions) was sufficient to attain good pulp strength of TCF bleached pulp. There was no total organically bound chlorine in the TCF bleaching effluent. The results of this study show that soda-anthraquinone pulping and TCF bleaching is a promising alternative to produce high-quality pulp from oil palm frond for writing and printing paper.

Mg(OH)2-based hydrogen peroxide bleaching of CMP pulps at high consistency

The objective of this study was to investigate the bleaching performance of a Mg(OH)2-based hydrogen peroxide process at a high consistency. In this work, an industrially produced chemimechanical pulp (CMP) was bleached via Mg(OH)2- or NaOH-based hydrogen peroxide processes at 10% and 25% consistencies. The results showed that the pulp bleached under the conditions of 1.5% Mg(OH)2 and 3% H2O2 at 25% consistency had a similar brightness to, a lower yellowness index, and a higher opacity than the pulp produced under the conditions of 2.1% NaOH, 3% Na2SiO3, and 3% H2O2 at the same consistency. The temperature (70 ºC) and time (150 min) of the bleaching were the same for both processes. Under the conditions stated above, the Mg(OH)2-based process had a higher yield than the NaOH-based process did. The bleaching effluent of the Mg(OH)2-based process had a higher residual H2O2, but a lower Chemical Oxygen Demand (COD) load and turbidity, compared with that of the NaOH-based process. However, the strength properties and water retention value (WRV) of the pulp bleached via the Mg(OH)2-based process were lower, while its bulk was higher than those of the pulp bleached via the NaOH-based process.

Development of model for treatment of pulp and paper industry bleaching effluent using response surface methodology

AbstractThe treatment of bleaching‐effluent from pulp and paper industry was investigated in a batch electrocoagulation (EC) reactor using aluminium as sacrificial electrodes. The response surface methodology (RSM) was employed to evaluate the individual and interactive effects of four independent parameters viz., current density (j: 5–25 mA/cm2), initial pH (pHo: 5–9), electrolyte (NaCl) concentration (c: 0–2 g/L) and electrolysis time (t: 10–30 min) on Colour, COD and BOD removal efficiency. The results were analyzed using Pareto analysis of variance (ANOVA). Based on RSM analysis, a second‐order polynomial regression model was developed and found to be good fit with experimental results. © 2011 American Institute of Chemical Engineers Environ Prog, 2011

Size-exclusion chromatographic study of ECF and TCF softwood kraft pulp bleaching liquors

Currently, elemental chlorine-free (ECF) and totally chlorine-free (TCF) bleaching systems are widely used for pulp production. Low and medium molecular weight lignin break-down products are known to have harmful effects on the environment. According to some recent results, also high molecular weight (HMW) material consisting mainly of lignin and carbohydrates may cause toxic effects to the environment. For these reasons, toxicity and structure studies of HMW materials are of great importance. This investigation is a part of a larger project to obtain more structure information of HMW materials and toxicity of ECF and TCF bleaching effluents. Size-exclusion chromatography (SEC) has been commonly used for the characterization of organic macromolecules such as lignin, but to our knowledge, no reports have appeared dealing with the comparison of SEC of ECF and TCF bleaching liquors. The aim of the present study was to get more information about the molecular weight distribution (MWD) of HMW fractions of waste liquors from ECF and TCF bleaching sequences by SEC. The MWDs of organic materials dissolved during different stages of ECF bleaching (O-D-EOP-D-ED) and TCF bleaching (O-Z-Q-P-Z-Q-P-P) of softwood (Pinus sylvestris) kraft pulp were determined and compared by SEC. All effluent samples from the above bleaching stages were ultrafiltrated using a membrane with a cutoff value of 1,000Da. SEC was performed on high and also low molecular weight fractions and non-fractionated effluents. In the SEC experiments, a Superdex 75 column was used with 0.1M NaOH solution as the eluent. Standards used for calibrating the SEC system were albumin, carboanhydrase, cytochrome C, tannic acid, dehydrodiacetovanillone, and vanillin. The chromatograms of liquors from TCF bleaching stages vary more than those from ECF bleaching. Peroxide and chelating stages contained mostly high molecular weight (HMW) matter whereas chlorine dioxide and ozone stages had more low molecular weight compounds. The lignin content in HMW matter was higher than in stages that consisted of low molecular matter. Bleaching effluents contained the highest amounts of HMW material, mainly lignin, in the beginning of the sequences; the amounts decreased towards the end of the bleaching sequence. Determinations of MWD by the SEC method showed that effluents from the TCF sequence contained more HMW material than those from the ECF stage. This might be due to peroxide stages (P) that dissolve HMW lignin effectively. However, the molecular weights of ozone stages (Z) were very low compared to other stages. Chlorine dioxide stages also dissolved mostly low molecular weight lignin. Ultrafiltration of bleaching liquors showed that high molecular weight fraction also included some low molecular weight compounds and vice versa. High polydispersity and high lignin content correlated with the amount of HMW material in ECF and TCF bleaching stages. Our liquor samples were studied by using a UV detector commonly used for lignin preparations; in upcoming investigations, it will be interesting to determine carbohydrates such as hemicelluloses. The results are applicable in papermaking in order to improve commonly used bleaching procedures, to test new potential bleaching systems, and to study chemical behavior of HMW materials in various bleaching liquors. The present results also form a good basis for toxicity measurements of ECF and TCF bleaching effluents and for more comprehensive spectroscopic and chromatographic experiments with samples taken from various bleaching stages. From the behavior of liquors studied, it appears that our other structure investigations by spectroscopic and chromatographic (NMR, Py-GC/MS, etc.) methods mostly correlate well with the present results.

Studies on coupled biological and photochemical treatment of soda pulp bleaching effluents from agro residue based pulp and paper mill

AbstractBACKGROUND: The effect of coupled biological and photochemical processes for treatment of bio‐recalcitrant effluents from chlorination (C) and first alkaline extraction (E1) stages of soda pulp bleaching in an agro‐residue based pulp and paper mill has been investigated.RESULTS: The work aims to evaluate the coupled effect on the extent of degradation in terms of chemical oxygen demand (COD) reduction. Biological treatment of C and E1 effluent resulted in 30% and 57% COD, respectively, after 1 day of treatment with acclimatized activated sludge. However, further increase in retention time did not show any significant change in degradation efficiency. Investigations on photocatalytic treatment (at 365 nm and 0.625 W) of C effluent resulted in 47% degradation under optimized conditions (3 g L−1 TiO2, pH 6.0 and 0.01 mol L−1 NaOCl), and E1 effluent showed 37% degradation under optimized conditions (2.5 g L−1, pH 4.0 and 0.03 mol L−1 NaOCl) after 6 h of UV irradiation. However, photocatalytic treatment of biotreated (1 day) effluents under similar optimized conditions significantly enhanced the degradation to 81% and 93% in C and E1 effluent, respectively, after 2 h irradiation.CONCLUSION: Coupled biological treatment followed by photocatalysis is an effective method for the reduction of total organic carbon (TOC) of the test effluents and it would facilitate closed circuiting of water used for soda bleaching in an agro pulp and paper mill. Copyright © 2011 Society of Chemical Industry

The decontamination of bleaching effluent by pilot‐scale solar Fenton process

A solar Fenton process was applied as post‐treatment to selectively eliminate organic pollutants and toxicants in bleaching effluents of kraft pulp mills. Experiments were conducted to study the effect of system parameters (pH, initial concentration of H2O2, molar ratio of Fe2+/H2O2 and solar‐UV irradiance) on the removals of chemical oxygen demand and colour. The results showed 92.8% of COD and 99.6% of colour were removed at pH 3.5, H2O2 30 mM/L, Fe2+/H2O2 1:100, solar‐UV irradiance 11070 mW/m2, reaction time 120 min. The first‐order kinetic model was used to study the dependence of the reaction rate on solar‐UV irradiance: a linear relationship was shown to exist between reaction rate constants and solar‐UV irradiance. The results of gas chromatography mass spectrometry analysis showed that the toxicity of the bleaching effluents was mainly derived from the presence of mononuclear aromatics, polycyclic aromatic hydrocarbons and organochlorides, which were all degraded into harmless organic acids under the attack of hydroxyl radicals generated from the solar Fenton reaction.

Simultaneous determination of chlorine dioxide and hypochlorous acid in bleaching system

This study has demonstrated a rapid spectroscopic method for the determination of chlorine dioxide and hypochlorous acid concentrations in the pulp bleaching processes. It was found that chlorine dioxide and hypochlorous acid have an isosbestic wavelength of 295 nm. The soluble lignin in such a system is the main interference, but can be corrected by determining the absorbances at 295 nm, 380 nm, and 480 nm. Thus, based on the spectroscopic measurements at 295 nm (the isosbestic point wavelength for chlorine dioxide and hypochlorous acid), 380 nm (absorbance wavelength of chlorine dioxide) and 480 nm (the acid soluble lignin absorbance wavelength), the chlorine dioxide and hypochlorous acid concentrations in the bleaching process can be quantified. However, hypochlorous acid was not detected in the real bleaching effluent for its low content. The present method is simple, rapid, accurate, and has the potential for on-line monitoring of the chlorine dioxide bleaching process.

BIO-CONVENTIONAL BLEACHING OF KRAFT-AQ PULP OF A. CADAMBA BY CRUDE XYLANASES FROM COPRINELLUS DISSEMINATUS MLK-03 AND EFFECT OF RESIDUAL ENZYME ON EFFLUENT LOAD

A new thermo-alkali-tolerant crude xylanase from Coprinellus disseminatus decreased kappa number by 34.38% and improved brightness and viscosity by 1.6 and 6.47% respectively after XE1-stage during prebleaching of Anthocephalus cadamba kraft-AQ pulp. At 2.4% chlorine demand, crude xylanase in a XECEHH (X= enzymatic prebleaching stage, E= extraction stage, C= chlorination stage, H= hypochlorite stage) bleaching sequence improved pulp brightness, tensile index, burst index, and double fold numbers by 3.66%, 4.78%, 6.38%, and 11.11%, respectively with a reduction in viscosity (10.59%) and tear index (10.77%) compared to the control. Combined bleach effluent of the XECEHH sequence mitigated adsorable organic halides (AOX) by 21% and increased chemical oxygen demand (COD), bio-chemical oxygen demand (BOD), and colour by 67.18%, 84.78%, and 97.53%, respectively, compared to the control. Residual enzymes that entered during enzymatic prebleaching stage decreased AOX, COD, BOD, and colour of combined effluent of the XECEHH bleaching sequence progressively and on 6th day, and these were reduced by 23.78%, 0.04%, 15.00%, and 0.61%, respectively, compared to the control.

Bio-conventional bleaching of kraft-AQ pulp of A. cadamba by crude xylanases from Coprinellus disseminatus MLK-03 and effect of residual enzyme on effluent load

A new thermo-alkali-tolerant crude xylanase from Coprinellus disseminatus decreased kappa number by 34.38% and improved brightness and viscosity by 1.6 and 6.47% respectively after XE1-stage during prebleaching of Anthocephalus cadamba kraft-AQ pulp. At 2.4% chlorine demand, crude xylanase in a XECEHH (X= enzymatic prebleaching stage, E= extraction stage, C= chlorination stage, H= hypochlorite stage) bleaching sequence improved pulp brightness, tensile index, burst index, and double fold numbers by 3.66%, 4.78%, 6.38%, and 11.11%, respectively with a reduction in viscosity (10.59%) and tear index (10.77%) compared to the control. Combined bleach effluent of the XECEHH sequence mitigated adsorable organic halides (AOX) by 21% and increased chemical oxygen demand (COD), bio-chemical oxygen demand (BOD), and colour by 67.18%, 84.78%, and 97.53%, respectively, compared to the control. Residual enzymes that entered during enzymatic prebleaching stage decreased AOX, COD, BOD, and colour of combined effluent of the XECEHH bleaching sequence progressively and on 6th day, and these were reduced by 23.78%, 0.04%, 15.00%, and 0.61%, respectively, compared to the control.

Treatment of pulp and paper industry bleaching effluent by electrocoagulant process

The experiments were carried out in an electrocoagulation reactor with aluminum as sacrificial electrodes. The influence of electrolysis time, current density, pH, NaCl concentration, rotational speed of the stirrer and electrode distance on reduction of color, COD and BOD were studied in detail. From the experimental results, 15 mA/cm(2) current density, pH of 7, 1 g/l NaCl, 100 rpm, 28°C temperature and 3 cm electrode distance were found to be optimum for maximum reduction of color, COD and BOD. The reduction of color, COD and BOD under the optimum condition were found to be 94%, 90% and 87% respectively. The electrode energy consumption was calculated and found to be varied from 10.1 to 12.9 kWh/m(3) depending on the operating conditions. Under optimal operating condition such as 15 mA/cm(2) current density, pH of 7, 1 g/l NaCl, 100 rpm, 28°C temperature and 3 cm electrode distance, the operating cost was found to be 1.56 US $/m(3). The experimental results proved that the electrocoagulation is a suitable method for treating bleaching plant effluents for reuse.

Impact of a secondary treated bleached Kraft pulp mill effluent in both sexes of goldfish (Carassius auratus L.)

The aim of the present study was to assess the toxic effects of a secondary treated effluent from a modern pulp mill processing Eucalyptus globulus on both sexes of goldfish. The effects in the exposed fish were investigated by measuring biomarkers which have been used to assess the effects of BKME on aquatic biota, with particular emphasis in fish. However, few studies have been focused on effluents from pulp mill plants processing eucalyptus. The relative proportion of cytochrome P450 (CYP1A) and vitellogenin (VTG) induction were analyzed in the livers of males and females goldfish, exposed to different concentrations of a bleached Kraft pulp mill (BKPM) effluent. Somatic indices such as HSI (hepatosomatic index) and GSI (gonadosomatic index) were calculated and a significant reduction was found in males GSI. A significant increase of HSI was recorded for both sexes exposed to 50% BKPM effluent. Exposure to BKPM effluent induced CYP1A synthesis in both sexes and decreased VTG synthesis in females according to the different effluent concentrations suggesting that the secondary treated effluent contained compounds that affect fish negatively. The results suggest that BKPM effluent can cause sex-related biochemical changes in xenobiotic metabolism.

Chemical components of bleached eucalypt kraft pulp effluent COD and treatment removal efficiency during normal mill operation and maintenance shutdowns

In order to meet increasingly strict Brazilian COD emissions limits, mills must understand the components that contribute to effluent COD, how these vary between normal mill operation and maintenance shutdowns, and how this variation affects treatment efficiency. To this end, primary and secondary effluents from a Brazilian bleached eucalypt kraft pulp mill activated sludge system were analyzed for COD, lignin, extractives, carbohydrates and AOX over a sixth month period that included two general maintenance shutdowns and four months of normal operation. Primary effluent presented significantly different compositions during periods of normal operation and mill shutdowns. During normal operation, the main components of effluent COD (909 mg/l average) were carbohydrates, followed by lignin. However, the lignin fraction was the main component of secondary effluent COD during both normal operation and mill shutdowns. Higher removal efficiencies for COD carbohydrates and AOX were observed during normal operation compared to shutdowns, while no difference in removal efficiencies of lignin and extractives was observed. Carbohydrate removal efficiency was significantly lower in one of the parallel treatment lines. The different removal efficiencies reflect not only variations in effluent composition, but possibly differences in system operational control which should be explored in greater detail.

The removal of COD from bleaching effluents by Fenton's reagent: Effect of system parameters and kinetic study

Fenton's reagent was now being used as a post-treatment procedure to eliminate the intractable pollutants in the bleaching effluents from pulp mills. To establish cost-efficient operating conditions for the application of Fenton's reagent, the effects of operating conditions such as the dosages of H2O2, the molar ratio of [Fe2+]/[H2O2], the value of solution pH and the system temperature on the removal of COD were evaluated systematically. The results showed a suitable operating condition that yields 88.7% of COD removal was obtained at [H2O2] = 4COD(mM L−1), [Fe2+]/[H2O2] = 20:1, pH3, 90°C, reaction time 120 min. A kinetic study was carried out using a modified pseudo-first-order model. The results showed the COD removal for bleaching effluents by Fenton's reagent followed the first-order reaction kinetics, and the activation energy Ea was detected to be 1.68 kJ mol−1 based on the experiments performed at different temperatures. In addition, the results of GC/MS analysis showed organic pollutants in effluents were degraded into organic acids of lower molecular weight under the attack of hydroxyl radicals generated in Fenton's reaction. The results of biodegradability assay indicated the bio-toxic substances in the bleaching effluents were eliminated. © 2010 American Institute of Chemical Engineers Environ Prog, 2011

A hybrid system combining self-forming dynamic membrane bioreactor with coagulation process for advanced treatment of bleaching effl uent from straw pulping process

An innovative hybrid system combining self-forming dynamic membrane bioreactor (SFDMBR) with coagulation process was developed to treat bleaching wastewater from pulping process. Pilot-scale experimental results showed that the average removal effi ciencies of chemical oxygen (COD) demand and lignin by the SFDMBR were over 80% and 51%, respectively. The fairly good treatment performances can be attributed partly to the high biomass concentration (i.e. -10 g/L) maintained in the SFDMBR, and partly to the more efficient solid-liquid separation achieved by the self-forming dynamic membranes. It was found that the self-forming dynamic membranes can be quickly created, normally less than 60 min, during the initial stage of SFDMBR operation. Effluent turbidity was lower than 5 NTU in most cases. Membrane fouling can be effectively controlled via 5 min online air backwashing at an intensity of 3.2m3·m−2·h−1, and the separation capacity can be restored within 30 min. The effluent of SFDMBR was subsequently treate...

Violuric Acid Catalyzed Electrochemical Delignification and Bleaching of Poplar Kraft Pulp

The electrochemically mediated delignification (EM) of lignocellulosic fibers is one of new methods for fiber pulp delignification and bleaching. The choice of mediator is a key to the foundation of the delignification system. In this paper, violuric acid (VIO) was selected as a mediator to catalyze this delignification process, and the catalyzing delignification was carried out as well as subsequent bleachability of delignified pulps. The catalyzing delignification efficiency was evaluated according to pulp kappa number and viscosity. The pollution loads of bleaching effuluents were discussed. The results showed that VIO is a suitable mediator for the electrochemical delignification system. At the optimum conditions, the kappa number reduces by 5.4, from 17.4 to 11.7, and viscosity decreases from 1144cm3/g to 1059cm3/g. The alkali treatment after EM can further reduce kappa number and increase brightness of resulting pulp. The kappa number of pulp treated with EME can be reduced to 8.9 with the delignification rate of 48.8% and viscosity only decreases to 1065 cm3/g. The brightness of EM E treated pulp increases from original 39.2% ISO to 50.7% ISO. The EM treated pulps can be bleached to 80%ISO with the subsequent QP or EQP bleaching sequences at 2.0% H2O2 charge. The bleaching effluents of EMQP have lower COD and BOD than that of OQP which is considered as environmental friendly bleaching sequence.

Opportunities for the integration of absorption heat pumps in the pulp and paper process

Implementation of absorption heat pumps (AHPs) in a Kraft pulping process was studied using a new methodology for the optimal integration of those devices in a process. Two generic opportunities were identified for an energy and water optimized mill: (i) integration of a double lift chiller in the bleaching chemical making plant to produce chilled and hot water simultaneously, using MP steam as the driving energy and, (ii) installation of a single stage heat pump to concentrate the black liquor and produce useful hot water by upgrading heat from the bleaching effluent and using MP steam as driving energy. The principles of AHPs operation and their efficient integration into a process are described. The simple payback time (SPB) and net present value (NPV) were used to evaluate the interest of such implementations. Considering 63 $/MWh for the steam price, SPB of 2.7 and 1.7 years have been estimated for the two cases.

Removal of recalcitrant contaminants from bleaching effluents in pulp and paper mills using ultrasonic irradiation and Fenton-like oxidation, electrochemical treatment, and/or chemical precipitation: A comparative study

This laboratory study aims at improving the treatability of bleaching effluents from local pulp and paper mills using individual physico-chemical treatments like chemical precipitation and electrochemical treatment. Advanced oxidation processes (AOPs) such as ultrasonic irradiation in combination with Fenton-like oxidation (Fe 3+/H 2O 2) or photo-Fenton degradation (Fe 2+/H 2O 2/UV) were also separately tested. Effects of operating parameters such as pH and oxidant's dose on the removal of organic compounds were investigated in terms of COD removal. Of the various treatments, chemical precipitation using 5 g/L of CaO was the most effective to remove organic compounds from the samples, as indicated by their decreasing COD concentrations up to 90% at an initial concentration of 1510 mg/L at pH 12. At initial concentrations ranging from 2.5 to 12.5 mg/L, this method could remove over 90% of target compounds such as abietic acid, linoleic acid, and β-sitosterol from bleaching effluents. Although each individual treatment could improve the treatability of bleaching wastewater, neither of them could generate treated effluents that met the strict requirement of COD limit of less than 200 mg/L, thus suggesting the need for integrating physico-chemical treatments and biological process to complement the degradation of recalcitrant compounds in the samples prior to their discharge.