Assessment of chemicals

Example of how to use the toolkit: a textile dye

 

Purpose

The purpose of this example is to outline how the OECD Environmental Risk Assessment Toolkit can be used to guide a review of the factors that may need to be taken into account when using the available hazard information for a substances in order to investigate the risk at a specific site using a substance. The example is based on a fictional textile dye but could equally be applied to other types of substances. As noted in the main toolkit description, this is an example of the steps involved in this type of assessment, not an actual assessment. It does not show in detail how the assessment was carried out, only the steps and the results and the tools used at each stage.

 

This example only considers the risk to the environment. For risk assessment for human health, please refer to the WHO Human Health Risk Assessment Toolkit project conducted by WHO/IPCs.

 

Background

A company had been trialling the use of a reactive dye for dyeing of cotton textiles in their existing plant and wanted to carry out an environmental risk assessment prior to entering full-scale use. The plant was located in Country X and effluent from the plant was discharged into a river with minimal treatment on-site before discharge. The company intended to carry out the dyeing of cotton textiles using an exhaust batch process. The plant typically processed 8 tonnes per day of textile but it was estimated that only around 30% of the textile on any one day would be treated with the reactive dye in question. The company decided to use the OECD risk assessment toolkit to develop the risk assessment. A summary of the overall process is given in Figure 1.

 

Environmental Hazard Assessment 
Gathering existing information
Following the OECD tool kit, the company carried out an information gathering exercise focussing on the effects of the substance in the environment. No specific information on the hazard and/or risks to the environment on the reactive dye were found to be available for the specific country and so the OECD tool kit was used to identify sources of information from outside of the country. The tool kit listed a number of sources of existing information on chemical substances including the OECD Existing Chemicals Database and the eChemPortal. A search was made for information on the environmental hazard for the reactive dye using both of these sources. However the search identified little or no information.

The company had obtained an up-to-date Safety Data Sheet (SDS) from their supplier of the reactive dye. This contained the following information on the environmental hazard of the dye.

 

Water solubility = 105 g l-1.

Octanol-water partition coefficient (log10 value) = -3.9.

The substance is inherently biodegradable.

96h-LC50 with fish >500 mg l-1.

48h-EC50 with Daphnia magna 176 mg l-1.

72h-EC50 with algae = >500 mg l-1.

The company also decided to carry out a search of the published literature in order to determine if any more recent data was available. This identified a toxicity test with Daphnia magna.

Evaluating the existing information
As most of the data available were from a SDS the company considered that the data could be used as the basis of the assessment. However the company decided to evaluate the Daphnia magna toxicity study identified in their own literature search using Chapter 2 of the OECD Manual for Investigation of HPV Chemicals. This revealed that the study was valid and the following toxicity data were reported.

 

·         48h-EC50 = 176 mg l-1.

·         21d-NOEC = 22 mg l-1.

The company concluded that the study was probably the same study as reported in the SDS and was appropriate for use in the risk assessment.

Generating new data
The company considered that sufficient basic information on the inherent properties of the substance was available to define the hazards and so it was not necessary to generate new data.

Conclusion on the inherent properties
The company’s assessment of the inherent properties concluded that the data in the SDS report, supplemented by the data found in a search of the published literature, were appropriate for use in their own risk assessment. The key ecotoxicity endpoint was identified as the 21d-NOEC for Daphnia magna of 22 mg l-1.

 

Environmental Exposure Assessment

Measuring or estimating releases to the environment
The company firstly carried out an in-house evaluation of the potential for release to the environment during the dyeing process. The following key steps in the process were identified (Figure 2).

  1. The dyestuff, along with the dyeing auxiliaries, is manually dosed to the dyeing machine at the required rate and the dyeing solution is brought up to the required temperature. This results in negligible emission of dyestuff to the environment.
  2. The fabric is introduced into the solution as a continuous loop and is circulated through the solution for one hour, during which time the is absorbed and fixed to the fibres. The process is well controlled and negligible emission of dyestuff to the environment occurs during the dyeing.
  3. At the end of the dyeing process, the spent dye solution is drained from the dyeing machine. This results in a discharge of the dyestuff from the process.
  4. The dyed textile is then washed in the dyeing machine to remove unfixed dye. The washings are drained from the machine resulting in a discharge of the dyestuff from the process.
  5. The dyed textile is then dried. Negligible emission of dyestuff to the environment occurs during the drying.
  6. The dyeing machine is cleaned with water in preparation for the next batch. The washings are drained from the machine resulting in a further discharge of the dyestuff from the process.

 

Figure 1 Summary roadmap for the textile dye example

 

No existing environmental release scenarios had been identified by the company and no specific emission figures were available for the process. The company therefore decided to develope an initial scenario based on the Emission Scenario Document for textile finishing from the OECD toolbox.

 

Figure 2 Outline of the batch dyeing process

 

The relevant equation from the Emission Scenario Document for calculating the emission to the environment from this process is as follows.

 

 

where    Elocalwater = Estimated emission to water (kg day-1).

            Qtextile = Mass of textile processed per day (t day-1).

            Fproduct = Fraction of the fabric treated with the dyestuff in question.

            Qproduct = Mass of dyestuff preparation per mass of fabric (kg t-1).

            Csubstance = Fraction of active substance (dye) in the dyestuff preparation.

            Ffixation = Degree of fixation.

The company consulted their records and operating procedures and found that the typical mass of textile processed per day was 8 t day-1 (Qtextile). The content of active substance in the dyestuff preparation to be used was known to be 0.65 (Csubstance) and the mass of dyestuff preparation intended to be applied to the textile was known to be typically 18 kg t-1 (Qproduct). The fraction of the fabric (Fproduct) to be treated with the dyestuff in question was estimated to be 0.3. The company did not have any specific information on the degree of fixation of the dye and so it was decided to use the suggested value of 0.75 (Ffixation) for reactive dyes with cotton fibres from the Emission Scenario Document.

Using these data about their specific process, the company estimated that the daily emission to the waste water from the plant was 7.0 kg day-1.

Environmental fate and pathways

The company considered that the main pathway to the environment from the process was from the effluent stream. The company records showed that the total daily amount of effluent from the whole plant was typically 1,600 m3 day-1 and that the effluent from the plant was treated in-house in a settling tank prior to discharge to a river with a typical flow of 45,000 m3 day-1. This resulted in an effective dilution factor of the plant effluent of around 28. The company had no information on how effective the settling tank would be at removing the reactive dye from the effluent stream and so the company decided to carry out the assessment initially assuming that little or no removal would occur.

Measuring or estimating concentrations in the environment
No measured concentrations for the reactive dye in the environment were available. The company looked at methods for estimating concentrations included in the OECD Manual for Investigation of HPV Chemicals  and the summary table of available tools [link to be added when developed] and decided that a simple dilution calculation was appropriate in this case. The company therefore estimated the concentration of the reactive dye in the effluent and the receiving water based on the estimated release of 7.0 kg day-1 and the known effluent and receiving water flow rates. This resulted in an estimated concentration of 4.4 mg l-1 in the effluent and 0.16 mg/l in the receiving water downstream of the effluent discharge point. No removal in the settling tank, adsorption to suspended matter or sediments, or subsequent degradation in the environment was taken into account in these calculations.

 

Risk Characterisation
A risk characterisation was carried out and a report prepared comparing the concentrations estimated in the receiving water with the available toxicity data. The estimated concentration in the receiving water of 0.16 mg l-1  was around 138 times lower than the no effect concentration for the most sensitive species tested (Daphnia magna). The company considered that this showed that there was no significant risk to the environment from the use of the reactive dye at the plant and that it was not necessary to further refine the assessment by, for example, obtaining further information on the removal during on-site treatment or measuring the actual concentrations in the environment resulting from the process.

 

Summary

A site specific assessment was carried out of potential risks to the environment associated with the use of a reactive dye for cotton textiles. The hazard assessment identified a NOEC of 22 mg l-1  Daphnia magna as a relevant ecotoxicological endpoint for carrying out the assessment. The exposure assessment identified the effluent stream from the plant as the most important pathway for release of the reactive dye to the environment. The release to effluent was quantified at around 7.0 kg day-1 using a combination of the OECD Emission Scenario Document for textile finishing and known process parameters. The concentration of the reactive dye in the river downstream of the effluent discharge point was estimated to be 0.16 mg l-1. The risk characterisation found that this estimated concentration was around 138 times lower than the relevant NOEC and so the company concluded that there was no significant risk to the environment from use of the reactive dye in the process.

 

 

 

 

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