Bacteriological quality of effluent submitted consecutively to a macrofiltration and ultraviolet light systems in the Tunisian conditions

Abstract

This paper deals with the study of bacteriological quality of effluents that have undergone consecutively different macrofiltration system (pressure sand filter or disc filter used as a secondary treatment) and UV254 irradiation process (used as a tertiary treatment). These two successive systems of treatment were evaluated to determine their possible application as commonly alternatives to the conventional system of wastewater treatment and disinfection before wastewater reuse. They both combined systems of wastewater treatment released effluent of excellent bacteriological quality, with almost total absence of feacal coliforms, of E. coli and of P. aeruginosa). However, if the bacteriological quality of the effluent remained constant in the case of macrofiltration system (disc filter or pressure sand filter); the UV disinfection process showed to deeply depend on the quality of effluent, particularly with regard to UV transmittance. The daily bacteriological monitoring of the secondary effluent at the exit of the pressure sand filter by UV reactor and by using a dose of 96 mJ/cm(2), corresponding to an exposure of 16 min, showed an average rate of inactivation of around 3 U-Log, for feacal coliforms, E. coli and P. aeruginosa, respectively. Therefore, the average bacterial concentration remaining in the water at the exit of the UV reactor is less than 1000 cfu/100 ml for feacal coliform and E. coli. For P. aeruginosa, the remaining number is less than 100 bacteria/100 ml. These two last values coincide substantially with the range recommended by several standardized international guidelines. Therefore, numerous authors reported that P. aeruginosa is very resistant to UV irradiation compared to the other bacterial indicators. In contrast, our study revealed that feacal coliforms and E. coli were more UV light resistant than P. aeruginosa. This finding could be explained by the fact that E. coli and feacal coliform forms aggregates in the treated effluent, while P. aeruginosa exists either as discrete cells or as cell pairs.

Keywords: Disinfection; Macrofiltration systems; Reuse; UV reactor; Wastewater.

Figure 1

Pst: Primary settling tank (Influent); Psf: Pressure sand filter; Df: Disc filter; Sst1 and Sst2: Secondary settling tanks; Se1 and Se2: Secondary effluent; Ut: UV treatment.

Figure 2

Comparison of UV inactivation rates for E. coli , feacal coliform and P. aeruginosa (log reduction) at the exit of the disc filter and of the pressure sand filter, respectively. y: Reduction = N/N ₀ with N; Number of micro-organisms at the instant T; N ₀; Number of micro-organisms at the instant T = 0; Dose (mJ/cm²) = X = It = UV Intensity (mW. cm⁻²). Time of contact(s).

Figure 3

UV inactivation rates for E. coli , feacal coliform and P. aeruginosa (log reduction). y: Reduction = N/N ₀ with N; Number of micro-organisms at the instant T; N ₀; Number of micro-organisms at the instant T = 0; Dose (mJ/cm²) = X = It = UV Intensity (mW. cm⁻²). Time of contact(s).

Figure 4

UV inactivation rates for E. coli , feacal coliform and P. aeruginosa obtained on the exit of the disc filter or pressure sand filter, respectively, and at exposure different times UV.

Figure 5

Effluent concentrations of E. coli, feacal coliform (FC) and P. aeruginosa in disc filter and pressure sand filter as a function of influent concentration of E. coli, feacal coliform (FC) and P. aeruginosa (Linear adjustment).

Figure 6

E. coli, feacal coliform counts (Log cfu/100 ml) and P. aeruginosa counts (Log bacteria/100 ml) in influent from disc filter, pressure sand filter and UV treatment. y: Reduction = N/N ₀ with N; Number of micro-organisms at the instant T; N ₀; Number of micro-organisms at the instant T = 0.