Influence of design and operating conditions on the removal of MS2 bacteriophage by pilot-scale multistage slow sand filtration

This paper summarizes findings from sixteen MS2 bacteriophage (MS2) challenge tests on biologically mature pilot-scale slow sand filters, conducted at varying water temperatures (<10° and >20°C), two hydraulic loading rates (0.1 vs. 0.4 m/h), and two bed depths (0.4 vs. 0.9 m). Few studies have quantified virus removal by slow sand filters at filtration rates on the high end and bed depths on the low end of typical practice, and none report virus removal below 5°C. The conditions investigated are important, because high filtration rates and low bed depths are sometimes seen as ways of making slow sand filtration more cost effective. MS2 removal increased with greater sand depth and warmer water temperature, but decreased at the higher hydraulic loading rate. Average MS2 removals ranged from 0.1 to 0.2 log in the roughing filters and 0.2 to 2.2 log in the slow sand filters. Shedding of MS2 was observed for up to 12 days after seeding was stopped. As a stand-alone process, slow sand filtration (with or without roughing filtration) may not provide adequate virus removal under some conditions and should be combined with a disinfection/inactivation step to provide robust compliance with regulatory requirements and protection of human health.