passive aeration of water from the growbeds

[Pat Chesney]

As some of you have noticed, I am designing a sustainable solar greenhouse. In my quest to limit the watt draw needed from the solar array and batteries (particularly at night) I have been searching all sorts of arcane methods for pumping water and using as little electrical devices as possible. One question I need some answers to concerns aeration. 

I have read that ebb and flow grow beds aerate the water passing through it. Does anyone know if I design a system that uses 2 grow bed sizes for 1 fish tank size how much aeration would a grow bed deliver to the fish? I am designing an Affnan bell siphon with an aerator attachment on the bottom to dump from the grow beds. I am going to build his shuttle valve design for an indexing valve.

I know that I will have to do a DO test to really see how much oxygen a grow bed delivers, but I wondered if anyone had already done anything with that.

When I get my pumping system built and documented, I will post documentation on it. I have found a method to pump large amounts of water using air alone as the force with a pump with no moving parts.

Actually, I have found and built prototypes of two different types of pumps that do this. I will be getting air from the pumping as well as the grow beds due to this. 

Thanks

[George]

Definitely, people have done it, using forms of cascades/sprays to aerate falling water.  When you break up or increase the surface area exposure of water to air, oxygen is added to the water.  Although I'm not using siphons, my water is aerated at a couple of  points while and after leaving my grow beds.  I don't own a DO meter but I believe I have plenty of aeration without using an aerator.  However, I do intend to add an aerator on separate power source for redundancy.  As media drains after being flooded, air is pulled down into the media but it doesn't seem likely to me that the air in the media would aerate the tank water much, if any.  Anytime water is falling, however, there is opportunity to add oxygen.  I wanted to do more in that regard but just couldn't feasibly get my beds any higher.  Good luck with your design. 

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I know that I will have to do a DO test to really see how much oxygen a grow bed delivers, but I wondered if anyone had already done anything with that.

[halemart]

Hi Pat,

I also have been working out ideas of how one could use less electricity in the system.    So far this is what I have come up with.

I run my pumps only 12 hours a day, this has been a big boost to my solar system and has given me more options of running more tiny air pumps.    Since I got my oxygen meter, I found that my oxygen levels were dropping very low in my fish tank,  this prompted me to alter my plan by putting filters into the grow beds to catch more solids, and now I run a 40 watt air pump into the tank 24/7   this works well for me.

[Chi Ma]

You can also make a Venturi for your pump.

[TCLynx]

The real key to helping aeration that flood and drain grow beds provide is that by draining and drawing air down into themselves, they allow the plants and bacteria access to air directly for their processes and therefore the grow beds "don't deplete" oxygen from the water the way a constantly submerged filter might.  There is a reason that many types of filters require huge amounts of aeration to keep them functioning properly.  Decomposing solids and plant roots need lots of dissolved oxygen if they are constantly submerged.

Now I'm not certain that using air lift of geyser pumps is going to use less wattage than simply hooking up a water pump but if the system is extensive with several points that need water lifted, then having a large blower to be able to cover all the needs instead of needing several small water pumps then maybe the air lift will make more sense.

Waterfalls and cascades and any extra splashing and disturbance of the water's surface will provide aeration.  I ran my 300 gallon system without supplemental aeration for the first 6 months and it was fine that way until the summer heat really kicked in.  Once the water temp was staying over 80 F I found the fish quit eating well until I placed supplemental aeration into the tank.  That system always has water flowing back into the tank from the grow beds.  As a grow beds reaches full flood it spends several minutes with water falling down the top of the stand pipe getting extra aeration on it's way back to the fish tank and since that system has six beds and an indexing valve the flow back to the fish tank is constant as there are always a few beds draining and the pump only turns off for a minute between each bed.  I will warn that you are never going to run an indexing valve directly off  an air lift or geyser pump.  Perhaps if the pump will lift the water 4 feet above the grow beds into a flush tank of sorts, you might be able to set up a header tank gravity flush system to run an indexing valve but then I doubt an air lift or geyser pump is going to fill that header tank fast enough to really keep up with more than a hand full of fish and probably wouldn't be worth the extra cost at that point.  FYI my 300 gallon system uses a 50 watt pump for water pumping plus a repeat cycle timer.  The air pump I added for the hot weather provides 1 cfm of air to that tank which has between 18-22 inches of water in it.

[George]

Which meter are you using?

halemart said:

Hi Pat,

Since I got my oxygen meter,

[halemart]

Hi George,

I got this one.

http://aquaponicscommunity.com/forum/topics/dissovled-oxygen-lookin...

I don't proclaim myself as expert, but having one sure has changed my AP approach.

[Jon Parr]

No moving parts to pump water? Does that include no moving parts to pump the air as well? It would have to to claim no moving parts. Interesting. Trees do it, right, it's called capillary attraction. A wick system keeps your plants watered with no moving parts, and a fish tank with low stock density and large surface area will keep the water oxygenated. Wallah. Dishes are done.

As far as electrical efficiency, you cannot begin to pump air as a drive mechanism to pump water cheaper than pumping the water itself. Air compresses, liquids do not. Compressed air gets hot, and that heat is rapidly lost to the environment. To compare, an air tool such as a sander will keep a 20 amp 220 V compressor running constantly, and doesn't hold a candle to the power of a 10 amp 110 V electric sander (using 1/4th the electricity). In my opinion, of course.

[Pat Chesney]

No, only the lift assembly uses no moving parts. The air must be supplied by some means. We are designing a windmill pump using a bellows and looking at air pumps within our design range of wattage to limit battery drain at night. (There is no problem during daylight hours.) The air lift assembly adds oxygen to the water, so it is not wasted. The power of the air lift is because it works by making the water in the assembly more buoyant. it is not depending merely on the pressure of the air. The water itself becomes the lifting mechanism with enough submergence.

In our first trial, we tested a 2" diameter air lift assembly with 7.5 ft submergence and a 2 ft lift. Supplying 2 CFM of air at the bottom of the assembly yielded 30 gal/min of water. My goal is 20 gal/min.

This air lift method is used to clean sludge from waste facilities while they move the nasty water. It will even lift small rocks out of the tank, so moving the fish waste up to the grow beds is not a problem. The "no moving parts" referred to the benefit of nothing in the tank to break, stop up, grind a fish, etc. Of course, the screening mechanism will still need to be small enough to not suck out the smallest size fish.

The issue I was addressing primarily was the search for passive aeration back to the fish tank. I had read that oxygen was imparted to the water by the ebb and flow in the grow beds. What I had not considered was the bacteria need that oxygen to do their part in the symbiosis and there may not be much left when leaving the grow bed. I think that the dumping action can induce oxygen into the tank water if we use methods to inject air into the stream, perhaps with a venturi or the like and then cascade the water down to the tanks. (Such as Affnan's siphon with aerator)

If a low stocking density could answer some of this, I wonder if anyone has charts or formulas to figure oxygen/density. This is most likely not in anyone's mode of thinking because they may not be looking at more passive systems like this.

[TCLynx]

so you can get 30 gpm from your set up.  How many watts is the air pump that can deliver 2 CFM at 7.5 foot of depth using?  Lets see to push water down to that depth you need at least 3.2 psi.  Hum, Ok I see an air pump that might do it for 90 watts and probably only cost an extra $130 per year in replacement parts.

So far in my experience, fish waste isn't solid enough to clog most water pumps, the screens keeping the fish out tend to clog first so the same would be true with the air lift.  When we talk about the solids in aquaponics they are not really solid, just slimy.

[Jon Parr]

SOB! I just typed a huge reply and lost it to a glitch. Dang.

Pat, would you please detail your measurement of 2 cfm of air lifting water 2' at 30 g/m? I may have to eat my words about electrical efficiency using air to lift water. If TC's math is right, and your cfm and lift rate is right, then it would take double the power to lift the water using a direct water pump. 30 g/m is 1800 g/h, which is more like 150-175 watts. Intriguing.

Couple points:
1- Agreed, TC, screen maintenance present in both methods, and if a diffuser is used it may be more subject to clogging than a water pump, especially to maintain cfm and lift
2- flood water need not be aerated in a flood and drain system. The drain cycle provides all the oxygen that roots and bacteria need, in my experience.
3- the dumping action provides all the oxygen the fish need too, in my experience. An air pump and diffuser provides cheap insurance during pump failure
4- Pat, you use the term passive. Wind mills, bellows, flood and drain, and aquaponics in general are not remotely passive. I think the term you are looking for might be 'off-grid'. Passive components would help achieve an off-grid goal, though, such as large-area shallow FT's and wicks in your GB's

I think the windmill and air-lift are great ideas, Pat. I'll follow your progress here. Good luck

[Pat Chesney]

Of course, you are correct that anything that uses some means to lift, move or otherwise do work is not passive.

I think I can clarify by restating my goals as "more sustainable". My group desires to perfect means to do many things in more sustainable ways. While I am not in love with solar panels and wind mills either from this perspective, it is closer to a sustainable situation once the infrastructure is purchased and installed. The only "natural" sustainable fish raising scenario would be a lake.  

What we desire to obtain is a system that will be more sustainable. That is why I am looking at fish that will winter over in our climate and solar power for water heating, electrical generation and the passive heat in the greenhouse. I know how to build solar chimneys for ventilation without fans and these sort of things. We do want our system to be off-grid. Or at least able to be sustainable off grid. See http://www.sustainlife.org/ to see some of what we are doing in several areas.

I have a spreadsheet on the formulas for the air lift assemblies if you would like to look at it, give me an email to send it to. I can be contacted at [e-mail] to share that privately.

The test was measured using a 5 gallon bucket filled in 10 seconds. I have not tested again since those numbers agree substantially with the industry specs on this type of thing. For a more exacting test, I would have to rig up a larger container to catch the flow since it filled my 5 gallon bucket in 10 seconds. My tank that I was pumping from is a 3000 gallon tank and the submergence would start to be affected slightly in a few minutes. It was also very cold and drizzling  outside and I went straight to the spreadsheets to do more calculations because it was time to retreat into the warmth for a hot laté.  

I was using a commercial air brush compressor that is rated at 2 CFM. It was running unrestricted by anything except the head from the depth. I know that is not very scientific, but I wanted to get an idea if the numbers I was seeing in the literature was remotely correct. I too was blown away from the idea of that much flow.

It was explained that the buoyancy of the water reduces the specific gravity of the fluid and it rises in the column just like a balloon would. That is why it moves so much water. There are lots videos of the air lift "pump" on youTube.com. A company in Belgium is doing great work with them for alternative energy projects. http://www.youtube.com/watch?v=fpfjOs0IZfk They are saying they are achieving up to 30 times the water flow per CFM of air. The video is amazing.  "The largest flow on the video - Air pressure 64 psig @ 9.9lt/s / Water flow 350lt/s / pipe diameter 490mm (internal) / Pipe Length 35m, 34m submerged."

They are very sensitive to submergence and air lift assemblies are rated in the ratio of submergence to lift. I was testing at 79% submergence to 21% lift.

Of course, this is a very specific application and you must have the depth in your system to get the submergence to get the lift.

Please tell me what you mean by "large-area shallow FT's" that term is unknown to me. Did you mean large area, shallow fish tanks?

Thanks for your response. And thanks TC for your response.

[TCLynx]

I expect that is what he meant "large-area shallow FT's" would be Fish tanks, because large surface areas exposed to air (and air movement) provide for much aeration and combine that with a little stirring of the layers and you get fairly good aeration provided the temperatures are not extremely high (which would probably be the biggest drawback of a large surface area shallow tank is that it will warm quickly in the sun and hot water doesn't hold as much dissolved oxygen.)

I too am curious about the details of the air lift numbers.  Sent a Friend request so we can message.

I've always been under the impression that air pumps running airlifts can't be more energy efficient than water pumps lifting the same amount of water.  Of course most people are dealing with fish tanks less then 3 feet deep and the less depth you have to work with the less an airlift will be able to lift above the water.