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

[Pat Chesney]

TC, those are valid considerations. It would be a normal storage cost vs battery life issue. However, at the time I built my solar system, I was planning a sustainable water pumping system that was geared to be somewhere that batteries might need to last up to 20 years without replacement. The scenario was that after the initial set up, batteries might not be available in this location.

Twenty years is not an impossible number to achieve if the rate of discharge is low. So, with a worst-case scenario in mind, I built a system that would supply 3 households with two solar pumps. One moved raw water during the daylight hours when there was lots of power and the other pressurized the tank with the ability to run some at night off the batteries. I built a slow-sand filter to purify the water that could deliver 700 gallons of pure water per day. That number was my design maximum water usage for the 3 homes combined.

Now, for this possibly constant use scenario in my greenhouse, I am having to re-think the draw from the batteries. I can set my controller to only allow discharge to a certain %. I am leaning to timing the pump furnishing the grow beds to be running at intervals and continuing to bubble air in the fish tank all night. I am re-calculating the depth of discharge vs life angle for this design and may settle on a deeper discharge rate.

Thanks again for your thoughtful comments.

[TCLynx]

Sounds like you have definitely done your homework.  I hope to hear more about your project and what you find that works well.

[Pat Chesney]

I appreciate the lively inputs from all of you. I will give an update once the dust settles.

Thanks

[Jason Eaton]

Great thread. I measured a small 12v bait airation pump only pulled 0.2 amps at 12v. While the 60g/m bilge pump pulled 3 amps at 12v. Its a huge difference when when your pulling from a 12v battery bank. Running at night with airation only +1.

Thanks Oz.

RupertofOZ said:

You can stop circulating to the grow beds overnight... I do so consistantly during our Australian winter... to keep tank temps as high as possible...

 

But.. and I say again...  BUT... you MUST have additional external aeration.... and you must have a battery backup system attached to it...

And IMO... the same applies.... if you rely on passive aeration from water movement...

[halemart]

My solution for extending my batteries has been to keep in the C20 range with the batteries, and to use desulfating to break up the normal build up on the plates.     In doing this I have bought used batteries about 8 years old, and have now a system that has been running for about 2 years.    I have had to weed out about 4 of the used batteries as they were bad but I got them for like $20.00 a peice for deep cycle Trojans.   I can't beat that deal anywhere yet.

Considering your sand filter idea, I was thinking of making a system that pulled out 1/2 of the water volume of the tank, and then running that thru a slow sand filter.    Then repeating the process with the water in the tank.   Thus making for water that could hold much more oxygen because the gunk is taken out of it.     

Looks like indeed you have done your homework.  

[Pat Chesney]

I have done work with slow sand filters for 2 years. They are great for purification of raw water to safe drinking quality. But, they will stop filtering very fast if there is any turbidity in the water that you don't remove before hand. The bio-waste from the fish should close them down very quickly. ( I did not try fish water, so this is my untested opinion) Also, they are great for destroying bacteria, protozoan life and even viruses (according to the WHO and the EPA). However the schmutzdecke (active bio-layer) might be overcome or replaced by the nitrogen cycle bacteria. If you don't get enough oxygen down into the water and sand, the bio-layer will die and you will have a stinking mess of anaerobic bacteria going.

I did build and test a fast sand filter for a fish tank and it became unusable in about 2 months. The anaerobic bacteria took over the filter and it was a stinking mess inside, even with backwashing. (I wish I had tested the water for ammonia and nitrite, but I must confess that I did not.) It finally would not pass enough water through and the smell of the sand told me all I needed to know. The experience was enough to wave me off sand filters because there was no way for the oxygen to keep the anaerobic reactions at bay.

Instead, I would use a clarifier-type filter to precipitate the solids and waste out rather than sand. My work with those showed that you could take out quite a lot of gunk. The problem is, you want the "gunk" to go into the grow beds if you are using those and not a raft system. (Look at the UVI clarifiers--these type of filters will work but you will be throwing away plant nutrients.) A grow bed is a huge filter.

However, in my experience, the problem we are discussing here is not the waste ( which generates ammonia, nitrite, etc) but lack of oxygen if you don't have aeration or agitation. I am using a pump right now that only draws 37 watts and moves 600 g/hr. This is hard to beat for moving water.

I am considering a short on/ long off cycle to move water during the night to conserve batteries with air pumps carrying the load of generation of oxygen constantly. During the day, the solar panel does it all with no problems. There can be a constant flow of water during the day. This is good because I think the plants need the water to circulate more during the day and the fish are more active and use more oxygen.

I have looked into oxygen generators to get a higher DO level, but they have a heavy amp draw. I would have to increase my solar array and battery bank and the trade-off doesn't seem to be worth the expense.

We are going to drill a well and there is a possibility that we can use a trompe to generate compressed air from the falling water after our windmill puts it up in a tank. The intermittent nature of wind power makes this a supplement and not a primary source.

Jason, any idea of the level of DO one of those bait agitators can produce? That is a pretty low amp draw. How did you get that number? Do you have a 12V amp meter or are you reading the face plate data? I will research that angle too, thanks.

[Jason Eaton]

Pat, I measured it with an amp meter. The packaging doesn't say what the amp draw is. It looks like your common small cheap plastic bilge pump. I was surprised at the amp draw. One thing a little different about it is the plastic screen is larger then the pump and it has a plastic mesh filter in it. Its a bait pump or for washing down your boat. Here is the pump.

 

http://www.amazon.com/Seachoice-19481-Livewell-Aerator-Kit/dp/B000X...

 

I'm sorry I don't have an oxygen meter to see how well it does. It just pumps water into a small piece of plastic pipe that has holes in it. I really don't know how affective that type is.

Best of luck.

[TCLynx]

I expect the talk about removing solids has more to do with reducing the BOD (biological oxygen demand) in a low power system but you are right, you remove it and you remove much nutrients for the plants.

How much dissolved oxygen a simple air pump type aerator can produce will be a function of not only the amount of air the pump will move at the depth (pressure) required of it but also the nature of the diffuser, the temperature of the water and the altitude and oxygen content of the air being pumped into the water.  Sorry I don't know the math for any of that stuff.

[Pat Chesney]

All true, it is complex. But, what isn't in life, eh? I gave up trying to measure DO because I can't afford a meter for that and the chemical test is too prone to error if you don't do everything just right. The best DO meter appears to be the fish themselves.   As long as you catch them before they stress too much. I am at 535 ft. So there isn't much altitude adjustment. My current tank is about 1/2 as deep as my new one will be so my pumps agitate the surface readily.  I will have to pay attention to the quality of the air pumps for the deeper tank. Temperature in the summer is a bummer here in Central Texas-scorching hot. My water was at 82 degrees yesterday morning before the sun came up. Time to get the shade cloth up. I can't even imagine what July and August will be like. The rain has already stopped and the drought cycle has set in again. It is going to be a long, hot summer.

[TCLynx]

Yea, and when the overnight water temps don't drop below 80 F, ya definitely need extra aeration for most types of fish under all but the absolute lowest fish loads.

[George]

Yes, sure is.  How much head will it pump that rate to?  What is the pump?

Pat Chesney said:

I am using a pump right now that only draws 37 watts and moves 600 g/hr. This is hard to beat for moving water.

[Brian Mouncer]

you guys should realy look inot nick iron batteries.

you can 100% discharge them, over chage them for weeks on end, and they will still last for 20+ years.

the only real catch is cost and they blow up with constant curent chargers, your need to use a constant volt charger.

 

Do a search for iron edison battery.

 

Brian.
 
TCLynx said:

In general the less deeply you discharge the batteries the longer their life will be but if you don't get to use the batteries then what is the point, kinda like maybe if you only discharge 15% you might get an extra few years off the batteries but if you need twice as many batteries in order to get 8 years out of them instead of 5 years, well is it really worth the extra cost to replace say 4 batteries every 5 years or 8 batteries every 8 years?    (I'm just tossing up numbers here as examples, they have no bearing on any research or reality, I'm only recommending looking carefully to avoid false economy.)

I sized my battery backup to hopefully never need to go deeper than a 50% discharge on my deep cycle battery.