Improved wastewater disinfection by ultrasonic pre-treatment

The objective of our work is to evaluate the scientific and economic potential of US application as a pre-treatment step in combination with UV to optimise the disinfection process of wastewaters. Ultrasound application of 20 s at low density of 30 W/l changed the particle size distribution (PSD) of the samples, the mean particle diameter dropped from 70 to 11 microm. Generally it is assumed that bioparticles bigger than 50 microm are difficult to disinfect by UV. We observed that the relevant particle size range >50 microm in samples taken from the primary clarifier was reduced by at least three-quarters by low ultrasound doses. As expected, these changes in PSD notably effect the disinfection efficiency of UV. Whereas UV treatment of secondary clarifier's effluents alone led to a reduction of fecal coliforms by 2.5 log units, pre-treatment by sonication (only 5 s at densities of 50 and 310 W/l) clearly enhanced the disinfection efficiency: reductions of CFU (colony forming unit) concentration now ranged between 3.3 and 3.7 log units. We noticed an influence of the bacteria's morphology on the disinfection efficiency of the combined process (US plus UV). Gram-positive streptococci seem less vulnerable to ultrasound exposure than thinner-walled gram-negative bacteria like the entire group of coliforms. The application of an ultrasound step might be also useful in terms of cost-effectiveness. In our lab-scale tests 30 s of UV treatment alone were required to reduce the number of fecal coliforms by 3.7 log units. When applied in combination, 5 s of ultrasonic followed by only 5 s of UV irradiation had the same result and energy consumption was only 43%.