|
FILTRATION Filtration is a physical process used to remove those suspended solids that would require unreasonably sedimentation times. This process is articulated in two phases: filtering and cleaning or regeneration. Wastewater is let pass through a column of granular media and particles are removed by impaction, interception, and physical straining. The suspended particles are accumulated on the surface of the filtering bed in some types of filter (slow filters), and/or are trapped in the interstices of the porous material in others (rapid filters), progressively clogging the filtering layer and increasing the head loss. When the head loss reaches a fixed value, filters are cleaned using a combination of water backwash and air scour. To optimise filter run-time and avoid rapid increases in filter head loss, the particle concentration (measured as TSS) applied to the filter should not exceed 20 mg/l. Filtration can be used downstream to the primary sedimentation or downstream to the secondary sedimentation. The percentages of removal that can be achieved by filtration are shown in the following Table . Since pathogenic organisms are attached to particles, filtration is effective at reducing pathogen concentration in wastewater providing an excellent pre-treatment for disinfection. Filtration is considered as a necessary treatment process in many applications of wastewater reuse to remove particulate matter that can compromise the effectiveness of disinfection. The disinfection of filtered effluent can remove or inactivate the 99.999% of the viruses, limiting the pathogenic content under measurable levels. Typical
filtration process removal* (Source: adapted from
WPCF, 1989)
* Values given in terms of percent removal from secondary effluent |