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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

In a reliable study according to OECD guideline 301B DEHP is demonstrated to be readily biodegradable fulfilling the 10-day window requirement.

Key value for chemical safety assessment

Biodegradation in water:
readily biodegradable

Additional information

Aerobic biodegradation:

Four reliable studies on biodegradation of DEHP are available.

In the key study, Diefenbach, (1994, unpublished), showed in an experiment according to OECD Guideline No. 301 B (Modified Sturm-test), that DEHP is “readily biodegradable” (82% after 28 days) and fulfilled the “10-days window” criterion. Test concentration of DEHP was 20.3 mg/l (dissolved in Vestinol) and temperature was 20-22°C. As the study is GLP, all validity criteria were fulfilled and no deviations were recorded, it is considered as reliable without restrictions.

This data is supported by 3 other studies:

- BASF AG (unpublished, 1984) in a Modified Respiration Test (BOD of ThOD) according to EC Guidelines 79/831 showed that at a test concentration of 100 mg DEHP/L and 10 mg/L emulsifying agent (Nonyl-phenol, 10 EO + 5 PO), 60-70% of the original DEHP was degraded after 28 days of incubation, at 20°C.

- Exxon (unpublished, 1995) in a manometric respirometry test (OECD 301F), studied the biodegradation in fresh sludge from a domestic STP. The test concentration of DEHP was 45.6 mg/l. The temperature was not reported. Biodegradation started after a lag-phase of 8 days, and after 28 days 63% was degraded.

Although of Klimish reliability 4, these two unpublished studies were validated in previous RAR (2008) and therefore considered as reliable and used as supporting study.

- With the same test procedure (OECD 301F manometric repirometry test, 35 mg DEHP/L) Stasinakis et al. (2008) showed a partial biodegradation of DEHP 57.7% (+/- 5.7) after 28 days with a lag phase of 4.1 days (half life of 6.9 days +/- 2.6 days). According to authors the strict definition was not fulfilled in the test conditions but DEHP showed similar range of biodegradation and is well described. This study is therefore considered as reliable with restrictions and considered as a supporting study.

The lag phases observed demonstrate the adaptation is of great importance for the test results. Due to the widespread use of DEHP, a major part of the domestic STP sludge can be expected to be adapted to the substance, and the strict requirement of unadapted inoculate for ready biodegradability testing may be difficult to fulfil. Since several test results have demonstrated rapid degradation, and since in reality, most industrial and domestic STP that receive DEHP are probably adapted to the substance, DEHP is assumed to be readily biodegradable fulfilling the 10 day window. Hence according to the guidance on information requirements and chemical safety assessment - chapter R16: Environmental exposure estimation (ECHA 2008) the biodegradation rate in STP to be used in this assessment is set to the default value of 1 (h-1) (i. e. DT50 -STP =0.029 d).

Anaerobic biodegradation:

Two experimental studies on biodegradation of DEHP in anaerobic conditions are available.

Ziogou et al, 1989 investigated the degradation of DEHP in anaerobic municipal sewage sludge. Test concentration was approximately 4 mg/l in the first stage of the study, where the samples were kept at 37°C without shaking for 32 days. During a second stage of the study, different concentrations were tested; 10, 1, and 0.5 mg/l (about 30 – 600 mg/kg dwt), and the samples were shaken during the incubation.

In both stages of the study, no decrease in DEHP concentration was observed during the 32 days under anaerobic conditions.

Ejlertsson et al. (1997) came to the same observation when they exposed 1 mmol/L of DEHP and other phtalic acid esters at 37°C during 92 days in a digester using a methanogenic butylbenzyl phthalate degrading enrichment culture.. While dibutyl phthalate, butyl 2-ethylhexylphthalate, and dihexyl phthalate were degraded, bis(2-ethylhexyl) phthalate (DEHP), dioctyl phthalate and didecyl phthalate were not.

The two studies were well described and considered as reliable with acceptable restrictions.

However, a new study on anaerobic degradation published in 2005 demonstrates thorough primary degradation of DEHP within 28 days: In a screening test similar to OECD 311 guideline, authors have tested the biodegradability of DEHP and DBP (alone or in combination) under anaerobic conditions (Chang B.V., Liao G.S. and Yuan S.Y., 2005). Two types of sludge were tested, a sewage sludge from WWTP at the Presidential Enterprise Co. and a petrochemical sludge from WWTP of the Chinese Petroleum Corp. (both located at, Taoyuan, Taiwan). All determined half-lives were for the petrochemical sludge. Only in the initial experiment both types of sludges were used to degrade a mixture of DBP and DEHP (probably 1 µg/g, each) demonstrating firstly faster degradation of DBP and secondly faster degradation of DEHP with petrochemical sludge (below LOQ within 21 days) than in sludge from Presidential Enterprise Co. (below LOQ within 28 days). No degradation was observed in sterile control incubations (>95% recovered for both sludges). Parameters followed for the degradation with petrochemical sludge were: CH4 production (coupled experiment with DEHP and DBP), DEHP and DBP concentrations, ORP (oxidation/reduction potential) values, and nitrate, phosphate and sulphate concentrations. Methane production was shown to increase concomitant to a decrease in both, DBP (faster) and DEHP. However, no control values for methane production are presented.

Under the multiple conditions tested, DEHP showed a complete biodegradation under anaerobic conditions with the sewage sludge. Half-lives (disappearance of parent) ranged between 3.3 days (0.5 µg/g for each of the phthalates, pH 7, 30°C and 16.1 days (1 µg/g for each of the phthalates, pH 5, 30°C) when tested in combination with DBP and 7 days when tested alone (30°C, pH 7 and 2 µg/g in concentration). These results are in contrast to previous studies mentioned above. Reasons for this might be the low initial concentrations applied or adaption of sludge microorganisms: Besides the petrochemical sludge which was used exclusively for the half-life studies obviously other industrial sludge had been used rather than municipal sludge. In any case, as authors tested various conditions for degradation it can be concluded that at least under certain circumstances rapid anaerobic degradation of DEHP in sewage sludge is possible. Support for these results comes from a long term study reproducing landfill conditions where anaerobic status is also encountered. From analysing leachates the authors concluded from the finding of MEHP and phthalic acid, several orders higher in magnitude than the parent, that primary biodegradation had occurred (Jonsson et al. 2003). This study is summarized in IUCLID section 5.2.4 and the discussion on simulation testing (water and sediments).

Value used for CSA: Biodegradation in water: readily biodegradable