Duckweed is a really good thing to grow if you have
water to clean up, when there's too much nitrogen. It is probably the best thing to grow in a
pond that is receiving the digestate from a biogas plant. The water is rich in nutrients and the duckweed sucks it up.
If you had two ponds, one could be managed for azolla production. You would not add nitrogen to this one, but instead make sure that phosphorus is in good supply. It will produce its own nitrogen and is also very high in protein. Animal manure produced from this can be used to fertilize a duckweed
pond. The two plants can coexist in the same pond, but one usually becomes dominant depending on nitrogen load and temperature. Duckweed can be dominant during the cold months, then in the spring as duckweed is removed and nitrogen levels drop, azolla can become dominant. When the azolla begins to turn reddish, it's time to add more phosphorus.