In order to evaluate the removal efficiency of an activated sludge plant operating under aride sahelian climate in Mauritania, a 15 months period bacteriological and chemical survey was undertaken. Motile aeromonads, Pseudomonas aeruginosa, faecal coliforms, faecal streptococci and the total heterotrophic bacteria abundances were systematically determined at three sampling points : E (wastewater plant influent), C (oxidation pond) and S (treated effluent). The total suspended solids (TSS) and the biochemical oxygen demand (BOD), temperature and pH were also systematically measured for each sample.Two seasonal qualitative analyses for the Aeromonas species dynamics have been undertaken on water samples obtained from the influent (E) and the effluent of the plant (S). Results showed removal efficiency of 0,45 U log, 0,41 U log, 1,16 U log, 0,59 U log and 0,47 U log for the motile Aeromonas species, Pseudomonas aeruginosa, total heterotrophic bacteria, faecal coliforms and faecal streptococci respectively. In regard to the chemical pollution removal efficiency, the results revealed an important removal of the total suspended solids and BOD. This removal seems to occur essentially (68 %) between the oxidation channel and the clarification pond.

The species dynamics analyses showed that A. caviae dominated in both raw wastewater and effluent. A. sobria strains were slightly present (17 %) in the raw wastewater and no A. hydrophila strains were detected in the treated effluent in the hot period campaign. The statistical non parametric analysis revealed significant differences (P<0.05) between the different bacterial abundances in the inflow (E) and the outflow (S) of the treatment plant, but have showed that the bacterial removal occurred mainly during the oxidation phase and that only a slight removal (23%) were originated from the clarification phase. Antimicrobial susceptibility tests have covered two hundred and twenty four-strains of motile Aeromonas species and E. coli isolated from the influent, the oxidation pond and effluent. Fifteen of the most frequently used antibiotics in the antibiotic treatment at the national level were tested on these isolates.

The statistical analysis showed no significant differences (P>0.05) in the resistance patterns between influent and effluent isolates. All strains were found to be monoresistant, primarily to Vancomycin. Nearly 4.5 % of A. hydrophila and 3.1 % A. caviae were resistant to Cefoperazone, while greater than 97.1 % of A. sobria were found to be susceptible to this drug. The overall resistance rates to Amikacin and Chloramphenicol did not exceed 4.6 and 10.6 % respectively.The results indicate that despite the important removal rate given by the treatment process, antimicrobial resistance incidence among pathogenic aeromonads has not been decreased and remains significant to potentially compromise the reclaim of the treated effluent in urban agricultural practices in the wastewater spreading area of Nouakchott where water reclaim permits to the urban agriculture to survive water scarcity.

Keyword:Activated sludges, Aeromonas, species dynamics, Antibiotics, Irrigation, Polyresistance, Rem

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