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The biodegradation is expressed as the percentage of CO2 produced corrected by the blank compared to the theoritical CO2 which is calculated from the test substance concentration and the Total Organic Carbon determined by elementary analysis.
OECD 301F test is the manometric respiration. The biodegradation is followed by oxygen consumption registered in real respirometer. The CO2 produced is absorbed by potassium hydroxide or an other suitable absorbent. The Oxygen needed for the biodegradation is produced electronically to compensate the change in pressure or in volume.
The biological oxygen demand is calculated from the oxygen uptake, which is corrected by the blank, and the quantity of test substance.
The Biological Oxygen Demand (BOD) is expressed in mg O2 per mg of test substance.
The biodegradability is expressed as the percentage of Theoritical Oxygen Demand (ThOD) which is the ratio between the Biological oxygen demand and the Theoritical oxygen demand.
Less satisfactorily the Chemical Oxygen Demand (COD) is used. why using Chemical oxygen demand is less satisfactorily and can lead to false (overestimated) results?
The calculation of the theoritical oxygen demand is possible if you know the composition of the test substance and the molecular weight. You mut at least know the content of Carbon, Hydrogen, Chlorine, Nitogen, Sodium, Oxygen, Phosphorous and Sulfur. The distinction has to be done for the ThOD without nitrification or with nitrification. For the compound:
CcHhClclNnNanaOoPpSs, The ThOD, without nitrification, would be: with nitrification:
The main restrictions in the applicability of the biodegradability test methods are due to the substance properties and some specific test criteria. The choice of a suitable test method is dependent of the test substance properties. First the biodegradability is dedicated to Organic compound. It is important to receive information on its composition, adsorption, water solubility, partition coefficient, dissociation constant in water and the presence of inorganic carbon or impurities.
where MW = molecular weight
The COD is an oxidation by potassium dichromate and sulfuric acid which are very strong oxidant. Nevertheless, in some cases, the complete oxidation do not occur leading to a chemecal oxygen demand lower than the Theoritical oxygen demand. In these cases, the COD can lead to false results and the biodegradability can be overestimated.
2.4. Factors which influence the aerobic biodegradation Some factors have a positive effect on the aerobic biodegradation:
• • • • • • •
Straight chain hydrocarbons Aliphatic short chain
Presence of unsaturation
Low molecular weight hydrocarbons. No toxicity on microorganisms. Oxygenates compounds. Poor hydrolitic stability.
2.6. Review of the specific test criteria For all methods the substance must not be toxic at the test concentration.
The OECD 301A and 301E methods which are based on dissolved organic carbon removal are not suitable for volatile substance, neither for poorly water-soluble substance, less than 100mg/l.
The organic carbon content should be known and preferably, the proportions of major components should be known.
The OECD 301 B based on CO2 production is not suitable for volatile substance. The Total organic carbon content should be known and preferably the proportions of major components. This method is not suitable for test substance with a high concentration of Inorganic carbon (for instance like carbonates) ; the inorganic carbon must be less the 5% of the Total carbon.
OECD 301C is based on oxygen uptake. In this method, precautions has to be taken for volatile substance. The main restrictions (difficulties) of this test method is the
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• •
• • • •
Favorable substrate conditions (binding sites)
Favorable environmental conditions: High temperature; Low pressure;
Favorable aqueous medium - presence of nutrients Neutral pH
Low interfacial tension with water.
Efficient microorganisms (enzyme for that substance to degrade, micro organisms adaptation).
2.5. Test methods applicability Test substances which are soluble in water to at least 100 mg/l may be assessed by all methods, provided they are non-volatile and non-adsorbing. For those chemicals which are poorly soluble in water, volatile or adsorbing, suitable methods are indicated in the following table.
The MITI method neither solvents nor emulsifying agents are to be used.
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LUBE MAGAZINE NO.126 APRIL 2015
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