Resistance and inactivation kinetics of bacterial strains isolated from the non-chlorinated and chlorinated effluents of a WWTP

Abstract

The microbiological quality of water from a wastewater treatment plant that uses sodium hypochlorite as a disinfectant was assessed. Mesophilic aerobic bacteria were not removed efficiently. This fact allowed for the isolation of several bacterial strains from the effluents. Molecular identification indicated that the strains were related to Aeromonas hydrophila, Escherichia coli (three strains), Enterobacter cloacae, Kluyvera cryocrescens (three strains), Kluyvera intermedia, Citrobacter freundii (two strains), Bacillus sp. and Enterobacter sp. The first five strains, which were isolated from the non-chlorinated effluent, were used to test resistance to chlorine disinfection using three sets of variables: disinfectant concentration (8, 20 and 30 mg·L(-1)), contact time (0, 15 and 30 min) and water temperature (20, 25 and 30 °C). The results demonstrated that the strains have independent responses to experimental conditions and that the most efficient treatment was an 8 mg·L(-1) dose of disinfectant at a temperature of 20 °C for 30 min. The other eight strains, which were isolated from the chlorinated effluent, were used to analyze inactivation kinetics using the disinfectant at a dose of 15 mg·L(-1) with various retention times (0, 10, 20, 30, 60 and 90 min). The results indicated that during the inactivation process, there was no relationship between removal percentage and retention time and that the strains have no common response to the treatments.

Figure 1

Reduction of CFUs at three temperatures (20, 25 and 30 °C) as a function of the product of the initial disinfectant concentration (mg·L⁻¹) and the contact time (min). (a) A. hydrophila; (b) E. coli; (c) E. coli PD3; (d) E. coli BL21; (e) E. cloacae.

Figure 2

Standard deviations observed in resistance tests that used temperature and disinfectant dose as variables. C = concentration (mg·L⁻¹) (a) T = 0 min; (b) T = 15 min; (c) T = 30 min.

Figure 3

Values from the ANOVA test and the average midline of all values.

Figure 4

Response of the bacterial strains to the treatment, as represented by the trend line and slope value. K. cryocrescens^a (y = −0.012x + 10.14). K. cryocrescens^b (y = −0.005x + 12.92). K. cryocrescens^c (y = −0.018x + 11.64). K. intermedia (y = −0.002x + 12.86). C. freundii^a (y = −0.013x + 12.68). C. freundii^b (y = −0.0412x + 13.524). Bacillus sp. FRC_Y9-2 (y = −0.022x + 12.33). Enterobacter sp. MS5 (y = 0.0057x + 11.92).