Pond UV Clarifiers: Clearing Green Water
A UV clarifier clears green water by clumping free-floating algae so the filter can remove them. Learn how it works, sizing, flow rate, lamp life and its limits.
A pond UV clarifier is the standard tool for clearing green water, the cloudy bloom of microscopic free-floating algae. Water is pumped past a UV-C lamp inside the unit, and the radiation damages the algae cells. It is important to understand that a clarifier does not filter the algae out itself; it makes the algae removable by the mechanical filter that follows.
How it works
Germicidal UV-C light, typically around 254 nm, damages the genetic material of microorganisms so they can no longer reproduce. In a clarifier, as algae cells pass the lamp the UV corrupts them and their cell walls start to break down, making them sticky so they clump together (flocculate). These clumps are then large enough to be trapped by the mechanical part of the filter, leaving the water clear. The same UV process reduces some free-floating bacteria, protozoa and other pathogens.
Sizing and flow rate
Two things determine whether a UV unit works: enough UV power for the pond volume, and the right flow rate. UV effectiveness depends on dose, which is the UV intensity multiplied by the exposure (contact) time. If water flows past the lamp too fast, the algae get too little contact and the green water is not cleared. Each unit has a recommended or maximum flow that gives the cells enough contact time, so match the pump to the clarifier rather than running it as fast as possible.
Clarifier dose vs. steriliser dose
The same hardware can act as a clarifier or a steriliser depending on the UV dose delivered. A lower dose (faster flow, less contact) is enough to flocculate algae and clear green water. A higher dose (slower flow, more contact) is needed to reliably inactivate a broader range of pathogens. So a unit run at clarifier flow rates targets green water, while disease-control sterilisation requires more contact time and usually a stronger unit.
Placement and lamp replacement
Position the UV so water passes it on the way through filtration, with the mechanical filter able to catch the clumped algae afterwards; clearer water also lets more UV reach the cells. UV lamps fade with use: their output drops over time even while the lamp still glows, so they must be replaced periodically. Lamps are commonly specified to reach 80% of their initial UV output at end of life, and a practical approach is to fit a fresh lamp before each spring and summer season, the period when green water is worst.