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.

Processing of the produced primary and secondary sludge during sewage treatment is demanding and requires considerable resources. Most common practices suggest the cotreatment of primary and secondary sludge starting with thickening and anaerobic digestion. The aim of this study is to investigate the anaerobic digestion of the primary sludge only and estimate its impact on sludge treatment and energy recovery. Within this context, the performance of the anaerobic digestion of primary sludge is explored and focused on practices to further enhance the methane production by using additives, e.g., a cationic polyelectrolyte and attapulgite. The results showed that the overall yield in methane production during anaerobic digestion of primary sludge alone was higher than that obtained by the anaerobic digestion of mixed primary and secondary sludge (up to 40%), while the addition of both organic polyelectrolyte and attapulgite enhanced further the production of methane (up to 170%). Attapulgite increased the hydrolysis rate of biosolids and produced relatively stabilized digestate, though of lower dewaterability. Moreover, the results suggest that single digestion of primary sludge may accomplish higher methane production capacities at lower digestors’ volume increasing their overall efficiency and productivity, while the produced digestates are of adequate quality for further utilization mainly in agricultural or energy sectors.

a b s t r a c tA 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 10 3 , 10 4 , 10 5 and 10 6 CFU/mL, the exposure time 1, 2, 3 and 4 h, the treatment temperature 20, 30, 40, 50 and 60• 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• C drastically impaired disinfection. Thermal inactivation with no regrowth predominated at 50• C and total inactivation of microorganisms was observed at 60• C in nonirradiated samples. Irradiation at 800 and 1200 W/m 2 much increased inactivation efficiency, especially at 50 and 60• C, proving sensitive light-temperature synergy at those temperatures. Total inactivation was achieved within 4 h under a range of treatment conditions, including all samples at 1200 W/m 2 , or 60• 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 • 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.

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