Mechanical Filtration 101 Chapter 2
External Mechanical Prefilters
Precisely because the smaller screens on “clean water” pumps clog more easily, the pumps are often paired up with larger external prefilters with much greater total surface area than the prefilter screen the pump comes with. The suction from the pump is spread out over so much larger an area that debris is less likely to get pulled into and trapped on the screen. These prefilters, properly sized, allow the pumps to go days, weeks or even longer without attention. The simplest of these larger prefilters is the pump bag, a simple open-ended bag typically made of 1/8” polypropylene mesh that the pump sits inside, and the bag cinched closed around the cord and tubing. Another type of External Prefilter isattached directly to the intake of the pump, and might carry a variety of filtration media besides just a simple screen or mesh.
This ability to add filter media makes theseSubmersible Filters very popular in ponds under 250 gallons, because the Prefilter can also function as a chemical filter with an activated carbon pad, and as a biological filter, where bacteria colonize media and metabolize ammonia and nitrites. Of course, if the filter media is not cleaned regularly, you don’t just lose filtration – you also starve the pump of water as the intake gets progressively more and more clogged, which can lead to overheating and eventual pump failure - talk about a double edged sword! Choose your External Prefilter on the basis of the balance you want to strike between effective filtration of fine particles, and length of time between cleanings. Many modern External Prefilters give up the ability to effectively filter the water in favor of the ability to keep a pump from clogging for weeks or even months.
Precisely because the smaller screens on “clean water” pumps clog more easily, the pumps are often paired up with larger external prefilters with much greater total surface area than the prefilter screen the pump comes with. The suction from the pump is spread out over so much larger an area that debris is less likely to get pulled into and trapped on the screen. These prefilters, properly sized, allow the pumps to go days, weeks or even longer without attention. The simplest of these larger prefilters is the pump bag, a simple open-ended bag typically made of 1/8” polypropylene mesh that the pump sits inside, and the bag cinched closed around the cord and tubing. Another type of External Prefilter isattached directly to the intake of the pump, and might carry a variety of filtration media besides just a simple screen or mesh.
This ability to add filter media makes theseSubmersible Filters very popular in ponds under 250 gallons, because the Prefilter can also function as a chemical filter with an activated carbon pad, and as a biological filter, where bacteria colonize media and metabolize ammonia and nitrites. Of course, if the filter media is not cleaned regularly, you don’t just lose filtration – you also starve the pump of water as the intake gets progressively more and more clogged, which can lead to overheating and eventual pump failure - talk about a double edged sword! Choose your External Prefilter on the basis of the balance you want to strike between effective filtration of fine particles, and length of time between cleanings. Many modern External Prefilters give up the ability to effectively filter the water in favor of the ability to keep a pump from clogging for weeks or even months.
