Submersible Vs. External UV Pond Filters

Ultraviolet sterilizers produce light that destroys bacteria, viruses, algae and protozoans. The ultraviolet light destroys the DNA in the cells of these living organisms. Ultraviolet sterilizers are cylinders through which pond water is pumped. These units can stand alone or can be submersible. External, standalone units are normally self-contained and are fitted with their own pump and mechanical filter. Pond water should always pass through a mechanical filter before entering the ultraviolet sterilizer to ensure as much dirt is removed as possible.

  1. Submersible Units

    • Submersible ultraviolet sterilizers are typically plumbed into the existing filtration piping or tubing. These units receive water from the main filter box and are not run by their own pump. Submersible ultraviolet sterilizers are cylinders that contain a treatment chamber and bulb, sealed in a quartz glass tube. Submersible ultraviolet sterilizers have their own power cord. By positioning these units downstream of the mechanical filter, large particles of debris can be filtered out of the water before it enters the sterilizer.

    External Units

    • External, standalone ultraviolet sterilizer units have their own dedicated pump and can be positioned anywhere. These units invariably also contain a mechanical filter sponge or similar material and can therefore function separately from the main biological filter box. The pump supplies pond water to the unit; the filter sponge removes particles from the water. This prevents the glass tube -- which contains the light source -- from becoming coated with detritus. These units are not dependant on gravity-fed water from the main filter box and can be hidden in shrubbery or behind rocks that surround the pond.

    Water Flow

    • The flow of water through an external ultraviolet sterilizer, run by its own pump, is balanced so the exposure time from the light source is sufficient for the volume of water that is pumped through the unit. Submersible ultraviolet sterilizers are normally connected to the existing filter piping and receive the volume of water that is pumped through the biological filter box. Water may therefore pass either too slowly or too quickly through the unit. In these cases, exposure time to the light source may not be optimal.

    Connections and Piping

    • External ultraviolet units are normally self-contained and are available with the necessary piping and connections. Submersible units, which need to be connected into existing piping, may require various fittings to ensure they are plumbed correctly. If the existing piping is too large, a reducer will be needed for the piping to fit the inlet and exhaust nozzles of the ultraviolet sterilizer.

    General Considerations

    • External ultraviolet sterilizers tend to be more expensive because they incorporate a dedicated pump and mechanical filter. Larger units, regardless of whether they are submersible or external, standalone units, will use more energy than smaller units. Submersible and standalone units are equally effective in destroying unicellular algae and disease-producing organisms, provided their lamps are replaced regularly. Both external and submersible ultraviolet units can be run all year, and both can safely be used in fresh and marine environments.