Comparison of drinking water treatment process streams for optimal bacteriological water quality

Four pilot-scale treatment process streams (Stream 1 – Conventional treatment (coagulation/flocculation/dual media filtration); Stream 2 – Magnetic ion exchange (MIEX)/Conventional treatment; Stream 3 – MIEX/Conventional treatment/granular activated carbon (GAC) filtration; Stream 4 – Microfiltration/nanofiltration) were commissioned to compare their effectiveness in producing high quality potable water prior to disinfection. Despite receiving highly variable source water quality throughout the investigation, each stream consistently reduced colour and turbidity to below Australian Drinking Water Guideline levels, with the exception of Stream 1 which was difficult to manage due to the reactive nature of coagulation control. Of particular interest was the bacteriological quality of the treated waters where flow cytometry was shown to be the superior monitoring tool in comparison to the traditional heterotrophic plate count method. Based on removal of total and active bacteria, the treatment process streams were ranked in the order: Stream 4 (average log removal of 2.7) > Stream 2 (average log removal of 2.3) > Stream 3 (average log removal of 1.5) > Stream 1 (average log removal of 1.0). The lower removals in Stream 3 were attributed to bacteria detaching from the GAC filter. Bacterial community analysis revealed that the treatments affected the bacteria present, with the communities in streams incorporating conventional treatment clustering with each other, while the community composition of Stream 4 was very different to those of Streams 1, 2 and 3. MIEX treatment was shown to enhance removal of bacteria due to more efficient flocculation which was validated through the novel application of the photometric dispersion analyser.

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► Removal of bacteria varied between the process streams with log removal values of between 0.9 and 2.7.
► Based on removal of bacteria, the streams were ranked: Stream 4 > Stream 2 > Stream 3 > Stream 1.
► FCM was superior to the HPC method with more definitive comparisons made between the streams.
► DGGE analysis revealed that the community of Stream 4 was very different to those of Streams 1, 2 and 3.
► MIEX enhanced bacterial removal through more efficient coagulation as determined by PDA indices.