Sand filters...

[JohnHuff]

OK, all the sand filters I see look like this:

I just got to thinking. Isn't this completely wrong?

When any crud flows through, all of it is completely blocked by the smallest layer on top. Everything, small or big particles get blocked by the sand. And the bigger layers really don't do anything since every particle bigger than sand is already blocked above.

Shouldn't the large stones be on top, then the size decrease with the sand at the bottom? This way, the biggest particles are blocked on top and smaller pieces of crud get blocked lower down.

Of course the problem here would be cleaning, since you would need to remove all the above layers to get to the sand. But that problem can be easily solved by separating the layers by mesh or containers, so you simply lift up the containers above to get to the sand below.

So, why are all the sand filters built the wrong way then?

 

[minnowman]

Sand filters are designed that way to allow for backflushing with a large volume of water from the bottom up. The sand fluidizes during backwash to eliminate solid waste. The gravel layers support the sand and distribute the water flow without plugging. This design is used in municipal water systems as a polishing filter. It doesn't work well as a biofilter because the sand gets stuck together by the biofilm and is lost during backwash. Fluidized bed sand filters can be used as biofilters, but usually contain larger sand grains to prevent loss of media. This type of biofilter can handle large amounts of ammonia, but does not collect any solid waste.

 

[dieselplower]

Some of these filters have the water enter from the bottom, so the water flows up through the big rocks thru smaller and smaller media as it goes up, and overflows out the top. That's how mine works. Not state of the art, but works very well.

 

[Catfishnut]

OK, all the sand filters I see look like this:

I just got to thinking. Isn't this completely wrong?

When any crud flows through, all of it is completely blocked by the smallest layer on top. Everything, small or big particles get blocked by the sand. And the bigger layers really don't do anything since every particle bigger than sand is already blocked above.

Shouldn't the large stones be on top, then the size decrease with the sand at the bottom? This way, the biggest particles are blocked on top and smaller pieces of crud get blocked lower down.

Of course the problem here would be cleaning, since you would need to remove all the above layers to get to the sand. But that problem can be easily solved by separating the layers by mesh or containers, so you simply lift up the containers above to get to the sand below.

So, why are all the sand filters built the wrong way then?

John,

There is one piece of the puzzle that you are missing here. The notion behind the "sand filter" is that the top layer of fine sand aloows for the growth of bacteria and what-not which erradicate or otherwise dispose of and expel all of the harmful compounds/chemicals/ammonia/bacterium/etc. The design of a sand filter or slow sand filter depends a great deal upon beneficial bacteria which reside in the very upper or topmost layer of the fine sand to "perform their function" of cleaning and purifying the water.

I think you may be analyzing these sand filter systems in the wrong way. I think you are percieving them to be more of a "mechanical" filtration system when they are actually and purely a biological system akin to a moving bed bio-conversion process. The beneficial bacteria and the biological breakdown of the harmful elements (compounds) occur in the top layer of the sand filter where the bacteria live and grow. It is the same process which goes on in a moving bed filtration system, but occurs or is carried out in a very slow and passive method.

The bacteria in charge reside in the TOP or upper layer of the fine sand. They do their job there and the layers of coarser sand and gravel below are simply support media or a "bed" for the finer sand.

A sand filter is more of a biological system then it is a mechanical system and this is where I believe that you may be misunderstanding the process at hand. Does this ring a bell or make logic for you?

Catfishnut

 

[Mucky_Waters]

Sorry Catfish, but I strongly disagree with you on this one. Most "sand" filters are primarily designed to function as mechanical filters, any bio filtration in sand filters for pond use is superfluous. There are probably more sand filters used in swimming pools then in any other other single application in the world, and any biological activity in these filters would be almost nil because the water is generally chlorinated as well.
If you wanted bio filtration sand would be a poor choice because it works so well as a mechanical filter and clogs easily. For bio filtration you'd be better off using something much more coarse to allow the water to flow more freely.
But that's just my opinion.

 

[Catfishnut]

Hi Randy,

I may have jumped onto a tangent here that was not related to the point which you were debating or contemplating. I looked at the "drawing" that you started this thread with and read your statement: "I just got to thinking. Isn't this completely wrong?" Please forgive me if that response seemed contrary to your direction with this thread and allow me to explain my statements
more thoroughly here.

The pictograph or drawing that you included is very similar to some of the drawings that I have stumbled across when researching "slow sand filters" for water purification. Your initial response was that there was something amiss with the concept, theory or design. However, my thought processes took me immediately to the notion that the drawing was a proper one for a different and totally unique purpose - water purification for drinking water - but not anything you would want to utilize for or incorporate into a pond filter system design.

I maintain my opinion that this sand filter system (as drawn in your original post) is for a totally different application (that of water purification), and is correct and works efficiently. I also agree with you that it is "completely wrong" as a concept or design for a mechanical pond filtration system for obvious reasons. I think you instinctively or subconsciencely caught this as it is seemed to me that your knowledge of filtration systems told you that it was incorrect or flawed.

What I was attempting to inform you is that the filter system depicted is, in my opinion, a slow sand filter for water purification - AKA for purifying drinking water - but not a mechanical filter for a pond system. Basically, I was agreeing with your assumptions that the design was improper, but it was improper only for the application that you were assuming that it was intended for.

Slow sand filters function extremely well for the bio-chem purification of drinking water from poor water sources. But, they call them "slow" sand filters for an obvious reason, the retention time required for such a filter is long, or highly extended, so that the bacteria can do their "thing" to purify the water coming through the system. Somewhat akin to a RO system for household drinking water supply.

I hope that this explanation seems more logical than my first reply. Beyond my entire statement, read the line at the very botom of the "advertisement" in the pix....

"A Useful Emergency Filter"

Catfishnut

 

[JohnHuff]

Apologies for the dilatory replies. I wanted to give it my full attention before reply.

I agree with you. These filters might be designed this way for backflushing, but since filters are only backflushed occasionally whereas they go the other way most of the time, it seems then that design is not very good. The sand will still pick up all of the solids and be plugged up quickly.

Sand filters are designed that way to allow for backflushing with a large volume of water from the bottom up. The sand fluidizes during backwash to eliminate solid waste. The gravel layers support the sand and distribute the water flow without plugging. This design is used in municipal water systems as a polishing filter. It doesn't work well as a biofilter because the sand gets stuck together by the biofilm and is lost during backwash. Fluidized bed sand filters can be used as biofilters, but usually contain larger sand grains to prevent loss of media. This type of biofilter can handle large amounts of ammonia, but does not collect any solid waste.

 

[JohnHuff]

Hi:
You've got this wrong. This is not a slow sand filter. Look at where the outlet is. It won't allow the sand to be continuously under water and form a schmutzdecke. This is a pure mechanical filter. Besides which the caption says emergency filter.

John,

There is one piece of the puzzle that you are missing here. The notion behind the "sand filter" is that the top layer of fine sand aloows for the growth of bacteria and what-not which erradicate or otherwise dispose of and expel all of the harmful compounds/chemicals/ammonia/bacterium/etc. The design of a sand filter or slow sand filter depends a great deal upon beneficial bacteria which reside in the very upper or topmost layer of the fine sand to "perform their function" of cleaning and purifying the water.

I think you may be analyzing these sand filter systems in the wrong way. I think you are percieving them to be more of a "mechanical" filtration system when they are actually and purely a biological system akin to a moving bed bio-conversion process. The beneficial bacteria and the biological breakdown of the harmful elements (compounds) occur in the top layer of the sand filter where the bacteria live and grow. It is the same process which goes on in a moving bed filtration system, but occurs or is carried out in a very slow and passive method.

The bacteria in charge reside in the TOP or upper layer of the fine sand. They do their job there and the layers of coarser sand and gravel below are simply support media or a "bed" for the finer sand.

A sand filter is more of a biological system then it is a mechanical system and this is where I believe that you may be misunderstanding the process at hand. Does this ring a bell or make logic for you?

Catfishnut

 

[JohnHuff]

Interesting. Why not build it the other way? It will be more energy efficient. The water and solids doesn't care if feed it up or down but it costs you more energy to feed it up.

Some of these filters have the water enter from the bottom, so the water flows up through the big rocks thru smaller and smaller media as it goes up, and overflows out the top. That's how mine works. Not state of the art, but works very well.

 

[dieselplower]

Interesting. Why not build it the other way? It will be more energy efficient. The water and solids doesn't care if feed it up or down but it costs you more energy to feed it up.

Well I reall don't know. I assume if I flowed the water in through the top and started with bigger rocks on top that the debris would work its way down due to gravity. I think eventually the larger media would end up on the bottom and the smaller on top anyway, would it not? Plus I imagine it would cost me energy or flow gph to pump the water up the extra few feet. Plus if the water output was on the bottom, the small media would flow out with it. Probably...?

 

[Catfishnut]

Hi:
You've got this wrong. This is not a slow sand filter. Look at where the outlet is. It won't allow the sand to be continuously under water and form a schmutzdecke. This is a pure mechanical filter. Besides which the caption says emergency filter.

John,

Sorry for not replying sooner. I get what you are saying here.... That the schmutzdecke won't be allowed to form because of the drainage / porosity of the design and that would eliminate it as a design (a good or proper design) for a slow sand water purification filter.

Then, back to your original assumption, I am not sure that it is a good concept as a mechanical filter, but for a different reason. The action and the layering is right to a point, but not entirely without another key element.... Backflushing! Most mechanical media filters of this concept use something heavy and larger like garnet or granite or marble or whatever on the bottom layers and the sand or finer media on the upper layer. But, you would have to backflush the whole system in order to make it work properly. So I think the layering is an acceptable principle of the design if you were to incorporate the buckflushing. Otherwise, it will simply clog up the upper layer of sand, eventually, and then the filter would be done.

So maybe what they are saying is that it is an emergency use "one-time" filter and since it is drawn as a small bucket, that would probably be functional, but I don't understand why you would then need the heavier / coarser media at all! Might as well have it all sand media and then toss it when it has clogged up.

Incorporate the backflushing and then it makes more sense because the lighter, smaller sand particles would be purged and properly distributed (there is a better term for that but I cannot think of it right now) and yet remain in the upper layers after the reverse flow of the back-flushing and the system would function as a replenishable filter. However, that doesn't seem to be the artists conception here - it appears to be lacking something quite important in the design.

Does this follow with your thinking?

Catfishnut