Filter Flow Rate

bonsaibill

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This follows on from a previous question I had. Do we really need to change the activated carbon in our filters so often? The reason for asking this was that I work in the water supply industry (as an electrical maintenance technician). I live in an area of intense agriculture so the water can contain quite high levels of pesticide and nitate from fertiliser. To assist in removing this all of our water treatment works employ Granular Activated Carbon in the final stages of filtration but it remains in service without being changed for years.
So back to my reason for this question. Speaking to a works controller the other day we were discussing the different types of filtration and respective flow rates. Some are slow sand filters which have a large surface area and layers of sand and carbon. As the name suggests these have a relatively low flow rate. The other type is known as rapid gravity and these have seperate sand filters and GAC filters. These have a much smaller surface area and a higher flow rate. The filtration beds on these are much deeper, however. This is beacause of the higher flow they need greater depth to allow the filter material the contact time to do it's job. It also transpires that the slow sands have bacteria that builds up to assist the carbon, this is something that does not happen in the Rapid Gravity Filters.
So, in view of this, my question is - 'can the water flow through a tank filter be too great for it to work efficiently, especially for the beneficial bacteria?'
To achive a certain litres (or gallons) per hour you could either have a filter with large surface area (or in other words large volume) with a slow flow through or a small filter with a much higher flow rate. My thoughts are that large volume filter and low flow rate are better than small volume and high flow, discuss!
 

AquaticJ

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I actually don't even use carbon in my tanks, so it's really up to you about how often they get changed. Too much flow through a hang on back filter can absolutely cause inefficient filtration. There's a lot of ways for water to just flat out avoid any kind of water polishing/cleaning media with hang on back filters, like the built in overflow. That's why I use canister filters. Water that enters cannot come back out without going through 5 stages of filtration.
 
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bonsaibill

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My thinking on this was that, presumably, a finite time is required for chemical exchange to take place. Therefore if the water is pushed through the filter too quickly then the desired reaction would not have time to take place. Any mechanical filtration would probably still work ok but chemical and biological filtration would be compromised.
 

bitseriously

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Cool question. I’m interested to see where this goes.
For my part, I’d like to know the relationship or ratio of surface area to volume filtered per unit of time for the slow filter type. I’m guessing you’d need something approaching (exceeding?) the size of the aquarium/tank itself to do the job, so it’s simply not feasible in a home aquarium setup.
Also, we always hear that for, say, hob filter u need 10x vol per hour; now I’m curious if that is related to the need to bring waste to the bb in the filter, or something else like oxygenation. In other words if I slow down the gph of my filter, but have high surface area in the filter itself, will the bb performance decrease, remain stable, or increase wrt removal of wastes?
 

FishFish221

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For biological filtration, the more surface area the better. The flow rate really depends on the type of filter you are using. If it is a sponge filter, the flow rate will not really matter. For motor driven filters such as sumps, canister filters and HOB filters, the flow rate should be at least 4x the tank size an hour to allow enough water to be "filtered" by the bacteria.

A low flow rate may not affect the biological filtration, but then there is also mechanical filtration. If you do not want your tank looking cloudy and dirty from debris and fish poop, you will need a mechanical filter with around 7-8x the tank size to allow most of the debris to be filtered out of the aquarium.
 
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