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Runoff water cleanup, using two pond system, for nitrogen and phosphorus extraction.

Posts: 9002
Location: Victoria British Columbia-Canada
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Runoff water cleanup, using two pond system, for nitrogen and phosphorus extraction.
It's too bad there's no way to easily separate the phosphorus in pig shit, from the nitrogen.
That's a quote from me, maybe a week ago when talking about using rich wastewater for irrigation. Then I thought about it, and figured it out.
It is completely possible to separate the stuff using very low-tech methodology. In the process we get a whole lot of extra protein.
Duckweed and azolla, in that order.

A two pond system that extracts first nitrogen, then phosphorus.
Duckweed is used in many places, to clean up the nitrogen in wastwater. When nitrogen levels become low, duckweed loses some of its green color. A simple visual cue. Roots are usually almost invisible. If they become one centimeter long, there is a nutrient shortage.

 Duckweed is a very high-value animal feed. Up to 40% protein, dry weight.

 Once it exhausts available nitrogen, there is usually excess phosphorus remaining in the water.
 Excess phosphorus in wastewater, causes huge algae blooms when that water enters natural systems. It's the reason that many countries have limited the amount of phosphates allowed in cleaning products. And it's so simple to clean it up.

Azolla produces its own nitrogen with the help of cyanobacteria. The limiting factor for growth is almost always the supply of phosphorus. Azolla will continue to double itself every 2 to 5 days, until phosphorus runs out. It then turns a crimson colour, which is a handy visual cue for pond managers.
By the time wastewater has gone through a duckweed pond and an azolla pond, the water is so nutrient poor, that not much lives in it. People who raise water lilies, water chestnuts and lotus, are advised to not allow too much azolla to cover their ponds because it sucks up all the nutrients.
A two pond system, starting with duckweed and ending with azolla, will remove most nutrients from water.
 My interest in azolla started because it can harvest far more nitrogen from the atmosphere than any other plant. About eight times as much per acre as is accomplished with soybeans. About 2% of all of the petroleum burned, is used to produce nitrogen fertilizers. This is how I want to bring nitrogen to my land.

In the process of cleaning up wastewater, we obtain a vitamin rich animal feed that is high in protein, without having to start with any nitrogen, which is the most expensive building block of protein.

I may throw my shoulder out of joint from patting myself on the back. :-)
In reading a lot of literature about wastewater treatment and duckweed, I didn't find anything about using azolla as a second step. Instead, they lament the other nutrients that escape and harm natural systems.  I think this may be because azolla thrives in warm water, and the majority of studies have been done in Europe and North America. They have space limits, being in populous areas. They don't have cheap labour, who can scoop animal feed from the ponds, at rates that far exceed the cost of employment.
I need to clean up nutrients from manure, in the tropics, where both species do very well. The duckweed and azolla are needed and there is no need to push a lot of water through the system, since the ponds will exist, mainly as a means of growing feed and storing irrigation water.
A few management notes

Some sunlight makes it past the layer of duckweed, which allows for the growth of free-floating algae and other organisms. When nitrogen levels become low, the algae produces its own nitrogen from the atmosphere. When water is transferred from the duckweed pond to the azolla pond, these organisms will travel with it.

Azolla grows much thicker on top of the water and blocks almost all light. It also starves the water of nutrients that other organisms need, so they die.

 Mosquito larvae can't live under azolla.

Both duckweed and azolla ponds, slowly build up a layer of dead material on the bottom of the pond. This sediment contains nutrients that are valuable to plants.

These ponds are likely to be needed during the dry season, for irrigation water. Whenever the pond dries out completely, this material dries up and it can be swept and scooped out, to be used as fertilizer.

When the ponds are empty, manure will be kept in a lagoon or in dry piles. When rain returns or the pond is partially filled from the aquifer, the next crop can be seeded. Both duckweed and azolla can grow in very shallow water, but for reasons of temperature regulation, it's best to be 6 inches deep or more. I expect to build ponds that hold water at a depth of four or five feet, in order to impound a lot of water, as reserve for dry periods.

Using pond water for irrigation
Many field crops and trees, will appreciate nutrient-rich water from the early stages of the duckweed pond. They would get a good dose of nitrogen and phosphorus and anything else that's in that water. If a little bit of duckweed is broadcast in the process, it will either be eaten by something or it will rot.
But certain crops have the ability to produce their own nitrogen and if it is given to them, they are less likely to maximize their own nitrogen fixation. I'm planning to grow thousands of small and large trees in the Pea family, specifically because they are nitrogen and biomass accumulators. The limiting factor for growth, is almost always phosphorus, along with some other micronutrients. If these plants need to be irrigated, it's best to use the nitrogen poor, and phosphorus-rich water that is ready to exit the duckweed pond.

Being trees, I don't expect to water them very often after they are established.

It's more likely that the majority of nutrients from the ponds will be distributed around the farm in the form of animal manure after duckweed and azolla are added to their diets.
Micronutrient fertilization. I intend to have my soil tested for all important nutrients. Some deficiencies will be dealt with by broadcasting the appropriate fertilizer. But if any of these same nutrients can help to make duckweed and azolla grow better and make them more nutritious, they will be added to the pond water. And then through manure spreading, those nutrients will still work their way around the farm.

Do we have any duckweed or azolla farmers among us? Does anyone have any other ideas to make this system better?
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