Mechanical Sand Filter

[Joe Pusdesris]

I am contemplating a design for a mechanical sand filter. I was reading about the use of mechanical sand filters for swimming pools as well as koi ponds and I figured the same idea could be applied to a reef, but for my reefs, it would need to be much much smaller scale.

The following design can probably be built for <$20. All that is needed is a 5g bucket, pvc pipe, pool filter sand, a divider (probably glass) , and a small pump.

The design is assuming that the device is above the aquarium or sump.

<table style="width:auto;"><tr><td><a href="http://picasaweb.google.com/lh/photo/FtflbfiVCmScNvRIEqA0Gg?feat=embedwebsite"><img src="http://lh4.ggpht.com/_xXnKvgKPgX4/SXi_yxks3hI/AAAAAAAAAbI/321N7KhZ9Gc/s144/mechanicalsand.png" /></a></td></tr><tr><td style="font-family:arial,sans-serif; font-size:11px; text-align:right">From <a href="http://picasaweb.google.com/deformative0/Diagrams?feat=embedwebsite">Diagrams</a></td></tr></table>

Yellow represents sand, red glass, orange bucket, black pvc, blue water.

Alternatively, if the siphon design is not trusted, (Both of my tanks have had this siphon to feed the sump for quite a long time now without a single event of siphon loss) the bucket could be drilled.

<table style="width:auto;"><tr><td><a href="http://picasaweb.google.com/lh/photo/OUbLkcILxAx3_VapoiWj3w?feat=embedwebsite"><img src="http://lh6.ggpht.com/_xXnKvgKPgX4/SXi_zGtNCnI/AAAAAAAAAbQ/Oa1Z-eZ6Tc0/s144/mechanicalsand2.png" /></a></td></tr><tr><td style="font-family:arial,sans-serif; font-size:11px; text-align:right">From <a href="http://picasaweb.google.com/deformative0/Diagrams?feat=embedwebsite">Diagrams</a></td></tr></table>

Upon water entering the first chamber, it would be forced through the sand by gravity and into the second chamber, in event of the sand getting clogged, (the goal) water would overflow the baffle into the second chamber, preventing flooding.

A bit of a elaboration on the remote deep sand bed. You may also notice that this design will have similar effects to those of an under gravel filter with mechanical and biological filtration. A key difference are particle size. Pool filter sand is made to precise size, optimal for catching particulate. Another is the fact that this design would be trivial to clean or replace, unlike an undergravel filter which requires nothing short of what could best be described as a complete breakdown.

Please ask questions or offer criticism.

 

[bwbixby]

I have contemplated similar ideas after going to aquarium of the pacific to donate a fish to one of their displays. they were using a jacuzzi brand pool filter on a couple of their reef tanks. their whole filtration is based on sand filters of one form or another needle wheel skimmers and injecting ozone into their skimmers. i was rather suprised at the simplicity of their systems. sometimes i think we over analyze things and make them harder than they really have to be.....

 

[jdieck]

Here are some and not expensive:

http://www.marinedepot.com/ps_Aquar...cs_rainbow-lifegard_fluidized_bed_filter.html

 

[Joe Pusdesris]

This is NOT a fluidized bed filter. A fluidized bed filter will force a lot of water through a small amount of sand, where this is the inverse. The "fluidized" sand eliminates the sand's compression that results in mechanical filtration. So a fulidized bed filter results in strictly biological filtration, rather than mechanical.

 

[Joe Pusdesris]

Also bwbixby, it is very encouraging to know that such a method is already employed and working. Even if my design doesn't work, it is good to know that the concepts are sound.

 

[billsreef]

You need to provide a method to backwash or otherwise flush the accumulated debris from your sand periodically. The large commercial ones you see used on pools and aquaculture systems have valves that allow for backwashing and rinsing, typically a once a day proposition in fish rearing

 

[bsbarton]

I have few things for you to consider in the design.

The first is over flow. Since this is primary a mechanical filter, clogging is an issue. As the sand is used and is slowly clogged. The water will back up and overflow. A high level bypass in the inlet side would solve this probable issue.

The second is why such an intense mechanical filter?

I have one more concern. With the water moving though this at a decent pace it will push waste into the sand. Nitrates?

 

[bwbixby]

No problem at some point in the future when i have a little more room to play with i am going to run a similar style filter on my tank and see how it effects my parameters after a set period of time. my thought is a 200 lbs of sand in a sand filter is going to reduce nitrates more than a similar sized fuge with macro algae could ever. and you never have to harvest or change the sand besides maybe a slight top off evry couple years. not saying a fuge doesnt have its place for pod growth.

 

[bwbixby]

what i saw on one of their tanks was the filter with approx 250 lbs of sand in it on a 1500 gallon system with close to 100 fish most mid sized in the 5 to 6 inch range as well as a etss skimmer and nitrates were undectable. fwiw

 

[Joe Pusdesris]

billsreef, interesting, I just figured that the sand would need to be replaced, but backwashing is even better, yet adds much more complexity.

bsbarton, if there is overflow, like I said, the water will overflow the baffle, skipping the sand and preventing flooding. That would be an indication that the sand needs to be cleaned.

The goal of the intense mechanical filter is to concentrate nutrients in one place, easy nutrient export. It should be cheaper than skimming and the lighting requirements of macroalgae.

Regarding nitrates, even with water being pushed through the sand, I still expect the ORP to drop below 550 mV. According to this randy holmes-farley article ( http://reefkeeping.com/issues/2005-12/rhf/index.php ) nitrate reduction takes place even int the presence of oxygen, so, if anything, nitrates would be reduced.

 

[bwbixby]

I dont see how it would not be biological filtration as well as mechanical. You would get the effects of the partical filtration of water being pushed through the sand, as well as bacteria forming in the sand causing nitrification. Not a biology or chemistry buff so maybe i am wrong, maybe Randy or someone else can chime in?

How I see it is your getting a mechanically filtered RDSB...correct?

 

[therealfatman]

A mechanical filter is a process that captures particles when the water flows though the media spaces and the spaces are small enough to prevent the particles from passing through but not stop the flow of the water, therefore the particles are seperated from the water. If his water was introduced at the top and drawn off from the bottom or vice versa it would be a bilogical mechanical filter if the nitrifying bacteria was allowed to establish. He has altered this by simply using a partition.

In essence his filter is a biological filter that that will support only nitrification bacteria. There would be to much water movement moving through it to have a zone with oxygen levels low enough to support denitrifying bacteria so it would not denitrify like a remote sand buket or a regular deep sand bed filter.

If it was a fluidized sand filter it would be a nitrifying filter but not a mechanical filter or denitrifying filter. A fluidized sand filter is a filter partially filled with sand that has water flowing through it that keeps the sand grains seperated from each other due to the flow and velocity keeping the particles seperated. The sand particles appear to be suspended or flowing hence the name fluidized. A fluidized sand filter is basically equivalent to a trickle filter that is flooded but with very liitle contact between the media. And what contact here is changes and is temporary.

A fluidized filter is less efficient than a trickle filter at nitrification but does not build the heavy biolayer and crud on its partcles surface that build on triclkle filters that then slough off in to the water in large pieces. It as with a trickle filter does not denitrify. It also requires less space than a trickle filter with the same nitrification capacity. Typically in the reef aquarium trades sand filters are not used as mechanical filtration as much as for denitrification.

Mechanical filtration is actually discouraged with most aquarium sand filters. Flushed sand filters are generally not used in the aquarium trade. Good flushing sand filters are not as easy to make as one might think. They are typically made up of layers of graduated sizes of sand and rock and have multiple flushing points at the bottom off the filter.

Flushing sand filters are really considered as finishing filters not primary filters. In small systems they are just not considered practical. For one thing you would have to shut off the water infeed to the filter and the flush water is generally disposed of. It also takes a lot of water to properly flush a sand filter. The sand filter in a marine tank should be flushed with marine water not fresh water. Flushing can be required as often as a few times a day if it is used as a primary mechanical filter as the top layer should be sugar fine sand in order to trap small particle on are near the surface whaere the are easy to flush into troughs during the frequent flushings. Flushing sand beds are discouraged from developing nitrifying bacteria by frequent flushing, however in a aquarium situation the dissolved organics would still assure that a flushing sand bed had enough nutrients to allow it to grow enough nitrification bacteria to function also as a biological filter.

 

[Joe Pusdesris]

bwbixby, Talking about the orp value was simply speculation on whether or not denitrification would take place. I did not mean to imply any sort of buffering.




therealfatman, to quote Eric Borneman's article: http://reefkeeping.com/issues/2004-11/eb/feature/index.php

However, denitrification can also take place in oxic areas, and some of the highest rates of denitrification have been found in the top 1 cm of sediments where nitrate and oxygen levels are highest (Oren and Blackburn 1979). Denitrifying zones can occur from the top millimeter down to 10-15 cm or more, such as in the sediment areas near the Bermuda shelf.

This would lead me to think that denitirification bacteria may thrive despite higher oxygen levels than typical in a deep sand bed.

 

[billsreef]

The trick to the top 1cm of sediment is that there is more than sufficient light penetration for removal of nitrates via algae. Makes it difficult to distinguish if the observed denitrification is bacterial in nature, or uptake via algae.

 

[Joe Pusdesris]

Billsreef, I would expect them to do such tests in a dark environment, but that article doesn't specify, so you could be right.

 

[therealfatman]

Eric Borneman also goes on to further define conditions and when describing the conditions in the anoxic sediments from 1/2" to 1" deep he use the f word mud so they d sediments he is in reality is referring to are likely more akin to silt or something like clay in consistency not crushed corals, sands or even sugar sand. Picture a fine powder that is wet and that also contains organics so that it is more like a sticky wet clay and that is a typical sediment where denitrification might very well take take place near the surface. If I emember right the bermuda shelf is known for its chalk sediments. That is very fine sediments. Like baby powder or they powder from chalk eraser. I do not believe anyone said the bucket would be filled with miracle mud.

I could also believe as Bill writes that the penetration of light and the subsequent algae growth are responsible for nitrate take up in the near surface sediments rather than denitrification taking place via bacteria and that even coarser sediments could respond be affected in that manner also.

Consider that the turbidity studies in the bermuda's almost always show very, very fine particles make up the bulk of the turbidity and it is then easy to see that the sediments are very likely chalk like sediments. Actually you will often see the turbidities in Bermuda turbidity studies referred to as chalk turbidities.