Why do I have green water even with a UV?

[kendragon]

For those running amalgam UV bulbs, be aware that they run hot. This one got so hot that the bulb connector caught fire. The quatrz sleeve extends past the water flow which allowed the connector to sit alway from the cooling affects of the water. Luckily this was outdoors. Very scary if this was under the tank in the house. The quartz sleeve on this one did crack adding more danger of flooding and electrocution.
I rebuilt this to a standard setup with a standard UV bulb. No need for amalgam to clear green water.

 

[duanes]

UV is all about detention time.
Unless whatever you are trying to kill spends enough time in front of the light to kill it, or scramble its organelles, UV doesn't do everything it claims to do.
Flow has to be slow enough to do the job.

Another aspect is turbidity, if the water is turbid, the light does not make enough contact to do the job.
So in some cases pre filtration is needed.

I attended a number of seminars on UV when working in the water filtration field, and tried it (not your exact type) , but never found it worked as well as claimed.

For my situation I found Foam Fractionation to be more effective at removal of the nutrients and algae spores that lead to green water.
koi pond fractionation
As you can see, the waste taken from the water column as foam, was significant.
I also found heavy planting that reduced nutrients helped.
I see you have papyrus, which I also used, but a heavier plant density compared to fish density was very important.
The there were only 5 adult fish in the 500 gal pond in these videos.
GOPR6064

 

[kendragon]

UV is all about detention time.
Unless whatever you are trying to kill spends enough time in front of the light to kill it, or scramble its organelles, UV doesn't do everything it claims to do.
Flow has to be slow enough to do the job.

Another aspect is turbidity, if the water is turbid, the light does not make enough contact to do the job.
So in some cases pre filtration is needed.

I attended a number of seminars on UV when working in the water filtration field, and tried it (not your exact type) , but never found it worked as well as claimed.

For my situation I found Foam Fractionation to be more effective at removal of the nutrients and algae spores that lead to green water.
koi pond fractionation
As you can see, the waste taken from the water column as foam, was significant.
I also found heavy planting that reduced nutrients helped.
I see you have papyrus, which I also used, but a heavier plant density compared to fish density was very important.
The there were only 5 adult fish in the 500 gal pond in these videos.
GOPR6064

I totally understand your points. I too have experienced the above over the years.

I was making a funny because this pond (not mine) had the UV go out and the pond went green. The fire was for real.
The pond was built for show and because of that the UV was needed to mask the down falls like filtration, water balance and plant coverage. They like to see the koi and have water clarity. As we know, clear water is not always good water quality. As the case here.

 

[aussieman57]

Put that green water to work and culture some daphnia.