Comportement des micropolluants prioritaires dans les filières conventionnelles de traitement des eaux usées

L’ensemble des données aquises par le programme de recherche OPUR sur les micropolluants au sein des filières de traitement des eaux usées ont été compilées afin de réaliser une synthèse sur le devenir des polluants prioritaires et émergents au sein des STEP. Cette synthèse a pour but d’analyser le devenir d’un large panel de polluants (n=104) au sein de deux STEP dont les filières de traitement sont différentes – décantation + boues activées vs. décantation physico-chimique + biofiltration. De plus, elle permet la comparaison des performances de deux types de traitements primaires (décantation classique vs. décantation lamellaire physico-chimique) et biologiques (boues activées forte charge vs. biofiltration), ainsi que la comparaison de deux filières complètes en normalisant les performances micropolluants à l’abattement de l’azote, paramètre impactant fortement le design des STEP.  vis à vis des micropolluants, est original car il ne se contente pas de comparer les procédés individuellement mais réalise également une comparaison à l’échelle de la filière de traitement en rapportant les performances à celles observées sur l’azote. This paper compares the removal performances of two complete wastewater treatment plants (WWTPs) for all priority substances listed in the Water Framework Directive and additional compounds of interest including flame retardants, surfactants, pesticides and personal care products (PCPs) (n=104). First, primary treatments such as physico-chemical lamellar settling (PCLS) and primary settling (PS) are compared. Similarly, biofiltration (BF) and conventional activated sludge (CAS) are then examined. Finally, the removal efficiency per unit of nitrogen removed of both WWTPs for micropollutants is discussed, as nitrogenous pollution treatment results in a special design of processes and operational conditions. For primary treatments, hydrophobic pollutants (log Kow > 4) are well removed (> 70%) for both systems despite high variations of removal. PCLS allows an obvious gain of about 20% regarding pollutant removals, as a result of better suspended solids elimination and possible coagulant impact on soluble compounds.

For biological treatments, variations of removal are much weaker and the majority of pollutants are comparably removed within both systems. Hydrophobic and volatile compounds are well (> 60%) or very well removed (> 80%) by sorption and volatilization. Some readily biodegradable molecules are better removed by CAS indicating a better biodegradation. A better sorption of pollutants on activated sludge could be also expected considering the differences of characteristics between a biofilm and flocs. Finally, comparison of global processes efficiency using removals of micropollutants load normalized to nitrogen shows that PCLS + BF is as efficient as PS + CAS despite a higher compactness and a shorter hydraulic retention time (HRT). Only some groups of pollutants seem better removed by PS + CAS like alkylphenols, flame retardants or DEHP, thanks to better biodegradation and sorption resulting from HRT and biomass characteristics. For both processes, and out of the 68 molecules found in raw water, only half of them are still detected in the water discharged, most of the time close to their detection limit. However, some of them are detected at higher concentrations (> 1 µg/L and/or > Environmental Quality Standards) what is problematic as they represent a threat for aquatic environment.

The fate of micropollutants in the environment has become an increasing issue last decades, especially in heavily urbanized areas. To struggle against water contamination, the European Community adopted a control policy strategy materialized by the European Water Framework Directive (WFD, Decision No. 2455/2001/EC) (EC 2001a). This regulation requires Member States to achieve a good ecological and chemical status in surface waters by 2015, 2021 or 2027. Concretely, 41 chemicals were defined as priority substances because they represent a significant risk for the aquatic environment, and Environmental Quality Standards (EQS) have been set for 33 of them. In parallel of WFD pollutants, a large number of molecules such as pesticides, pharmaceuticals and personal care products (PPCPs), flame retardants, etc. are detected in the environment and also represent a potential threat for it (Heberer 2002, Jorgensen et Halling-Sørensen 2000, Rogers 1996). treatments. For primary treatments, primary settling (PS) and physico-chemical lamellar settling (PCLS) are commonly used. While PS has been initially and widely employed, PCLS is more and more frequent since this technique operates more compactly and allows a wider flexibility of configuration and use. To our knowledge, some papers exist on one or another technology (Alexander et al. 2012, Carballa et al. 2005, Choubert et al. 2011) but there is no study neither comparing both technologies nor examining the impact of coagulant and flocculant on the pollutant removal at industrial scale.