Articles and Information

Aquaponics Miserly Water Use
by Geoff Wilson

Aquaponics appears to be the world’s most productive food system in terms of water use efficiency. 

That can be expected to be a political ace for aquaponics science and technology as the world’s fresh water supplies come under increasing pressure. 

Depending on where it is practiced, much aquaponics takes only about half the volume of water to produce $100 worth of food in the form of fresh fish and fresh vegetables or fruit – whatever the currency – than inorganic hydroponics.   

The accompanying table uses Australian figuring. It shows just how badly other food production systems perform. Inorganic hydroponics is the only close contender for the title of “Most Miserly Water User.” 

The best Recirculating Aquaculture Systems (RAS) generally use around 700 to 800 liters of water for about $100 worth of food (with American, Canadian and Australian currency comparisons being roughly the same).  

But add organic hydroponics as a “back-end” to grow food plants from fish wastes, and there’s an immediate reduction in water use as waters from fish tanks are cleaned and recycled back to these tanks.  This is the key to why aquaponics is such an efficient system. 

It is “Mother Nature” at work – where, over countless millennia, the wastes of one process have become the raw materials of the next.  Bumbling humans are only just discovering the great good sense of  linked polycultures such as those in aquaponics – instead of using monocultures that gobble up water, fossil-fuel energy and nutrient resources (mostly created from, or using, fossil energy). 

I predict that, with increasing awareness of the advantages of aquaponics and hydroponics in political circles, where the painful economics of water use allocations are being thrashed out in many countries, there will be a very logical favoring of aquaponics and hydroponics development. 

My other prediction is that this miserly water use, combined with small space needs in cities, will make urban aquaponics and hydroponics the dominant food security technologies that will emerge from this century of humankind’s development. 

Small signs of this can be seen already among intensive aquaculture producers who are able to harvest fish wastes for additional profit rather than send them down drains at a cost --  or not at all as environmental protection laws continue to tighten. Further signs are evident in the advances being made in hydroponics, especially close to city markets, or within cities. 

I asked the world’s leading aquaponics scientist, Dr James Rakocy, to come up with his aquaponics water use figures in US dollars, based on his 25+ years of experiments and practical running of aquaponics at the University of Virgin Islands. Here’s what he said of his situation at St Croix in the Virgin Islands:

“For tilapia/lettuce it is 210 liters per US$100, and for tilapia/basil it is 85 liters per US$100.  

He explained: “I took the total revenue generated from the tilapia/lettuce and tilapia/basil enterprise budgets and divided this by US$100 to the get the total number of US$100 units.”  

"I then took the initial system volume (110 cubic meters) plus the annual daily makeup water (0.015 x 110,000 liters x 365 days). This came out to 712,250 liters. I then divided this by the number of US$100 units.” 

“The total annual revenues from tilapia/lettuce were US$333,855 or 3338.55 US$100 units and from tilapia/basil were US$840,375 or 8403.75 US$100 units.”  

Dr. Rakocy said: “This is based on the economics in the Virgin Islands where the price for fish and lettuce is very high. This is also based on gross revenue, not profits,” Dr Rakocy said.

But even a most conservative calculation of water use to revenue earned comes up with figures that

are as impressive as the best inorganic hydroponics. 

My back-of-the envelope figuring based on the UVI aquaponics production figures translated to Australian prices and fish species indicates that Dr. Rakocy’s aquaponic unit, if it were transported to Australia, would have a water use efficiency of less than 500 liters per A$100 of output.  

The Australian pricing of the UVI unit’s production is thus:

· Fish output value - 5 tons x $11/kg = A$55,000 ($41,489 USD)

· Lettuce output value - average of 35,000 heads by A$1/head = $35,000 ($26,398 USD).

· Basil - five tons - A$10/kg = A$50,000 ($37,712 USD).

· Okra - two tons - A$3/kg = A$6,000 ($4,525 USD) 

Estimated total value of output = A$146,000    Divided into 700,000 liters of water used each year this is about 479 liters of water used per A$100 of production.  

But if I chose to grow basil only, and was able to achieve A$10/kg from the 35 or so tons it would be possible to grow from the barramundi wastes, then the situation changes dramatically: 

· Fish output value – 5 tons x $11/kg = A$55,000 ($41,486 USD).

· Basil – 35 tons x $10/kg = A$350,000 ($264,004 USD). 

Estimated total value of output is then A$405,000 ($305,491 USD) -- which represents water use efficiency of around 173 liters/A$100 of production. 

This compares very favorably with the Australian commercial hydroponics figure of 600 liters of water used per A$100 of production. 

But, in comparison to fairly static figures for hydroponics, the aquaponics figures are extremely variable and will fluctuate wildly according to species of fish and food plants chosen.  

All that can really be said is that aquaponics can be expected to be the most miserly user of water to grow food in most practical circumstances. Hydroponics is nearly as good – and is well proven. 

That is surely enough at the moment.  

Some additional questions posed to Dr. Rakocy include:   

· Does the UVI unit produce year round ?

· Can its water-use productivity be increased ?

· How much water comes from (a) the plastic rainwater catchment and (b) rooftop runoff? 

Dr. Rakocy answered: “Yes, we produce year round. Yes, water-use efficiency during washing of the filter tanks could be increased. We could recycle all the water in these tanks. However, I have not included cleaning water, so maybe this should be left alone. All the water comes from rainwater catchment, so we are not depleting any current water supplies.”   

“If I can remember correctly from a paper we did on a 3-year lettuce trial, about half of the 1.5% daily make-up water replaces water lost to splashing, evaporation and transpiration while the other half replaces water lost during sludge removal,” Dr. Rakocy said. 

I plan to go into the water use experience of other aquaponics growers next issue.  

In the meantime, the preceding figuring will give many growers who use water to grow food, an incentive to think a little more deeply about the aquaponics directions they could be taking to better use the world’s water – and to become the owners of a better business.   

About the Author: 

Freelance journalist Geoff Wilson has just launched “UrbanAg Online”, a news and information service on 29 different topics in urban agriculture – including aquaponics. For more information, visit:  www.urbanag.info

Geoff is convener of Aquaponics Network Australia.  He can be contacted by email at: geoff@nettworx.info