This is Chapter 4 of The Urban Aquaponics Manual – 4th Edition.
In a Chapter 2, we looked at how aquaponics works from a basic microbiological perspective…and I said a properly-designed aquaponics system was a recirculating aquaculture system (RAS) to which growing systems were (loosely speaking) attached. Consistent with that direction, Chapter 3 looked at the filtration methods that are at the heart of a RAS.
Then I revealed that there was this creature called the basic flood and drain system…where media grow beds allegedly doubled as the filtration system.
Here’s where I explain what I meant when, back in Chapter 2, I referred to “informed decisions” – and here’s where you get to make what is arguably the most important choice that you will make with respect to aquaponics.
First the explanation…
The Basic Flood and Drain System
The Basic Flood and Drain System (which I also refer to as the Speraneo model) comprises a fish tank, a pump and a grow bed that contains media like gravel, expanded clay pebbles or lava rock.
The water is pumped from the fish tank up into the grow beds. Once the water reaches a predetermined level it drains back into the fish tank.
It’s simple to understand, easy to build and operate – and (within particular constraints) it can work.
It should come as no surprise, therefore, that the basic flood and drain system is the most commonly used backyard aquaponics system in the world.
Tom Speraneo inadvertently discovered that he could take a gravel grow bed (long used in hydroponics circles) and adapt it to:
- capture and mineralise the fish solids.
- facilitate nitrification
- aerate the water
- grow plants.
It all sounds very positive, so far. So, what’s the problem?
Well, there are several actually but, before we get into those, it’s appropriate that we should learn a bit more about how the Speraneo model came into being.
Aquaponics Biggest Mistake
Many people who are interested in aquaponics know that Missouri farmers Tom and Paula Speraneo popularised what is commonly termed as flood and drain aquaponics.
For the uninitiated, flood and drain aquaponics in its simplest guise comprises a fish tank and one or more media (usually gravel) grow beds. Nutrient-rich water is pumped from the fish tank into the gravel grow beds before draining back into the fish tank.
What far fewer people know is how the Speraneos came to be involved in aquaponics and where the idea for their basic flood and drain system originated.
In the mid-1980’s, Dr Mark R McMurtry invented the Integrated Aqua-Vegeculture System (iAVs) – the first successful ‘closed loop’ production of vegetables using the metabolic wastes of fish.
iAVs comprises a fish tank and sand biofilters (in which the plants are grown). It’s simple to understand, easy to build and operate – and it definitely works.
Following the completion of his PhD dissertation at North Carolina State University, McMurtry undertook a series of trips to showcase iAVs and its benefits for allied faculty staff, students and aquaculture industry professionals.
In December 1989, one such trip to Arkansas put McMurtry in contact with Tom and Paula Speraneo at the University of Arkansas in Little Rock.
A week later, he facilitated a 3-day interactive discussion/workshop at the Meadowcreek Project in Fox, Arkansas for the usual mix of faculty, staff, students and other interested parties – including the Speraneos.
The Speraneos returned home keen to construct an integrated aquaculture system based on what they’d learned from its inventor.
As it turned out, they weren’t able to afford the sand that was central to iAVs’ effectiveness, so they dug up their gravel driveway for use in their system bio-filter.
Let’s remember that the efficacy of iAVs relies on the use of sand (not gravel) so this was a significant change and one that would have serious implications for iAVs – and aquaponics.
Meanwhile, oblivious to the fact that his work was about to be usurped by a mistake, McMurtry had begun a promotional tour of sub-Saharan Africa and Middle Eastern countries.
When he returned, he became aware of the Speraneo’s substitution of gravel for the sand and he counselled them at length about their choice – but they persisted. This aberration would subsequently be popularised as the flood and drain aquaponics system.
This “mistake” – subsequently to become wilful ignorance – was what best-selling author Malcolm Gladwell would later describe as a “tipping point” – one that would have profoundly negative implications for aquaponics.
The sand bio-filter is the heart of the iAVs “living machine.” The substitution of gravel for sand impacted the design in several ways including:
- a dramatic reduction in mechanical filtration capability
- a dramatic reduction in soil microbial types and population numbers
- reduced aeration of media bacteria and plant root zone
- reduced nutrient utilization and system stability
- a significant reduction in feed conversion rate and fish growth
- increased capital costs with reduced fish and plant yields
- increased risk profile
- increased operating cost per unit of production
One of the key features of the iAVs design is its versatility. A backyard farmer – or an impoverished villager – or a protected cropping greenhouse operator could use the same system design.
The first casualty of the change in media was iAVs‘ commercial potential. The basic flood and drain system never gained commercial traction because gravel does not lend itself to the mechanisation and automation that is a feature of controlled environment agriculture. Sand, by contrast, had been used in hydroponic greenhouse culture for decades – subject to all of the usual constraints associated with greenhouse culture.
The iAVs could be built and operated by a humble villager with some seeds and relatively little guidance. The basic flood and drain system, by contrast, requires a connection to the grid, a pump (or two) and ongoing access to mineral supplements. The basic flood and drain system also required greater skills and knowledge to offset the heightened risks that it poses.
As an aside, the Speraneos (who initially gave credit to McMurtry for their introduction to what was yet to become known as aquaponics), eventually used their utilisation of gravel as a point of sufficient difference (in their minds at least) to assume ownership of the concept.
This process of taking a system design and “tweaking” it (with a view to assuming ownership of the idea that underpins it), was to become a recurring theme in aquaponics.
Anyway, the Speraneos developed an information package and promoted their system through an Internet mail list (the fore-runner of the discussion forum).
Interestingly, when this information package first became available, purchasers were asked to agree (by way of a binding legal instrument) not to market their own information packages. It seems that the Speraneos were not keen to have done to them what they had done to McMurtry.
This requirement obviously lapsed at some point because, in 2005, Joel Malcolm bought the Speraneo’s information kit and “tweaked” it into an Australian context. Australia’s ABC Gardening TV program ran a segment on Malcolm’s home-based system and the basic flood and drain system enjoyed a new surge in popularity. Regrettably, however, the “new” flood and drain system had the same basic flaw – the media particle size.
Various other kit makers (including Murray Hallam and Sylvia Bernstein) adopted the Speraneo flood and drain system and, while they “tweaked” the model too, none of them managed to grasp the toxic tipping point – the gravel instead of sand.
To summarize, the substitution of gravel (or clay pebbles) for sand was not just a minor detail – it was the aquaponics difference between chalk and cheese. The iAVs is a living machine whereas the basic flood and drain system is, given a convergence of common (indeed likely) events, a killing machine.
In terms of its filtration efficacy, McMurtry has characterized the use of gravel to capture solids in the biofilter as “attempting to catch BB’s with a basketball hoop.”
It’s important to understand that the difference between iAVs and the Speraneo model is much more than one being usurped by the other…or any philosophical notion.
The basic flood and drain aquaponics system was/is nothing more than a big mistake – an unfortunate mutation with nothing like the productivity, resilience and versatility of its iAVs predecessor.
Anyway, this manual is about aquaponics and, since iAVs is not aquaponics, it’s time to focus on the technical issues of the Speraneo model.
Earlier, I said that the basic flood and drain system relied on the gravel grow beds to:
- capture and mineralise the fish solids.
- facilitate nitrification
- aerate the water
- grow plants.
The simple fact is that the capture and mineralisation of fish solids in the gravel grow bed is at odds with the nitrification and aeration functions of the grow bed.
In other words, particulate matter consumes oxygen – and, in certain circumstances, inhibits the conversion of ammonia into nitrite and (subsequently) nitrate. The greater the quantity of this particulate matter, the greater the amount of oxygen that is required to deal with it.
For an understanding of how this happens, let’s hark back to what we said about ammonia when we looked at the aquatic nitrogen cycle.
“As the fish digest food, they produce solid wastes – and they include urea, uric acid and faeces. Uneaten food also contributes to the solid wastes in the system.
These solid wastes eventually yield ammonia – through a process known (not surprisingly) as ammonification.
The family of bacteria that facilitate this conversion of organic nitrogen into inorganic ammonia are called heterotrophs.”
Not bloody Heterotrophs again?
Heterotrophs are as essential to the operation of any aquaculture/aquaponics system as autotrophs – the nitrifying bacteria – however the relationship between the two types of microorganisms is not without its problems.
The first issue is the rate at which their numbers grow – relative to each other. Autotrophs multiply relatively slowly – where heterotrophs multiply very rapidly.
This means that heterotrophs can overwhelm autotrophs – indeed eat them – to the point where nitrification is stalled.
OK, so what is likely to cause heterotrophs to multiply to the point where they might actually inhibit nitrification?
The answer is solid wastes – in the form of urea, uric acid, faecal matter and uneaten food.
More solids = more heterotrophic activity.
The other issue is that rapidly multiplying heterotrophs consume large quantities of oxygen from the water.
So, the problems for the fish are twofold – they can run out of oxygen and/or, in the event that nitrification is stalled, they’ll be affected by ammonia toxicity.
We’ll look at the role of oxygen in aquaponics, in depth, in the section titled “Managing Water Quality.” At this stage, it’s sufficient to know is vital to the survival and wellbeing of fish, plants and beneficial bacteria.
Once dissolved oxygen in the water drops to sub-lethal levels, fish begin to die – quickly. Even if they don’t quite reach that point, low dissolved oxygen levels stress fish – and stressed fish are more prone to disease and parasitic infestation.
In fact, low dissolved oxygen levels (or stressors arising from low DO) are the leading cause of fish deaths in aquaponics systems.
OK…so what’s the solution to the solids issue?
The best way to deal with sedimentary and suspended solids is to capture and remove them from the water column.
Now, this viewpoint flies in the face of aquaponics fundamentalists who argue that the solids contribute to the overall nutrient mix in an aquaponics system. They contend that the solids will be trapped in the grow bed, mineralized by composting worms and eventually become part of the nutrient mix.
While I don’t argue with the basic mineralization proposition, here’s why I suggest that solids be removed:
- Bio-filters (including grow beds) function more efficiently when solids are removed.
- Both fish and nitrifying bacteria require oxygen. Fish wastes and uneaten food consume oxygen and, in extreme situations, will drive dissolved oxygen levels down to the point where fish can no longer survive.
- Built up fish wastes create pockets of anaerobic (without oxygen) activity resulting in de-nitrification – the opposite of what we’re trying to achieve.
- Grow beds will require less frequent maintenance if solids are removed. Regardless of how many worms you have in a grow bed, there will still be some sediment left in the bed. Over time, this sediment will (unless removed) build up and will eventually impair the biological functionality of the bed.
- Solids irritate the eyes and gills of the fish – and stress them. Stressed fish become more susceptible to disease.
- Solids can harbour harmful pathogens.
- Working with clean grow beds (and clean hands) is a more pleasant task.
OK…but, by removing the solids, aren’t we wasting nutrients that would otherwise be available to our plants?
First, we need to understand that there are three types of solid wastes – dissolved, suspended and sedimentary. The dissolved solids – and the smaller fraction of the suspended solids – remain in the water and undergo ammonification and nitrification.
Indeed, up to 75% of the wastes produced by the fish in the system – having passed across their gills – are in the dissolved form. Put another way, up to 75% of the nutrients in the water are in dissolved form.
Second, the simple fact of removing the solids ought not infer that we are wasting them – quite the contrary.
The solid wastes can/should be processed so that they deliver up any remaining nutrients. The nutrient-rich water is then decanted from the sludge and returned to the aquaponics system.
The remaining sludge contributes nothing useful to the system. In fact, it can harbour harmful pathogens and irritate the fish’ eyes and gills and the best place for it is the compost heap or the worm farm.
OK…..so what prompted the confusion around the removal of solids in the first place?
A Matter of Dogma
Earlier I said that the basic flood and drain aquaponics system was the most commonly used backyard layout in the world.
That begs the question…“If it’s so problematic, why are so many people using it?”
Fair question…and here’s the answer…
- Few people knew about it’s iAVs heritage. For several reasons, Mark McMurtry wasn’t around to defend the iAVs method and, while the Speraneos knew about it, they obviously decided that it wasn’t in their interests to press the facts around iAVs.
- Its inherent simplicity appeals to people. It’s easy to understand, build and operate…and it works (right up to the moment that it doesn’t).
- During its rise to worldwide prominence, kit manufacturers owned three out of the four largest aquaponics discussion forums in the world, and they promoted it as the aquaponics ideal. They exploited the fact that it’s easier to sell something if you don’t confuse the purchaser with all of the things that could go wrong.
- Most of the people who set out to build the layout didn’t understand its pitfalls – they got caught up in the hype and simply didn’t know what they didn’t know.
As far back as 2007, I argued in support of the use of dedicated mechanical and biological filtration in media-based aquaponics systems. I met with such a barrage of criticism from aquaponics fundamentalists that I built four basic flood and drain systems side-by-side. Over a period of nine months, I trialled three Australian native species – and a diverse range of plants – in these units.
The pretty picture belies the biological unhappiness that’s happening in the fish tanks. This is the truth of the basic flood and drain system – it’s presentation as a sustainable way to grow food is largely an illusion.
To summarise the outcome, most of the plants grew very well. Regrettably, the fish suffered almost from the outset.
Plant production was no issue with the basic flood and drain systems that I built but, the presence of sedimentary and suspended solids means that there’s a heightened risk of disease and death for the fish.
To summarise…the basic flood and drain system is a very bad idea…particularly for fish. It was the product of wilful ignorance and it continues to be aquaponics’ biggest mistake.
For those who are attracted to the basic approach, my advice is to learn about the Integrated Aqua-Vegeculture System (iAVs), build the real deal…and reap the benefits.
The Improved Basic Flood and Drain System
For those who want to persist with gravel (or clay pebbles, lava rock or other coarse media) grow beds, I still recommend that they be thought of as hydroponics growing units to be attached to a recirculating aquaculture system.
The inclusion of mechanical and biological filtration will make a vast difference to a basic flood and drain system. From the fish’ perspective, it’s probably the difference between life and death.
So long as you have an effective means of capturing the sedimentary and suspended solids, you can utilise the gravel grow bed as a biofilter. There are still advantages, however, to including a dedicated biofilter into your design and we’ll explore those in detail in the next section.
So, having put what I hope is a reasonable case for using a recirculating aquaculture system as the basis for all aquaponics systems, it’s time to head over to Chapter 5 – Designing Your Recirculating Aquaculture System.
In the meantime, I invite you to comment…to express any concerns that you may have…and to provide ideas or suggestions that you feel will improve the book – or add value to it.