A few months ago I came across a concept that hasn't seemed to hit the urban homsteading movement by and large but has some really exciting implications for it: Aquaponics.
Aquaponics is a combination of Aquaculture (The cultivation of fish for human consumption, usually in tanks.) and Hydroponics (The growing of edible plants in a water/nutrient solution). Separately, each item has its own challenges and difficulties. In aquaculture, fish produce waste that must be filtered out to keep the fish alive. Hydroponics requires outside (sometimes expensive) nutrients to feed plants and promote plant growth. By combining these two in a cycle, the waste produced by the fish become the nutrients that feed the plants which then filters out the waste cycling clean water back to the fish. The system becomes its own life cycle that produces both vegetables and high quality protein for the person running it. And the beautiful part of it is that an entire system can be fabricated for relatively little money.
An aquaponics system has several components:
1) The fish tank: This can be any container from a small aquarium to a fully lined pond. One of the more popular choices for aquaponics tanks right now, is an IBC container. They're standardize, uniform, and come with a metal cage for reinforcement. They are designed to hold large quantities of liquid for shipping and used food grade containers can be found for about $100 (you just have to rinse out whatever was in it previously).
2) The pump(s): All Aquaponics systems have at least one pump that carries water from the fish tank to the growing beds. Some systems have a second "sump pump" that carries water from a sump tank back to the fish tank.
3) The growing beds: Usually plastic containers of some type, they are filled with gravel or hydroton (a spherical clay aggregate). They are usually 6-12" deep (the deeper the better). Gravel is a fine, inexpensive option as a growing medium, and hydroton has the benefit of being more uniform in shape and much lighter in weight, but far more expensive.
4) The syphon: This is one of the cooler parts of the system. Plant roots need both water and oxygen. The syphon creates a cycle where the water fills the bed up to a certain level, and when it reaches that level it creates a vacuum in the syphon causing all the water to be quickly sucked out of the growing bed, when the water is emptied to a certain level, it breaks the vacuum and no more water leaves the bed until it's full again. This creates a respiratory affect for the plants and any other organisms living in the growing medium.
5) Other potential equipment can include an aerator to maintain dissolved oxygen levels in the fish tank (which can also be accomplished through certain plumbling designs), A sump tank (most often used when you have growing beds with drains that are at a lower level than the tank and cannot be gravity fed back.), Aquarium testing kits to regularly test PH levels as well as Nitrogen and Ammonia levels in the fish tank. A water heater and lighting may also be required depending on whether the system is indoors or outdoors). Some people also have earthworms in their grow tanks that eat particulate waste and create their own fertilizer.
There are several types of fish that can be used in an Aquaponics system. Goldfish, and koi are often used in small systems (not intended for human consumption) and larger systems often use Tilapia or catfish or perch. Tilapia is the most favorable option in most systems. They grow fast, they're delicious. They are hardy (tolerating poorer quality water) and they're easy to acquire. But they also require warmer water (at least 70 degrees F) which would require the system be indoors or have a heater for the fish tank. Some systems even have shrimp or prawns to eat additional wastes in the water. Fish are usually fed some kind of commercial or home made fish food but many fish (like tilapia) can eat a variety of foods including shredded vegetable scraps, or duckweed (which is high in plant protein).
There are many types of vegetables that can be grown in aquaponics. Leafy greens make best use of the available nutrients but fruit bearing plants like tomatoes, beans, peppers are also good (but MAY require supplemental nutrients like seaweed extract). Root vegetables like radishes, carrots, turnips are not recommended for this type of system.
What is great about this is that people who might otherwise not want to grow their own meat (because of the unpleasant task of killing a usually cute furry critter) might have no objection to growing their own fish, which can still be an excellent source of protein.