The Effect of Chlorination Effluent Recycle on Toxicity and Organically Bound Chlorine in Softwood Kraft Pulp Bleaching

G.D. FARR, K.L. PINDER and L.R. GALLOWAY

Factorial experiments were con- ducted to investigate the effect of chlorina- tion filtrate recycle on toxicity and organically bound chlorine in the first two stages of softwood kraftpulp bleaching. The effects of the mode of chemical addition, the substitution of chlorine dioxide, and the kappa factor were also studied. A continuous laboratory-scale (6.0 o.d.g/min) chlorina- tion apparatus was designed to rapidly pro- duce suficient efluent for trout bioassays. An increase in recycle equivalent to 18 m3/adt caused the first-stage toxic emis- sion factor to decrease by 314 TU.m3/adt. Thus, toxicity data from chlorination eflu- ents that are produced in the laboratory without recycle are not representative of the industrial process. A regression equation was formulated for the organically bound chlorine (AOX) results, which did not de- pend on recycle.

INTRODUCTION Much of what is known about pulp

bleaching has come from laboratory experi- ments. Unlike the industrial process, nearly all laboratory bleaching is performed on a batch basis. The simplest technique involves performing the reactions in plastic bags. In

G.D. Farr, K.L. Pinder ’P and L.R. Galloway* ‘s Dept. Chem. Eng’g. Univ. British Columbia 221 6 Main Mall Vancouver, BC V6T 124

Now with: H.A. Simons Ltd. 425 Carrall St. Vancouver, BC V6B 2J6

contrast, the batch apparatus of Reeve and Earl [l] features an adjustable reaction chamber with high-shear mixing, gaseous- or liquid-chemical addition, and computer- ized process monitoring. The only continu- ous laboratory-scale bleach plant reported is that of Jamieson [2]. This apparatus received limited attention because it utilized reactors (stirred tank, short residence time, low con- sistency) that were vastly different from those used in industry.

It has become common industrial practice to recycle bleaching filtrates coun- tercurrently for brownstock dilution and pulp washing. Sepal1 [3], Histed and Nicolle [4], and Wartiovaara [5] all built lab-scale bleach plants designed to simulate these operations. This was accomplished by recy- cling filtrates from repetitive batch bleach- ing runs.

There are several difficulties with the use of batch recycling. A considerable effort is required to produce effluents at steady state because of the number of batch bleach- ing runs needed. The length of time taken to yield such effluents casts some doubt on their relevance. It also tends to limit experi- mentation. Another disadvantage of the technique, when applied at the lab-scale level, is the inability to produce enough ef- fluent for troct bioassays.

Despite the widespread use of filtrate recycle in industry, there is a distinct lack of information regarding its effect on the char- acteristics of bleach plant effluent. Thus, the primary goal of this research was to deter- mine the effect of chlorination filtrate recy- cle for brownstock dilution on the formation of toxicity and organically bound chlorine in the first two stages of softwood kraft pulp bleaching. The variables of mode, substitu- tion, and kappa factor were studied in com-

bination with recycle to determine the ef- fects of interaction.

EXPERIMENTAL Apparatus

In order to overcome the problems associated with recycling effluent on a batch basis, a continuous lab-scale chlorination apparatus was fabricated. The apparatus, shown schematically in Fig. 1, was designed to allow the recycle of chlorination filtrate on a continuous basis, thereby rapidly yield- ing effluent at steady state. It was also de- signed to rapidly produce sufficient effluent to allow the use of trout bioassays. Detailed specifications of the equipment are given elsewhere 161, but a brief description is given here.

Brownstock pulp, diluted with tap water to a consistency between 0.35 and 0.40%, is pumped from the stock tank to the constant-head tank. The stock is then pumped from the head tank to the brown- stock decker headbox at a rate of 6.0 0.d.g /min (0.01 adt/d). Excess stock in the head tank overflows back into the stock tank. The brownstock decker (horizontal-belt design) then thickens the stock to approximately 10% consistency with the aid of two press rolls and suction from the blower. The pulp is discharged as a coherent mat into the top of the downflow section of the bleaching reactor (U-tube design). Dilution filtrate, separated from the brownstock by the decker, is collected in the decker seal tank and discharged to the sewer.

Upon entering the reactor, the un- bleached-pulp mat is disintegrated by a jet of heated dilution filtrate composed of a mixture of distilled water and recycled chlorination filtrate. The pulp is then broken up further by the upper impellers of the first

J144 JOURNAL OF PULP AND PAPER SCIENCE: VOL. 21 NO. 4 APRIL 1995

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24. CAVE, I.D., “The Anisotropic Elasticity of the Plant Cell Wall”, Wood Sci. Tech. 2(4):268-278 (1969).

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27. SCHULGASSER, K. and PAGE, D.H., “In- fluence of Transverse Fiber Properties on the In-Plane Elastic Behaviour of Paper”, Com- posites Sci. Tech. 32(4):279-292 (1988).

28. BAUM, G.A., “Subfracture Mechanical Properties”, Tenth Fund. Res. Symp., Oxford, UK, 1-126 (Sept. 1993).

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30. JENTZEN, C.A., “The Effect of Stress Ap- plied During Drying on Some of the Proper- ties of Individual Pulp Fibers”, Tappi 47(7): 412418 (1964).

31. SETH, R.S. and PAGE, D.H., “The Stress- Strain Curve for Paper”, 7th Fund. Res. Symp., Cambridge, 421442 (1981).

32. VAN DEN AKKER, J.A., “Some Theoretical Considerations on the Mechanical Properties of Fibrous Structures”, Bolam, F., Ed., For- mation and Structure of Paper, Vol I., Trans. Symp. Tech. Sect. Br. Paper and Board Mak- ers’ Assoc., London 205-241 (Sept. 1961).

33. VAN DEN AKKER, J.A., LATHROP, A.L., VOELKER, M.H. and DEARTH, L.R., “Im- portance of Fiber Strength to Sheet Strength”, Tappi 41(8):416-425 (1958).

34. HELLE, T., “Some Aspects on Fibre Strength and Fibre Bondings in Sulphate and Sulfite Paper”, Svensk Papperstidn. 66(24): 1015- 1030 (1963).

35. PAGE, D.H., TYDEMAN, P.A. and HUNT, M., “A Study of Fibre-To-Fibre Bonding by Direct Observation”, Bolam, F., Ed., Forma- tion and Structure of Paper, Tech. Sect. Br. Paper Board Makers’ Assoc., 171-193 (1962).

36. PAGE, D.H., TYDEMAN, P.A. and HUNT, M., “The Behaviour of Fibre-To-Fibre Bonds in Sheets Under Dynamic Conditions”, Bo- lam, Ed., Formation and Structure of Paper, Tech. Sect. Br. Paper Board Makers’ Assoc., London, 249-276 (1962).

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KOLSETH, P. and DE RUVO, A., “The Mea- surement of Viscoelastic Behaviour for the Characterization of Time-, Temperature- and Humidity-Dependent Properties”, Mark, R.E. and Murakami, K., Eds., Handbook of Physical andMechanical TestingofPaperand Paperboard, Marcel Dekker Inc., 1:255-322 (1983). SALMEN, L., KOLSETH, P. and RIG- DAHL, M., “Modeling of Small-Strain Prop- erties and Environmental Effects of Paper and Cellulosic Fibers” Mat. Sci. Mono. 36 Salmh, L., de Ruvo, A., Seferis, J.C. and Stark, E.B., Eds., Composite Systems from Natural and Synthetic Polymers, 21 1-223 (1986). BAUM, G.A., BRENNAN, D.C. and HABE- GER, C.C., “Orthotropic Elastic Constants of Paper”, Tappi 64(8):97-101 (1981). BAUM, G.A., “The Elastic Properties of Pa- per: A Review”, Swedish Pulp Paper Res. Inst., STFI-meddelande A 969, Design Crite- ria for Paper Performance, 1-27 (1987). BAUM, G.A., HABEGER, G.G. and FLEISCHMAN, E.H., “Measurement of the Orthotropic Elastic Constants of Paper”, 7th Fund. Res. Symp., Cambridge, 453478 (1981). NISKANEN, K.J. and ALAVA, M.J., “Planar Random Networks with Flexible Fibers”, Finnish Pulp Paper Res. Inst., Paper Sci. Cen- tre, PSC Communications 69 (1994).

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TABLE I FACTOR LEVELS -____.

Coded Level Factor - 0 + Recycle (m3/adt) 4.5 13.5 22.5 Mode (s) 40 0 40 Substitution (“10) 20 50 80

--

Kappa factor 0.18 0.20 0.22

chlorination mixer before it reaches the top of the pulp slurry at approximately 2.2% consistency. Solutions of chlorine and chlorine dioxide are then added through injection

TABLE 111 VARIATION IN FACTOR LEVELS

Factor 95% Confidence Interval Recycle f l .3 m3/adt Mode e . 7 s Substitution 10.5% Kappa factor 80.003 Excludes data from run 0.

ports on the reactor. These ports enable either simultaneous addi- tion or sequential addition with a delay of 40 or 60 s. The bleaching chemicals are mixed with the pulp by the impellers located near the ports. The pulp, now at 1.9% con- sistency, then flows through the horizontal and upflow sections of the reactor aided by the second and third chlorination mixers, respectively.

The stock spends most of its 24 min residence time in the upflow section where the inside diameter of the reactor increases from 5 to 10 cm. At the top of the upflow section, the pulp is diluted to approximately 0.5% consistency with chlorination filtrate from the chlorination washer seal tank. The stock then overflows into the dilution vat, where larger flocs are broken up by a mixer.

The stock is separated from the chlorination filtrate on the chlorination washer (horizontal-belt design) and washed with distilled water that is applied from the washer headbox. The washed pulp is then discharged as a coherent mat at roughly 12%

consistency. The chlorina-

tion filtrate is col- lected in the chlorination washer seal tank and recy- cled to both the inlet and the outlet of the reactor. Excess filtrate overflows from the seal tank.

Variable-speed peristaltic pumps are used to meter the various process streams. Instrumentation consists of three thermistors and a pH electrode.

Experimental Design A full 24 factorial design with three

centrepoints was chosen for the experimen- tation. The four factors studied were: 1 . Recycle (R). The rate of recycle of chlori-

nation filtrate from the chlorination seal tank to dilute the brownstock pulp was based on unbleached pulp-throughput.

2. Mode (M). The mode of chemical addi- tion in the first stage was represented by

A. STOCK TANK C. DECKER E. BLOWER 0 DILUTION VAT B. HEAD TAM: D. SEAL TANK F. REACTOR H. WASHW I. HEAT EXCHANGER

Fig. 1. Flowchart of the continuous laboratory-scale pulp bleaching apparatus.

JOURNAL OF PULP AND PAPER SCIENCE: VOL. 21 NO. 4 APRIL 1995

the time delay between the addition of chlorine and chlorine dioxide. The se- quential addition of chlorine dioxide be- fore chlorine, also referred to as DC mode, was designated with negative val- ues. Conversely, the addition of chlorine before chlorine dioxide, also known as CD mode, was defined with positive val- ues. Simultaneous addition involved no delay.

3. Substitution (S). The level of substitution of chlorine dioxide for chlorine was de- fined as the percentage of total active chlorine added in the first stage that was supplied by chlorine dioxide.

4. Kappa factor (ZQ. The kappa factor rep- resented the charge of active chlorine used in the chlorination stage expressed as the percentage on oven-dry un- bleached pulp and divided by the kappa number of the unbleached pulp.

Factor levels, which are given in Table I, were selected to represent those achievable in industry. A total of 24 runs, numbered 0-23, were performed in random order according to run number. Runs 0- 4 were used to solve experimental problems. Runs 5-23 comprised the 24 factorial design with three centrepoints. Coded factor levels for each run are given in Table 11. Replicate runs included the centrepoint (runs 7,12, and 18) and four factorial points (runs 1 and 20, 2 and 22,3 and 2 1, and 4 and 23). The results of a postrun analysis of the deviations in the levels of the four factors are shown in Table 111.

Three parameters associated with the environmental impact of bleaching effluent were chosen as the primary response vari- ables. The first two, toxicity and organically bound chlorine, were measured in both the chlorination and extraction stages and are examined in this paper. Chlorate, the third

J145

parameter, was measured in the first stage and is discussed elsewhere [6]. Both kappa number and viscosity of the extracted pulp were selected as secondary response vari- ables. These pulp quality characteristics are discussed with the chlorate results. In addi- tion, a comparison was made between the levels of the response variables produced from the laboratory-scale experiments and those produced from an industrial-scale bleach plant.

Materials An industrially produced softwood

kraft pulp (kappa number 29.2, viscosity 34.7 mPa-s) was used for the experimental bleaching runs. Roughly 600 kg of brown- stock pulp at 12.5% consistency was sam- pled from the brownstock decker of the Celgar Pulp Company pulp mill at Castlegar, BC. Celgar Pulp, which began operations in 196 1, produced fully bleached pulp using standard kraft pulping practices and a five- stage (DC) (EO) DED bleach plant. The mill has since been modernized and expanded. The chip furnish was 35% hemlock, 30% fir and larch, 15% cedar, 10% spruce and bal- sam fir, and 10% lodgepole pine. Over the course of the experimentation, the supply of pulp was stored at 4°C without degradation.

To carry out the industrial compari- son, samples of effluent from the first two stages of bleaching were also taken from Celgar Pulp. These samples, collected under stable operating conditions, were shipped refrigerated and were received the day after sampling. Sample treatment was identical to that used on the experimental effluents.

During effluent sampling, brown- stock (kappa number 31.0, viscosity 37.1 mPa.s) entered the chlorination stage at a rate of 650 adt/d with 12 kg/adt as black liquor carryover (Na2S0,). Chlorination was performed for 40 min at 43°C and 3.1% consistency with a kappa factor of 0.21. Chlorine dioxide was added 30 s before the chlorine at a level of 30% substitution. The residual chemical amounted to 2.3% of the active chlorine added. Chlorination filtrate was recycled for brownstock dilution at a rate of 7.2 m3/adt. The flow of chlorination effluent at a pH of 1.5 was 26 m3/adt, which accounted for 4 1 % of the total bleach plant effluent flow.

Oxidative extraction was performed for 48 min at 74°C and 11 % consistency. The chemical application was 2.8% NaOH and 0.5% O2 on an oven-dry pulp basis. The flow of extraction effluent at a pH of 10.5 was 25 m3/adt, which accounted for 39% of the total effluent flow from the five- stage sequence.

Laboratory Experiments Chlorination

Before each run, the peristaltic pumps were fitted with new tubing and calibrated. The bleaching solutions were made by ab- sorbing the gases in distilled water. Chlorine

dioxide was prepared by passing gaseous chlorine through a bed of sodium chlorite. Both solutions were analyzed iodometri- cally.

Pulp was run through the reactor for 45 min before the addition of bleaching chemicals. After 90 min of chemical addi- tion, a steady state was assumed to have been reached. A dynamic model of the laboratory process [6] indicated that this period would be sufficient. The model was corroborated by measuring the step change response of the process.

The effluent was then collected in 26 Lpolyethylene carboys. With wash water applied at a rate of 9.0 m3/adt and recycle rates of 4.513.5, and 22.5 m31adt, theefflu- ent flows were 286,226, and 166 " i n , respectively. Chemical residuals were io- dometrically determined in the collected ef- fluents before being reduced with sodium thiosulphate. The mean residual for all 24 runs was 1.2% of the active chlorine added (SD = 1.8%).

Temperature and pH data were re- corded at 15 min intervals throughout each run. The mean temperature in the horizontal section of the reactor was 33.2"C (SD = 023°C) for the 24 runs. The mean pH in the chlorination seal tank was 1.9 (SD = 0.2).

Extraction The second stage of bleaching was

carried out on a batch basis on representative samples of pulp from the first stage. Extrac- tions were performed at 70°C and 10% con- sistency for 90 min using a chemical charge of 3% NaOH on an oven-dry pulp basis. Before extraction, the chlorinated pulp was stored in heat-sealed polyethylene bags at 4°C after being sampled from the chlorina- tion washer without further washing or re- sidual chemical reduction. The mean pH of the E stage effluents was 11.0 (SD = 0.2). The extraction effluents were adjusted to pH 2.0 with concentrated HNO, before taking samples for analysis. Pulp shrinkage after the chlorination stage was not considered in the calculation of extraction stage toxicity and organically bound chlorine results.

Analyses Toxicity

Separate toxicity tests were com- menced on both the chlorination and extraction effluents within 24 h of their pro- duction. Before testing, the chlorination fil- trates were stored at 4°C in sealed 26 L polyethylene carboys after residual chemi- cal reduction. The extraction filtrates were stored at 4°C in sealed 500 mL amber-glass bottles after the pH was adjusted to 2.0. These conditions have been shown to pre- serve the toxicity of kraft pulp bleaching effluent [7]. All effluents were tested at a pH of 7.0. Caution was exercized in neutralizing the chlorination effluents to avoid highly alkaline conditions capable of converting part of the organically bound chlorine to

chloride. The toxicity of the chlorination efflu-

ents was measured towards rainbow trout (Oncorhynchus mykiss) according to stand- ard methodology [8]. The mean fish length was 4.7 cm and the mean weight was 1.2 g. Static bioassays with aeration were used at a mean temperature of 13.4"C (SD = 1.4"C). Dilutions and controls were made with dechlorinated tap water from the city of North Vancouver, BC. Effluents from runs 0-4 were tested to determine the 96 h me- dian lethal concentration (LC50) using a test volume of 30 L. LC,, values were deter- mined by plotting mortality versus log con- centration and interpolating to 50% mortality. Effluents from runs 5-23 and Cel- gar Pulp were tested at 100% concentration to determine the median lethal time (LT50) using a test volume of 25 L. LTs0s were determined by either of the related methods of Litchfield [9] and Bliss [lo]. LC50 values (96 h) were then calculated via the average slope of the relationship between concentra- tion and median lethal time that was deter- mined for runs 18,19, and 23 [6]. Thus, the effluents from these runs were also tested at 62% and 38% concentration. The small quantities of pulp that were extracted pre- cluded the use of trout tests on extraction effluents.

The toxicities of both the chlorination and extraction effluents towards lumines- cent marine bacteria (Photobacterium phos- phoreum) were measured at 15°C using a Microtox Model 2055 analyzer. A modified form [11] of the standard method [12] in- cluding colour correction was used. Version 5.0 of the Microtox Calculations Program (Microbics Corporation, Carlsbad, CA) was used to calculate values for the 15 min EC50 (the median effective concentration causing a decrease in luminescence).

Lastly, all toxicity measurements were converted to toxic emission factors ac- cording to

TEF = (100%/LC50) x Q

where LC50 is the 96 h median lethal con- centration ( ~ 0 1 % ) and Q is the effluent flow rated on unbleached pulp throughput (m3/adt). The term 100%/LC50 is known as the number of toxic units (TU). For the tox- icities to bacteria, the LC50 in Eq. (1) was replaced with the 15 min median effective concentration (vol %). The TEF is the total dilution needed to make an effluent non- toxic. It enables the toxicity of effluents cre- ated at different rates of water consumption and pulp production to be compared.

Some sensitivity was undoubtedly lost in the analysis of toxicity to trout by using an average slope that described the relationship between concentration and me- dial lethal time. However, for the mean LT50, the TEFs that were calculated using the larg- est and smallest magnitude slopes were within the 95% confidence limits of the TEF that was calculated using the average slope.

5146 JOURNAL OF PULP AND PAPER SCIENCE: VOL. 21 NO. 4 APRIL 1995

Organically Bound Chlorine The chlorination and extraction efflu-

ents were analyzed separately for total or- ganically bound chlorine as AOX. Analysis was performed according to the microco- lumn technique of APHA Method 506 [ 131 using either a Dohrmann or a Mitsubishi analyzer. C stage samples were taken after the reduction of residual bleaching chemical but before pH adjustment for bioassays. E stage samples were taken after the pH was adjusted to 2.0.

Black Liquor Carryover Sodium, a common indicator of black

liquor carryover, was determined for each run. Samples of brownstock decker filtrate were thus analyzed via atomic absorption spectrophotometry. The mean carryover ex- pressed as Na2S04 was 0.16 kg/adt. Due to the method in which the unbleached stock is handled by the lab apparatus, the amount of carryover was very low compared to indus- trial levels.

The extractives content of the pulp entering the chlorination stage, another mea- sure of black liquor carryover, was deter- mined using acetone extraction. The mean level of extractives in the three pulps that were tested was 1.5 kg/adt. In addition, the extracts were derivatized with diazomethane and quantified individually using gas chro- matography. This analysis in conjunction with the corresponding 96 h LC,, values for juvenile salmonids enabled a theoretical TEF to be calculated. Consequently, the maximum combined TEF due to the extrac- tives (eight resin acids and two fatty acids) in the black liquor carryover was found to be 22 TU.m3/adt. Thus, black liquor carryover had a negligible effect on the toxicities of the experimental effluents.

RESULTS AND DISCUSSION Apparatus

Because of scale-down effects, there are several important differences between industrial chlorination and chlorination as it is carried out with the continuous lab-scale apparatus: 1.

2.

3.

4.

5.

In industry, brownstock is stored at 12% consistency and partially diluted before being pumped to the tower. In the lab it is stored and pumped at 0.4% consis- tency and thickened to 10% just before the reactor. Pulp moves through industrial towers due to the head produced by centrifugal pumps located at the inlet. The lab appa- ratus uses a series of impellers located throughout the reactor. In industry, chlorine is added as a disper- sion of gas bubbles in either water or recycled filtrate. This equipment uses a solution of gas in distilled water. Chemical mixing is done industrially with both static and high-shear mixers In the lab equipment, marine-type impellers are used. Industry chlorinates pulp at 3% consis-

tency, whereas the lab apparatus operates at 1.9%. Thus, at 3% con- sistency the lev- els of recycle that are equivalent to those used in the lab equipment (see Table I) are 2.8, 8.4, and 14.1 m3/adt, re- spectively.

6. The industrial process responds to brownstock changes through automatic feed- back control. In the lab, the con- trol strategy re- lies on a constant flow of brown- stock at a con- stant kappa number.

In addition, because the lab ap- paratus is operated as an isolated bleaching stage, the countercurrent recy- cle of filtrate from later stages is absent. In industry these recycle flows are primarily responsible for chlorination temperatures being over 40°C, since filtrate from later stages is typically near 70°C. This thermal effect is simulated in the lab equipment by passing the recycled chlorination filtrate and dilution water through a heat exchanger.

Toxicity Rainbow Trout

The chlorination stage TEFs for trout are listed in Table IV. The corresponding factorial effects, which were calculated using the TEFs from runs 5-23, are given in Table V. The pooled standard deviation from replicate runs was 120 TU.m3/adt (5 df). Using the 95% confidence interval, the only significant effect was that of recycle.

Increasing the recycle from 4.5 to 22.5 m3/adt caused the C stage TEF to de- crease on average by 314 TU-m3/adt. This result coincides well with the previous find- ing [ 141 that increasing the level of filtrate reuse reduces the formation of chloropheno- lic compounds, which have been implicated [ 15-19] as being partially responsible for the toxicity in softwood bleaching effluent. It is clear that recycling filtrate in the first stage enables toxicants to be degraded by the ac- tion of fresh bleaching chemical. Two stud- ies [14,20] have found that chlorophenolic compounds can be oxidized by active chlo- rine.

The only previous study on the effect of filtrate recycle on toxicity is that of Nikki and Korhonen [ 141. Direct countercurrent washing was found to give lower TEFs

towards Daphnia magna than jump-stage countercurrent washing. A comparison with open washing could not be made because the resulting effluents were too dilute to yield an acute toxic response. In addition, the effects of recycle for dilution and that for washing could not be differentiated as separate phe- nomena.

Voss et al. [21] found the toxicity (Daphniapulex) of both C and E stage efflu- ents increased as the pH in the first stage decreased, especially below a pH of 1.5. The decrease in pH associated with increasing recycle was not great enough for this effect

JOURNAL OF PULP AND PAPER SCIENCE: VOL. 21 NO. 4 APRIL 1995 5147

to have been noticed here. The relatively low consistency used in the chlorination appara- tus kept the pH slightly elevated.

There are several important ramifica- tions of recycle having a large effect on the toxicity of chlorination filtrate: 1. It is clear that toxicity data from chlori-

nation effluents that are produced in the laboratory without recycle are not repre- sentative of the bleaching process as it is typically carried out in industry.

2. The model used by Wong et al. [22] to predict the toxicity (rainbow trout) at steady-state conditions of effluent reuse for pulp dilution and washing is not ap- plicable in the case of chlorination efflu- ent recycle for brownstock dilution. Toxicity cannot be viewed as an inert substance that simply builds up to an equilibrium as filtrate is recycled.

3. Industrial bleach plants presently operat- ing with minimal levels of chlorination filtrate recycle might be able to reduce toxic discharges by increasing the amount of recycle used for both brown- stock dilution and chemical addition. More research needs to be conducted to see if the effect of recycle can be detected in combined mill effluent. A survey [23] of bleach plant operating practices in Canada during the early 1980s found that only 11 of 19 mills utilized chlorination filtrate for brownstock dilution.

Although kappa factor, substitution, and mode were not significant factors at the 95% level of confidence, their effects (see Table V) are in general agreement with the literature. Voss et al. [21] found that the toxicity (Daphnia Pulex) of chlorination ef- fluent showed a maximum as a function of the kappa factor. In addition, the majority of the literature [14,21,22,24] found that toxic- ity decreases as the level of chlorine dioxide substitution increases. Lastly, Liebergott et al. [24] found that CD mode produced less toxic effluents than DC mode to both Ceri- odaphnia afinis and fathead minnows.

Only two of the experimental filtrates had TEFs for trout larger than that of the chlorination effluent from Celgar Pulp (see Table IV). Undoubtedly, the difference in black liquor carryover between the mill (1 2 kgladt) and the experimental runs (0.16 kg/adt) was partly responsible for this. The limited use of chlorination filtrate recy- cle for brownstock dilution was another cause of the hi h toxicity in Celgar’s efflu- ent. The 7.2 m /adt level of recycle repre- sented only 33% of the total i-equirement io dilute the unbleachedpulp from 12% consis- tency in storage to 3.1% consistency in the chlorination tower.

9

Luminescent Bacteria The TEFs for bacteria are given in

Table IV for both bleaching stages. The chlorination stage factorial effects, which were calculated using the TEFs from runs 5-23, are listed in Table VI. Results from replicate runs had a pooled standard devia-

tion of 104 TU.m3/adt (6 df). None of the effects was found to be significant with the use of the 95% confidence interval. How- ever, the four largest effects all involved recycle.

The extraction filtrates were much less toxic than the corresponding chlorina- tion filtrates. This agrees with results found elsewhere [14-16,251. Although several of the extraction stage factorial effects were found to be statistically significant, the mag- nitudes of these effects were much smaller than those from the chlorination stage. For this reason, a combined effluent toxicity would likely follow the effects which ema- nated from the first stage. This ignores the detoxification that can occur when C and E stage effluents are mixed.

Celgar Pulp’s extraction stage TEF was very similar to those from the experi- mental runs. It was also much lower than that from the chlorination stage.

It is clear by comparing the C stage factorial effects for the two bioassay organ- isms (see Tables V and VI) that the responses were not alike, aside from the effect of recy- cle. Not surprisingly, a logarithmic plot of TEFs (trout versus bacteria) gave a correla- tion coefficient ( r ) of 0.30. In a millwide study, Firth and B a c k ” [26] found a stronger correlation between the same two test organisms. However, other studies [ 17,251 that involved only bleach plant toxi- cants failed to find relationships between the responses of all of the organisms that were investigated.

Organically Bound Chlorine The organochlorine data are listed in

Table VI1 as AOX production based on unbleached-pulp throughput. The total pro- duction is given as the arithmetical sum of productions from the C and E stages. The pooled standard deviation for total AOX production, which was deter- mined from replicate runs, was 0.20kg/adt (6 df).

and Larsson [27] were the first to pro- pose that the total amount of organi- cally bound chlorine produced from a bleaching sequence is a linear function of the consumption of total elemental chlo- rine (TEC). Earl and Reeve [28] bleached a softwood kraft pulp using a (DC)E sequence and de- scribed the resultant production of AOX

Germgard

(kg/t) with

AOX = k (C1, + 0.526 C10,) (2)

where C1, and C10, are the consumptions (kg/t) of chlorine and chlorine dioxide, re- spectively. The constant k was found to be 0.10 kg AOX/kg TEC. For the data from runs 5-23 of the present study, k was deter- mined to be 0.096 kg AOXikg TEC. This analysis, which gave a coefficient of deter- mination (R’) of 0.92, utilized the total elemental chlorine added since chemical re- siduals were low.

The same data were regressed to give a more extensive relationship for the AOX (kg/adt) produced from the first two bleach-

5148 JOURNAL OF PULP AND PAPER SCIENCE: VOL. 21 NO. 4 APRIL 1995

ing stages

AOX = 0.96 + 19.4 K - 0.199 KS

- 0.00595 M (3)

where K , S, and M are the previously defined experimental factors. The regression coeffi- cients were tested and found to be significant 0, < 0.001). The fit to Eq. (3) gave an R2 value of 0.97 and a residual standard de- viation of 0.21 kg/adt (14 df). Run 19 was deemed via residual analysis to be an outlier and was therefore excluded from the regres- sion. This trial had a combination of decreas- ing brownstock flow and increasing chemical flows that resulted in an AOX level that was much higher than expected.

Equation (3) has several important limitations. Firstly, the kappa factor used in the equation should be that which is con- sumed. Secondly, because the production of AOX depends on the lignin content of the unbleached pulp in addition to the kappa factor consumed, the factor should be pro- rated for pulps with kappa numbers that are different from that used here (29.2).

Figure 2 shows Eq. (3) as a plot of AOX production versus chlorine dioxide substitution at three combinations of mode and kappa factor. Confidence intervals (95%) are given as error bars for several regression estimates from Eq. (3). Equa- tion (2) is also shown at one of the combina- tions for a pulp with a kappa number equal to that of the brownstock used here. There is good agreement between the two equations.

The inclusion of mode in Eq. (3) makes its effect on AOX production easy to assess. Figure 2 shows that, with an interval of 30 s, CD mode produces 0.4 kg/adt less AOX than DC mode. This is in good agree- ment with the literature [24,29]. However,

unlike these pre- vious results, Eq. (3) describes a contin- uum of AOX with re- spect to the time delay between chemical additions. Thus, simultaneous addition was not found to be equiva- lent to CD addition as Liebergott et al. [24] found. Because the formation of AOX is closely linked to the degree of delignifica- tion and since simultaneous addition delig- nifies less than DC addition but more than CD addition, it is logical to believe that AOX production would follow a similar trend. Undoubtedly, there exists a time delay be- tween chemical additions beyond which an increase in the effect on AOX production is not seen. This has not yet been demarcated.

Factorial analysis of the data showed that the recycle of chlorination filtrate had no effect on the formation of organically bound chlorine. This result contradicts the litera- ture.

Histed and Nicolle [4] found that chlorination filtrate recycle for brownstock dilution substantially increased the amount of bound chlorine in the first stage. It was argued that recycle lowers the pH thereby increasing the proportion of chlorine that reacts via substitution as opposed to oxida- tion. This in turn would result in more bound chlorine. It should be noted [6] though that this indirect result came from inorganic chlo- ride measurements that were used in an in- complete mass balance.

In contrast, Mattinen and Wartiovaara [30] found that chlorination filtrate recycle for brownstock dilution markedly decreased the formation of organically bound chlorine in the first stage. Since, in each experiment,

M e_q - 2 ----- 0.22 -30 3 --- 0.22 -30 3 - 0.1 8 -30

- K 5 -

a 2 =

1 -

20 30 40 50 60 70 80

Chlorine Dioxide Substitution (%)

Fig. 2. The effects of substitution, mode, and kappa factor on the production of AOX from the first two stages of softwood kraft pulp (kappa number 29.2) bleaching. Error

1 month of batch bleachings was needed to reach a steady state and different applica- tions of bleaching chemicals were used, this result lacks some rigour. Because recycle decreases the formation of chlorophenolic compounds [14], it has been postulated that the production of organically bound chlorine would be similarly affected.

The AOX produced from the C and E stages of Celgar Pulp was measured at 5.8 kg/adt, whereas that predicted by Eq. (3) (S = 30%, M = -30 s, K = 0.22 prorated from 0.21 at a kappa number of 31.0) was 4.1 kg/adt. Possible explanations for the dis- crepancy of the mill result include the chlori- nation of black liquor carryover, the chlorination of extraction filtrate used to wash the chlorinated pulp, and the counter- current flow of AOX from the last three bleaching stages.

Finally, the relationship between tox- icity and organically bound chlorine was explored. Table VI11 gives the level of corre- lation between the concentration of AOX and the number of toxic units. Because the levels of AOX in the industrial effluents were considerably higher than those in the laboratory effluents, the results from Celgar Pulp would have carried a disproportionate amount of significance in the analysis and were consequently excluded. Not surpris- ingly, the relationships were all weak. Pre- vious attempts [24,26] to find a relationship between AOX and toxicity have also had limited success.

SUMMARY 1. A continuous laboratory-scale apparatus

for pulp chlorination was developed to study chlorination filtrate recycle for brownstock dilution and to rapidly pro- duce sufficient effluent at steady state for use in trout bioassays.

2. Of four factors studied, the recycle of chlorination filtrate had the largest impact on the toxicity of chlorination- stage effluent to rainbow trout. The first- stage TEF decreased 011 aveiage by 314 TU-m3/adt as the level of recycle increased from 4.5 to 22.5 m3/adt. Thus, toxicity data from chlorination effluents that are produced in the laboratory with- out recycle are not representative of the industrial process.

3. A regression equation, which includes the effects of substitution, mode, and kappa factor, was formulated to describe the AOX produced from the chlorination and extraction stages. The recycle of

JOURNAL OF PULP AND PAPER SCIENCE: VOL. 21 NO. 4 APRIL 1995 5149

chlorination filtrate had no effect on AOX.

4. The toxicity of chlorination effluent to luminescent marine bacteria did not cor- relate well with the toxicity to rainbow trout. The correlation between toxicity and AOX was also weak.

ACKNOWLEDGEMENTS This work was carried out with finan-

cial support from the Natural Sciences and Engineering Research Council of Canada, Paprican, and Microbics Corporation. The contributions of R.J. Kerekes (Paprican), G. van Aggelen (British Columbia Ministry of Environment), R. Watts (Environment Can- ada), and J. Browne and R.W. Zaitsoff (Cel- gar Pulp) are greatly appreciated.

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