Impact of different light intermittence regimes on bacteria during simulated solar treatment of secondary effluent: Implications of the inserted dark periods

In this study, the effect of light intermittence on solar disinfection of secondary treated wastewater was investigated. Synthetic secondary effluent was spiked with Escherichia coli and submitted to 3 different light intermittence regimes by circulating the effluent between a dark storage tank and three in-series illuminated reactors. The relative influence of the recirculation rate on bacterial inactivation was studied, in short (3-7 min) light regimes and a dark-to-light ratio of 2.04. Lower recirculation rates resulted in poorer disinfection results, showing the detrimental effect of longer dark storage periods on the removal efficiency. Also, longer time intervals were employed in batch tests, to investigate the effect of 1, 2 and 3-h dark intervals, during recreated solar disinfection conditions; fourteen different scenarios were tested. Three hours of continuous or cumulative illumination were proven enough to provide the necessary dose to damage bacteria irreparably, while interruption during these hours favored bacterial resistance. Finally, absence of regrowth was observed in all cases that derived from samples with null bacterial counts. However, when a fraction of viable bacteria was present at the end of the solar treatment, survival was favored. (C) 2013 Elsevier Ltd. All rights reserved.

Bactericidal action of illuminated TiO2 on pure Escherichia coli and natural bacterial consortia: post-irradiation events in the dark and assessment of the effective disinfection time

César Pulgarin

Bactericidal action of illuminated TiO2 on pure culture of Escherichia coli K 12 and bacterial consortium was studied. Photocatalytic E. coli inactivation rate was dependent on the biological parameters such as: physiological state, generation and initial concentration of bacteria. The behavior of the bacterial suspension during the subsequent dark period was discussed in order to estimate the potential of using the photocatalytic treatment process in a real water disinfection Situation. The effective disinfection time (EDT) was defined as the time required for total inactivation of bacteria without re-growth in a subsequent dark period referenced at 48 It. An increase of E. coli concentration was observed after illumination of bacteria without TiO2 (named here as photolysis). In the presence of TiO2 (photocatalysis), the decrease of bacteria continues in the dark, and no regrowth was observed within the following 60 h. The extent) applied during the illumination period. Thus, of this "residual disinfecting effect" was dependent on the light intensity (400 or 1000 W/m(2)) EDT was reached in photocatalytic but not in photolytic treatment. Bacterial consortia present in two different wastewaters were phototreated with and without TiO2 and a significant difference in bacterial sensitivity to both phototreatments was observed. Enterococcus sp. appear to be less sensitive than coliforms and other Gram-negative bacteria. Considerable differences in photoreactivity were observed for both samples taken at the same place but at different date. The first sample named as wastewater 1, exhibited a bacteriostatic but no bactericidal effect after both phototreatments; the addition of TiO2 accelerated the solar disinfection. In the second sample, wastewater 2, a bactericidal effect was observed in photolytic and photocatalytic experiments and the EDT was attained in both cases, even if the bacterial inactivation rate was the lowest in the presence of the catalyst. The chemical oxygen demand (COD) decreases during photocatalytic treatment, but not during the photolytic one; whereas the Dissolved organic carbon (DOC) increases in both cases. COD/DOC ratio decreases during phototreatrnents. (C) 2003 Elsevier B.V. All rights reserved.

2004

The antagonistic and synergistic effects of temperature during solar disinfection of synthetic secondary effluent

Stefanos Giannakis, César Pulgarin

A 4-factor, multilevel, full factorial design of 240 experiments was performed in order to investigate the effect of temperature on the inactivation efficiency of spiked Escherichia coli in simulated solar disinfection of a synthetic secondary effluent. The initial population of the microorganisms was 103, 104, 105 and 106 CFU/mL, the exposure time 1, 2,3 and 4h, the treatment temperature 20, 30, 40, 50 and 60 degrees C and the sunlight intensity 0, 800 and 1200 W/m(2). Radical changes in bacterial behavior, process efficiency and remaining populations were observed, while treating effluents in discreet temperatures. Elevating treatment temperature from 20 to 40 degrees C drastically impaired disinfection. Thermal inactivation with no regrowth predominated at 50 degrees C and total inactivation of microorganisms was observed at 60 degrees C in nonirradiated samples. Irradiation at 800 and 1200 W/m2 much increased inactivation efficiency, especially at 50 and 60 degrees C, proving sensitive light-temperature synergy at those temperatures. Total inactivation was achieved within 4h under a range of treatment conditions, including all samples at 1200 W/m(2), or 60 degrees C samples at 800 W/m(2). Also, 99.9-100% efficiencies and final population below 1000 CFU/100 mL were obtained at 800 W/m(2) and temperatures of 50 degrees C and above. Treatment time, temperature and intensity are the critical parameters for the disinfection process, while initial population is insignificant for removal efficiency. An explanation of the mechanism of the process as well as a general linear model predicting the outcome of the experiments is also suggested. (C) 2014 Elsevier B.V. All rights reserved.

Elsevier2014

Bactericidal action of illuminated TiO2 on pure Escherichia coli and natural bacterial consortia: post-irradiation events in the dark and assessment of the effective disinfection time

César Pulgarin

2004