Comparative Study of the Microbial Quality of Greywater treated by Three On-Site Treatment Systems

Abstract

This paper analyses the performance of a pilot scale treatment plant, treating light domestic greywater. The treatment included three parallel treatment units: stand-alone sand filtration (SFEB), RBC followed by sand filtration (SFRBC), and an MBR equipped with UF membranes (MBR). The performance of the SFEB unit was rather poor. The RBC and MBR units produced effluent of excellent quality, with COD of 42 and 40 mg l−1, BOD of 1.8 and 1.1 mg l−1, and turbidity of 0.6 and 0.2 NTU respectively. The SFEB failed to remove heterotrophic microorganisms (HPC), while the SFRBC and the MBR exhibited 2.1 and 3.6 logs removal, leading to effluent concentrations of 1.1×105 and 8.8×103 cfu ml−1 respectively. Faecal coliforms (FC) counts were 3.4×105, 1.4×105, 1.1×103 and 3.5×102 cfu 100 ml−1 in raw greywater, and in the SFEB, SFRBC and MBR effluents respectively. Further, in 60% of the samples no FC were detected in the MBR effluent. In order to simulate residence times in full scale systems, effluents were disinfected and stored for 0.5 h, 3 h, 6 h (normal operation), and one week (extreme event). The average chlorine demand was 8.1, 3.8 and 2.9 mg l−1 for SFEB, SFRBC and MBR effluents respectively. Low residual chlorine (0.15–0.22 mg l−1) remained in all effluents even after a week-long storage. Disinfection reduced HPC by 5, 2 and 2 orders of magnitude in the SFEB, SFRBC and MBR effluents respectively, with no regrowth in short contact times (up to 6 hours). Some regrowth was observed after a week-long storage leading to 106, 104 and 103 cfu ml−1 (SFEB SFRBC and MBR respectively). Disinfection reduced FC counts in all three types of effluent to 0 cfu 100 ml−1, whilst no FC regrowth was observed after week-long storage. The results show that both RBC and MBR treatment units are viable options for on-site greywater reuse. The disinfection experiments strongly indicate that the health risk associated with the reuse of these effluents is minimal even after long period of storage.