The selective serotonin reuptake inhibitors citalopram, sertraline, paroxetine, fluvoxamine and fluoxetine have been investigated in 101 anaerobic lab-scale digesters with continuous stirring and mesophilic conditions at 37 degrees C to investigate whether they would be reduced or accumulated in sewage sludge depending on whether the bacteria present were able to use the SSRIs as a carbon source or not. The total SSRI concentration had a significant reduction in concentration during the anaerobic treatment process from theoretically 0.58 mg/l to 0.21 mg/l after 17 days. However, large differences in the reduction of the different compounds were found. Paroxetine and citalopram were found to be almost completely reduced at day 24 with reductions of 85% (citalopram) and 98% (paroxetine). Reductions of 32% (fluoxetine), 53% (fluvoxamine) and 38% (sertraline) indicate that these three compounds have a higher potential for accumulation. None metabolites of these compounds were found in the samples. (C) 2012 Elsevier Ltd. All rights reserved.