Onder Ayyildiz

Effect of ultrasonic pretreatment on chlorine dioxide disinfection efficiency

Ultrasound (US) and chlorine dioxide (ClO(2)) were combined sequentially in an aim to improve wastewater disinfection. Results demonstrated that the combined effect of US and ClO(2) on Escherichia coli (E. coli) and total coliform (TC) inactivation in raw wastewater was synergistic involving high removal of bacteria from the solution. A sequential combination of US (150 or 300 W/L) and ClO(2) (2 mg/L) provided about 3.2-3.5 log reduction in the number E. coli and TC in raw wastewater, while the sum of log reductions by the individual treatments were 1.4-1.9. However, the measured inactivation rate with the combination of ultrasound and ClO(2) in synthetic wastewater or secondary effluent was the same as the sum of the log inactivations with individual treatments. The enhancement attained by combined US and ClO(2) disinfection methods was attributed to the presence of high concentration of particles in raw wastewater and their break up under shock sound waves.

Impacts of water organic load on chlorine dioxide disinfection efficacy.

This study has examined the bactericidal effect of chlorine dioxide in untreated artificial and domestic wastewaters and secondary effluent of various organic loads. Results indicated that the inactivation of Escherichia coli in artificial wastewater was similar with that in real municipal wastewater. Among three waters, the bactericidal effect of chlorine dioxide was lowest in secondary effluent. The bacteria log inactivation increased by up to threefold when the COD concentration of raw wastewater was decreased by half. An unfavorable COD effect was also observed for the disinfection of secondary effluent. To explain the COD effect on bacteria inactivation, chlorine dioxide residuals were measured with time through each disinfection process. Results from statistical analyses have revealed that, in comparison to the correlations using CT values, the inactivation data can be better correlated with the ratio of COD to ClO(2) concentrations. The results of this study would be a useful guide for many municipalities and communities in determining chlorine dioxide dosages for water and wastewater disinfection systems.

Ultrasound-assisted activation of zero-valent magnesium for nitrate denitrification: identification of reaction by-products and pathways.

Zero-valent magnesium (Mg(0)) was activated by ultrasound (US) in an aim to promote its potential use in water treatment without pH control. In this context, nitrate reduction was studied at batch conditions using various doses of magnesium powder and ultrasound power. While neither ultrasound nor zero-valent magnesium alone was effective for reducing nitrate in water, their combination removed up to 90% of 50 mg/L NO3-N within 60 min. The rate of nitrate reduction by US/Mg(0) enhanced with increasing ultrasonic power and magnesium dose. Nitrogen gas (N2) and nitrite (NO2(-)) were detected as the major reduction by-products, while magnesium hydroxide Mg(OH)2 and hydroxide ions (OH(-)) were identified as the main oxidation products. The results from SEM-EDS measurements revealed that the surface oxide level decreased significantly when the samples of Mg(0) particles were exposed to ultrasonic treatment. The surface passivation of magnesium particles was successfully minimized by mechanical forces of ultrasound, which in turn paved the way to sustain the catalyst activity toward nitrate reduction.