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Jims Pond Blog

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This page covers the following topics (click to jump to them):-

A What?

A venturi is a device that injects air into your pond water to ensure adequate pond aeration.

It is made very simply from pipework and has a restrictor inside and an air tube which extends above water level (you can see a diagram below). Water is pumped through the venturi, and the restrictor creates a vacuum that sucks air in from the tube above water level, and then mixes with water inside the venturi to cause bubbles that aerate the pond water. It is used to improve pond aeration and movement of water in the pond and increase the oxygen level in the water.

To the right is the final version of my homemade DIY mini bio-filter venturi being tested without the filter media in the tank. Plenty of bubbles!

In-Pond Venturis

Normally a venturi is used directly in the main body of the pond, to provide oxygen and water movement for the benefit of the fish (some current to swim into). However I thought that a venturi dedicated solely to the bio-filter would provide it with additional oxygen, which "aerobic bacteria", vital for a bio-filter to work, require in order to live and work efficiently.

Some experts don't recommend venturis, as they can be inefficient devices because it forces the pump to push water through a restrictive venturi tube. Not only can the airflow produced usually be too small unless the device is placed right at the surface, but the final pump flow rate can be reduced by as much as 2/3rds! This means you would need a much bigger pump to power the system than required, just to power the venturi. This increases the initial cost of the pump, the running costs, and can reduce the life of the pump. This is true of "in-pond" venturis.

But venturis come in all types and sizes for different purposes. Industrial strength venturis, pumped using large electric motors for use in reservoirs, sewage farms, and water treatment plants, down to smaller scale for use in ponds. Venturis are also used to create the petrol/air mix in petrol engine carburettors.

When used directly in the pond, a venturi does require a considerable force to create good water movement, and a reasonably high pressure to "squeeze" the water and air together so making thousands of very fine tiny bubbles. Click here for a more heavy-duty venturi design specifically for use directly in the pond.

Note: My venturi is intended to help oxygenate the bio-filter, not the pond.

You might consider having a seperate larger venturi fed from another pump dedicated to pond aeration itself, I do. To understand why it is a good idea, learn a little more about oxygenation at this link: http://home.att.net/~oxymax/aerate.htm


Mini Bio-Filter Venturi

For my purpose I don't want a powerful jet pushing into the bio-filter because it would disturb the filter media too much. All I need is a gentle flow of water with air bubbles, 24 hours a day, to help improve the oxygen level in the bio-filter. However the pressure in the device has to be sufficient to create the "venturi" effect in the first place.

Alternatively I could have purchased an electric air-pump, a length of tubing and a couple of air-stones to put in the bottom of the bio-filter. However, a venturi can be contructed easily, for virtually no cost, and has no moving parts to wear out, while an air-pump would have been another £10 or £20 depending on its size.

Since I had some piping left over after building my homemade bio-filter I thought I would give it a go.

After building my venturi I agree that the pressure has been reduced, but only very slightly, more like just one quarter. My waterfall and stream which are gravity fed from the bio-filter, still have a nice convincing flow (at the time using a Hozelock Cascade 4000 mains powered pump - 4000 litres/hour).

Water Turnover & Pressure Points

The most important thing to remember with waterfalls and venturis is that it is not only the stated flow rate which is important, but the wattage of the pump. There is a big difference between a 2000 galls/hour flow rate @ 95 watts and a pump delivering 2000 galls/ hour @ 220 watts. The former is classed as a water mover, and whilst excellent for filtration flow, may not have sufficient power for a venturi or waterfall where back pressure and head are significant factors. The second pump mentioned would deliver the rated flow, overcoming back pressure and would pump up to considerable heights as it is more powerful, but of course more expensive to run.

My pond is only about 400 gallons (as mentioned before I am not a large-scale koi-keeper, I just want to improve the quality of my water). The Hozelock Cyprio Cascade pump I originally used can pump 4000litres per hour (or 880 gallons per hour) at its maximum rated capacity, and 616 gallons/hour at 1 metre head.

My new Hozelock Titan Solids Handling Pump can push 8000 litres/hour, and this powers my new DIY pre-filter, venturi and bio-filter, and also has a bypass valve going to the top of my waterfall.


Design Ideas

The venturi detailed here went through several test phases and rebuilds before it was functioning as desired. I will show you the final design first, then later on I go over the earlier prototypes which didn't make the grade, and why they didn't work!

First I started out searching the Internet for information on how a venturi is constructed. I came across the following page and basic design which I tried out first:-


The design looked simple enough, but I found that my pump pressure was too weak for the size of piping suggested (3/4" pipe), because really the purpose of the venturi suggested at the above site is intended for a good strong flow into the pond itself.

I also found the following novel illustration of the principles and pressures involved in venturi design, and although probably beyond our humble purpose of pond water aeration (it is aimed more at engineering techniques for precision venturis and demonstrates the mathematics and pressures), it is quite interesting to look at because it is an interactive Java Applet which you can play around with! Give it a go!

With kind permission and courtesy of Prof. S.A. Kinnas, Ph.D., Ocean Engineering - Ocean Eng. Group, EWRE, Civil Engineering, University of Texas, Austin.

During the first three weeks of my bio-filter being in action it didn't really get much chance to build up any bacteria. I just wouldn't leave it alone, because I was taking it to pieces every 3 or 4 days to experiment with different configurations of piping and air-inlet pipe sizes in my efforts to get the venturi operating nicely.

How to build the Venturi

The final configuration is as shown in the diagram below. It is fashioned out of 22mm UPVC water pipe, with a corresponding 22mm T-piece. The central cone shaped "restrictor" hole is fashioned out of - wait for it - plasticine! Yes, I used ordinary kids plasticine because it was dead easy to mould while warm, and yet when it is cooled by the water it becomes stiff enough not to lose shape.

The restrictor hole cone shape was carefully formed in the plasticine by using a 10mm drill bit hand held (not in a drill!), first by rotating in a drilling fashion to make the initial hole straight through. This had to be done in small stages because the plasticine sticks to the drill bit, so you have to drill a little, remove and clean the bit, and repeat several times until you're through.

Next, again using the drill bit, constantly and slowly rotated, held loosely in your fingers, rotate it in a circular conical see-saw kind of motion with its axis at the centre of the T-piece, so that the plasticine is slowly squished out from both ends and then excess carefully cut off with a modelling knife. You have to be careful to make sure the excess plasticine is trimmed sufficiently far back into the T-piece because if you don't, when the 22mm piping is push-fitted into the T-piece, it will squish the plasticine in again so ruining the shape of the cone just created. You may have to repeat the process several times:- mould, shape, trim excess, until you are happy with it.

I would recommend not making the middle aperture larger than 10mm initially, so that the increase in pressure causes the velocity of the water to be quite fast as it goes through and passes the air-intake, so causing a good strong vacuum. Its easy to squish plasticine out, but not so easy to put it back in again without having to totally remake the restrictor. Only if the water pressure is reduced too much on the exit side should you consider increasing the diameter of the restrictor bore. It is surprising how much volume of water can pass through the venturi under pressure.

The air-intake pipe is made from electricians yellow/green coloured earth-sheathing tubing normally used to cover the earth-wire in domestic electric switch and socket connections. It was about the only kind of tubing that I could find readily to hand that was thin enough (about 3mm diameter) to cause a nice draw of air. This is because the vacuum in the venturi has to overcome the backpressure of the water trying to go up the tube. The bottom of the tubing was cut at a slight angle (about 20 degrees) to match the cone angle of the moulded plasticine, and it must be positioned exactly flush with the cone shape of the venturi for maximum effect. Again drilling through the plasticine vertically to accomodate the earth-sheathing is a bit tricky, because it squishes the plasticine out of shape again, so you might have to do some re-forming of the cone shape again, but this time with the air-intake pipe in place, while being careful that the plasticine does not get into the narrow air-intake pipe and block it up.

The plasticine is a little tricky to work with, but with patience you can do it. And like this you can re-form it until you have the best size and shape for it to work nicely.

Once you know what works you can, if required, re-fashion the venturi cone out of something more permanent like epoxy-resin. I did consider doing it with epoxy-resin at the start, but then I thought about the problems involved;

  • How would you form the resin into the correct cone shape before it sets?
  • Alternatively you could fill the T-piece completely with epoxy-resin to make a solid lump and drill after it has hardened, but then how do you drill a nice smooth cone shape in the resin if you don't have engineering tools?
  • How do you put epoxy into the T-piece without getting it all gunked on the inside which after it has hardened would cause problems pushing the 22mm piping into the T-piece? We all know how incredibly sticky and stringy epoxy resin can be while working it, and how hard it is once set!

My main objective in creating the venturi was to try different things out until they worked, and what simpler way than to use good old plasticine? And if it works - well don't knock it!

Anyway after 2 years of use, so far the plasticine version has functioned perfectly well, and also operates far better at lower pressures or deeper water than Zik's venturi design, while Zik's is a real water mover and mixes air into the water more efficiently.

The pre-filter prevents any large solid particles going through the venturi which might block or mess-up the shape of the plasticine.

Inspect the design diagram below and note the following very important points to ensure the venturi works:-

  • The air-intake must be positioned on the exit side of the restrictor just after the narrowest section of the restrictor, not in the centre of it.
  • VERY IMPORTANT: The air-intake should be quite narrow (about 3mm) in comparison to the water inlet (22mm) and the bore of the restrictor (approx 10mm), so that the vacuum on the exit side of the restrictor can act easily to pull air in. This is the main mistake people make when constructing a venturi. The larger the diameter of the air-intake pipe, the faster must be the flow of water through the venturi, especially if you want it to pump into deep water.
  • The air-intake must be cut at a slight angle (about 20 degrees) to match the cone shape.
  • The more narrow the bore of the restrictor is (start with about 8-10mm), the faster will be the flow of water through the restrictor resulting in a more powerful vacuum to suck air down the air-intake pipe, but the trade-off will be less water pressure out of the venturi into the bio-filter (this isn't a bad thing since we want a controlled flow through the bio-filter to give it enough Retention Time for bacteria to work), but this might also mean any water feature following on would not look very effective and it could put a strain on the water pump.

Diagram of How a Venturi Works

Figure 1. Venturi design.


Finished Venturi

The picture below shows the final venturi design in operation and as can be seen there are plenty of bubbles being output from the swirler at the bottom of the bio-filter. The black cap is just a suitably sized threaded cap fitted to the top of the 22mm pipework to protect the innards of the venturi.

Obviously the air-intake tube should not be pushed or pulled because it relies on the plasticine holding it in place, and its location in the venturi is critical. However as I explained it has now been in operation for 2 years with no problems. These pictures show the bio-filter with the filter media removed for testing.

And here it is with filter-media replaced, and the air bubbling up nicely through the media. In my earlier designs I had the venturi at the bottom of the bio-filter tank (see pics further below). Now with it at the top, it is much easier for me to remove it for inspection and cleaning, or even to upgrade to a better design if necessary.

With the finished design, after flushing out my bio-filter as a part of routine cleaning maintenance, when I turn the pump back on again there is some slight spouting of water out of the air-intake pipe due to large bubbles being pumped through from the pre-filter, but as the air bleeds out of the system and it settles down the venturi effect starts to work and pulls the air in properly giving a constant sucking sound and air bubbles are properly fed down into the bio-filter. So when you start it for the very first time, expect it to act a bit like a "coffee perculator" until the air in the pre-filter and piping has bled through and out!

Put your ear next to the air-intake and you should hear a constant draw of air into the venturi, and of course should see the bubbles rising to the surface.


Earlier Prototypes - What didn't work!

This picture shows the very first design with the venturi T-piece positioned right near the bottom of the bio-filter, with a long upriser tube.

The first venturi design was attempted with a piece of copper tubing normally used to convert 22mm pipework to 15mm. It was glued in place with Fernox jointing compound.

When the T-piece was fitted on it was pushed on only far enough so that the narrow end of the copper converter was halfway through, so that a length of plastic tubing could be pushed down in front of the opening.

Another view.....(note that there is nothing else to fill/block this pipe or hold the narrower air-pipe in place - this was a mistake and one of the reasons why this design didn't work for me initially).

Looking into the exit or outflow end of the T-piece (below) you can see the plastic tubing in place, with its end cut at an angle and facing with the direction of the water flow (the water comes out of this end).

Below is the complete assembled (non-working!) venturi.


Back to the Drawing Board!!

Well the initial prototypes didn't work. There were several factors causing these designs to fail:-

  1. The force of the water exiting from the copper tube was insufficient to create enough vacuum to suck the air down the clear plastic air-intake tubing. When the water was turned on and the bio-filter started to fill, it was drawing air in as expected, But as the tank started to fill, the level of water became higher, until eventually the vacuum was unable to overcome the backpressure of the water as it was trying to go up the air-intake tube instead!
  2. There was nothing surrounding the air-intake tubing at the bottom of the 22mm pipework where it entered the T-piece, therefore nothing blocking and preventing water from rising up and around the air-intake tube inside the 22mm pipework. So the vacuum was also having to pull the water down in the main 22mm riser tube as well. It just wasn't going to happen!
  3. The clear plastic tubing was too wide, so again the vacuum had too much volume of water to pull down the air-intake before it could even start drawing air in.
  4. The copper tubing did not narrow enough, and didn't form any decent union with the air-intake pipe, which just flopped around in front of it with nothing to hold it in place.
  5. All of these were exascerbated by the depth at which the venturi T-piece was located, about 12 inches underwater.

Three different types of tubing were tried. 8mm clear plastic tubing. 6mm tubing (blue), and finally electric Earth-sheathing which is about 3mm diameter.

With my earlier designs when I was testing them out (the ones that didn't work very well) I was listening with my ear right next to the air-intake to hear how well it was working, and you guessed it - I got an ear full of pond water! LOL.

Even though the Earth-sheathing was attempting to work better, it would once again stop as the filter filled up and the depth of water increased. Bear in mind that in this design there was still nothing to block the water coming up the main riser tube. i.e. there was NO plasticine in this model.

So it was a case of "back to the drawing board" to have a rethink about the problems, and come up with a better design.


Was it worth it?

The final design shown at the start of this page works quite well. No doubt there is room for improvement, but.....

During the first 2 weeks of experimenting the bio-filter didn't really get much chance to settle down, what with me pulling it apart every couple of days to re-arrange the venturi. It is said that a bio-filter requires about 6 to 8 weeks to mature, while the bacteria builds up, and a balance is created in the ponds eco-system.

In the third week, with the early prototype venturis not working very well, there was very little happening, and while the water was beginning to look cleaner due to the pre-filter, it was still VERY green.

But in the fourth week, about 4 days after I got the final venturi design working mixing air into the water nicely, the water was starting to clear noticeably. Up until now the fish could only be seen clearly if they came within about 3 inches of the surface, any deeper and they became a green blur. But after 6 days of oxygenating the water going into the bio-filter the fish could be seen progressively deeper, now down to 6 to 8 inches, and the vivid green was weakening.

Now about 8 weeks after the whole bio-filter and venturi setup has been fully operational we can once again see our fish. The water is much clearer and cleaner. The bio-filter has healthy looking gunk in it, and the pre-filter does not seem to be blocking up as quickly - I still clean it regularly, but there is just not as much build-up of muck in it.

Clean, clear, healthy water = happy fish
Now my pond is back to normal, with visible fish, thats a novelty!! And done the natural way without chemicals or ultra-violet lights. The big ghost-koi in the right hand picture is right at the bottom of the pond about 30 inches deep.
Looking into the deepest part of the pond, the 2 ghost-koi can be seen at the bottom

Another advantage of having the venturi just before the bio-filter is that if the pump is turned off, either for maintenance or due to power failure, it prevents syphoning back into the pond, because the air is drawn back down the pipe rather than the water, so immediately killing any syphoning effect. This is good on two counts; 1) it prevents any sediment held in the settlement chamber from backwashing right back into the pond, and 2) the bio-filter will not accidentally empty, so keeping the bacteria in water which although not flowing will at least keep the bacteria alive for a few hours until water flow is restored.

The success of the whole setup is not down to any one thing. It is a combination:-

  • Re-adjusting mental attitude for regular cleaning of filters
  • Oxygenating/aerating the water has a very positive effect on water quality, and efficiency of aerobic bacteria in the bio-filter
  • Feeding the fish only as much as is required
  • Cleaning other muck out such as leaves to minimise decaying matter
  • Adding nitrate loving and oxygenating plants to increase bacteria breeding surface area elsewhere in the pond, and reduce the level of nitrate available to phyto-plankton.

This is a continuously evolving project, and I have made some alterations to my setup. To read about the modifications I have made to my venturi in 2005 click here.

If you have any comments or suggestions about this project please contact me:

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