Can UV flow rate be to slow?

[exnisstech]

I'm having trouble getting my head around uv flow rate. Slow flow for bacteria and fast flow for algae. Wouldn't slow flow kill algae and bacteria then? I'm running a lifegard pro max 55 Watt on a 200ish gallon system and am starting to get cyano on the upper part of my back wall. UV has been running for a couple of months but I just decided to check flow.
I know it should have been the first thing I did but sometimes I work backwards. I'm getting 132 gallon an hour flow. I know this is slow which brings me back to the original question, wouldn't longer contact time be better? Thanks

 

[Jay Hemdal Verified Community Expert]

I'm having trouble getting my head around uv flow rate. Slow flow for bacteria and fast flow for algae. Wouldn't slow flow kill algae and bacteria then? I'm running a lifegard pro max 55 Watt on a 200ish gallon system and am starting to get cyano on the upper part of my back wall. UV has been running for a couple of months but I just decided to check flow.
I know it should have been the first thing I did but sometimes I work backwards. I'm getting 132 gallon an hour flow. I know this is slow which brings me back to the original question, wouldn't longer contact time be better? Thanks

Longer contact time will have a higher kill rate percentage and will kill larger organisms. However, there is also the intangible “dwell time” to consider - while a UV is running more slowly, a smaller percentage of the water in the tank is being processed. Slow enough and the target organism is able to reproduce faster than the UV is killing it.
Jay

 

[exnisstech]

Thank you Jay! Makes perfect sense to me now. I Googled all morning and could not find an explanation that simple. Looks. Like I need to step up my flow. Thanks again

 

[Malcontent]

Imagine a microorganism has 100 hit points and a system has a turnover rate of 1X per hour resulting in an exposure duration that delivers exactly 100 damage.

Double flow rate and the exposure halves but turnover doubles. The microorganism takes 50 damage twice per hour and dies. Same result.

Now halve the flow rate and only half the water passes through the UV per hour. But the microorganism gets 200 damage with each trip. Well, you don't get any benefit from killing it twice and only half the water is treated this time.

UV manufacturers most likely don't have any engineers or scientists on staff and they're certainly not performing any research. They just contracted out the design decades ago and only have marketing people on staff. Any data they have is from other sources and assume a single pass because that's how UV is used when it treats drinking water.

The experiments involving multiple passes come from sterilizing food (drinks, specifically, that likely can't be heat pasteurized) and they found that below a certain flow rate, performance decreases but above that there's no decrease if you increase flow rate.

When I suggested that slow wasn't the only way to go years ago, it was supreme heresy. Illusory truth effect and all...

 

[exnisstech]

Imagine a microorganism has 100 hit points and a system has a turnover rate of 1X per hour resulting in an exposure duration that delivers exactly 100 damage.

Ha ha I love it! An explanation an old pc gamer like myself can understand

 

[exnisstech]

I was able to get the output up to 275g an hour. Total volume is a little over 200g, that should be sufficient no?

 

[Jesse571]

I'm having trouble getting my head around uv flow rate. Slow flow for bacteria and fast flow for algae. Wouldn't slow flow kill algae and bacteria then? I'm running a lifegard pro max 55 Watt on a 200ish gallon system and am starting to get cyano on the upper part of my back wall. UV has been running for a couple of months but I just decided to check flow.
I know it should have been the first thing I did but sometimes I work backwards. I'm getting 132 gallon an hour flow. I know this is slow which brings me back to the original question, wouldn't longer contact time be better? Thanks

How did you check your flow rate please?

 

[Jay Hemdal Verified Community Expert]

How did you check your flow rate please?

One way is to run the system into a measured bucket and time how fast it fills. If a one gallon jug fills in 10 seconds that works out to be 60 gallons per minute or 3600 gallons an hour.
Jay

 

[Jesse571]

I was able to get the output up to 275g an hour. Total volume is a little over 200g, that should be sufficient no?

Following...

 

[exnisstech]

How did you check your flow rate please?

I have silicone tubing on the exit side of the uv. Since all the plumbing is in the sump I just hold a one gallon container under the hose tell my Gal when to start the timer and when to stop and that is the number I use to calculate exactly like Jay said.