Biodegradation process kinetics in water at environmental concentrations

Estimates of kinetic rates of biodegradation of organic chemicals under different environmental conditions are needed for chemical risk assessment. Next to the emission rate, very commonly, the rate of biodegradation turns out to be the most important parameter governing the long term environmental fate and thus the exposure of humans and populations within ecosystems. Current biodegradation testing practise does not generate kinetic data for this purpose, as studies are performed in a quite different kinetic regime of high test concentrations of several mg/L while at environmental low concentations (ng - ug/L range) the kinetics is substantially different. In the low concentration regime, the chemical is degraded as a secondary substrate while the main energy and carbon flux in the system relates to metabolism of naturally occurring carbon sources. A procedure for determining degradation rates in surface water, developed by Niels Nyholm, is currently being evaluated by the ISO (International Organisation for Standardization) and has reached the stage of a CD (sommittee draft). The project deals with both basic scientific aspects of biodegradation kinetics and provides specific scientific support for the ISO method. Emphasis is being put on the interaction between chemical and natural substrates, the occurrence of threshold concentrations for adaptation and growth, the influence of (solid) particulate phases (sediment and sludge), and the link to standard biodegradability tests performed at high concentrations in synthetic media with no other carbon substrates. Degradation in activated sludge sewage treatment plants is also considered. Also the OECD has shown interest in the proposed method, and presentations of the work have been invited by the SETAC (Society og Environmental Toxicology and Chemistry) at a special symposium on biodegradation kinetics and at a general conference in the U.S.