June 8th, 2012


The Duckponics system has just received its final touches to make it fully operational and ready for sampling.

A diagram of the duck ponics system

Andrew: I have added in a diagram of Duck-Ponics that I drew up for Daniela during our intial correspondence. Here is the descriptions I offered for the various numbers on the diagram.

  1. Windward's main potable water tank is continuously feed by our well. The over flow from the main tank feeds a "buffer tank" which then feeds the 1200 gallon duck pond via a float valve. This ensures there is always a source of water to keep the level of the duck pond high through the summer, both to dilute the nitrates and to ensure the ducks can get in and out of the tank with ease.
  2. A pump set to a timer runs approximately every two hours, filling a 100 gallon “flood tank” which gravity feeds two 10ft diameter grow beds and two shallower duckweed tanks. The water flows down hill back into the duck pond.
  3. The nutrient-rich overflow from the duck pond travels downhill to a “green water” storage tank where it is then used to irrigate our main garden and courtyard orchard area.

The ducks are the only input of nutrients into the system. The Duck tank is being continuously refreshed by the main water tank. This means that the system is not “closed”. There is a constant flow of nutrients out of the duck-ponics system via the overflow from the duck pond into the main garden.

Before discussing these finishing touches, I will give a simplified overview of how the system works and what I am looking to accomplish with my research. The ducks play around in the duck pond, and enrich it with nitrogen in the form of ammonia (NH4) with their droppings. This ammonia-enriched water is then pumped into the grow beds every four hours.

These grow beds have bacteria living in them that ultimately convert the ammonia into nitrate, which is the form of nitrogen that is useable by the plants for their growth and life processes. Plants have certain nitrogen needs and will either grow too fast if supplied with too much or suffer from nitrogen deficiency if supplied with too little.

As part of my internship, I am executing an experiment to see if there needs to be any adjustment to the amount of nitrogen the plants need; and, if necessary, how much to adjust the flow in order to supply the plants with the right amount of nitrogen.

At the top is one of the grow beds, the tub in the middle with only gravel is the control bed, and the tub to the right of it contains a local population of duckweed.

I have been continuously devising and revising my plan of execution, and after much discussion with professors and Lindsay, one of Windward's stewards, decided that it was best to create a “control bed” in which there are no plants, only the gravel inoculated with the nitrogen fixing bacteria from the grow bed. This will help me measure amount of nitrogen that is converted by the bacteria into nitrate in the control bed, which I can then use to estimate how much nitrate is being produced by the bacteria in the plant containing grow beds.

Subtracting that estimated amount from the tested total concentration of nitrate in the grow beds will give the amount of nitrate that the plants are consuming. Through research on the nitrogen needs of the various plants in the system, it can then be determined if the amount of ammonia, flowing into the beds from the duck pond should be increased or decreased to meet those needs.

Having sorted this out, I recently just finished putting together the control bed system, which was rather fun. I enjoyed sawing the pipes to the correct size and figuring out how to put them together to get the water where it needed to go. I found that a valve was needed to make sure that the control bed did not monopolize the water that was also flowing to the grow beds, since they were all drawing from the same source.

Also, a boiling pot of water was used to help make the hoses pliable enough to connect to the drainpipes. With a certain amount of convincing, the PVC and drainage pipes went into the right places, and the water was finally able to flow to the control bed on the same automated schedule as the grow beds.

Close up of duckweed

Because there was also a location for a second tub, we made that into a duckweed growing pond. The duckweed was collected from a local population that Lindsay had found near the river, and currently seems to be doing well. This duckweed will help filter some of the water before flowing back into the duck pond, as well as help feed the tilapia, ducks, pigs, and other animals on the property.

Seeing the system fully set up and running is an exciting stepping stone, and now I get to embark on the next exciting step: calculating the volume of the beds and the discharge of water flowing into them. More information on that will come in a future article.