Filter retension time vs. turnover rate

[Scorponok]

Filter retension time = Volume of water in the filter/pump rate.

The higher the turnover rate, the lower the retension time. How to overcome it besides a bigger sump? And how important is the retension time?

 

[dawnmarie]

Not sure how critical "retention" time is in a sump. I assume you are refering to Bio filter not mechanical filter. Higher turnover rates would theoretically improve mechanical filtration Short of a larger sump you would need to divert a portion of the total flow around the sump or reduce the flow rate. My guess is that would defeat the purpose.

 

[KaiserSousay]

Not sure I understand you.
What would be the purpose of having the water slow through your filter, over having it make a number of trips through it.

 

[kjfan1]

I'm lost. more explaining please.

 

[greenterra]

Personally I don't find retention/dwell time as important as flow rate. When have you ever had or heard of a high enough flow rate that stopped your biological bacteria from doing their job? If you have no ammonia or nitrite readings your BB are doing their job making Retention/dewll times a mute point IMO.

 

[Scorponok]

"The longer the better
Broadly speaking, the effectiveness of biological filtration is improved the longer the 'polluted' water is held in the filter - i.e. the longer the retention time. The most time-consuming process in filtration is the breakdown of dissolved organic carbon compounds into simple inorganic compounds. These compounds are ultimately incorporated back into living organisms. This complex chain of processes is not instantaneous and will, even under ideal circumstances, take some time. If insufficient filtration time is available, intermediate products will be pumped out of the filter back into the pond. This is clearly undesirable and rather defeats the object of having a filtration system. Indeed, this may well be the reason why excessive algal growth occurs in some ponds, with the filter merely producing an endless supply of plant nutrients!"-- link below.....

http://www.fishdoc.co.uk/filtration/koi5flow.htm

 

[squint]

I don't think this is something that you can figure out just by sitting down and thinking about it. It's something you'd have to setup a carefully designed and controlled experiment to determine.

That being said, I don't think the conversion of waste to ammonia, nitrite, and then nitrate occurs at a high rate. The concentrations are relatively low and I think would be distributed evenly throughout the entire body of water. There isn't a plume of nitrite, for example, streaming from the filter. I don't think you would find any difference in ammonia/nitrite/nitrate levels before or after the filter or any stages of filtration. You'd have to slow the flow rate to almost zero for any step of the nitrogen cycle to become localized. Think about how long a filter has to be deprived of flow for detectable levels of hydrogen sulfide to build up. Even the slightest flow would prevent that.

 

[dawnmarie]

"The longer the better
http://www.fishdoc.co.uk/filtration/koi5flow.htm

Interesting link. Hopefully we will get some feedback from the major players.

I read the link and here are the high points and an example application.

a retention time of ten minutes, possibly longer, will usually suffice.

filter retention time = filter size/pump rate

Application: 240 gallon tank,50 gallon sump,1000 GPH = 4.16 Turnover rate.
Using the formula and recommendation of the link : 50 devided by 1000 = .05 X 60 = 3 minutes.
So a given sample will take three minutes to pass through the filter.
The link suggests 10 minutes.
In order to achieve this you would have to slow the Turnover rate to 1.37 times per hour.
I did the math a couple of times it always comes up the same.
Are you suggesting we reduce our turnover rates this drastically or is my math faulty ?

 

[Pharaoh]

Keep in mind that not 100% of the BB within the system is contained in the filter. BB inhabits the entire tank. The filter is just designed to be the optimum place to add additional surface area.

 

[squint]

I don't think the turnover rate or flow rate really affect biological filtration. A high flow rate only traps debris in the mechanical filter sooner. The higher currents would also lessen the dead spots where debris collects on the bottom of the tank.

It would be interesting to experiment with...perhaps when cycling a new empty tank with ammonia. After the filter is seeded and mature, divert the outflow from a filter into a bucket and compare its ammonia levels with that of the tank. I don't think there would be a difference.