Need help with selecting UV water filter

[Lepke]

I use a UV light after my watermaker. It's only on when I make water. The bulbs have a lifespan. After that even though the bulb puts out light, it isn't killing anything. My bulb gets changed yearly.

The bulbs don't degrade with reasonable on cycles, just change them when they hit the rated hour limit.
Even though my water tanks only receive UV sanitized water, in time the tanks will become contaminated. Probably the organisms come in the air. About yearly I add chlorine in low amounts to my tanks. I have a filter at the galley that removes any chlorine taste.

[tomfl]

Originally Posted by CarlF

My understanding is that the standard UV bulbs are best left on. If you turn them on and off, they fail quickly.

Do look at the Pentex Floplus10 mentioned earlier. $15. Fits a standard filter housing. Far superior to a standard “charcoal filter”. Lasts a long time.

Not sure how you define 'fail quickly' but one of mine is over two years old and still works after more off/on cycles than I can count.

Did look at that filter.

[Cheechako]

Originally Posted by Westcliffe01

Tom, I don't think you understand the mechanics very well. UV sterilizers work by having a high output UV source and then making a very small annular water passage around the bulb, thus putting whatever is in the water in close proximity (thousandths of an inch) from the powerful UV rays. Unless the UV irradiation level (watts/unit of surface area) is high enough it wont kill anything.

So, putting a UV source in the water tank wont work. 1: because water is a very good attenuator of light, let alone blue light. Thus the UV wont penetrate more than a few inches and would be so weak it couldn't kill anything. 2: There is an effect called the inverse square law, which without considering the transmission properties of the fluid, just based on distance in air, if you double the distance from the light source, you get 1/4 of the irradiation level. So if your tank is 40" as a maximum dimension and you put the light source in the center, and in the UV sterilizer the water is not further than 0.010" from the bulb, you jut changed the distance to the sides of the tank by a factor of 20/0.010=2000x By doing this (only based on distance) the irradiation level would be 1/(2000^2) = 1/4/1000000 (a quarter of a millionth) of the original power.


For the system to work, the water has to flow over the surface of the light source and it has to be forced through that small opening.


Usually, in manufacturers literature, the UV sterilizer is the last item in the water purification system, so being positioned right before the faucet. If you imagine that the 55W light source is on all the time, which it usually is, water in that last section of line will heat up and get pretty how in between the times the faucet is opened. But once you open the faucet, this hot water is flushed out pretty quick and then cooler water will follow. In a recirculation system (polisher) the heat input would only be a fraction of the bulb wattage and even if you ran it day and night it would ultimately only raise the temperature of the tank a few degrees. But as described, you would probably only have to run it a few hours a day to get the desired reduction in bacterial growth.


Unless sunlight can reach the inside of your tank, you should never be seeing green algea. The slimy algea we are talking about has no color and is recognized by its slimy consistency when you touch it.

Good stuff, I will add a couple things though:
-The hot water in the UV housing takes quite a while to flush out actually. A minute or more. (we had ours in our house and used it daily) So think about if you want to run a minute or two of water down the drain on a boat.
-UV housings should pass water through at a rate that will work for the UV strength. I think ours was 2-3 gallons a minute. Some of the water passing through it can be 1-1.5" away from the light source. So I'm not sure how close the water really needs to be to the source. But not super close.
-As far as putting it in a tank, I think it may work. The reasoning being water in the tank doesn't just stay put, it moves around, as the water passes the UV it gets sterilized. SO if this goes on continually or a lot of time, it should clean the tank I would think. Just like a small fuel polisher returns clean fuel to a dirty tank, over time it gets the job done.(conjecture)

[NorthCoastJoe]

Have you given any thought to an Ozonator? I live in the country on well water we use ozone to purify our water. It should work in any tank less than four feet deep and under 1000 gallons.

[Westcliffe01]

The sterilizers that I used had a pretty close fitting glass tube that the water flowed over.

If you put the light source in the center of the tank you will never kill the algea that clings to the wall of the tank and that is the largest source. If you kill a little algea in the tank and you don't filter it out, you are going to have decaying organic matter in your water, not a good idea. Sterilizers are made the way they are for the best efficiency and work best by continuously removing contamination from the water all the time. If you were converting an existing system you would have to scrub out and bleach the existing tank first, then you would be able to make a fresh start and the steriliser would prevent a buildup from occuring in the first place and keep organic material from accumulating in the tank.

Originally Posted by Cheechako

Good stuff, I will add a couple things though:
-The hot water in the UV housing takes quite a while to flush out actually. A minute or more. (we had ours in our house and used it daily) So think about if you want to run a minute or two of water down the drain on a boat.
-UV housings should pass water through at a rate that will work for the UV strength. I think ours was 2-3 gallons a minute. Some of the water passing through it can be 1-1.5" away from the light source. So I'm not sure how close the water really needs to be to the source. But not super close.
-As far as putting it in a tank, I think it may work. The reasoning being water in the tank doesn't just stay put, it moves around, as the water passes the UV it gets sterilized. SO if this goes on continually or a lot of time, it should clean the tank I would think. Just like a small fuel polisher returns clean fuel to a dirty tank, over time it gets the job done.(conjecture)

[a64pilot]

Originally Posted by NorthCoastJoe

Have you given any thought to an Ozonator? I live in the country on well water we use ozone to purify our water. It should work in any tank less than four feet deep and under 1000 gallons.

Don’t do that if you have any kind of metal tank though.

[Cheechako]

Originally Posted by NorthCoastJoe

Have you given any thought to an Ozonator? I live in the country on well water we use ozone to purify our water. It should work in any tank less than four feet deep and under 1000 gallons.

Good thought, they are used a lot. Though they may have a limited life. My ozone air gen burned out after a few years.

[bcboomer]

I think UV on a boat is a solution looking for a problem. We've used water from our plastic tanks for 7 years with no issues except a bit of a "plastic" taste. We thought it was the tanks so we sterilized them. No change. Added a household sized carbon block filter and the water tastes great.
Ozone sounds like a better idea but I know the ones used in hot tubs have a very short effective life span.

[Westcliffe01]

Up North in cold water, you may be right. In the tropics it's a different story.

[tomfl]

Guess my Culligan carbon water filter I have been using since I bought the boat in 2012 and replacing the carbon filter every three months as recommended along with regular doses of bleach and flushing the tank once a year along with shocking with a gallon of bleach is what I will keep doing.

[Tellie]

Antique technology. Get a SeaGull and forget it.

[Rose Li]

I used to be a UVC lamp worker. I really like the functions provided by relatively new technology. The bleach or chlorine in my water solves the problem, but I hate the smell of the bleach in the water. I think the water treated like this It cannot be consumed directly.

[Lepke]

My UV light runs after the watermaker and only runs when I'm making water. The bulb last about a year making a few thousand gallons. After the bulb life hours, it no longer sanitizes. The light is probably within 1/4" to 1/2" of the water and kills at that distance. I test my water about once a year.

[Cheechako]

Another thought, just get a household undersink RO unit. The low pressure kind that works off your pressure pump. It's quite small really and uses it's own small accumulator. No heat or bulbs to deal with. No electricity required. The downside being it takes a few minutes to make up a gallon after you use some, so probably best for drinking and cooking, not washing dishes etc. They are cheap and take up maybe one cubic foot max. Cheap.