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Solids Management in Backyard Aquaponic Systems

Do We Need to Remove the Fish Solids?

“Do I need a solids filter?” is one of the most frequent questions I get from newcomers to aquaponics after they've seen my aquaponic system on YouTube clips. 

The topic can be a bit of a contentious subject amongst many “gurus” and enthusiasts that inhabit online forums and social media groups. If you choose to discard your solids, many will argue that they are needed, and you will end up leaving your plants malnourished.   

While others say that it’s preferable to remove solids, due to the issues that can be caused by solids slowly accumulating in the grow beds of your backyard aquaponics system.

 I prefer to take a more balanced approach, as I think it all comes down to what sort of a system YOU want to run.

A quick note on nutrients

To those that think it’s all about the poop, let me set the record straight, most of the ammonia (the precursor to the plant available nitrate) is released from the respiratory system of the fish, through the gills. 
A small amount leaves the cloaca along with other nutrients in the form of solids. These solids can be over 1cm/¼”, down to less than a few hundred microns in size and contain the nutrients needed for healthy plant growth.

A paper called Aquaponic nutrient model -A daily material flow analysis  by Thomas Peterhans, shows that a good deal of all elements present in the fish waste can be found in the water column.

Hopefully, we will see more studies on this subject, to help growers better fine-tune their systems.

What’s in these solids?

To begin with, let’s get our heads around what these solids are, and how they are useful to the system.

The main form of solids we’re dealing with in aquaponics is waste from the fish with some uneaten fish food making up the balance. This solid waste contains some of the un-metabolised nutrients provided by the fish feed, plus organic matter.
In these solids, you’ll generally find some nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and some other macro and micro-nutrients. Some of these elements, like nitrogen, are in greater amounts than others, like calcium & potassium.

These solids are removed from the fish tank via a Solids Lifting Outlet (SLO) or pump, depending on how your system is configured and delivered out to the grow beds.  Here they settle out amongst the media where they are mineralised by microorganisms into plant-available forms. Not all the solids can be broken down into the plant available inorganic forms, so some will be left and will build up in the media beds over time. 

Solids Build-up in media beds, A Worst-Case Scenario.

Media beds make great biological and solids filters. The media surface is colonised by waste processing microorganisms, and the voids between the media are perfect areas for the solids to accumulate.

Solids in aquaponics grow bed

When you add plant roots into the equation, you end up with a very efficient solids filtration system. Solids will slowly build up over time impeding the amount of fresh water and oxygen available to the bacteria and plant roots. 

These oxygen-deficient areas are called anoxic or anaerobic zones and will greatly reduce the ability of the grow beds to process the waste produced by the fish.

They can even create a denitrification zone, where the nitrate (NO3) can be converted back to nitrite (NO2), which can poison your fish, particularly if the right safeguards aren’t in place.

Denitrification can also cause a sudden rise in pH levels, and depending on the size of the anoxic zone, can also affect some plants as well as impact the health of your fish.       

Now, these are extreme situations that will occur in your system if solids are allowed to build up to critical levels in the media beds.

I've had to deal with two denitrification events in our system. These happened when I first started out, and since that time I’ve chosen to run a solids filtration component in my aquaponic system, as an extra safeguard and would recommend everyone else does so too.

kang kong roots in aquaponics

Plant selection can also play a large part in solids accumulation. Growing longer living plants with more extensive root systems, like tomatoes and Kang Kong (left), can cause issues. These larger root systems are very efficient at trapping solids into pockets of the beds where they can build up to dangerous levels over time as mentioned above.

Now there’s nothing wrong with growing these crops in your media beds, as thousands of us aquaponicists do. It is a good idea to rotate these plants every few months so the root systems don't get a chance to bind up too many solids in the media. One rotation method is to clone the existing plants by striking a cutting or two. That way you can ensure that you get a continual harvest while reducing the risk of creating large anaerobic root zones throughout the system.  

How can we manage solid waste?

How you manage these solids depends on what type of system you want to run.

 Are you only looking to grow a handful of herbs? Do you want to raise enough greens for your morning juices? Or maybe you’re wanting to provide your family with enough veggies to feast on daily?

 The reason I believe this is important comes down to the size of the system needed, the type of plants you want to grow, and the style of grow bed you want to use.

Chop and flip aquaponic ballel system

 A small system with a media-based bed, built to grow a couple of small herbs, will have very few solids accumulate, as they are generally run with a small amount of fish in them.  The solids can be captured by a small sponge under the water inlet to the grow bed which can be cleaned weekly. 

If the solids do build up enough to become an issue, these small grow beds can be manually cleaned out relatively quickly.

A larger media-based grow bed system with several beds, operated with the proper fish to plant ratio, may not have an issue with large amounts of solids being accumulated in a single 6-8 month growing season. This is especially so if planted out with fast-growing, high turnover crops like salad and Asian greens.

You could get away with cleaning the solids out of the beds at the end of a growing season after all the fish are harvested but it's just easier to remove the solids via a filter before they are able to build up in the media. Cleaning the beds will also reduce the bacterial colonies and could potentially release toxic compounds that could put your fish at risk if there are any left in the system.
I personally think it is just easier to add some form of solids filtration to your aquaponic system to make life easier for yourself and keep the water cleaner for the fish. 

When removing all solids is a must

If you are growing with high fish to plant ratios; meaning more feed entering the system than the plants can use, it is recommended that you add in some form of solids filtration. Filters will lower the volume of solids accumulating in the beds which can result in toxic build-ups as mentioned above.

 Many backyard growers I know will tend to have a higher fish to plant ratio, and as a result, they generally get enough nutrients for their plants even after the solids have been removed.

**PLEASE NOTE - You still need adequate biofiltration to convert the ammonia to nitrite, then to nitrate, or the ecosystem will collapse and you will kill your fish.**

For those that are considering using Nutrient Film Technique (NFT) or Deep-Water Culture (DWC) growing methods, solids removal is a must in their system build. The reason being is the roots of the plants will quickly pick up fine solids, reducing their ability to take up nutrients and oxygen.

Aquaponic deep water culture Lettuce Roots

If left to build up, this can lead to issues with diseases.

These growing methods require a solids filter/settler (e.g. radial flow settler) and some form of fine solids removal like a sock filter at the DWC bed inlet. This will help keep the plant roots clean, oxygenated and healthy.  

DWC and NFT growing methods also require some form of biofiltration. This will provide the bacteria with the biological surface area they can colonise so they can convert the ammonia produced by the fish into plant-available nitrate. The biofiltration can be achieved by including media-based grow beds into the system or a stand-alone biofilter.

How to remove & use the solids

DIY solids settlers/filters are probably the easiest way to keep a large amount of muck from entering the grow beds in first place.

Radial Flow Settler explanation

These settlers are best located on the pipework leaving the fish tank, allowing the bulk of the solids to be collected as soon as possible. There are a few different DIY filters you can build yourself, with the radial flow and swirl settlers being the two most popular used in backyard aquaponic systems. These are generally emptied once or twice a week, with the removed waste used around the patch to feed soils and plants.

For a good reason, my preference is for radial flow filters but will get into greater depth as to why in another article looking at DIY filters.

A very handy little DIY mechanical filter for smaller systems is the canister style filter. They do a fantastic job at collecting fine solids and can be scaled up to whatever size system your build requires.

These filters would be handy for folks wanting to grow using DWC or NFT. I would also recommend them to those that are running the basic, single loop, fish to grow bed style systems, like the IBC Chop and Flip aquaponic system. They do a great job at removing the fine solids created when the fish waste gets macerated into fine particles by the pump when moving water directly to the grow beds.

They do require a bit more time to maintain, as the material used to capture the solids needs to be cleaned out regularly.

Utilising removed solids

Now, if you do remove the solids from the system, there is no need to discard the remaining nutrients that are in there. I have always used the waste from our radial flow settler to fertilise different garden beds and fruit trees in the patch. I know some folks that strain the water out for plants in the veggie patch and feed all the solids to the compost worms.

Mineralisation tanks are a great way to recover the remaining nutrients from waste collected from your filters and add them back into the aquaponic system for the plants to take up.

Aquaponic Mineralization Tank

 Mineralisation tanks are a vessel that is large enough to take around a month’s worth of discharge from your solids settler/filter. The collected “sludge” is actively aerated and sometimes fed with carbon-rich feed sources like flour or sugar to provide the heterotrophic bacteria with an extra energy source to further break down or mineralise the waste into plant-available forms.​

Once a week the air is turned off so the solids can settle to the base of the tank.  Clean, nutrient-rich water can be decanted off & added back into the aquaponic system.

I have not run a mineralisation tank on our system but will be posting about them when I build our first one in the coming months. 

“What about Compost Worms? They’ll eat all the solids!”

Adding compost worms to the grow beds to “eat all the solids”, is a bit of a furphy I see continually recommended to beginners.  I’ve also received this gem dozens of times myself under clips where I’m talking about solids in our aquaponic system.

It would be great if it were true, but it is unfortunately not the case.

Compost worms will only break down the organic material they consume to a certain point. They will assimilate some of the nutrients for their own growth and make some elements available to the plants. They will also create organic waste that the plants can’t take up which will remain in the media beds and build up over time.

Compost worms in aquaponic grow bed

I am a fan of having these little fellas in the beds though but for a different reason. Compost worms do an excellent job at consuming plant matter that is left behind in the grow beds reducing the risk of exacerbating issues like plant disease and restricting water flow through the beds. 

I hope this article has helped you budding backyarders to see the reasons solids management is something you should take into consideration when designing your system. You may find the video and the beginners playlist link below helpful as well.

Cheers and Happy Growing.🥬🐟🌱🍓


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