Microbial and operational response of an anaerobic fixed bed digester to oleic acid overloads

The effect of oleic acid overloads on biomass accumulation and activity in an anaerobic filter was investigated. An anaerobic fixed-bed reactor specia lly designed to allow the regular withdrawal of accumulated biomass was use d for that purpose. Organic and hydraulic shocks were performed during four days, by stepwise increasing the substrate concentration from 4000 to 20 0 00 mg COD/1 or by reducing the hydraulic retention time from 16 to 3.2 h. D uring the organic shock, operational performance was more affected than in the hydraulic one, which was the result of the higher degree of inhibition detected in the acetoclastic, hydrogenophilic and syntrophic activities. Th e ratio adhered/total biomass remained between 17 and 32% during the hydrau lic shock, and between 13 and 60% during the organic shock, suggesting a mo re stable biofilm during the hydraulic shock. A long time (900 h) after the hydraulic shock, hydrogenophilic and syntrophic activities recovered to hi gher values than before the shock, but after the organic shock only acetocl astic activity recovered pre-shock values. Hydraulic shock induced an incre ase in tolerance to oleic acid toxicity, evidenced by an increase in the to xicity limit (IC50) from 140 +/- 30 to 215 +/- 25 mg/l. (C) 2001 Elsevier S cience Ltd. All rights reserved.