Improving chlorine disinfection of wastewater by ultrasound application

The presence of soluble organic material as well as high concentrations of suspended matter in waters and wastewaters affect the efficiency when chlorine is used as disinfection agent. The objective of our work is to explore to which extend ultrasonic treatment can facilitate wastewater disinfection with chlorine in order to bring down doses of ecologically questionable chlorine and to shorten contact times. Sewage treatment plant (STP) effluents with different concentrations of suspended solids are exposed to sonication in combination with chlorine dosage. We observed that enhancement of chlorine efficiency is better for samples with higher concentrations of suspended matter. For samples with a TSS concentration of 50 mg/L chlorination efficiency (2 mg/L) can be doubled from 0.7 to 1.4 log when treated simultaneously with 20 kHz ultrasound for 5 minutes, i.e. levels of indicator organisms can be brought down to numbers that conventionally require far higher doses of chemical disinfectants. As subsequent sonication/chlorination does not have the same significant effect as simultaneous application of these two means, ultrasound does not just have a declumping effect; it seems that ultrasound application provokes a better chlorine dispersion in the aqueous media which improves the fast chemical and bactericidal reaction.