טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentGilboa Ben-David Yael
SubjectStudy of the Effects of UV Disinfection on the
Microbial Quality of Greywater
DepartmentDepartment of Civil and Environmental Engineering
Supervisor Professor Eran Friedler
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

The research examines the microbial quality of raw and treated greywater, assesses the efficiency of UV disinfection of greywater effluent and studies its kinetics, and provided an insight into the behavior of selected microorganisms in onsite greywater reuse systems for toilet flushing.


The research was performed in a pilot plant that treated light greywater from 14 flats. The plant consisted of two parallel sub-systems: An RBC (Rotating Biological Contactor) followed by sedimentation, and an MBR (Membrane bio-reactor). Effluents of the two sub-systems were disinfected by UV irradiation and pumped to two simulative reuse systems.


Residence time distributions and the flow regimes in the UV reactors were determined by tracer experiments at various theoretical residence times. UV radiation intensity was experimentally measured and calculated by actinometery (photochemical reaction).


The required UV dose was determined by batch experiments which examined the kinetics, survival and regrowth of selected bacteria. At low doses (up to 69 mW·s/cm2) fecal coliforms (FC) were signaled as the most resistant bacteria, followed by HPC (Heterotrophic Plate Count), P. aeruginosa sp., and S. aureus sp. Microscopic examination indicated that FC tend to aggregate in greywater effluent. These aggregates may provide shelter to FC and therefore contribute to their increased resistance to UV irradiation. Nevertheless, unlike the other bacteria, HPC was not totally removed from the effluent even at high UV doses, exhibiting "tailing" kinetics.


HPC regrowth was proven to be statistically significant (t-test, p<0.05) for undisinfected effluent and after irradiation with high doses (147, 439 mW·s/cm2). At high doses, regrowth resulted from growth of resistant bacteria due to decreased competition with other bacteria, which were eliminated by the irradiation. FC, P. aeruginosa sp. and S. aureus sp. did not exhibit regrowth at these experiments.


The optimal UV dose, as determined by batch experiments, was implemented in the continuous reuse systems (69 mW·s/cm2). In the reuse system (pipelines and tanks)  HPC regrowth occurred. This included regrowth of opportunistic pathogens such as P. aeruginosa sp. Therefore, addition of residual disinfectant or moving the UV units closer to the reuse point is needed.


The health risk associated with disinfected greywater reuse for toilet flushing was found to be insignificant. Furthermore, the microbial quality of the disinfected greywater was found to be the same or even better than the microbial quality of "clean" water in toilet bowls flushed with potable water. The UV disinfection units totally removed viral indicators spiked into the system.