Filter retension time vs. turnover rate
- Thread starter Scorponok
- Start date
The science says that higher flow increases a filter's ability to nitrify. practically, you also need contact time otherwise nitrite may come out of the filter due to the second stage in the nitrification process not being completed. This is why trickle filters for instance are better if they are taller. This does not mean that slower flow through a trickle filter improves nitrification, quite the opposite in fact, but rather that there needs to be enough filter media so that the nitrite can also be converted in one pass through the filter. These principles are important in an intensive aquaculture setting but rarely cause a problem in most aquariums/private ponds as the stocking density is rarely that high ie 15 kg fish and upwards/1000L.
In most aquariums/ponds it doesn't matter if a minimal ammount of nitrite is in the water due to low filter retention time as it will quickly be converted to nitrate with the next pass through the filter (all filter types). This is because most aquarium/pond filters have a high turnover rate. If you are running a filter with a low turnover rate of say 4 x per day or less, then contact time becomes more important when considering nitrification.
Heterotrophic bacterial balance in a filter is improved with longer retention time but is only really important if the filter is supposed to eat the mulm like with a Hamburg mat filter. Heterotrophic bacteria populations in filters that are regularly cleaned usually find a state of balance pretty quickly regardless of retention time/turnover.
Generally, the finer the mechanical filter material is, the cleaner the water will be.
Mechanical filtration does however improve for a given filter cross sectional surface area with slower flow rates due to improved adsorbtion. Adsorbtion in this case is the process by which mulm particles are attracted to a filter surface through weak electrical charge. This means that a more coarse mechanical filter material with slower flow rates can sometimes out perform a finer filter material using faster flow rates. So the optimal balance for mechanical filtration retention time versus turnover is filter material specific. A minimum optimal turnover does need to be maintained but this figure is usually much lower that most people assume and is reliant on the individual variables of each aquarium /pond system.
So what does all this mean?
In an aquarium/private pond setting you are unlikely to have a problem around nitrification rates or heterotrophic bacterial growth with 99% of diy or shop bought filters regardless of differences in retention time or turnover rate if the turnover rate is at least once per hour. The mechanical filtration is really the defining factor. To achieve optimal water quality choose a filter with an oversized fine mechanical filter and pump the water through at a moderate rate for that filter but at a rate that still allows the turnover to be multiple times per hour. This may not be economical in a pond setting so usually a balance must be struck.
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In most aquariums/ponds it doesn't matter if a minimal ammount of nitrite is in the water due to low filter retention time as it will quickly be converted to nitrate with the next pass through the filter (all filter types). This is because most aquarium/pond filters have a high turnover rate. If you are running a filter with a low turnover rate of say 4 x per day or less, then contact time becomes more important when considering nitrification.
Heterotrophic bacterial balance in a filter is improved with longer retention time but is only really important if the filter is supposed to eat the mulm like with a Hamburg mat filter. Heterotrophic bacteria populations in filters that are regularly cleaned usually find a state of balance pretty quickly regardless of retention time/turnover.
Generally, the finer the mechanical filter material is, the cleaner the water will be.
Mechanical filtration does however improve for a given filter cross sectional surface area with slower flow rates due to improved adsorbtion. Adsorbtion in this case is the process by which mulm particles are attracted to a filter surface through weak electrical charge. This means that a more coarse mechanical filter material with slower flow rates can sometimes out perform a finer filter material using faster flow rates. So the optimal balance for mechanical filtration retention time versus turnover is filter material specific. A minimum optimal turnover does need to be maintained but this figure is usually much lower that most people assume and is reliant on the individual variables of each aquarium /pond system.
So what does all this mean?
In an aquarium/private pond setting you are unlikely to have a problem around nitrification rates or heterotrophic bacterial growth with 99% of diy or shop bought filters regardless of differences in retention time or turnover rate if the turnover rate is at least once per hour. The mechanical filtration is really the defining factor. To achieve optimal water quality choose a filter with an oversized fine mechanical filter and pump the water through at a moderate rate for that filter but at a rate that still allows the turnover to be multiple times per hour. This may not be economical in a pond setting so usually a balance must be struck.
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I think this is also why ehiem compares with the fx5 flow rates. I believe to a certain extent this is very true. But like pharaoh said, your tank his a whole BB filter. So i wouldn't give it to much thought.
LoL edit: what justin said. Wow great read.
LoL edit: what justin said. Wow great read.
I like this, some good points have been made.
More practical applications? How about an amalgamation of flow rate and retention time. I’ve experimented previously with extra pumps in a sump. Pumps that serve to return water from the final stages of the sump back to the initial stages. This happens while overflows are returning tank water to the sump, and the sump pumps are returning sump filtered water to the tank. You just need a bit of residual water in the system.
It didn’t make much noticeable difference. I did feel better for doing it. I wouldn’t recommend doing this with UV lamps. In UV filtration, all water needs to go through the UV lamp as regularly as possible. We’ve mentioned before that BB exist throughout the tank and nitrite and ammonia can be broken down all over the tank albeit at a lower rate than in the sump.
I don’t see MFKers reducing turnover rate and starting threads on “Who has the highest retention rate on here??”
Any other thoughts comrades ??
More practical applications? How about an amalgamation of flow rate and retention time. I’ve experimented previously with extra pumps in a sump. Pumps that serve to return water from the final stages of the sump back to the initial stages. This happens while overflows are returning tank water to the sump, and the sump pumps are returning sump filtered water to the tank. You just need a bit of residual water in the system.
It didn’t make much noticeable difference. I did feel better for doing it. I wouldn’t recommend doing this with UV lamps. In UV filtration, all water needs to go through the UV lamp as regularly as possible. We’ve mentioned before that BB exist throughout the tank and nitrite and ammonia can be broken down all over the tank albeit at a lower rate than in the sump.
I don’t see MFKers reducing turnover rate and starting threads on “Who has the highest retention rate on here??”
Any other thoughts comrades ??
the average amount of time water spends in contact with the filter media is not effected by turnover rate or retention time. using the 50 gallon sump example on a 240 tank and 1000 gph pump the turnover rate is 20 times per hour, once in 3 minutes, in the sump and about 4 times per hour, once in 15 minutes, in the tank. 3 minutes out of every 15(20% of the time) an average gallon spends in the sump. as you change the turnover rates and retention times change but the percent of time spent in the filter remains the same. 500gph 6 minutes out of 30(20%), 2000gph =1.5 minutes out of 7.5(20%).
Bio filters will treat basically the same amount of water in a given time regardless of retention time as long as the flow rate is not so fast that bacteria cannot attach to the media
Bio filters will treat basically the same amount of water in a given time regardless of retention time as long as the flow rate is not so fast that bacteria cannot attach to the media
yeah,
if the water is travelling at a lower velovity, contact time is increased
it the water is travelling at a higher velocity, contact time is decreased, but contact frequency is increased.
this increased velocity brings overall contact time back up to levels of lower velocity rates.
but if u have extra sump pumps to recirculate sump water in the sump, u increase contact frequency again do you not ??
if the water is travelling at a lower velovity, contact time is increased
it the water is travelling at a higher velocity, contact time is decreased, but contact frequency is increased.
this increased velocity brings overall contact time back up to levels of lower velocity rates.
but if u have extra sump pumps to recirculate sump water in the sump, u increase contact frequency again do you not ??
yes you can increase frequency by recirculating in the sump but you are also increasing flow rate. there is a net gain because it is sump water being pumped through but it's a law of diminishing returns deal.
