bio media - does slower flow rate give better filtering

[Jgray152]

I really appriciate your knowledge and experience being shared.

I am going to quote 3 paragraphs from what you said and break it down into the velocity. You will be surprised to know that the velocity of your 150000 LPH (39,624 GPH) through a sqaure meter (36"x36") cross section is not much different than the Fluval Fx5.

In scientific studies (no I can´t source them because i last saw them 10 years ago while studying) which I have read, the increase in nitrification didn´t slow down with extremely high flow rates (much higher than we are talking about here).

My personal experience is with many different types of filters but the highest flow rates i´ve used on trickle towers was 150 000L / square meter cross section. For a trickle filter under an aquarium with a cross section of say 40x40cm (~16x16 inch) that means a flow rate of about 24 000L (~6350 american gal) / hour.

For a comparison with filters with immersed filter material, I´ve run rapid sand filters as biomechanical systems with flow rates of 100 000L / square meter cross section. For an Fx5 (not sure of exact diameter - I used 30cm/12 inch) this would mean about 7000L (~1850 gal) / hour

Im going to calculate the sqaure meter cross section into the cross section of a cylinder. Easier to get the velocity for. So a 36" x 36" sqaure meter = 1296 sq in.
A cylinder with a diameter of 40.64" has the same cross section area of the sqaure meter.

Conversion Reference
150,000 LPH = 39,625 GPH
24,000 LPH = 6340 GPH
2271 LPH = 600 GPH

Fluval Fx5
Fluval Fx5's velocity at 600 gph through a 6" cross section area. The bio chamber in the Fx5 is only 6" across.

600 GPH through a 6" cylinder equals = 1.36 inches per second.

Ehiem 2080 Pro3 (Cycles 2 days earlier than a Fx5, yes other factors play a roll)
I don't know the exact dimentions of the Eheim 2080's media baskets but im going to guess they are around 11" considering the out side dimentions are 12" x 12.75".

I will also convert this to cylinder dimentions. 11x11 = 121 sq in.
A cylinder with a diameter of 12.42" has the same cross section area as above.

With that in mind. 318 GPH through 12.42" diameter = .168 inches per second

Your sqaure meter cross section
36x36 = 40.64" diameter.

39625 GPH through a cylinder diameter of 40.64" = 1.96 inches per second

Trickle filter 16"x16"
16x16 = 256 sq in cross section
Cylinder diameter of 18.04" has the same cross section area as above.

A cylinder diameter of 18.04" at a flow rate of 6340 GPH = 1.59 inches per second

Hagen AC110
3" x 9" = 27 sqin cross section
Cylinder diameter of 5.86" has the same cross section as above.

500 GPH through a diameter of 5.86" = 1.18 inches per second.


With all the said, you can notice a trend here. Velocitys through the above setups never exceed 2 inches per second which is not fast at all.

The Velocity is one thing. Now the contact time.

What was depth of the bio media with your sqaure meter setup? Also, the depth of the 16x16 setup?

Fluval Fx5
The Fx5, if I remember correctly, each tray is 3.5" deep. 3.5 x 3 = 10.5 / 1.36 (velocity) = 7.7 seconds. That means there is 7.7 seconds of conctact time.

Eheim 2080
I wish I had the dimentions of the 2080's trays and how many trays are full of bio media so I could do a calculation of that as well. I am going to do a rough calculation to the depth of the media. The 2080 hold 12L of bio media. At 11x11x1 there is 121 cubic inches which equals 1.98 liters. 12 / 1.98 = 6.06 inches of depth. 6.06 / .16 (velocity) = 37 seconds of contact time. AMAZING!!

It has been known, through experience that the Ehiems were always better for bio filtration. ^ That is why.

Can some one measure the 2080s media trays???????

Hagen Ac110
The AC110 has about 1" of depth so the water only has about 1.18 seconds of contact time. No very much for 500 gph. There is about a 1/4 gallon of water passing through the media in the AC110 every 2 seconds.

I have tried differing flow rates down to about 1500lph (395 gal/h) and haven´t noticed any difference in nitrification rates.

This is no surprise if the bio load does not exceed the bio filtration available at the given flow rates. If you tanks bio load is filtered just fine at 300 GPH, it will not filter any better at 500 GPH or 3000 GPH (depending on velocity and contact time)

 

[Jgray152]

To amend to the above post.

When you were testing with a 36x36 cross section area, it makes sence that you would get better nitrification at much higher flow rates considering the oxygen in the water and nutrients would probubly be consumed before the water could ever pass through the entire bio media.

 

[justin guest]

Hi jgray152

i´m not disagreeing with you that Eheim filters may be better than Fluval filters but as I´ve already said there are too many variables that affect the performance of a filter. That´s why i´m not willing to accept that lower velocity is better for nitrification just because the filter comparison you used showed that the filter with a lower velocity was better. Apart from that there is also nitrification occuring in the outer chambers of the fluval Fx5 where velocities are also much lower which doesn´t really back up your argument. This means that your calculations for contact time also need to be adjusted. As I´ve already stated others have already done the science (and yes i´m critical of scientific studies as well) showing higher velocities to be better for nitrification. My personal experience fits to the scientific findings as well though.

If you are interested in what type of systems I´ve designed and built you can check out the details in the following thread;

http://www.monsterfishkeepers.com/forums/showthread.php?t=158808

I´m happy to answer any further questions if you can´t find enough info in the thread.

j<><

 

[Jgray152]

i´m not disagreeing with you that Eheim filters may be better than Fluval filters but as I´ve already said there are too many variables that affect the performance of a filter.

Without this even being said. I specifically took your data that you supplied to us that explains how you were running over 39K GPH (Which was jaw dropping by the way, congrats to you!)through a 36"x36" cross section surface area and you were telling us that this flow is where you found the optimum nitrification to be. GREAT! That backs up what I have been saying. The volume of flow doesn't mean anything by its self. You need to know the velocity and with that, the contact time. Will 4ppm of ammonia be removed quicker and more thorough from a flow contact time of 3 seconds, or 14 seconds at 1200 GPH? See what im getting at?

You can't design a filter soley on just the volumn of flow. You then wouldn't know pressure drops, velocities, contact times, etc..

That´s why i´m not willing to accept that lower velocity is better for nitrification just because the filter comparison you used showed that the filter with a lower velocity was better.

The Eheim 2080 performs better than ALL other filters with a higher velocity and much less contact times. The amount of media contained in the filter is directly proportional to the contact time. The more media, the more contact time. Increasing the flow will lower the contact time. Thats why having 12L of bio media on very heavily stocked tanks works better than 2 liters of bio media running the same volume of flow.

Apart from that there is also nitrification occuring in the outer chambers of the fluval Fx5 where velocities are also much lower which doesn´t really back up your argument.

Actually the velocity will be higher in some parts and lower in others. Even at that, yes it does backup my argument. It does not back up yours. You are telling us a VERY high velocity is what you want, reducing contact time substaintially. Its exactly the opposite.

This means that your calculations for contact time also need to be adjusted.

I won't argue that there can be some fine tuning of the figures since they are just estimates. With that said, flow mathematical calculations are very close to the actual flow properties in reality.

As I´ve already stated others have already done the science (and yes i´m critical of scientific studies as well) showing higher velocities to be better for nitrification.

I have not seen any proof of this. You have shown zero data containing anything that tells us in which higher velocities are better. You keep going on about volume of flows (GPH or LPH) but that will not tells us anything about what is going on inside the filter, such as contact time and velocity.

3000 GPH in a 30" diameter cylinder is TOTALLY different from 3000 GPH in a 6" diameter cylinder. Now there really wouldn't be much of a difference in this if I said, 30" diameter cylinder with 14 liters of bio media compaired to a 6" diameter cylinder with 14L of bio media, even though velocity will be much higher, you still have the same amount of media meaning you have the same contact time. So this means you also can't soley talk about velocity as a way of describing nitrification, it has to be contact time.

The more contact time, the better right? No. There is a point, which no one can say when because there are too many variables in play, to which oxygen levels and nutrients are depleted.

My personal experience fits to the scientific findings as well though.

Im waiting to see your scientific findings. The experience you have brough to us is GREAT! It really is but it does not back up your claims. Now does the sites I was brough to by another member. All of which said the higher the flow, "within limits" the better.

Tell you the truth, I was actually jaw dropping after looking at the flow numbers you were runnning and thinking, wow, maybe I am way of base here but then I started to included your information in with my findings and well, they were the same mathematically.

 

[Jgray152]

BTW, how many liters of bio media were you using in the square meter setup? Just wondering.

 

[justin guest]

Hi jgray 152

Don´t take this the wrong way but I feel like we´re going around in circles and that our discussion isn´t helping mystic.bertie much (it is his thread). We obviously have different views on nitrification and i´m happy to accept that.

The Largest trickle filters we are using are 800l volume. I gave the flow rates in square meter cross section because that makes the numbers easier to crunch. the filters are around 60cm wide x 80cm long each with a pump with around 75 000lph. Contact time is approx. 4 seconds.

j<><

 

[Jgray152]

Pretty impressive filtration numbers! I must say.

So I took some calculation from one filter at 800Liters capcity with a cross section area of around 744 sq inches at a flow rate of 19812 GPH. The velocity is about 6.87 inches per second, pretty quick. With 800 liter of media though, thats a contact time of about 9 seconds.

 

[Jgray152]

I do think this can help the OP since we are going into more specifics than most.

Now, regaurdless if you have nine seconds of contact time. What if you kept 800L of bio media but created a larger cross section of area?

Instead of 23.6"w X 31.5" L which will produce a velocity of 6.87 inches per second with about 9 seconds of contact time.
What if you doubled the size to 47.2"w X 63" L which would have a velocity of 1.7 inches per second with the same contact time of around 9 seconds.

Is this where you would say the filter with the higher velocity would work better? In all honesty. To me, 6.87inches per seconds is not very quick but is a lot quicker than 1.7 inches per second which I suppose could help with the "bio film" issue being spoken about in some writeups.

Im going to be doing some more reading on my own vut I found this article which was a bit hard to follow for me but tell me what you think its saying to you. Sounds like "water flow" (could it be water flow path or velocity or volume?) has little effect on nitrification but a lot of effect on denitrification but doesn't really go into to deep in flow velocity.

Effects of water flow velocity on nitrification, denitrification, and the metabolism of dissolved oxygen and inorganic carbon in macrophyte-epiphyton complexes were investigated in the present study. The metabolic rates were measured in microcosms containing shoots of Potamogeton pectinatus L. with epiphytic biofilms in the light and dark with no flow or with the flow velocities of 0.03 and 9 cm s-1, Photosynthesis and respiration increased with increasing water flow velocity. Rates of oxygen respiration were positively correlated to the oxygen concentration of the water. Nitrification was not significantly affected by flow velocity, but nitrification was higher in light than in dark at 0.03 cm s-1, but not at 9 cm s-1. Denitrification was higher in stagnant water and at 9 cm s-1 than at 0.03 cm s-1 in the absence of oxygen, possibly due to complex effects of water flow velocity on the supply of organic matter to the denitrifying bacteria. Denitrification was always inhibited in light, and negatively correlated to the oxygen concentration in dark. Epiphytic denitrification occurred only at low oxygen concentrations in flowing water, whereas in stagnant water, denitrification was present in almost oxygen saturated water. Therefore, because there are little of water movements and high oxygen consumption in dense stands of submersed macrophytes, significant rates of epiphytic denitrification can probably be found within submersed vegetation despite high oxygen concentrations in the surrounding water. In conclusion, this study shows that the water flow and oxygen metabolism within submersed vegetation have minor effects on nitrification, but significantly affect denitrification in biofilms on submersed macrophytes.

Link here
http://cat.inist.fr/?aModele=afficheN&cpsidt=1105393