Nowadays, mesophilic anaerobic processes (under 35ºC) predominate but they show to be unable to accomplish the new specifications. Consequently, only the migration to higher temperature conditions, i.e. thermophilic operation, seems to be capable of producing stabilized sludge that meets the new restrictions related to the presence of harmful bacteria. In addition, the effect of anaerobic digestion on organic micropollutants has scarcely been studied. Moreover, the temperature impact on these compounds in anaerobic digestion of sewage sludge has also not been elucidated.

The overall objective of this work is to study the fate of some organic micropollutants during raw sewage sludge (RSS) anaerobic digestion and the effect of thermophilic temperature conditions on treatment efficiency, when compared with mesophilic anaerobic. To this purpose, a mesophilic and thermophilic anaerobic digester were operated using Waste Activated Sludge (WAS) as inoculum and the effects of Hydraulic Retention Time (HRT) reduction and its influence on the fate of the selected organic micropollutants (namely, PAH, DEHP, AOX, PCB, NPE and LAS) were studied. Furthermore, the effect of a complementary treatment, namely the ultrasonic pretreatment, on the biodegradability of the organic feed and the behaviour of the subsequent anaerobic digestion was also examined, focussing on the micropollutants reduction.

The main conclusions of every set experimental are presented:

Start-up and HRT reduction

· The use of WAS as inoculum for anaerobic digesters start-up represents a good option specially under thermophilic conditions.

· Thermophilic anaerobic digestion is a good alternative to mesophilic digestion in order to improve specific biogas production.

· The similar values of organic matter removal (VS) registered in both digesters.

· 8 days was found to be the minimum HRT for the thermophilic digester, since VFA accumulation and pH decrease inhibited the process when working at a lower HRT.

Organic micropollutants removal

· Organic micropollutants removal efficiency was enhanced under thermophilic conditions.

· The HRT influence was clearly observed for DEHP and PCB, independently of digestion temperature. However, it was temperature dependent for PAH, AOX, LAS and espcially for NPE.

· NPE and DEHP are identified as the more recalcitrants contaminants.

· The thermophilic conditions were not ensured the sludge safety conditions for agriculture use.

· The accumulation of some compounds as NP and LCB was observed.

· NP was stated as the intermediate compound of NPEO anaerobic degradation. Moreover, LCB were generated during HCB reductive dechlorination during anaerobic digestion.

· Monitoring the metabolites of cited contaminants can be elucidated their biodegradation mechanisms and prevent any undesirable effects.

Ultrasound-anaerobic digestion combined treatment

· Organic matter removal, SBP and naphthalene removal were enhanced using the combined treatment in both digesters.

· High organic matter removal, SBP and napthalene removal was registered in the thermo-digester.

· Higher was enhancement of organic matter removal, SBP and naphthalene removal in the meso-digester than in the thermo-digester.

· Pyrene removal was nearly the same with and without pretreatment.

· It seems that the ultrasonic pretreatment affect more the mesophilic than the thermophilic digestion.

· The results obtained in this combined treatment (ultrasound + anaerobic digestion) are promising and suggest the convenience of other investigations of other combined treatments to enhance the stabilized sludge quality.