View Full Version : Mechanical Filtration for 7/18 chat

07-18-2004, 01:04 PM
Below is a brief article to act as a starting place for tonight's (7/18) chat at 7pm CDT. Please, no questions on this thread until after the chat session.

Mechanical Filtration
(Prepared for Koiphen.com 7/18 chat)

As most know, there are 2 basic types of filtration, mechanical and biological. There are many approaches to both of these very necessary forms of filtration. We will attempt to limit our discussion to the mechanical side of the equation but is is important to understand first that Mechanical filtration will help our biological filtration perform better by removing solids that will otherwise hamper or biofilters’ abilities to perform at their peak efficiencies.

As stated above, mechanical filtration is the removal of solids from the water column. The two primary ways to accomplish this is through 1) separation and 2) trapping of these solids. In either case it becomes imperative to actually “remove” the solids from the system once separated or trapped. Although both of these methods work, separation is preferable to trapping of the solids. Separation allows the solids not to interfere with the movement of the water through the system while trapping collects the waste while the water is still being forced through it. Fine matting, bead filters and such are examples of methods to trap solids while vortexes, sump pits and mechanical screens (when positioned properly) are examples of separation systems.

Before we get further into mechanical filtration, its also important to understand the types of solids and their properties inorder to more fully understand why some mechanical filters are better than others and what types are more effective in varying installations.

The movement of solids through our ponds and filtration systems is dictated by a number of factors, most importantly, the solids relative buoyancies. Think of buoyancies as Negatively buoyant (sinking solids), Nuetrally buoyant (stay in suspension) and positively buoyant (float). Now, please remember that although under normal conditions negatively buoyant solids sink and positively buoyant solids float but are ponds are NOT normal conditions. Currents from returns, bottom drains and skimmers all effect the movement of solids as well and floating solids can and are pulled downward as well as sinking solids being pulled upward depending on the forces applied on the relative buoyancy of a given solid (infinitely variable).

For sake or ease, we will assume solids are collected in 3 possible locations…..bottom drains, mid-level pickups and skimmers, each targeting a specific type of buoyancy for delivery to the mechanical filter for collection and ultimate removal.

Now there are two fundamental methods of mechanical filtration, passive and active. Passive mechanical filtration would allow for a solids buoyancy to separate it from the water column for removal…..sink to a collection point, or float to a collection point. This is done by reducing the force being applied on the solid and let it fall or rise from suspension…..with the idea that the forces in the pond kept these solids in suspension to move them to the mechanical filtration to begin with. The reduction in this force is typically the reduction in the current strength that is holding those solids in suspension. A settling chamber and or vortex is an example of this method of passive mechanical filtration. Peter Waddington who made the vortex common place in the Koi hobby once thought that this method of mechanical filtration accounted for 80% of the solids in our systems. Peter eventually discovered that his estimation was a tad off too. Another person once claimed that her 400 gallon farm tank separated solids down to the 80 micron level. Well, considering the variable buoyancies, both positive and negative along with the neutrals, this is a physical impossibility! Thus a few years back, micro screens became readily available to the Koi hobby…even though that have actually existed for years. Peter quickly figured out that the vortex was NOT separating as large of a percentage of solids from the system as he once thought.

The micro screen is an example of an “active” separation system that physically separates all particulates down to the given screen size.

By removing these solids from the system, they will not have the opportunity to collect on the surface area of our biological filters, thus allowing them to be more efficient.

But a word of caution. As I attempt to communicate to all ponders, you have to design an entire system to work in conjunction with each other. Simply having the best of this, may and often times does, negate the best of another component piece. The pond, its delivery system, mechanical filtration, biological filtration must all work together to get the best synergistic effects.

Steve Childers

luke frisbee
07-18-2004, 01:24 PM
Nice steve.....

07-18-2004, 04:30 PM
Great thread Steve. I am having such problems...and hopefully can make it to the chat. I've got the big green machine in full throttle :(
(very ill grandfather :( may prevent me from jioning in).

Anyway, I would be interested in all that is said. Thank you for doing this, and hope to see you there.


07-18-2004, 04:41 PM
Pond Design….Bottom drains, TPRs, diffusers skimmers and Midlevel pick ups.
By Steve Childers

Many theoretical discussions have occurred on the NI Board concerning the effectiveness of each of these components and their optimal positioning. Along with these components, pond shape plays a major role. Since our primary goal is to remove solids from the pond so that they can be separated out in the filtration stage, lets talk first about bottom drains….the avenue in which normally sinking particles are removed to filtration. The flow rate into the bottom drain determines the suction action and effective range of such from the drain for the almost infinite combinations of size and weight of the particles. The drain can effectively pull in smaller lighter debris from a larger radius around the drain as compared to larger heavier objects from a smaller distance. Friction of the object settling on the bottom is the force to be over come once the solid reaches the bottom beyond its specific capture radius to allow the drain to suck the object in. So, by over coming this friction we supplement the flow rate to the drain with additional current, inwards to the drain, but not necessarily in a straight line!

Fig. 1
Figure one represents the increasing radius away from the pond’s bottom drain

Also it might help to understand “buoyancy” of solids. There is practically an infinite range of buoyancy for solids, but to keep it simple let’s discuss a theoretical 2.

Fig. 2
For particulates “A” and B” we have lines of varying length to depict the rate at which these both negatively buoyant solid move towards the bottom of the pond. The left side depicts, calm water while the right shows the effect of a “current”, “X” applied to the horizontal (inward to the drain). Although the lengths of “A” and “B” respectively are the same in the left had and right hand drawings, the vertical distance “fallen” has been reduced. The stronger the current, the sharper the angle. Although this is overly simplistic in that with force “X” would not create the “same” angle for lines “A” and “B” it does show the concept of how current effects buoyancy or at least the vertical distance covered in a given time frame.

Now keep in mind as the effective radius is increased for heavier solids outward from the drain through additional inward current (created by a mushrooming current effect due to air diffuser domes mounted on the top bottom drain, we have increased the current at the drain beyond the suction capabilities of the drain itself and thus what once might have settled there may not (depending on the “layer” within the current itself, weight, size etc). But keep in mind, there is still no settlement within this effective radius for our “targeted” size and weight solids. Granted, drop a brick into the pond and it will settle with our “normal” suction at the drain. We could however move that brick to the bottom drain with enough inward current to the drain! But at that rate, not much else would have a chance to “settle at the drain”.

The concept is to actually get “target” solids to settle “out at the drain” or at least far enough into the suction action of the drain to be drawn into it. We’ll call this the “capture zone, keeping in mind that each particle’s capture zone varies based on its own specifics…size, weight etc.. This allows a higher percentage or particles to enter the drain for removal. But there are trade offs as we increase current at the sides to keep the particles in suspension and not settle to the bottom at the outer radius, the current increases at the drain…correct? Think of this diagram in comparison to the isobar maps used in weather forecasting…the closer the lines are together, the faster the current. As the radius from the drain increases the proportionate current decreases over the widening area. If the current created due to the suction of the pond was represented by a 3-D version on figure 2’s lines and the capture zone for our targeted “sinking solids” was radius “C”.

Fig 3
Figure three would represent an equal flow of water from all points along the perimeter of the pond wall…in this case, a circular pond. As these lines get closer, they represent increased current.

Now there are two ways to increase the effective range of the drain, increase the current at the outer edges to keep particles in suspension as they move inwards or reduce the friction on the bottom so that the inward current is enough to overcome the friction of the settled solids on the pond bottom. Sloping the bottom in a bowl design with the bottom angling up at an increasing rate mimics the loss of current in a widening radius. Thus, in wider ponds it is advisable to put drains closer to each other than to the bowled outer sides. Yet as our “target” particles increase in size/weight the bowling in and of itself may not be enough to move particles to within the “capture zone.” The capture zone being the “area” for a specific buoyancy particle within the current. The height in the flow to the drain being in relation to the buoyancy of the particle.

Diffuser drains create a great source of up to down ..and back upwards rotational current….like a mushroom. This in turn has an effect of increasing that downward and thus inward current from the sides. Additional inward current equals an expanded range of effective radius for any given sinking solid. BUT, since the current is increased at the outer radius and drawn inward, towards the drain, less particles (smaller less dense, less negatively buoyant) have an opportunity to be swept by the “inner” effective suction of the drain itself! BUT, once again, since the effective range of the drain has been increased, less of the pond bottom has any settlement on it. Thus, a once sinking solid now becomes a suspended or “moving” solid due to the current

The concept now becomes a little more complicated, how to keep “target” particles from settling at all on the pond bottom. This can be accomplished again by designing a “system” with each component part when combined adding synergy to the over all actions of the individual components. Bowling the bottom, increasing the current at the outer edges while maintaining and expanding the pull from the center minimizing any added pull away from the drain created by the upward pull of the diffuser. You have to love trade-offs!

Its been suggested to shoot the TPRs directly toward the drains. That would increase the current at the outer edges but also create undo current at the drain and add to the upward current created by the diffuser….counter productive. Now, to fully understand the complimenting forces created by TPRs pointed along the perimeter, you have increased the outer current to keep settlement from occurring, WITHOUT increasing the inner current. Couple that with the mushroom rotation created by the diffuser increasing the inward draw and you now have a 3 dimensional “cyclonic” current. Now for the hard part and I don’t have the definitive answer here. Balancing the mushroom current to the draw of the drain itself. Too much air can increase the upward current at the drain. As that current increases our “target” particle size and shape and weight changes, increasing the density/buoyancy necessary to fall into the upper to lower, now “shrinking in height” “particle specific” capture zone due to the increased velocity of the current to the diffuser.

Fig. 4

Figure 4 now represents this same flow, equal to that of the suction of the drains…but entered tangential along the walls. The same number of lines = the same volume of water, but since they are entering tangential they are closer together thus equaling a stronger current at the outer radius without increasing the current at the bottom drains themselves! Now add the third dimension of the mushrooming current created by the diffuser drain and you not only accelerate the currents further (good at the perimeter, but bad at the drain itself) and you have the “cyclonic” current effect, giving a better over all circulation throughout the system. .

As for the actual placement and angles of these TPRs, I am not in the same “camp” as our editor or as one of my main mentors who prefer an inward and down approximately 30 degrees from the corners placement. I would suggest that the TPRs be placed only slightly down, if not level and only slightly inward. The mushrooming current caused by the diffuser will more than make the downward and inward portion of the current while the TPRs themselves are more effective at creating the rotational effect and thus increased current along the perimeter. Thus not adding to the anti productive inward current beyond the capacity of the drain itself. This is not to say that the increased velocity current at the drain itself is not a necessity., but once again there are trade-offs.

Please keep in mind that all of these concepts of capture zones effective radius’ etc vary for every particle, again based on each particle’s own characteristics.

If you’ll recall in the first paragraph when I talked about bottom drains, their purpose is to take, “ normally sinking particles are removed to filtration”. In addition they will also carry away suspended solids which are within the water flow into the drain. The more current at the drain, the less that falls into this flow beyond what is already in it.

Now that “sinking solids” removal is covered, lets talk some about suspended solids. Solids which are neutrally buoyant will stay in suspension, heavier solids will sink and lighter will float…this is in calm water! Add current to the water and the “range” expands…in both directions…depending on an upward or downward current.. So, with added current created by drains, diffusers and TPRs, we have handled the sinking issues…for those particles heavy enough to get to their capture zone or be suspended within the water flow entering the drain…or even suspended in the water flow entering a skimmer …along with the floating particles. So, how do we increase the effectiveness of removing the suspended solids from the pond? Since I know of no way to concentrate suspended solids within a given area without reducing overall circulation to all other parts of the pond, it becomes fairly simple actually….full circulation which evenly mixes the suspended particles throughout while increasing the turnover rate of the pond! This means more suction points…drains, mid level pick ups or skimmers…all with adequate filtration to remove them from the recirculated water. The faster the turnover is beyond the build up of suspended solids, the cleaner and less suspended solids there are in the pond. Now you can also get down to the level beyond the capabilities of the typical mechanical filter and start talking about DOCs….but that is a whole different issue and worthy of an article by someone way more familiar with it than I am.

So, with all of this, the real question is, “What do we expect our bottom drains to accomplish?” In other words, what is the actual “Target” particle that we expect the bottom drain to be able to remove? I would suggest that the target particles would be those most negatively buoyant that can be held in “suspension” or moved through the drain line to the mechanical filter. Anything with more negative buoyancy than what that current through the line can move is useless!

As for non-circular ponds….we can talk about that in another article…hopefully in the next issue of NI.

Figures 1-4 added....hope I got them in the correct order!

Jeff R.
07-18-2004, 04:59 PM
Hey Steve:
Where are Figures #1 - #4? You are a smart aleck mister! You edited.
Jeff R. :D

07-18-2004, 05:43 PM


07-18-2004, 07:03 PM
Steve, I do have one question that is relevant to your article, for us DUMMIES.

What is TPR? From the article it seems to be additional inlet pipes, mid level in pond, circulating water around the pond. Am I correct? :confused:

Thanks for doing this "class". I've already learned a lot. ;)

07-18-2004, 07:15 PM
Tangental pond returns...I just tried to upload some pics but they wouldn't go through, sorry


07-18-2004, 09:44 PM
I thought it went well and hope that I may have helped a few along the way. Please feel free to post any additional questions on this thread now or even pm me if you wish.

Thanks again,
Steve Childers

07-18-2004, 09:48 PM
Steve, I can't begin to thank you enough!

07-18-2004, 09:50 PM
Did someone hopefully follow Momma's instructions and save the engagement into a word processing program? Say Yes!:)

07-18-2004, 09:51 PM
Stephen, I'm hoping someone figured out how to do it. We couldn't copy it.

07-18-2004, 09:54 PM
Wouldn't work for me....too bad Roddy wasn't there....he has a photographic memory...Oops.......Pass the Gas X please! :)


07-18-2004, 09:55 PM
Did someone hopefully follow Momma's instructions and save the engagement into a word processing program? Say Yes!:)
This is gonna be cool.
Hey K... I behaved !...only cause I was outa my league.;)

07-18-2004, 09:56 PM
Wouldn't work for me....too bad Roddy wasn't there....he has a photographic memory...Oops.......Pass the Gas X please! :)

Bad!;) Very Very Bad!;) :( :mad: :D k:cool:

07-18-2004, 09:57 PM
Wouldn't work for me....too bad Roddy wasn't there....he has a photographic memory...Oops.......Pass the Gas X please! :)

Steve Good Lord, Steve... PM doctor Kalfman immediately...:D

07-18-2004, 09:57 PM
Can anyone post a "transcript" of the chat? I was not able to copy it, and it contained some really useful information, as does this thread. Thanks to all involved!


07-18-2004, 10:16 PM
Thank you Steve for the chat tonight, your filtration knowlege is astonishing! I think my way smarter than me DH is now armed to build us one smokin mech filter! Thanks! :D

07-18-2004, 10:28 PM
Thanx so much for your time. That was very informative. I am proud to
be here, associated with such smart sharing folks. It will take some time for all that to soak in.

........and everyone behaved rather nicely:D:D:eek:

Thanx again

07-18-2004, 10:54 PM
Thanks Steve!:) Great Job! I hope you will be there for me Friday at my gig to back me up! I will field questions with lame replys and defer to you!



07-19-2004, 06:42 AM
Don't sweat it....no matter what newbie thing you've done, the experienced probably did it too....that is why they are "experienced! LOL


12-12-2018, 04:44 PM
Thank you!