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

Abstract

In this study, the effect of light intermittence on solar disinfection of secondary treated wastewater was investigated. Synthetic secondary effluent was spiked with E. 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-hour 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.

Monitoring the post-irradiation E. coli survival patterns in environmental water matrices: Implications in handling solar disinfected wastewater

“…Marugan et al [43] have underlined the importance on Mg and K salts for the maintenance of bacterial life and Caballero et al [44] highlighted the aid nutrients provide in the growth of bacteria in water matrices. Therefore, optimal support conditions for bacterial development is expected, as seen in previous works with the same wastewater matrix [40,61]. Generally, increasing the dose decreases bacterial survival.…”

“…Water matrices 2.1.1. Synthetic wastewater preparation Analytical preparation of the synthetic secondary effluent and the specifications concerning the microorganism strain used in this research, are described in detail elsewhere [40]. In summary, the synthetic wastewater composition was 160 mg/L peptone, 110 mg/L meat extract, 30 mg/L urea, 28 mg/L K 2 HPO 4 , 7 mg/L NaCl, 4 mg/L CaCl 2 2H 2 O and 2 mg/L Mg 2 SO 4 7H 2 O, with 250 mg/L COD.…”

Temperature-dependent change of light dose effects on E. coli inactivation during simulated solar treatment of secondary effluent

“…The bacterial preparation, have been described analytically elsewhere (Giannakis et al, 2013) Then, a 1/10 dilution of the effluent was performed, in order to simulate 90% BOD removal, a typical value in secondary treatment of municipal wastewater. Contrary to actual secondary effluent samples, this synthetic wastewater contained only small amounts of suspended solids.…”

Ultrasound enhancement of near-neutral photo-Fenton for effective E. coli inactivation in wastewater

“…1, the configuration permits the sequential treatment of the synthetic wastewater; US/photo-Fenton treatment was taking place (or vice versa). The same configuration was used in one of our previous works, (used in [29], similar to the set-up used by Mendez-Arriaga et al, [34]), synthetic wastewater from a cylindrical double-wall glass vessel (400 mL) was pumped by a peristaltic pump through three glass reactors (diameter 3.8 cm, effective irradiation surface 214.8 cm 2 ), connected in series, of total volume 230 mL. Temperature was regulated by water recirculating around the reactor and connected to a thermostat.…”

“…The recirculating flow of these reactors creates a gap in the illumination for as long as water is present in the (dark) storage tank, allowing bacterial defense mechanisms to deploy [28]. Literature indicates a variety of light-to-dark distributions (Table 1), which materialize this difference [17,18,23,26,[29][30][31].…”