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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Key value for chemical safety assessment

Biodegradation in water:
under test conditions no biodegradation observed
Type of water:
freshwater

Additional information

Three closed bottle tests (OECD 301D) are available (Hüls AG, 1991a; RBM, 1996a; Slovnaft VÚRUP, a. s., 2005a); the percentage of biodegradation observed in these studies ranged from 0 to 9.24 %. Therefore, MTBE is not considered readily biodegradable in the aquatic environment according to the standardised aerobic ready-biodegradation tests. As no test results from standard inherent test systems for aquatic biodegradation are available, non-standard tests were considered (Shell, 1981, Mo et al., 1997).

Certain adapted micro-organisms are capable of degrading MTBE (e. g. Kharoune et al., 2002). Thus, a well-adapted industrial sewage treatment plant (STP) is able to degrade the substance. High degradation rates have been observed in non-standard tests using special types of inoculum, pure cultures and mixed cultures (Shell, 1993; Salanitro et al., 1994; Steffan et al., 1997; Cano et al., 1999; Fortin & Deshusses, 1999; Fortin et al., 2001; Kharoune et al., 2001; 2002). These studies show that at least some microbial species are capable to degrade MTBE and to use it even as their sole carbon source.

The main responsible enzyme for the first steps of MTBE metabolism are P450 enzymes (e.g. Kharoune et al., 2002; Pruden et al., 2004), which are widely present in all living organisms and have even been found in viruses. Many P450 enzyme families and subfamilies have been reported in 905 bacterial species and 2570 fungal species (Nelson, 2009), which can explain why microbial populations are able to degrade MTBE.  Furthermore the bacterial pathways of metabolism of MTBE are well known, as described in the degradation following pathway of MTBE in the EAWAG Biocatalysts/Biodegradation Database (EWAG, 2015).

While there is no doubt that bacteria and fungi are generally able to degrade MTBE, for biodegradation of MTBE, adaptation is needed and would usually be induced by simultaneous exposure to alkanes and MTBE. In biodegradation simulation studies according to EPA methodology, the half saturation concentration, at which the growth of the MTBE degrading microbial organism is 50% of the maximum, was found to be between 0.07 and 0.13 mg MTBE per litre (Cano et al., 1999).

It could be assumed that where there are continuous releases of MTBE to a STP, such as for large production and processing sites, sewage sludge will have become adapted to the substance and in these cases, the substance could be considered as readily biodegradable. For professional and consumer releases and releases on the regional scale, where adaptation may not occur, the non-standard test data available indicate that MTBE degradation might not fulfil the test criteria (of OECD 302), and a slower degradation categorisation as “inherently biodegradable, not fulfilling criteria” could be considered.

However, owing to the lack of ready biodegradation seen in the available standard screening tests, and as mandated by ECHA via a substance evaluation decision, a conservative approach to the available data means that the conclusion ‘under test conditions no biodegradation observed’ has been used for exposure assessment purposes.

References:

EAWAG (2015). Methyl tert-butyl ether Graphical Pathway. Available at http://eawag-bbd.ethz.ch/mtb/mtb_image_map.html (accessed May 2015)

 

Fortin, NY and Deshussses, MA (1999).Treatment of methyl tert-butyl ether vapors in biotrickling filters. 1. Reactor startup, steady-state performance, and culture characteristics. Environ Sci Technol 33, 2980-2986.

 

Fortin, NY, Morales, M, Nakagawa, Y, Focht, DD and Deshusses, MA (2001). Methyl tert-butyl ether (MTBE) degradation by a microbial consortium. Environ Microbiol 3(6), 407-416.

 

Nelson, DR (2009). The Cytochrome P450 Homepage. Hum Genom 4, 59-65.

 

Pruden, A and Suidan, M (2004). Effect of benzene, toluene, ethylbenzene, and p-xylene (BTEX) mixture on biodegradation of methyl tert-butyl ether (MTBE) and tert-butyl alcohol (TBA) by pure culture UC1. Biodegradation 15, 213–227.