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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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In a test similar to OECD 303A (laboratory activated sludge unit) hydroquinone was removed by >=99.9%, independently of the influent concentration at concentrations between 50 mg/L and to 275 mg/L. Based on this extensive removal under relevant conditions (HRT 12 hours, independently of the kind of synthetic sewage used), a first order rate constant for the degradation in sewage treatment plants of 144/d is assumed for the chemical safety assessment.

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In the work by Harbison and Belly (1982), biodegradation of hydroquinone was investigated in a thorough manner (experiments E1 to E3). Assessed were (E1) the extent of mineralization by radioisotopic labelling and determination of 14CO2 (74% within 5-10 days; n=5); (E2) the biodegradation pathway of hydroquinone, identifying metabolites and investigating their possible accumulation (low concentration only, no accumulation); (E3) the extent of metabolization in a laboratory activated sludge unit (equivalent to the set-up of OECD 303 A: Simulation Test – Aerobic Sewage Treatment) considering different synthetic sewages and analyzing for important metabolites.

With regard to E3, the extent of degradation in a 6-liter laboratory activated sludge unit (equivalent to the set-up of OECD 303 A: Simulation Test – Aerobic Sewage Treatment) was assessed, considering different synthetic sewages and analyzing for important metabolites. The hydraulic retention time was 12 h instead of 6 h required by the guideline. However, according to Simple Treat v. 4.0 (Struijs, 2014), the default hydraulic retention time for sewage treatment plants with primary clarifier and without primary clarifier is 11.5 hours and 18 hours, respectively. Thus, 12 h retention time is a very good approximate for real life STPs. As a result, after an acclimation period of typically 2 days hydroquinone was removed by >=99.9%, independently of the influent concentration at concentrations between 50 mg/L and to 275 mg/L. This high value of degradation was observed independently from the synthetic sewage used, i.e. basal salts, only; synthetic domestic sewage; synthetic photographic sewage; or a 80:20 mixture of synthetic domestic and photographic sewage. The extent of TOC removal was between 70% and 92% for hydroquinone containing experiments, while it was lower (69%) in the control experiment without hydroquinone and synthetic domestic sewage, only. Thus, hydroquinone did rather stimulate than inhibit TOC removal in the STP simulation tests. Further, effluents were also analyzed for important metabolites of hydroquinone. But neither 1,4-benzoquinone could be detected (independently from the presence of sulfite, contained in synthetic photographic waste), nor 2-hydroxy-1,4-benzoquinone. Together with the supporting experiments on hydroquinone mineralization and metabolism described above, it can be assumed with high certainty that the high extent of hydroquinone removal of≥99.9 % observed in the simulation tests for aerobic sewage treatment plant can be attributed to mineralization and possibly microbial assimilation, and an accumulation of stable metabolites can be excluded.

Based on this extensive removal under relevant conditions (HRT 12 hours, independently of the kind of synthetic sewage used), a first order rate constant for the degradation in sewage treatment plants of 144/d is assumed for the chemical safety assessment. This rate constant is based on the following considerations: Using SimpleTreat model v. 3.1 with a sludge loading rate of 0.1 kg BOD per kg dwt per day (updated default according to Strujis et al., 2014) a biodegradation of 98.3% is resulting (98.4% total elimination from waste water) using the rate constant of 144/d, which is associated with a hydraulic retention time of 10.4 hours in STPs with primary settler. Thus, biodegradation extent is still below the value of >=99.9 reported by Harbison and Belly (1982). Further, this work conclusively demonstrated that biodegradation is not limited to primary biodegradation but thorough, and metabolites do not accumulate (none detected).

Struijs, J. (2014)

SimpleTreat 4.0: a model to predict the fate and emission of chemicals in wastewater treatment plants.Background report describing the equations. RIVM Report 601353005/2014

RIVM, National Institute for Public Health and the Environment, Bilthoven, The Netherlands