Notes from Windward: #64

The Aquaponics Department

      Aquaponics involves the marriage of aquaculture, the growing of fish, with hyrdoponics, the growing of plants in a nutrient rich solution. The fish waste fertilizes the water for the plants, just as the plants purify the water for the fish. When everything's in balance, the only inputs are sunshine and fishfood in exchange for which you get fresh vegetables and fish for the table.

      That part's easily observed; what isn't so easily noticed is that the system relies on one type of bacteria to break down fish poop into compounds called nitrites, and another type to then break down the nitrites into nitrates which are then utilized by the plants. The planting beds are filled with tiny rocks which provide support for the growing plants, and a home for the bacteria.

      At regular intervals, the grow beds are flooded with water pumped up from the fish tanks. Gradually the water drains back out of the grow beds dripping back into the fish tank where it awaits the next cycle.

      The first thing needed to make an aquaponics system viable here in south Washington state is a greenhouse since both the plants and the fish need a warm environment in which to function. While a standard greenhouse can work if one is willing to invest the BTUs needed to heat it through the winter, what we're striving for is something more sustainable; i.e. a solar greenhouse that will serve as a winter home for both fish and plants.

      The solar greenhouse design we're going to be using is different from the traditional, stand-alone structure in that it's longer than it's wide with the long axis oriented east-west. Another feature is that the lower part of the north side of the greenhouse is earth-sheltered, while the upper part of the north wall, as well as the north half of the roof, are all solid and insulated. Indeed, the only part of our solar greenhouse that will be transparent will be the southern wall and the southern half of the roof.       The reason why a solar greenhouse is built this way is because in winter, the northern side only receives one tenth as much energy as the southern side of the greenhouse does. If both sides were constructed the same, then both sides would radiate heat to the outside at the same rate. Given the small amount of solar energy which would be gained during those short winter days, and the substantial amount of heat which would be lost during those long winter nights, an earth-sheltered, insulated design is the energy-efficient way to keep the greenhouse warm.

      Given the low angle at which sunlight will hit the greenhouse in mid-winter, the centerline of the roof is brought forward and the two sections of the roof are sloped at different angles. The solid, insulated northern section of the roof is sloped at 30° while the translucent southern side is sloped at 46° (Windward's latitude).

      We already have most of the concrete retaining wall poured, so we know that our contained space is going to be some 1,200 square feet, an area which should allow us to put in some 800 square feet of plants and still have room for the fish who'll live in a pair of twenty-four foot long insulated tanks sunk into the floor along the north side of the greenhouse.

      The reason the tanks are insulated is because tilapia, the fish we're most interested in growing, does best between 80° and 85° and start to die when the water temperature drops below 60°.

      The key to bringing a sustainable system on line is to seek out the simplest manifestation possible, get that up and working, and then make incremental changes as you work from where you started out to where you want to go. The classic mistake that folks make in working with these complex, dynamic systems is to take too big a bite to start with, and then choking as the variables gang up on them in real time.

      For example, we'll actually start out raising trout instead of talapia since trout are available locally and won't need supplemental heat in order to make it through a winter in the greenhouse.

      The first step is to get at least one of the 24' long, 4' wide by 3' deep tanks installed. Once it's in the ground and properly backfilled, it gets hooked up to a pump and timer. While the system is proving that it can reliably pump water in a circle, the next step is to expand the circle by installing four 24' long, 2' wide grow boxes filled with pea gravel.

      Once the gravel beds are in place, filled with gravel and plumbed into the system, we'll let the system cycle for a while to insure that the system is mechanically sound, that the water goes where it's supposed to go when it's supposed to, and to leach out any chemicals left over from the assembly of all that PVC piping.

      Once we're satisfied that the system is working reliably, we'll drain the water and refill with two thousand gallons of fresh well water and a gallon of urine.

      The life of the system depends on effectively balancing the ability of the bacteria to break down fish waste into compounds that the plants can utilize. If the gravel beds are seeded with bacteria before there's anything for them to digest, they'll die, but if we add too many fish too soon, then the water quality will tank before the bacteria and plants have a chance to get up to speed.

      In order for us to coordinate this process, we have to be able to accurately measure the ammonia, nitrite and nitrate levels in the system, so this is the time when we develop our analytical skills as we watch for the ammonia level to fall, since a fall in the ammonia level indicates that the bacteria are starting to do their job.

      As the ammonia level starts to fall, and the nitrate level starts to rise, we'll begin the process of transplanting fast growing plants like leaf lettuce into the grow beds. Once they're established, and the nitrate level stabilizes, then it's time to start introducing live fish into the system.

      From this point on, it's a process of fine tuning the balance between the fish and the plants so that the nutrient levels are high enough for the plants to prosper, and low enough that the fish can prosper. Additionally, we have to make sure to establish and maintain adequate levels of micro-nutrients such as calcium, potassium, phosphorus, iron and copper, as well as the inclusion of buffers such as oyster shell in order to make sure that the system pH remains stable.

      As you can see, there's a good deal of science involved in the efficient operation of an integrated system such as this, but that's true of most of sustainable systems. The more data we have, the better we can operate, and efficiency is key to sustainability.

      At this point, the major system input will be fish food. One of the reasons for growing fish is that because they're cold blooded, i.e. the fish don't have to burn calories to keep themselves warm like we do, and since they float, they don't have to expend additional calories in order to keep themselves erect and to move around. Consequently, conversion ratios of 3:2 are obtainable; in other words, three pounds of feed can produce two pounds of live fish which in turn will yield a pound of edible meat.

      With fish food at $0.50 USD a pound, that translates into fish fillets at $1.50/lb, a product which our local Safeway sells for $6.00/lb. The aquaponics system we're building will have the capacity to produce about 6,000 pounds of live fish a year, so we're talking about a department which can produce lots of food for our kitchen, as well as a considerable amount of cash flow.

      And that's not counting the value of the fresh, organic vegetables being grown by 800 square feet of grow beds. One of the challenges encountered when growing plants in a greenhouse is the need to maintain adequate levels of carbon dioxide since plants consume carbon dioxide as part of the photosynthesis process. In aquaponics, the fish insure that the plants have all the carbon dioxide they need in order to prosper.

      So far, I've been describing a standard commercial system, and that's where we need to start. However, once it's up and running, that's when we start the process of making the system sustainable. In order for that to happen, we have to deal with the questions of input and output.

      Since the system is housed in a solar greenhouse, most of the energy needs are met by passive design features, and a run of photovoltaic panels built into the roof will provide more than enough electricity to operate the pumps that move the water around.

      The primary input we need to come up with to make this work is a steady source of wholesome, unmedicated fish food. As noted above, that doesn't come cheap; better to make our own.

      Tilapia are, for most of their lifecycle, content to eat plants, and pelletized alfalfa is a good start, but it doesn't have as much protein as the fish would like, so we'll mix in earthworms and Black Soldier Fly larva to raise the percentage of protein. In order to grow an adequate supply of those, we're building what we call "Vermidise," a fourty foot by twenty foot hoop-type "greenhouse" that will house 360 square foot of beds for the earthworms, and a run of old chest freezers to grow the BSF larva.

      While the Vermidise will look a lot like a greenhouse, it could more accurately be called a "cool house," since it's designed to optimize the conditions needed to grow earthworms, not plants. For example, it's sighted in a location that's shaded for most of the summer day by a ring of oak trees, and it's orientation is north-south, not the east-west that's best for greenhouses in this latitude.

      For most of the year the Vermidise will be covered with shade cloth, not clear plastic, since earthworms don't like light at all, and if exposed will burrow earnestly downward until they're out of sight again. The plastic will go on in the late fall and stay on until spring in order to keep the worm beds warm enough to keep the worms from becoming dormant, but for the most part, the worms don't like it too warm.

      Black Soldier Fly larva offer a marvelous way for us to utilize kitchen scrap without having to keep a pig. While some kitchen scraps can be composted, some food scraps such as meat, doing do well in a compost pile. No matter, BSF larva will be happy to eat it all.

      Unlike the common house fly, Black Soldier Flies don't have any mouth parts, so once they break out from the puppa stage, for the few days they have left, all they're interested in doing is mating and laying their eggs. The larva that hatch out of those eggs are voracious eaters that will convert two pounds of scraps into a pound of the sort of fat larva that any fish would fight for.

      Perhaps the factor which makes BSF larva so remarkable is their ability to climb a 45° incline whereas the larva of the common housefly can only handle an incline of about 30°. Consequently, the old freezer that holds the scraps and the larva is fitted with an exit ramp set at 40°.       When the BSF larva reach maturity, something which can take between two weeks and three months depending on temperature and how much food scraps they have access to, the larva empty their stomachs and leave the food pit in search of the ideal spot in which to puppate into the adult form.

      Unfortunately for the larva, but fortunately for the fish, the 40° exit ramp leads to a collection bucket which holds the mature larva until the next day's fish feeding.

      When up and running, the vermidise will convert waste bedding, leaves, sawdust and shredded junk mail into earthworms, and kitchen scraps, butcher waste and the occasional dead chicken into BSF larva, all of which will be mixed with ground up alfalfa and go to feed the fish.

      The outputs from the aquaponics greenhouse are fish and vegetables, part of which are edible, and part of which aren't. The head, tail and guts of the fish go to the BSF larva to consume, as do the vegetable scraps. And as the plants in the greenhouse are thinned and trimmed, the leaves just get tossed into the fish tanks where they'll be eaten by the ever hungry talapia along with any algae growing anywhere in the system since instead of being a nuisance, algae is a food input which the tilapia seine out of the water.

      Well, that's an overview of how the aquaponics department will function. It's an exciting concept which will go a long way towards enabling Windward to produce a bounty of fresh vegetables and tasty fish year round.


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Notes From Windward - Index - Vol. 64