This is Chapter 6 of the Urban Aquaponics Manual.
Designing any food production system starts with the question… “How much food do you plan to produce?”
The design of an urban aquaponics system begins with questions. too…
How Much Fish?
We’re going to keep it simple for the purposes of this design discussion so let’s assume that we’re going to grow enough for one person to eat fish once a week…so we’ll need about 50 fish. We’ll also assume that each fish will be around 500 grams (one pound) at the time of harvest. That’s 25kg (50lbs) of fish per year.
Once you establish how many fish you want to grow each year, you’ll want to know…
How Much Fish Tank?
We can accommodate our 50 fish in a cubic metre…about 1000 litres (250 US gallons)…of water.
Over time, you’ll come to appreciate that everything in aquaponics starts with feeding the fish and that leads us to…
How Much Feed?
Fish in recirculating aquaculture systems are most effectively fed a percentage of their bodyweight – each day.
Fingerlings may be fed up to 8% of their bodyweight but that figure decreases over time to the point where they may only be getting 1% at the time of harvest.
Each fish will, at the time of harvest, be around 500g and will (based on a daily feed rate of 1%) be eating 5 grams of feed per day, Fifty such fish will be eating 250 grams (0.5 pounds) per day.
Assuming a feed conversion ratio of 1:2 – one kg of fish biomass for each 2kg of feed provided – we can expect that our 50 fish (each weighing 500g) will consume a total of 50kg of feed throughout the growing period.
While it’s interesting to know how much feed we’ll use in total, the more important number, for our immediate purposes, is the maximum daily feeding rate of 250 grams…because that figure will allow us to calculate the size of the filtration system that we are going to require to deal with the metabolic wastes of our 50 fish – with a total weight of 25kg (50lbs).
How Much Filtration?
Back in Chapter 3 – Understanding Filtration, we looked at all manner of different mechanical and biological filtration devices. For what it’s worth, the list of filtration devices that I chose to ignore is far bigger than the one that I provided.
While choice is a wonderful thing, introducing too many choices into a learning situation becomes confusing so, from this point on, I’m going to focus (based on my experience) on what I think will work best for you rather than attempting to cover every possibility.
Our filtration system will comprise three elements:
- a radial flow separator – to capture sedimentary solids
- a packed media filter – to capture suspended solids
- a moving bed bio-reactor – to nitrify dissolved solids
If this is all sounding pretty complex, let me assure you that, behind each of these fancy names, is a simple blue plastic barrel. It’s how we fit out each barrel that determines its function – and name.
We’re going to build these filters in the next chapter so, for now, all we need to do is work out how much filter media we need. Once we know that, we’ll be able to determine the size of the barrels we’ll need.
The radial flow separator contains no media so that one’s simple enough. The packed media filter is almost filled with media so that one is easy, too. That leaves us with the moving bed bio-reactor.
Manufactured plastic media is very effective, is self-cleaning and will deal with a predictable solids loading so I’ll be using AnoxKaldnes K1 filter media for this design model.
AnoxKaldnes K1 manufactured plastic media – excellent bio-media.
How Much Bio-Media?
The manufacturer of K1 claims that each 50 litres of media will deal with the metabolic wastes arising from the use of 750 grams (0.75kg or 1.6 pounds) of fish feed per day. That figure applies to industrial wastewater treatment and commercial aquaculture and it assumes that there is some heavy duty filtration equipment upstream of the moving bed biofilter.
Our design will feature some inbuilt redundancy…so we’ll be using 50 litres of K1 to deal with the wastes from our 50 fish…based on a maximum daily feed rate of 250 grams (0.25kg or about 0.5 pound).
When sizing a moving bed biofilter, I calculate the amount of media to be 60% of the total filter volume. So, if our filter was going to be, for example, 100 litres we’d use about 60 litres of media. Since we’ve already decided that we need 50 litres of media (a standard shipping volume, by the way), a 100 litre plastic barrel will suit us just fine.
In fact, we’ll use three 100 litre (25 gallon) barrels to house our entire filtration module.
One More Thing…
This particular design will feature something that we haven’t spoken about previously – a sump tank. We’ll look more closely at the sump tank, what it does and its capacity in the next chapter.
Now that our system has taken on a physical dimenstion, it’s time to address the two things without which no fish can live…water and oxygen.
How Much Flow?
Implicit within the notion of a recirculating aquaculture system is the idea that water flows from the fish tank through the filtration modules and back into the fish tank. That flow is created by a water pump.
When calculating the size of the water pump to be used in a RAS, I take the total system volume and double it. What we are looking to do here is to move the entire capacity of the system through the filtration unit twice per hour.
The fish tank, filters and sump tank contain 1400 litres (around 370 US gallons). If we double that figure, we’ll be looking at a total volume of 2800 litres (or 750 US gallons).
Pumps are rated in terms of the volume of water that they will pump – per hour – so that would suggest we need a pump that will move around 3000 litres/hour.
For reasons that I’ll clarify as we get into the construction of this system, we are going to want a bit more than than that amount. so, for now, I’ll be proposing that our pump will have a capacity of 4000 – 5000 litres (1000 – 1250 US gallons) per hour.
How Much Oxygen?
Our fish need oxygen..and so do the microbial organisms that facilitate nitrification.
Having said that, the matter of how much oxygen we’ll need depends, to some extent, on the plants we grow – and how we grow them – since plants need oxygen, too.
Suffice to say, at this stage, oxygen is as fundamental to recirculating aquaculture as water. Quite simply, without it, nothing of value to us will live. We will, however, address the matter of how much oxygen we need – and how we’ll provide it – when we get into selecting our system components.
OK, let’s take a look at what we’ve got so far.
RAS Design Specification
Our proposed recirculating aquaculture system will:
- raise 50 fish to a harvest weight of 450 – 500 grams (one pound) in about 30 weeks – subject to fish species.
- utilise a fish tank with a capacity of around 1000 litres (250 US gallons).
- require around 250 grams (0.5 pounds) of fish feed per day by harvest time.
- feature a filtration system – comprising a radial flow separator, a packed media filter and a moving bed bio-reactor – each housed a 100 litre (25 US gallon) plastic barrel. Fifty litres of AnoxKaldnes K1 (or similar) will be adequate to nitrify the metabolic wastes from the 250 grams of feed that we will feed our 50 fish by the time that they reach harvest.
- utilise a 100 litre (25 US gallon) sump tank.
- turn over the entire volume of the system – about 2800 litres (750 US gallons) – twice per hour.
- use a water pump with a capacity of 4000 – 4500 litres (1000 – 1100 US gallons) per hour.
While it doesn’t look like much yet, this little RAS will yield lots of clean, fresh fish. It will also provide some other valuable outputs – about which we’ll talk more later.
OK, so what if you want/need something bigger – or smaller?
The amount of fish to be produced can be doubled – or halved – by simply doubling or halving the specification numbers.
Indeed, you could scale this system up to provide five times as much fish by making proportionate adjustments to those numbers. A system of that size is, for most people, on the upper limits of a family fish production unit.
In that situation, my preference would be to have two (or more) smaller units rather than one larger 5000 litre system. I have good reasons for feeling this way but I’d like to address larger systems in greater detail later in the manual.
In the next chapter, we’ll find ourselves something to use as a fish tank and filtration modules…and all of the other bits ‘n’ pieces that we’ll need to build our very own 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.