July/August – 2005
Author: Wilson Lennard
WILSON LENNARD outlines his research to develop a successful recirculating aquaponic system to produce Murray Cod and lettuce, with significant savings in water use and zero environmental impacts.
I came to the world of aquaponics from the other side , that is, the aquaculture side. Freshwater aquaculture is moving into a new phase and a lot of people are now turning towards using large, environmentally controlled indoor recirculation systems to grow fish. These systems are a self-contained unit, usually located in an insulated shed. These are high intensity systems, with tons of fish being produced annually on very small land areas. The advantage of recirculating fish farming is that water is recycled through the system, and is therefore used to its full advantage.
The downfall of any aquaculture operation is that fish produce waste, and this waste needs to be disposed of in a way that won’t impact on the environment. Fish waste is nutrient rich and if it is disposed directly to the environment, it can have negative consequences. This is where aquaponics and I come into it.
I was looking for a way to filter the nutrient-rich fish waste out of aquaculture systems. Solid fish waste is constantly removed from these systems and is usually composted, so it is not much of a problem. It is the water-bound fish waste that is a problem.
Around 70% of fish waste is actually water-bound, arising from the gas exchange of ammonia-type waste that the fish excrete across their gills. It is this water-bound component that I was looking to treat. So, from my point of view, I was looking for a way to remove water-bound waste from our fish culturing systems. This is required because fish farmers at present change approximately 10% of their water every day, to counteract this build-up of waste. Ten percent may not sound like much, but in a system containing 100,000 litres of water (which is not a large system), that means removing 10,000L of water a day, finding a way to dispose of it, and replacing it with 10,000L of clean, fresh water. So, as I said, I was coming at the problem from a fish culturalist perspective.
The great thing about this water-bound fish waste is that it is mainly nitrates and phosphates. As all hydroponic plant growers know, these are some of the main nutrients used for hydroponic plant culture. So the question arose, can these fish wastes be used as plant nutrients? This is where I started after obtaining a PhD scholarship through the Rural Industry Research and Development Corporation (RIRDC).
I set about designing and building an aquaponic system to integrate fish culture with hydroponic plant culture. I had to design a very small-scale system, as I needed to be able to replicate my experimental situations for scientific purposes. So I eventually ended up with 12 aquaponic units that were identical to each other.
A unit consists of a 100L fish tank with an associated biofilter. The biofilter is very important to the fish’s health, as it converts harmful ammonia released by the fish into harmless nitrate. Above the fish tank is a shelf containing a hydroponic gravel bed. Water can be pumped from the fish tank, up to the hydroponic gravel bed, and then returns to the fish tank. That’s it. It’s pretty basic, but it really works.
The theory behind aquaponics is this:the fish live in a tank, eat fish food, and produce two types of waste; solid waste (fish poo) and water-bound waste. As I said earlier, solid waste is routinely removed and generally composted. The water-bound fish waste is actually the same nitrate and phosphate hydroponic farmers add to their systems using inorganic salts that they purchase.
What was fish waste, is now plant nutrient. The water from the fish tank is pumped to a hydroponic plant culturing component and the nitrate and phosphate from the fish is used to feed the plants. The water, now ‘cleaned’ of nutrients, is then returned to the fish tank and the whole cycle begins again.
If the amount of waste the fish produce can be balanced with the amount of nutrient the plants require, then we should have a system where we can perpetually grow fish and plants in the same water, with no water replacement required, other than that used to replace transpiration from the plants.
So I set about running a number of experiments to develop the idea within an Australian context. Some of the questions that arose were:
- does this aquaponics thing really work?
- can Australian fish species be used?
- what pumping rates are required?
- what hydroponic systems can be used (gravel bed, floating raft, NFT etc.)?
- are there any nutrient deficiencies in the plants?
- is the system productive in a commercial sense?
The question of does the aquaponic process actually work was answered with my first experiment. One kilogram of fish was placed in the tanks and 20 lettuce seedlings planted. The fish were fed, the system monitored and the fingers crossed. It is an amazing thing to inspect an aquaponic system daily and watch both the fish and plants grow and thrive. After three weeks in the aquaponic system, I had harvest size lettuce (around 120g, Green Oak fancy heads), fish that had grown, and water with 80% less nutrient in it than the fish-only controls.
This was definitely a good way to start for a PhD student -success! The fish were healthy and had grown at a rate the same as the industry standard, with no side effects. In fact, they actually seemed to like their new, cleaner environment. The lettuce plants were full of head and a beautiful, rich green, with no signs of nutrient deficiency.
At this point I was wondering, is there really three years of research in this? What followed was two and a half years of further experiments and trials to try and optimise the system for better plant growth and better nutrient stripping from the system.
WHERE ARE WE AT THEN?
Well, we now have a system that is fully optimised and is ready for commercial trials. Some of the variables that you may be interested in include:gravel beds work better on a constant flow water delivery regime.
Past hydroponic research has suggested that a ‘reciprocal flow’ (water is pumped to the gravel bed now and then, instead of a constant flow) was better as it aided water oxygen levels and distributed nutrient to plants better. This may be true in standard hydroponics, but we always require oxygen above 5mg/L for our fish, so oxygen is always above what the plant roots require (around 2mg/L for lettuce).
Our constant flow gravel bed system grows lettuce about 20% better than a reciprocal flow. Gravel beds and floating rafts are about 15-20% more efficient than NFT. My experiments have proven, within an aquaponic context, that NFT is less efficient at plant growth and nutrient stripping.
The last key finding is that we need to use a potassium and calcium-based buffer system. Fish farmers use sodium bicarbonate and similar basic salts to make sure the pH doesn’t drop. Fish systems are the opposite to hydroponic systems – as fish eat and metabolise feed, the water pH drops.
To counteract this pH drop, we use buffers to keep the pH up around 7. If we use potassium and calcium-based buffers, we can add the potassium and calcium to the system that the plants require for good growth.
KEY RESEARCH FINDINGS
So I had better tell you of the key findings. Fish (we used Murray Cod) and plants (we used lettuce) can be grown in an integrated aquaponic system. If the correct balance is met between plants and fish, no nutrient build-up occurs in the system, and the plants get all the nutrients they need.
We don’t get conductivity build-up or drop-off;it stays constantly at about 500mS/cm. This is because the fish renew the nutrients every time they are fed, which can be as high as 3-4 times per day, and the plants constantly use those nutrients.
A combination of potassium and calcium is used to buffer pH and provide the other essential plant nutrients. We also add a little chelated iron, as fish food is lacking in iron and the plants require it to produce chlorophyll. That is all we add to the system – fish food, a little buffer each day, and a little iron once a week. All the micronutrients required for the plants are in the fish food, so we don’t need to add any of these.
WHAT ARE THE ADVANTAGES OF AQUAPONICS?
There are several advantages; some relate to the fish and some the plants. Because we can balance the nutrient output of the fish with the nutrient uptake of the plants, we never need to exchange water. We do need to replace any water lost through plant transpiration, but this is a small amount. We are now saving above 90% of the water a normal recirculating fish farm would use. So, the system is very water friendly. We have no nutrient-rich waste output, we use our nutrients to feed the plants, and we have zero environmental impact.
Our fish grow just as well as they do in any other fish system. The best outcome is that we grow healthy, strong plants that yield at the same rate as they would in standard hydroponics. That’s right, our lettuce grew just as well in our aquaponic system as they did in our hydroponic controls. So, the advantages are:
- excellent fish and plant growth
- zero environmental impact
- efficient water use
- yields as good as the prospective stand-alone industries, and
- the ability to grow two cash crops (fish and plants) off the one food source.
I am now building a commercial-scale aquaponic system. We will have the ability to grow around 500kg of fish a year and harvest 3,000 lettuce per week. We will have no environmental impact and will use less than 10% of the water a normal recirculating fish farm would use. The only other question is whether we can obtain ‘Organic’ certification? If we can achieve this, we believe we are on a definite winner. But more about that in a coming issue.
About the Author
Wilson Lennard is in the final stages of his research into aquaponics at RMIT University, Melbourne, Australia. He believes aquaponics is a new and emerging industry that will fill a defined niche in the aquaculture/hydroponics market.