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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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Description of key information

The degradation rate for polysulfides in water and sediment is  estimated using the default value according to the TGD.

Articles published in peer reviewed journals are used to show  that the removal of sulfide in biological treatment systems is also representative for thepolysulfide removal. Exampels of sulfide removal  in different reactor types / treatment systems are given.  

Key value for chemical safety assessment

Additional information

Half life in water and sediment

Experimental data on the biodegradation in water and sediment of polysulfides are not available. The degradation rate is therefore estimated using the default value according to the TGD, assuming that no degradation takes place in the bound phase.Microorganisms are capable of growth on polysulfides in the presence (aerobic) and absence (anaerobic conditions) of oxygen. Therefore the same degradation rate is used for the aerobic sediment layer and the anaerobic bulk sediment.

The partitioning of polysulfides in sediment was calculated using the default Koc for Non-hydrophobics. Based on this Koc a partitioning coefficients for sediment Kp sed = 1.7 L/kg was calculated (EUSES 2.1.1; Kolck 2010). The suggested half-life for a readily degradable substance with a Kpsed in the range ≤100 l/kg is 30 days for the aerobic sediment layer and the anaerobic bulk sediment.

Biological wastewater treatment.

 

Biological treatment of hydrogen sulfide is widely used due to efficient conversion. In these treatment systems, sulfide is oxidized by microorganisms to for instance elemental sulfur or sulfate. The biodegradation kinetics of polysulfide and sulfide are comparable because both substances are metabolized by chemolithoheterotrophic and chemolithoautotrophic bacteria at acidic, neutral and alkaline conditions. The chemical oxidation of polysulfide is high compared to sulfide oxidation at slightly alkaline conditions. The removal of sulfide in biological treatment systems is therefore also representative for polysulfides.

Removal of sulfide with oxygen as oxidizing agent has been investigated in three reactor types i.e. a continuously stirred tank reactor with polyurethane particles as carrier material, a biorotor with polyurethane particles and an fixed film submerged up-flow reactor also with polyurethane particles. The fixed film submerged up-flow reactor and biorotor performed best. At hydraulic retention times of < = 1 h removal percentages of > 90% were achieved (Buisman et al, 1989). Sublette (1987) cultured Thiobaccillus denitrificans aerobically in a continuous flow reactor and found 100% removal of sulfide. Biological treatment of sulfide in a reverse fluidized loop reactor allowed 90 to 100% of sulfide removal from wastewater. This reactor was operated aerobically with an hydraulic retention time of only 90 minutes (Krishnakumar et al, 2005).

In a pilot-scale plant sulfide-containing wastewater from petrochemical industries was treated using autotrophic denitrifying bacteria. Denitrification is nowadays also widely used in publicly owned wastewater treatment plants. The hydraulic retention time in the reactor was only 0.5 days. A removal of >99% of the sulfide was found in the reactor fed with influent containing sulfide concentrations of up to 110 mg/L (Vaiopoulou et al, 2005).