Pharmaceuticals in soil leachates after raw and treated sludge spreading : impact of sludge treatments
Résumé
Sewage sludge contains a huge diversity of organic contaminants including pharmaceuticals (1). These compounds may interact with the organo-mineral environment prevailing in such complex matrix through various sorption/complexation mechanisms. These interactions could be modified by the treatment applied to sludge and this reactivity modification could thus influence the contaminants fate after sludge disposal onto soil. In order to study the impact of sludge treatments on the fate of pharmaceuticals after sludge spreading on soil, we have conducted lab-scale leaching experiments with 4 types of sludge sampled on an industrial sludge treatment line including anaerobic digestion, drying and composting. Batch and column assays (based on EU standards XP CEN ISO/TS 21268-1, XP CEN ISO/TS 21268-2, XP CEN/TS 14997 et XP CEN ISO/TS 21268-3) and lysimeters experiments were conducted to assess contaminant release from 4 sludge: a thickened sludge (S), the same sludge after anaerobic digestion (DS), then after drying (DDS) and finally composted with green wastes (C-DDS). The release was also assessed from soil-sludge mixtures. Carbamazepin (CBZ), ciprofloxacin (CIP), sulfamethoxazole (SMX) and ibuprofen (IBP) were quantified in the raw and treated samples and in the leachates of the assays. CIP and IBP transfer from a non-contaminated soil was also assessed by means of laboratory column experiments. CIP was present at the highest concentration in the 4 sludge samples, the three other compounds were present at concentration around and below 100 μg/kgDM. During all the leaching experiments, only CIP, CBZ and IBP were detected. Experiments carried out on sludge showed that different sludge treatments can lead to different contaminant releases. Indeed, the dried anaerobic digested sludge and the composted sludge led to a better retention of CIP and CBZ as shown during batch and column experiments. Sludge thickening and composting led to a greater retention of IBP as shown in batch experiments. However, the sludge exhibited the same behavior in column experiments concerning IBP. In addition, CIP was neither detected in batch nor in column experiments carried out with soil-sludge mixtures whereas IBP and CBZ were either quantified or detected. In batch experiments IBP was quantified in leachates coming from soil-DS mixture and only detected in leachates of other mixtures. Under the same batch conditions, CBZ was quantified in each leachate with a lower level in the soil-C-DDS. In the column experiments, IBP was quantified in all soil-sludge mixtures as well as CBZ. In addition, column experiments with the non-contaminated soil showed that IBP can migrate in soil. But, a fraction of IBP seemed irreversibly sorbed to the soil. On the contrary, CIP showed no migration in laboratory column experiments. In lysimeter vessel under unsaturated conditions, CIP was not detected in leachates. IBP was quantified in only one leachate coming from the lysimeter filled with the soil-DS mixture and was also detected in some other leachates. CBZ was quantified in all leachates, the lowest concentration was found for the soil-compost mixture. These results confirmed what was already observed during batch and column experiments: CIP is highly retained in soils whereas IBP and CBZ can be mobilized. These results are also in accordance with results obtained on field-experiment where CBZ and IBP were detected at low frequency in the soil-leachates (2). According to the concentrations found in the raw and treated sludge, it seems that ibuprofen is preferentially released compared to carbamazepin and ciprofloxacin. Ciprofloxacin is highly retained in the soil. Ibuprofen release is nevertheless moderate. The releases of ciprofloxacin and carbamazepin are lower is the case of compost supply.