Reliability of models that predict the fate of organic trace pollutants in municipal activated sludge plants

<p>The production, use and disposal of many compounds inevitably leads to their presence in the environment as organic trace pollutants. Although their concentration may be low, these trace compounds can present an environmental hazard associated with their toxicity for human beings, their potential to accumulate in biota and ecosystems and in some cases their function as a catalyst in the destruction of the ozone in the stratosphere.</p><p>In many cases these organic trace compounds enter the sewage system and will finally appear in municipal activated sludge plants. Several processes determine their distribution in these plants. Hydrophobic compounds can partition to the sludge and in this manner may create a potential hazard associated with sludge disposal. Volatile organic compounds are amenable to air stripping and surface desorption and therefore are frequently found in the off-gas of activated sludge plants. Whereas sorption and volatilisation merely rearrange the distribution of compounds among the different environmental compartments (sludge, air and water), only a destructive process such as biodegradation can actually remove them from the environment.</p><p>During the last two decades several activated sludge fate models have been developed. These models are used (i) to predict the environmental exposure to specific organic trace compounds expected to appear in municipal wastewater's, (ii) to optimise the design and control of treatment plants with respect to the removal of these pollutants and (iii) to establish limits on treatment plant influent loads based upon allowable effluent loads. The reliability of these models only has been demonstrated to a limited extent and therefore their applicability remains uncertain. A validation study may help to gain some confidence in these models, but also can reveal some of their shortcomings.</p><p>Based on an extensive literature review such a validation study was set-up. The fate of three test trace compounds in a pilot-scale municipal activated sludge plant was investigated: the C <sub>12</sub> -homologue of linear alkylbenzene sulfonate (LAS-C <sub>12</sub> ), nitrilotriacetic acid (NTA) and toluene. The sludge retention time (SRT) of the plant and influent concentration of the trace compounds were varied. In addition, for each of the test compounds their biodegradation kinetics by activated sludge were assessed with a method which was specifically developed for this purpose. The results showed that existing models do not yield accurate predictions, in particular because the biodegradation kinetics they employ are incorrect. A generic assessment of the fate of organic trace compounds in activated sludge plants therefore should not rely on model calculations alone. Still, existing models can be used for a first screening of new priority compounds or to design field monitoring studies for existing priority compounds. More accurate models can only be developed if more fundamental research is directed towards biodegradation of mixtures of organic substrates by mixed microbial cultures such as activated sludge.</p><p><em>Keywords</em> : activated sludge, organic trace pollutants, fate modelling, wastewater</p&gt