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ECF and TCF: Effluent Toxicity
An Analysis of Recent Published Data

The Alliance for Environmental Technology

October 1994


Table of Contents




Summary

Recent publications and technical presentations claim:

    "...Biological tests show TCF EFFLUENTS LESS TOXIC.."1

    "Our tests show...the TCF methods at our mills produce the least toxic effluents..."1

    "the TCF effluents in all the tests have shown lower TEF values compared to the ECF effluent.... The only difference between ECFand TCF production is the bleaching process..."2

Critical analysis of the data and methodology used to make these claims show they are misleading and incorrect. The data used to make these claims are insufficient and the comparison methodology is flawed for the following reasons:

  • The observations were not consistent. Neither primary treated whole mill effluent produced using an ECF bleaching sequence nor primary treated whole mill effluent produced using a TCF bleaching sequence exerted a toxic response for three of the five biological tests used;

  • The comparison cannot account for confounding factors, such as the contribution of toxic substances from other sources within the mill, because the biological tests were performed on whole mill effluent;

  • Bleaching effluents are not compared. Rather whole mill effluents were tested, therefore biological responses due to changes in bleaching practice cannot be assessed with certainty;

  • Toxicity comparisons use Toxicity Emission Factor (TEF). This is not a valid methodology for comparing non-toxic effluents. It was designed to normalize toxic effluents only, and is meaningless when no toxicity is observed; and

  • Biological tests of other pulp mill bleaching effluents produced from both ECF and TCF processes have shown no significant difference in toxicity in other investigations.

Further levels of investigations of the environmental impact of pulp mills are best conducted by analysis of receiving ecosystems exposed to realistic concentrations of individual mill effluents. Until results of model ecosystem studies or analyses of the receiving ecosystem are peer reviewed, validated and published, conclusions regarding which mill has the least adverse impacts on the environment are premature. Furthermore, claims of reductions in toxicity of bleaching processes should be based on toxicity measurements of bleaching effluents only.

In the final analysis, the risk that a particular mill presents to the environment is dependent on the processes used in the mill as well as the performance of those processes (i.e., spill control, degree of recovery, type of effluent treatment, etc.)



Background

A recent publication1 used laboratory biological test results, summarized in Figure 1, to suggest that primary treated whole mill effluents from a mill operating with a "Totally Chlorine-Free" (TCF) bleaching sequence may be relatively less toxic than effluent from the same milloperating with an "Elemental Chlorine-Free" (ECF) bleaching sequence. (Data for another mill operating with a "Chlorine" (Cl) process is also presented.) The data compares the relative toxicity of primary treated whole mill effluent when operating with the ECF bleaching process to effluent from the same mill operating with the TCF bleaching process.

The TCF pulping and bleaching process incorporates extended cooking, oxygen delignification, chelation (EDTA) treatment and hydrogen peroxide. The ECF pulping and bleaching sequence incorporates extended cooking, oxygen delignification, chlorine dioxide in the first stage of bleaching followed by oxygen reinforced alkaline extraction, and two chlorine dioxide brightening stages with an intermediate extraction stage. It is important to note that the analyses are of primary treated whole mill effluent. The mill does not have secondary biological treatment.

The relative toxicity compares Toxicity Emission Factor (TEF).

TEF = (100/V) x F

V = Effluent concentration, (% by volume) at "EC50," "LOEC," etc.
F = Effluent flow, (m3/ADt)

From this equation it follows that if the effluent is not toxic, i.e., EC50 > 100% by volume, then all that can be said about the TEF value is that it is less than the effluent flow rate, F.

TEF was designed to normalize the toxicity of effluents with respect to effluent flow rates. However, when V > 100%, the expression is meaningless. A non-toxic effluent is non-toxic regardless of flow rate.



Toxicity Emission Factor Comparison and Analysis

Another publication2, presents similar data and is reproduced in Figures 2, 3, 4, 5, 6. The figures compare the Toxicity Emission Factor for a variety of biological tests. These figures are designed to show that when operating with the TCF bleaching sequence, the TEF for the primary treated whole mill effluent is consistently lower than when the mill is operating with the ECF sequence.

Presenting the data in this manner creates the impression that the effluent from the mill when operating with the TCF bleaching process is "less toxic." However, for non-toxic effluents, all the charts identify is difference in effluent flow rate, which is not a valid indicator of any environmental impact.

Such charts are thus meaningless when the effluents tested are non-toxic as in three of the five biological tests: Ceriodaphnia Reproduction (Figure 2); Ceriodaphnia Acute Toxicity (Figure 4); and Selenastrum Growth Inhibition (Figure 5.)

Using such a methodology, it cannot be said that one effluent is less toxic than another. The only conclusion from Figures 2, 4 and 5 is that the specific effluent volume when the mill was operating with the TCF bleaching sequence was less than when it was operating with the ECF bleaching sequence, a fact that may be totally unrelated to the bleaching process used and is environmentally irrelevant, at least with respect to toxicity. To suggest otherwise is misleading.

Figures 3 and 6 do show toxic responses from effluents produced when the mill was operating with the TCF and ECF bleaching sequence. A more detailed discussion follows in later sections.



Biological Effect Concentration Comparison and Analysis

While the TEF approach for comparing effluents is technically invalid here, the effluents can be compared by examining the effluent concentration required to produce the biological effect examined in each separate test. Using the published2 Toxicity Emission Factor data and the specific effluent volume data, the originally measured effect concentrations can be back calculated.


Table 1
Biological Test and Mill Production Data
(Ref. Lövblad, R and Malmström. 1994 International Non-Chlorine Bleaching Conference.)
Case Production
ADt/d
Flow
m3/t
Flow
1000 m3/d
(Calculated)
TEF
(Reported)
Effect
Conc., %
(Calculated)
Test
ECF-1
ECF-1
ECF-2
ECF-3
ECF-3

TCF-1
TCF-2
TCF-2
TCF-3
TCF-3
800
800
900
800
800

880
930
930
980
980
111
111
95
133
133

95
93
93
89
89
89
89
86
106
106

84
86
86
87
87
152
<111
173
<133
<133

<95
106
103
<89
<89
73
100
55
100
100

100
88
90
100
100
Microtox
Selenastrum
Zebra Repro
Cerio Repro
Cerio Acute

Selenastrum
Microtox
Zebra Repro
Cerio Repro
Cerio Acute


When the mill was operating using the TCF or the ECF bleaching sequence, 100% effluent concentration exerted no toxic or observable effects for: 1) Selenastrum Growth Inhibition (Fig. 7); 2) Ceriodaphnia Reproduction (Fig. 8); and 3) Ceriodaphnia Acute Toxicity (Fig. 9.) This indicates that both processes, in this pulp mill, produce non-toxic primary treated whole mill effluent.

The biological tests did show differences between the effluents for Microtox and Zebra fish reproduction, with lower effect concentrations for the ECF case (Figs. 10 & 11) and therefore greater TEF values (Figs. 3 & 6 .)

Therefore any discussion of differences in "toxicity" should be restricted to effect concentrations for Microtox and Zebra fish reproduction.



Zebra Fish Reproduction Toxicity Comparison and Analysis

The Zebra fish reproduction test results in Figure 11 show the primary treated whole mill effluent, when the mill was operating with a TCF bleaching sequence, did not have an observable effect until the effluent concentration was 90%. On the other hand, when the mill was operating with an ECF bleaching sequence, an observable effect occurred at lower concentration, 55%. At face value, these results suggest that whole mill effluent identified as ECF-2 was more toxic to Zebra fish eggs/fry than whole mill effluent identified as TCF-2.

However, in another publication, based on mill experience, comparing softwood bleaching effluents using ECF and TCF sequences, results do not show that ECF bleaching effluent is more toxic to Zebra fish than TCF bleaching effluent. For example, the following figure, prepared from published data3 shows ECF and TCF biological tests performed on bleaching effluent only. This more accurately depicts the impact of the bleaching process.

Observations from this data set are:

  • There is no significant difference between bleaching effluents produced with an ECF bleaching sequence or a TCF bleaching sequence on Zebra fish acute toxicity, egg hatching or larvae survival.

  • The effluent from the last washing filter, (brown stock decker), prior to bleaching, is much more toxic in all tests compared to bleaching effluent. This would indicate that a small change in washing losses, can make a significant change to toxicity test results performed with whole mill effluents. This further calls into question the TCF versus ECF comparisons by Lövblad et al., as carryover of washing filter effluent to the bleach plant could be a confounding effect.

The importance of Figure 12 is that it compares bleaching effluents, not primary treated whole mill effluents. The influence of effluent from other pulp mill sources on toxicity, i.e., wash filters, is avoided. In addition, the data shows the importance of efficient washing and recovery of black liquor. Perhaps most importantly, as the authors stated: "... It also implies that toxic substances released/emitted as a result of Pulp production need not originate in the bleach plant... "3

Finally, since the previously discussed data analysis1,2 was for primary treated whole mill effluent, which included all effluent sources, it is not possible to conclude with certainty that the differences seen in the Zebra fish reproduction effect concentrations or the Microtox data are due solely or even in part to a difference in bleaching technology. The differences observed could be from other sources, such as increased washing losses,even though the reported COD discharges were essentially the same2.

Therefore, based on the data provided and:

  • by only analyzing primary treated whole final mill effluent;
  • by not accounting for other possible sources of toxic substances; and
  • by not testing bleaching effluents

the claims that:

"... Biological tests show TCF EFFLUENTS LESS TOXIC... "1

"... Our tests show ... the TCF methods at our mills produce the least toxic effluents... "1

"... the TCF effluents in all the tests have shown lower TEF values compared to the ECF effluent.... The only difference between ECF and TCF production is the bleaching process ..."2

are not supported by the data.



Conclusions

Critical analysis of the data and methodology shows that the claims made regarding the relative toxicity of ECF and TCF are misleading and incorrect. The data used to make these claims are insufficient and the comparison methodology is flawed for the following reasons:

  • The observations were not consistent. Neither primary treated whole mill effluent produced using an ECF bleaching sequence nor primary treated whole mill effluent produced using a TCF bleaching sequence exerted a toxic response for three of the five biological tests used;

  • The comparison cannot account for confounding factors, such as the contribution of toxic substances from other sources within the mill, because the biological tests were performed on whole mill effluent;

  • Bleaching effluents are not compared. Rather whole mill effluents were tested, therefore biological responses due to changes in bleaching practice cannot be assessed with certainty;

  • Toxicity comparisons use Toxicity Emission Factor (TEF). This is not a valid methodology for comparing non-toxic effluents. It was designed to normalize toxic effluents only, and is meaningless when no toxicity is observed; and

  • Biological tests of other pulp mill bleaching effluents produced from both ECF and TCF processes have shown no significant difference in toxicity in other investigations.



References

  1. "Response from Södra Cell". Spring 1994. pp.6-7.

  2. Lövblad, R. and Malmström, "Biological Effects of Kraft Pulp Mill Effluents-A Comparison Between ECF and TCF Pulp Production". Proceeding, 1994 International Non-Chlorine Bleaching Conference. March 1994.

  3. Flink, J., Grundelius, R. and Swan, B., "Modern ECF and TCF Bleaching". Proceedings, 1993 Non-Chlorine Bleaching Conference. March 1993.



Acknowledgment

This report was reviewed by Dr. Keith Solomon, Environmental Toxicologist, Centre for Toxicology, University of Guelph and Dr. Stella Swanson, Aquatic Biologist, Golder Associates. Their comments were incorporated into the document. The Alliance for Environmental Technology (AET) gratefully acknowledges their contribution.