Evaluation of dechlorination mechanisms during anaerobic fermentation of bleached kraft mill effluent

War. Res. Vol.27, No. 8, pp. 1269-1273,1993 Printed in Great Britain. All rightsreferred

0043-1354/93$6.00+0.00 Copyright © 1993PergamonPress Ltd

EVALUATION OF DECHLORINATION MECHANISMS DURING ANAEROBIC FERMENTATION OF BLEACHED KRAFT MILL EFFLUENT W. J. PARKER~O, E. R. HALLzO, and G. J. FARQUHARI 'Department of Civil Engineering, University of Waterloo, Waterloo, Ontario N2L 3GI and 2Environment Canada, Wastewater Technology Centre, 867 Lakeshore Road, Burlington, Ontario, Canada L7R 4A6 (First received August 1992; accepted in revised form February 1993)

Abaraet--The impact of nucleophilic substitution of biologically mediated sulfide as a potential mechanism for dechlorination during anaerobic treatment of kraft mill bleach plant effluents was investigated. Batch anaerobic biodegradative studies employing a prepared solution of chlorinated organic matter with a low concentration of anions revealed that sulfate supplementation, with subsequent production of sulfide by anaerobic fermentation, did not significantly increase the extent of dechlorination. Competition with other degradative mechanisms, such as reductivc dechlorination was concluded to be responsible for the reduced impact of sulfide on AOX removal in biological studies. Substantially more inorganic chloride was produced than could be accounted for by AOX reduction during the batch tests. Subsequent analyses revealed that the total chlorine concentration in the original effluent was greater than the sum of the AOX and chloride concentrations. It was concluded that the excess chloride produced in the batch tests resulted from the degradation of chlorinated organic matter which was not detected by the AOX technique. Key words--kraft bleach plant effluents, adsorbable organic halogen, nucleophilic substitution, sulfide, chloride, neutralization, reductive dechlorination

INTRODUCTION

Chlorinated organic matter present in effluents from kraft mills employing either chlorine or chlorine dioxide in the bleaching process has become a matter of concern due to its recalcitrance to biological degradation, toxicity to aquatic species, genotoxicity and potential to accumulate in the fatty tissues of a variety of organisms (Suntio et al., 1988). Anaerobic treatment of these effluents can result in substantial degradation of chlorinated organic matter as indicated by removal of adsorbable organic halogen (AOX) (Parker et al., 1993; Lafond and Ferguson, 1991; Randle et al., 1991). Kraft mill bleach plant effluents contain a variety of chlorinated compounds which encompass a spectrum of physical and chemical properties (Kringstad and Lindstrom, 1984). The degradation of these compounds during anaerobic treatment has been attributed to reductive dechlorination. In theory, the electronegative nature of chlorine substituents acts to increase the oxidation level of the organic compounds, making them more susceptible to reduction reactions. For a given compound, a greater number of substitutions will result in a higher oxidation level, and hence, a greater tendency to be reduced. This behaviour has been observed during the anaerobic fermentation of several chlorinated phenolic compounds. As expected in reductive dechlorination reactions, chlorinated phenolics with multiple substi-

tutions were well removed, while lower substituted compounds were either poorly removed or accumulated during anaerobic treatment (Parker, 1991; Hakulinen et al., 1981; Hakulinen and SalkinojaSalonen, 1982). However, chlorinated phenolics typically account for only 1% of the total chlorinated organic matter in bleached kraft mill effluents (Parker, 1991; Kringstad and Lindstrom, 1984). Reductive dechiorination has been presumed by extension to be the mechanism by which the majority of chlorinated compounds are transformed by anaerobic treatment of kraft bleaching effluents. It is however apparent that other mechanisms contribute to the dechlorination of kraft mill bleach plant effluents during anaerobic treatment. Fitzsimons et al. (1991) reported that 10-15% of the AOX of a mixed bleach plant effluent disappeared with neutralization to a pH of 7.0. Milosevich and Hill (1991) demonstrated 50% reduction in AOX by treating bleach plant effluent with lime mud and NaOH to a pH of I0. Confirmatory experiments in our laboratory have demonstrated a 25% reduction in AOX by pH adjustment to 7.0 with NaHCO3 (Parker, 1991). The most fikely degradation mechanism, accounting for these observations, is the substitution of hydroxyl (OH-) ions for organochlorine; commonly known as alkaline hydrolysis (Soiomons, 1980). Other possible mechanisms for dschiorination during anaerobic treatment have been virtually

1269

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W.J. PARKERet al.

ignored. An example of such a mechanism for dechlorination is nucleophilic substitution as follows: N u - + R - - X ---, R - - N u

+ X-

where N u - -- nucleophile R - - X = halogenated organic R - - N u = nucleophile substituted organic

X - = inorganic halide. Typical nucleophilesfound in wastewaters include OH-, HS- and S 2- with the nature of the nucleophile having a significanteffecton the potential rate and extent of substitution.The type of chlorinated organic compound isalso known to influencethe degree of nuclcophilic substitution.Aliphatic compounds, with the exception of vinylicgroups, have been found to be reactiveto nucleophilicsubstitution(Solomons, 1980). Halogenated aryl structures have however, been found to be non-reactiveto nucleophilicsubstitution unless a strong electron withdrawing group is positioned ortho or para to the halogen. A nucleophilc which is likely to be present in anaerobic reactors treating kraft bleaching effluents is the bisulfide(HS-) ion. This ion is formed as a product of the biologicalreduction of residualsulfate present in the wastewatcr from the kraft pulping process. Schwarzenbach et al. 0985) have indicated that nucleophilic substitution for the halides on halogenated aliphatics, by biologically generated bisulfide,occurred in groundwater below a leaking waste retention facility.This has been confirmed by Barbash and Reinhard (1989) in laboratory studies. Milosevich and Hill (1991) have demonstrated the disappearance of A O X upon the addition of sulfide to segregated kraft mill bleach plant effluents at various p H levels. In an anaerobic system treating kraft mill bleach plant wastewaters, it is possible that nucleophilic substitutionof sulfidemight improve dechlorination by substitution for chloride on compounds which would not otherwise be degradable biologically.In the following study, the contribution of biologically generated sulfideto the dechlorination of kraft mill bleach plant chlorinated organic matter is examined. By employing a source of chlorinated organic matter which was representativeof kraft bleaching effluent but with a reduced concentration of chloride,the fate of chlorinated organic matter is assessed through a mass balance of chlorine around the anaerobic fermentation. MATERIAI~ AND METHODS Wastewater source

The wastewater studied originated from a kraft mill bleaching pine and spruce pulp in a sequence of CDEoDF-qJ). (C0, chlorine with chlorine dioxide substitution; E, sodium hydroxide; D, chlorine dioxide; P, hydrogen peroxide) The chlorine multiple employed resulted in an elemental chlorine charge of 4--6% in the Cn stage with a CIO2 substitution of

17-25%. The chlorination stage effluent stream was sampled by mill personnel and shipped jn 225 I. plastic barrels to Environment Canada's Wastewater Technology Centre in Burlington, Ontario where all laboratory research was performed. The samples were stored in the barrels for periods of up to 2 months at temperatures ranging from 4 to 10°C. Approximately 15% of the AOX disappeared during this period. Preparation o f chlorinated organic matter

A procedure for preparation of chlorinated organic matter with low concentrations of inorganic anions was adapted from the total organic chlorine (TOCI) technique (Sjostrom et al., 1982). This procedure involved adsorption of chlorinated organic matter onto a synthetic adsorptive resin, rinsing of the resin and subsequent desorption with a solvent. The chlorinated organic matter was prepared by sequentially contacting a 4.0 !. batch of chlorination stage effluent, at a pH of 2.0, with 1000 g each of Amberlite XAD-4 and XAD-7 synthetic adsorption resins. The resins were prepared by sequentially washing 1000 g of resin with 4.0 i. of distilled water, 1000 ml of methanol and 4.01. of distilled water respectively. The wastewater was first contacted with the XAD-4 resin in a stirred flask for a period of 12 h. The adsorbent was then separated from the liquid by filtration and the filtrate was contacted with XAD-7 resin in a similar fashion. The contacted resins were washed with distilled water to remove chloride and sulfate and a volume of 1000 ml of methanol was added. Elution of the organics was allowed to occur for 12 h, after which the methanol was separated from the resin by filtration. This operation yielded 2.01. of eluent from a volume of 4.01. of the original chlorination stage effluent. The volume of eluent was reduced by evaporation under vacuum at 60°C to a volume of 150 ml of concentrate. This concentrate was then dissolved in distilled water to the original volume of 4.0 !. to form an isolate of organochiorine with a background chloride concentration of less than 25 mg/l. Approximately 500 mg/l of residual methanol, after evaporation, was present in the AOX solution. The preparation procedure is summarized in Fig. 1. An average recovery of 51% of the AOX was obtained with this process. Biological serum bottle studies

Glass serum bottles with a total volume of 2.0 !. were filled with 500, 50 and 5 ml of AOX solution, biomass and nutrient solution, respectively. Sulfate amended bottles received 180 mg of solid K2SO4. Each case described was prepared in duplicate. The studies were performed using the reconstituted AOX solution previously described, that had been neutralized to a pH of 6.8 with solid NaHCO 3. The biomass concentration in the serum bottles, as indicated by volatile suspended solids, was approx. 2000 mg/l. The stock nutrient solution for the serum bottle study was selected to be low in chloride and sulfate and consisted of 10,000 mg/I of methanol, 5500mg/i of (NH4)3PO( and 100 mg/l of yeast extract prepared in distilled water. The serum bottles were inoculated under anoxic conditions which were maintained by bubbling a gas mixture consisting of 70% N, and 30% CO, through all liquids and through the serum bottles prior to inoculation, to purge the contents of oxygen. Immediately after the addition of all components, the bottles were capped with butyl rubber septa and placed in a 35°C room Where agitation was provided by stir bars on a magnetic stir table. A sample volume of 25 ml was removed with a modified syringe capable of passing solids with a diameter of 2.5 ram, from a port located near the bottom of the bottles at 0, 4, 8 h, l, 4, 7, 14 and 21 days after inoculation. It was necessary that the inoculum be low in sulfate and chloride to eliminate background sources of anions. Therefore, prior to use, granular biomass obtained from a dormant sludge blanket reactor was placed in an acrylic column

Dechlorination of bleached kraft mill effluent

1271

4000 mL C-ST/~E EFFLUENT

19oog ..~-~

;

lOOOg ADSORPTION~

ADSORPTION

--~

!1 I 1000 mL METi~OL .j

-I

spE.

1000 mL F" METHANOL

I

I

1

1

8PENT

x~

t I

ROTARY EVAPORAllONi

t

DISTILLED WATER

Fig. 1. AOX isolation procedure. reactor with a 5.0cra diameter and a height of 31 cm. Inorganic ions were washed from the biomass by feeding the reactor a solution containing 1% v/v of the previously described nutrient stock, prepared in distilled water, for a period of 7 days. A hydraulic retention time (HRT) of 24 h was maintained during this period, thereby allowing seven bed volumes of liquid to pass through the column. Substantial methane was detected in the off-gas of the reactor indicating that the biomass was actively methanogenic.

Analytical methods Filterable AOX was assayed by the adsorbable organic halogen (AOX) technique (AWWA, 1989) using a Dohrman DX-20 AOX analyzer. Many of the samples analyzed for organic chlorine content contained biological solids which were separated from the liquid phase by filtration though 0.45 pm cellulose acetate filters. Inorganic halides were then washed from the solids with a 0.5 M KNO3 solution. The washed solids were analyzed for chlorine by neutron activation in a Slowpoke reactor, with a neutron flux of 5 x 1012 neutrous/cm 2s, at the Saskatchewan Research Council. Total halide was measured using the Dohrman DX-20 analyzer. A volume of 0.1 ml of sample was injected into a liquid boat which was then placed directly into the analyzer furnace. The resulting total chlorine consisted of the original inorganic chloride present in the sample, as well as the chloride produced from combustion of the wastewater chlorinated or~*nic matter. Samples for chloride and sulfate analysis were pre-filtered through glan fiber filters and stored at 4°C. The analysis was performed by ion chromatography with a Waters HPLC equipped with a Waters model IC-PAKA anion exchange column and a Waters model 430 conductivity detector.

RESULTSANDDISCUSSION The abiotic removal of AOX with sulfide addition suggests that sulfide, generated from the anaerobic reduction of sulfate, might transform chlorinated organic compounds through nucleophilic substitution. However, the impact of sulfur compounds on the degradation of A O X during anaerobic biological treatment of bleach plant efliuent cannot be directly determined since sulfur compounds are present in this stream as a residual of the kraft pulping process. Therefore, it was necessary to isolate chlorinated organic matter with low associated concentrations of inorganic anions. The behavior of this material was studied in batch anaerobic biological reactors for a control case without sulfur compounds and for a test case with sulfate supplementation to a concentration of 150 mg/l in the form of K2SO4. Sulfate concentrations were non-detectable in the control assay. The isolated AOX also contained a low background chloride concentration relative to the organochlorine concentration ( < 2 5 mg/l). This allowed detection of changes in inorganic chloride concentration which would result from dechlorination of the chlorinated organic matter, thereby permitring a mass balance on chlorine to be calculated for the system. The individual and average concentrations of AOX, sulfate and chloride over time are presented for

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W.J. P A t m et al. 80

Table i.

Summaryof measurements in batch anaerobic studies Sulfate-free

Parameter

60

AOX (rag/I) Chloride (rag/I) AOX + chloride Sulfate (rag/I)

4O

2~

°'oo

8

I

Sulfate-amcaded

Initial

Final

Initial

Final

32.9 20.5 53.4 ND

16.8 58.5 75.3 ND

34.3 16.0 50.3 142.5

15.6 53.5 69. I ND

ND, not detected. NA, not ~amured.

formed at a 5% level of significance revealed that the initial and final concentrations of AOX in the sulfateFig. 2. Chlorine concentrations in anaerobic batch tests free and sulfate amended experiments were identical. As is apparent from Fig. 3, the sulfate added to the without sulfate supplementation. serum bottles had been completely reduced by day 7 of the experiment. Although sulfide was not the sulfate-free and sulfate-amended cases in Figs 2 measured due to analytical difficulties, stoichiometric and 3, respectively. The average initial and final reduction of the measured initial concentration of concentrations of AOX, chloride and sulfate are sulfate (Table 1) in the serum bottles would have summarized in Table 1. In both cases, it is apparent resulted in the production of 49mg/I of sulfide. that degradation had ceased after approx. 7 days of Milosevich and Hill (1991) reported a 25% reduction incubation. In the sulfate-free tests, the AOX concenin AOX with the direct addition of 50 mg/l of sodium tratious decreased from 32.9 to 16.8mg/l, while in sulfide. As there was no substantial increase in the the sulfate-amended tests a decrease from 34.3 to removal of AOX with sulfide present it would appear 15.6mg/! was observed over the time of the study. that the compounds which are subject to abiotic The extent of AOX removal observed in the serum dechlorination are also dechlorinated biologically. bottles in this study was similar to that observed in Any transformation of biologically recalcitrant comother studies where mixed bleach plant wastewaters pounds by nucleophilic substitution would have prewere treated in continuous flow anaerobic treatment sumably resulted in a greater overall removal of AOX reactors (Parker et al., 1993). This would suggest that over that when sulfide was absent. It was difficult the composition of the chlorinated organic matter to positively identify the mechanism(s) responsible isolated by the resin adsorption and methanol elution for the dechlorination of the chlorinated organic method, was similar in nature to that of the whole matter. However, the production of chloride under wastewater. anaerobic conditions would suggest that reductive Although not quantified, there was substantial dechlorination is the primary mechanism responsible methane production during the fermentation. This for dechlorination. indicated that the oxidation-reduction potential of As demonstrated in Table 1, a mass balance on the reactor contents was well below that required chlorine based upon the sum of chloride and AOX for sulfate reduction. The pH of the serum bottle revealed that more chlorine was measured at the end contents remained at neutral during the experiments. of the batch biological tests than was apparently The removal of AOX due to alkaline hydrolysis present at inoculation. The chloride present at inocuduring the anaerobic fermentation was therefore lation of the serum bottles was a residual of that likely insignificant. present in the original effiuent. In the sulfate-free Biologically produced sulfide did not increase the tests, on average, 53.4mg/i of chloride and AOX removal of AOX in the experiment, t-Tests perwas present at inoculation while upon termination, 75.3 mg/! was measured. Similarly, in the sulfate100 0 t l l f e t e I:mm. 1 5 0 amended tests 50.3 and 69.1 mg/l of AOX and chlor• Olmx~leI=eM. [ O t~0(OHa. 125 ide were measured at inoculation and termination, respectively. It was first hypothesized that the additional chloride resulted from release of intracellular chloride o • •• ~ 75 from the inoculum. To investigate this possibility further, 10 ml of the original biomass was suspended in 100 ml of distilled water for a period of 14 days at 20 35°C. Samples of the water were analyzed for chloride at the beginning and end of the experiment and ' ; '15 " ~° 5 10 20found to have chloride concentratiom of less than the Time (d) analytical detection limit of 10 mg/l. This test indiFig. 3. Chlorine and sulfate concentrations in sulfate- cated that release of chlorine from the inoculum was amended anaerobic batch teats. not responsible for the excess chloride measured. A

°o

~

,'o

,'5

Time (d)

2'o

~

Dechlorination of bleached kraft mill effluent

1273

mass balance based upon 1.5 g of biomass as VSS, the Acknowledgements--This study was funded by Environreactor liquid volume of 555 ml and an excess 20 mg/I ment Canada, the Natural Sciences and Engineering Reof chloride produced, revealed that a chloride concen- search Council of Canada, the Federal Panel on Energy R&D (PERD) and by the MISA Program of the Ontario tration in the biomass of 7.3 mg chloride/g VSS Ministry of the Environment. would have been required. Tests revealed a biomass chloride concentration of less than 1 mg chloride/g REFERENCES biomass. These results therefore confirm the observed negligible chloride production in the washout AWWA (1989) Standard Methods for the Examination of Water and Wastewater. American Water Works Associexperiment. ation, Washington, D. C. An alternative hypothesis was that the additional Barbash J. E. and Reinhard M. (1989) Abiotic dehalogenachloride resulted from degradation of chlorinated tion of 1,2-dichloroethane and 1,2-