Enhancement of ultraviolet disinfection of wastewater by high pressure water jet pretreatment: preliminary results and a new approach

The shield or shadow effect of large particles on bacteria undergoing irradiation from ultraviolet light in effluent of treated wastewater would greatly reduce the effectiveness of ultraviolet disinfection. The objective of this work is to evaluate the newly proposed method of applying a high pressure water jet (HPWJ) as a pretreatment step in combination with ultraviolet irradiation to improve the disinfection efficiency of wastewater by breaking up the large particles (reducing the shield or shadow effects). Pretreatment experiments with two HPWJ units having different water pressure output (6.9 and 13.1 MPa) and nozzles were conducted in the study on primary clarifier overflow (PCO). The experiment results revealed that the particle size distributions (PSDs) in the test samples were apparently shifted toward the small size range and the total number of particles has been greatly increased after undergoing treatment with the HPWJ. In general it is assumed that bioparticles larger than a certain size (10 [micro]m) are difficult to disinfect by ultraviolet. After the HPWJ treatment the relevant particle size range > 12.5 [micro]m in the test samples of the PCO was reduced on average by half for tested HPWJ settings. Also, a primary comparison showed that under a similar amount of energy consumption, the HPWJ performed better than the ultrasonic method did in terms of effectively breaking up large particles. As expected, the changes of PSD notably affect the efficiency of ultraviolet disinfection as tested with the collimated beam. The E. co/i and total coliform tests showed that pretreated PCO by the HPWJ enhanced the efficiency of ultraviolet disinfection by more than 1 log unit compared to treatment by ultraviolet alone when the ultraviolet dose was in the 20-30 mJ/[cm.sup.2] range. Multiple pretreatment with the HPWJ was also tested, which showed the advantage of breaking up slightly more particles, but with less enhancement of ultraviolet disinfection compared to a single pretreatment. This could possibly be due to the reduced ultraviolet transmission in the test sample caused by the presence of a larger number of suspended particles. The E. coli test results indicated that the well known 'tailing region' of typical patterns of the ultraviolet dose-response curve was eliminated or decreased after the PCO was treated by HPWJ. The application of a HPWJ step might also be beneficial in terms of cost-effectiveness. With HPWJ pretreatment it only takes about one-third of the ultraviolet light exposure time to achieve the same disinfection results as ultraviolet treatment alone in the collimated beam test. The experimental results in this study showed some degree of variation for similar experiments. This is not unusual for this kind of study, due to many uncontrolled factors such as characteristics of tested PCO samples, random error in the microbiology tests and so on. However, these experiments clearly showed that the proposal of using the HPWJ to pre-treat wastewater could be a very attractive alternative to enhance ultraviolet disinfection efficiency. DOI: 10.1061/(ASCE)EE.1943-7870.0000076 CE Database subject headings: Disinfection; Particles; Water jets; Viruses; Wastewater management.