Mechanisms implied in Escherichia coli removal during wastewater treatment

Abstract

The wastewater treatment reduces the assimilable organic fraction and the number of microorganisms of the effluents due to biological treatment and to the concentration of bacteria in sludge after settling. Recycling of sludge as an organic fertilizer is environment friendly but some pathogens could be concentrated in it. To make an integral tracing of E. coli during the activated sludge treatment, the fate of gfp-tagged cells were analysed in batch and pilot plant experiments. The exposure of E. coli to wastewater in absence of microbial population did not induce the entry into the viable but nonculturable state. The wastewater microbial populations showed a different relation with E. coli survival process. The presence of bacteriophages did not affect the survival while decrease in population was related with the presence of protozoa. Moreover, the wastewater bacteria behaved as predation-escaping prey and maintained their population density, while the E. coli were predated. Wastewater pilot plants prove an accurate model of a large scale plant. In our experiments, when pilot plant reached equilibrium, E. coli counts in aqueous fractions were stabilised about 104 cells ml-1 and in flocs or in sludge about 108 cells g-1. When addition of inoculated wastewater was stopped, the plant continued working with non-inoculated influent. Number of gfp-tagged E. coli in aqueous fraction diminished progressively and, after 2 d, it was below the detection limit. However, for the same period, 106 cells g-1 remained adhered to flocs and sludge. In conclusion, despite the efficacy of the protozoa removing E. coli from wastewater, this bacterium is not totally eliminated by treatment but mainly concentrated in sludge.