12. Integrated Plant-Animal System

In this post I am going to outline (as simply as possible) the integrated plant-animal system that I designed for my Agroecology module. The idea is that plant and animal production systems compliment each other and by combining the two you can reduce the need for external resource inputs (e.g. manure can be used as fertilizer for plants).

My system aim:

Feeding the global population whilst not crossing environmental planetary boundaries is one of the greatest challenges that we are faced with today (Rockström et al. 2009). We need to be able to grow more food on less land, whilst using fewer resource inputs. A shift to a more sustainable diet is arguably going to be essential to help us overcome this problem. As the world develops, more and more people have meat and dairy heavy diets, which are much less sustainable than plant based diets due to the loss of energy up the food chain from one trophic level to the next.

However, not everyone wants to become vegetarian and the environmental impact of all meat production is not equal as different animals have different nutritional and energy requirements. My project aims to find and produce a more sustainable animal based protein source, which is integrated with plant production systems.

What is my system?

I have designed an aquaponic system consisting of three primary biological components:

1. Nile Tilapia - a species of fish

2. Duckweed (Lemna minor) - a species of floating aquatic plant

3. Aerobic bacteria

Aquaponics refers to the integration of aquaculture (raising fish) with hydroponics (the soil-less growing of plants). Aquaponic systems recycle nutrients (primarily nitrogen) between fish and plants to reduce the need for external nutrient inputs. At the most basic level, fish excrete ammonia (a plant unavailable form of nitrogen). Bacteria convert ammonia in the fish wastewater to nitrates (a plant available form of nitrogen). This nitrate rich water is used to grow plants. This water is then recycled back into the fish tanks. Excess ammonia is toxic to fish and therefore, this process also reduces the need for water detoxification (Rakocy et al. 2006).

Usually, both the fish and the plants grown in an aquaponic system are marketable outputs and the fish are fed using external inputs of feedstock (often fishmeal made from ground up fish). However, in my system the plants grown will not be a cash crop but instead will be used as feedstock for the fish. Although this reduces the profitable outputs of the system, it also reduces the expensive and arguably less sustainable inputs to the system, thus creating a more self-sufficient nutrient loop.

The output product of the system is tilapia fish. These grow quickly and have unusually high feed conversion ratios (close to 1:1), meaning they use feedstock energy very efficiently (Ridha, 2000) Tilapia are omnivorous and will be fed predominantly on duckweed: a fast growing, environmentally tolerant floating aquatic plant with a protein content of up to 40% (Leng et al. 1995)!!! The use of duckweed reduces external feedstock inputs, thus reducing cost, increasing self-sufficiency and reducing the environmental impact of feedstock production. Feeding the fish plants as opposed to ground up fish also reduces energy losses up the food chain.  The nutrient rich wastewater from the fishponds will be fed through an aerobic digester or ‘bioreactor’ that will convert organic plant unavailable nitrogen to inorganic plant available nitrogen. This water will then be fed into a system of shallow pans that will be used to grow the duckweed. The duckweed will then be harvested with a simple net and fed wet to the fish.

Infrastructural system design:

• The whole system will operate within a heated greenhouse.
• Tilapia will be grown in large insulated tanks.
• A swirl separator will be used to separate solids out of the fish tank wastewater.
• The liquid from the swirl separator will then be fed into a bioreactor containing the aerobic nitrifying bacteria. 
• The nitrate rich water from the bioreactor will then be pumped to a series of shallow stacked plastic trays that will be used to grow the duckweed. (Duckweed is a floating plant and therefore can grow in a little as 3mm of water!! Their shallow size and stacked setup maximize the surface area available for growth).
• Water from these trays will then be fed back into the fish tanks.

References:

Leng, R., Stambolie, J. & Bell, R., 1995. Duckweed - a potential high-protein feed resource for domestic animals and fish. Livestock Research for Rural Development, 7(1).

Rakocy, J.E., Masser, M.P. & Losordo, T.M., 2006. Recirculating Aquaculture Tank Production Systems: Aquaponics — Integrating Fish and Plant Culture. Southern Regional Aquaculture Center, (454).

Ridha, M., 2000. Preliminary study on growth, feed conversion and production in non-improved and improved strains of the Nile Tilapia. Aquaculture, Fisheries and Marine Environment Department, Kuwait Institute for Scientific Research.

Rockström, J., Steffen, W. & Noone, K., 2009. Planetary boundaries: exploring the safe operating space for humanity. Ecology and Societty, 14(2), pp.32–65.