User Guide to Thresholds and Classification - Environmental ...

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366User Guide for Thresholds and Classificationsprogram predicts a substance to be degraded ‗slowly‘. In this case, the substance can be regarded as notrapidly degradable.The same conclusion was reached in the US EPA/EC Joint Project on the Evaluation of (Q)SARs by use ofexperimental and QSAR data on new substances in the EU. The evaluation was based on an analysis ofQSAR predictions on 115 new substances also tested experimentally in ready biodegradability tests. Only 9of the substances included in this analysis were readily biodegradable. The employed QSAR methodology isnot fully specified in the final report of the joint US EPA/EC project (OECD, 1994), but it is likely that themajority of predictions were made by using methods which later have been integrated in the BiodegradationProbability Program.Also in the EU technical guidance document (EC, 2003) it is recommended that estimated biodegradabilityby use of the Biodegradation Probability Program is used only in a conservative way, that is, when theprogram predicts fast biodegradation, this result should not be taken into consideration, whereas predictionsof slow biodegradation may be considered.Thus, the use of results of the Biodegradability Probability Program in a conservative way may fulfil theneeds for evaluating biodegradability of some of the large number of substances for which no experimentaldegradation data are available.19E.3 Factors influencing degradability in the aquatic environmentIntroductionInterpretation of test results on biodegradability of organic substances has been considered in DetailedReview Paper on Biodegradability Testing (OECD, 1995).The conditions in the environment are typically very different from the conditions in the standardised testsystems, which make the extrapolation of degradation data from laboratory tests to the environment difficult.Among the differences, the following have significant influence on the degradability:organism-related factors (presence of competent micro-organisms);substrate-related factors (concentration of the substance and presence of other substrates); andenvironment-related factors (physico-chemical conditions, presence of nutrients, bioavailability of thesubstance).These aspects are discussed further below.Presence of competent micro-organismsBiodegradation in the aquatic environment is dependent on the presence of competent microorganisms insufficient numbers. The natural microbial communities consist of a very diverse biomass and when a ‗new‘substance is introduced in a sufficiently high concentration, the biomass may be adapted to degrade thissubstance. Frequently, the adaptation of the microbial population is caused by the growth of specificJanuary 2012 EPA0109
367User Guide for Thresholds and Classificationsdegraders that by nature are competent to degrade the substance. However, also other processes asenzyme induction, exchange of genetic material and development of tolerance to toxicity may be involved.Adaptation takes place during a ‗lag‘ phase, which is the time period from the onset of the exposure until asignificant degradation begins. It seems obvious that the length of the lag phase will depend on the initialpresence of competent degraders. This will again depend on the history of the microbial community, that is,whether the community formerly has been exposed to the substance. This means that when a xenobioticsubstance has been used and emitted ubiquitously in a number of years, the likelihood of finding competentdegraders will increase. This will especially be the case in environments receiving emissions for example,biological wastewater treatment plants. Often more consistent degradation results are found in tests whereinocula from polluted waters are used compared to tests with inocula from unpolluted water (Nyholm andIngerslev, 1997; OECD, 1995).A number of factors determine whether the potential for adaptation in the aquatic environment is comparablewith the potential in laboratory tests. Among other things adaptation depends on:initial number of competent degraders in the biomass (fraction and number);presence of surfaces for attachment;concentration and availability of substrate; andpresence of other substrates.The length of the lag phase depends on the initial number of competent degraders and, for toxic substances,the survival and recovery of these. In standard ready biodegradability tests, the inoculum is sampled insewage treatment plants. As the load with pollutants is normally higher than in the environment, both thefraction and the number of competent degraders may be higher than in the less polluted aquaticenvironment. It is, however, difficult to estimate how much longer the lag phase will be in the aquaticenvironment than in a laboratory test due to the likely lower initial number of competent degraders.Over long periods, the initial concentration of competent degraders is not important as they will grow upwhen a suitable substrate is present in sufficient concentrations. However, if the degradability in a shortperiod is of concern, the initial concentration of competent degrading microorganisms should be considered(Scow, 1982).The presence of flocs, aggregates, and attached micro-organisms may also enhance adaptation by, forexample, the development of microbial niches with consortia of micro-organisms. This is of importance whenconsidering the capability of adaptation in the diverse environments in sewage treatment plants or insediment or soil. However, the total number of micro-organisms in ready biodegradability tests and in theaquatic environment are of the same orders of magnitude (104–108 cells/mL in ready biodegradability testsand 105–106 cells/mL or more in surface water (Scow, 1982). Thus, this factor is probably of minorimportance.When discussing the extrapolation to environmental conditions it may be valuable to discriminate betweenoligotrophic and eutrophic environments. Micro-organisms thriving under oligotrophic conditions are able tomineralise organic substrates at low concentrations (fractions of mg C/L), and they normally have a greaterJanuary 2012 EPA0109
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