UV disinfection of RBC-treated light greywater effluent: Kinetics, survival and regrowth of selected microorganisms

The microbial quality of raw greywater was found to be much better than that of municipal wastewater, with 1.6×107 cfu ml−1 heterotrophic plate count (HPC), and 3.8×104, 9.9×103, 3.3×103 and 4.6×100 cfu 100 ml−1 faecal coliforms (FC), Staphylococcus aureus sp., Pseudomonas aeruginosa sp. and Clostridium perfringes sp., respectively. Further, three viral indicators monitored (somatic phage, host: Escherichia coli CN13 and F-RNA phages, hosts: E. coli F+amp, E. coli K12) were not present in raw greywater. The greywater was treated by an RBC followed by sedimentation. The treatment removed two orders of magnitude of all bacteria. UV disinfection kinetics, survival and regrowth of HPC, FC, P. aeruginosa sp. and S. aureus sp. were examined. At doses up to 69 mW s cm−2 FC were found to be the most resistant bacteria, followed by HPC, P. aeruginosa sp. and S. aureus sp. (inactivation rate coefficients: 0.0687, 0.113, 0.129 and 0.201 cm2 mW−1 s−1, respectively). At higher doses (69–439 mW s cm−2) all but HPC (which exhibited a tailing curve) were completely eliminated. Microscopic examination showed that FC self-aggregate in the greywater effluent. This provides FC an advantage at low doses, since the concentration of suspended matter (that can provide shelter from UV radiation) in the effluent was very low. FC, P. aeruginosa sp. and S. aureus sp. did not exhibit regrowth up to 6 h after exposure to increasing UV doses (19–439 mW s cm−2). HPC regrowth was proven to be statistically significant in un-disinfected effluent and after irradiation with high UV doses (147 and 439 mW s cm−2). At these doses regrowth resulted from growth of UV-resistant bacteria due to decreased competition with other bacteria eliminated by the irradiation.