My pursuit of an energy efficient system continues. Yesterday after about 6 months of flawless operation my bell siphon failed to drain. This inspired me to find something simpler, less expensive and more dependable. I remembered this conversation on
AquacultureHub
By simply providing small drain holes at the bottom of the media bed a Hybrid Flood & Drain / Ebb & Flow can be created. I have long been a proponent of Ebb & Flow due to the energy savings.
Here's how it works:
It's pretty simple. There's a stand pipe that keeps the water from
exceeding a set level about 1-1/2" below the gravel, and a weep hole
(about 5/16") that drains the water out. The pump comes on just a few
times per day. While the pump is filling the grow bed the water seeps
out. But the pump fills the tank much faster than the water leaves, so
it fills. The timer need only be set on long enough for the grow bed to
fill.
In the Summer you may need to run the pump more often, but I run mine
3 times per day all year long. The gravel stays moist and the water is
aerated sufficiently for the fish.
By running the pump only a few times per day you save utility costs,
and there is no siphon to fail. I inserted a capped 1" pipe at the
bottom of one of my grow beds and drilled my weep hole in it so that I
could see it and this makes cleaning the hole very easy. I can remove
the cap to get inside if need be. I've been running my timed flood and
drain for several months and the hole has not clogged, but it does not
hurt to inspect it once in a while.
The timer I use is just a cheap $3.00 indoor timer with a plastic
food storage container over it. I poked a couple holes through the
plastic, and plugged the timer in. It stays nice and dry this way, and
easily accessed by removing the cover of the container.
Here are the conversations between
John Burgess, aka
Glenn Martinez, and
J. Linden Rose.
System Design Considerations
Many people use gallon of fish tank to gallon of grow bed ratios.
This has a minor flaw, because nutrient uptake and removal by plants is
directly related solar surface area, and only indirectly related to grow
bed volume assuming root development plays a large role in your
particular crop. But before you can say something like 3 sq. ft. of grow
bed per gallon of fish tank, you need to think about stocking density
and
feed conversion ratio (FCR).
In this industry everyone is taught to sell their tilapia when they
are no more than a year old. The reasoning is that when fish are small
and young, the FCR is smaller, and less feed becomes more fish faster.
That's useful thinking in the aquaculture world, but lacks a certain
breadth of consideration in the AP world. You might consider an
aquaculture strategy that focuses on growing larger, older fish.
Remember that AP systems produce about $4 of produce for every $1 of
fish in a Rakocy style system which has been optimized around fish
production. By lengthening the grow-out period for your fish, the FCR
will increase, but it will also stabilize allowing you to have better
control over the nutrient stream to the much more important produce
side. Additionally, higher FCR means more nutrients end up in the plants
than in the fish... which is where the money is. This in turn should
imply that larger fish lead to larger grow beds.
With that in mind, its probably a good idea to see your system as one
that will go thru many changes when it is new. Then as your fish
mature, and your nutrient stream stabilizes, you will be able to fine
tune the grow bed surface area to match.
Gravel beds, a type of fluidized bed reactor, are the oldest and most
established form of modern aquaponics, and were first used in the work
of Woods Hole researcher John Todd at his New Alchemy Institute, and Dr.
Mark McMurtry at NC State.
Dr. Jim Rakocy, a RAS
(Recirculating Aquaculture Systems) aquaculture specialist at the University of the
Virgin Islands in St. Croix, abandoned gravel beds because he wanted to
work around the issue of sludge build-up. To avoid sludge issues, he
developed raft culture, which in the absence of a bed reactor, required
settling and degassing tanks. This system falls short of optimal for
several reasons: 1) you're spending money on nutrients your plants need
that you then remove from the system wasting resources 2) his floating
rafts blocks most of the air-water interface for gas exchange
intensifying the need to waste electrify on compensatory aeration, and
3) this system design is fish-centric, obsessed with how many fish you
can cram into a barrel, when Jim's own publications showed that the
aquaculture side was a minor contributor to revenues, and 4) only a very
limited number of plants can tolerate having their roots immersed in
water 24/7. Typically plants with low nutrient requirements and low
ability to scrub nutrients from the water.
Tom Speraneo, is credited with the first use of tilapia in AP
systems, but also is credited with cycling the water level in the gravel
beds, creating enormous amounts of temporary thin water surfaces for
gas exchange, and facilitating aerobic RAS waste processing. But the
other interesting development was that it made the beds capable of
sustaining active vermiculture, which further breaks down and decomposes
the solid wastes, and restores lost nutrients instead of wastefully
removing them. This seems to have been confirmed by the systems
popularized in Australia by Joel Malcolm and Murray Hullam, with Murray
claiming to have systems that have run without any waste removal being
necessary for over 3 years. Similar results have occurred in Hawaii
with Glenn Martinez's system.
Using the archaic meaning of vermiculated of "worm eaten", we can call these vermiculated fluidized
bed reactors, and their use should free up a great deal more nutrient
from the same amount of feed input, allowing you to focus on the much
more profitable produce production side of an AP system.
With
that in mind, Wilson Lennard has shown that raft cultures seem to be
slightly more productive with low nutrient crops like lettuce, whereas
most crops cannot be grown in rafts at all. Since lettuce has a very
fast seed to crop cycle, it can provide stead cash flow while waiting
for more valuable crops to mature in the gravel beds, and with all the
extra nutrients available, there's plenty of reason to have both in your
system design. This will also increase the total system volume, which
is good for fish, and if you design your deep water wells with tops that
are not floating on the water, you'll have greatly enhanced gas
exchange surfaces.
It would probably be a good idea to avoid NFT (
Nutrient Film Technique)
completely if your system includes fish production, because a thin layer
of water spread out over long distances is basically a heat exchanger,
which will cause diurnal temperature swings in your fish tank greatly
adding to their stress levels.
