Gary's mythconceptions #4 - question on solid wastes

[bob]

Hi. New here, still learning.

Thanks to Gary for the several useful mythconceptions notes. On the processing of solid wastes, I am wondering about a couple of points:

1) Gary mentions that mineralisation of removed solids by

Anaerobic digestion ... may provide a more useful effluent..

than would, presumably, aerobic digestion. Does "more useful" here mean more fully converted or converted more efficiently in some sense, or converted to a form more bio-available to plants, or something else? Grateful if he or anyone could elaborate.

2) Gary also points out that

As the solids (or the residue of spent solids) build up, anaerobic patches are created within the grow bed. This leads to denitrification...

It seems to me that anaerobic conditions in an aquaponics water column could arise in two ways. System-wide, if the DO is somehow allowed to drop too low, the entire microbial population would soon evolve toward a mainly anaerobic one. But of course in this case the fish are floaters pretty quick anyway, so so denitrification is the least of our problems.

On the other hand, if in isolated corners and spots solids accumulate in sufficient concentrations to choke out the O2 needed by the aerobic populations, one could get anaerobic pockets. These are what I am wondering about.

Referring back to my first query about side processing of removed solids, wouldn't a tank doing anaerobic digestion of removed solids also lead to a denitrified effluent unsuited to replacement into the aquaponics water column? Or in other words, if an anaerobic condition is a danger where it occurs in the system, why wouldn't it have similar problems if used in an separate digester for removed wastes?





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[GaryD]

Hi Bob,

Welcome to APHQ.

Does "more useful" here mean more fully converted or converted more efficiently in some sense, or converted to a form more bio-available to plants, or something else? Grateful if he or anyone could elaborate.

While aerobic digestion occurs much faster, anaerobic digestion does not require the energy input of aerobic digestion.....through aeration. I also have a faint recollection that anaerobically digested effluent has a higher nutrient value.....but I'd need to check on that.

It seems to me that anaerobic conditions in an aquaponics water column could arise in two ways. System-wide, if the DO is somehow allowed to drop too low, the entire microbial population would soon evolve toward a mainly anaerobic one. But of course in this case the fish are floaters pretty quick anyway, so so denitrification is the least of our problems.

Catastrophic failure of a system (due to low DO levels) can be caused by a single event (like power failure) but it is often the consequence of a convergence of circumstances.......like high rates of feeding, algal blooms, high stocking densities, equipment failure or high ambient temperatures.

On the other hand, if in isolated corners and spots solids accumulate in sufficient concentrations to choke out the O2 needed by the aerobic populations, one could get anaerobic pockets. These are what I am wondering about.

Referring back to my first query about side processing of removed solids, wouldn't a tank doing anaerobic digestion of removed solids also lead to a denitrified effluent unsuited to replacement into the aquaponics water column? Or in other words, if an anaerobic condition is a danger where it occurs in the system, why wouldn't it have similar problems if used in an separate digester for removed wastes?

Anaerobic conditions in a grow bed are different from anaerobic conditions in a digester in terms of intent.

Anaerobic conditions in a digester are encouraged for particular ends......biogas, sludge (soil conditioner) and a value-added effluent.

Anaerobic conditons in a grow bed, however, signify a problem the symptoms of which include......biogas (stench), sludge and effluent (low DO).

Placing an air stone in a drum of the digester effluent will quickly stall all anaerobic activity. It will also assist toxic gases (like hydrogen sulphide) and unhelpful ones (like carbon dioxide) to disperse.

Once the effluent has been heavily aerated it can be reintroduced into the aquaponics system.....heavily diluted.

Don't pour a stream of undiluted effluent directly back into the system - it will be a culture shock to any fish that swims through it. Trickle feed the effluent back into the system.

Setting up a small anaerobic digester can be an interesting learning experience and will mineralise the solids.....but is a protracted and rather more complicated process than tipping the solids into a bucket and placing an air stone it with them.

Generally speaking, biogas digesters are not practical for backyard purposes.

Aerobic treatment is, in my view, the much easier way to go. It happens a lot faster and it is much more in harmony with the aquaponic marriage of fish and plants.

Gary

[GaryD]

Hi,

I also have a faint recollection that anaerobically digested effluent has a higher nutrient value.....but I'd need to check on that.

I did check on it and, as it happens, I got it wrong. Aerobically-digested effluent has a higher nutrient value that anaerobically-digested effluent.

For those who might be interested, I've just put up a post on my blog.......7 Good Reasons to Remove Solids.

Gary

[bob]

hhhmmmm...

still wondering what best to do with solid and suspended waste after removal. I know it is often used for composting for soil enhancement and so forth, but I would like to aim at a pure aquaponics system for now.

I have read elsewhere that at least 25% of the biomass introduced as feed becomes solid waste of one sort or another, which should be removed as Gary points out. Twenty-five percent seems quite a sizeable number in this context.

Which gets me to thinking: could such removed wastes be bio-converted in some way into plant-useable molecules, (aerobically or anaerobically, in a side system so any detrimental effectes can be isloated from the fish) and could the appropriate parts of the transformed materials be then separated and re-introduced to the water column, this might offer significant potential to get more nutrients to the plants. This would thus allow for a larger plant/fish component ratio and more useful output from the same inputs. At least in theory, anyway.

Any thoughts, anyone ? Or am I dreaming?

[GaryD]

Hi Bob,

.....could such removed wastes be bio-converted in some way into plant-useable molecules, (aerobically or anaerobically, in a side system so any detrimental effectes can be isloated from the fish) and could the appropriate parts of the transformed materials be then separated and re-introduced to the water column, this might offer significant potential to get more nutrients to the plants. This would thus allow for a larger plant/fish component ratio and more useful output from the same inputs. At least in theory, anyway.

Any thoughts, anyone ? Or am I dreaming?

Yes, the solids can be removed, treated externally and the nutrient-rich liquor (minus the sludge) can be reintroduced back into the system.....and that's exactly how I suggest that it be done.

The best way to process the solid is to use a simple aerobic digester (a plastic drum with a couple of airstones). You bubble the watery solids for a while and then shut off the air and allow the sludge to settle out before decanting the nutrient-rich liquor.

After I'd typed all of this out, I realised that I had covered the same ground a couple of posts back.

Anyway......bottom line is yes you can and yes you should.....in the best water quality interests of the fish.....and the nutrient interests of the plants.

The sludge is great compost starter or worm food.

Gary

[GaryD]

Hi,

Interestingly, one of the loudest noises against the removal of solids from aquaponics systems, has been quietly developing and trailling (wait for it).....solids removal devices.

While one of his experimental filters has failed to capture solids and has been abandoned, the other device is showing more promise.

Based conceptually on a filter module that a colleague and I installed during the retrofit of a conventional flood and drain system, the filter device comprises a sedimentation space and filter pads.

The idea is that the water entering the filter loses velocity and the settleable solids precipitate out. As the water moves through the filter pads, the suspended solids are trapped on the bio-film which forms on the filter pad matrix.

Never mind that the new device operates back to front.....the water goes through the pads first and then into a sedimentation space.....it is heartening to see experimentation of this type from such a champion of the outhouse (solids retention) approach to aquaponics.

While the new device is said to be for raft systems, I confidently predict that it (or something like it) will find a similar application in flood and drain systems.

“Every truth passes through three stages before it is recognized. In the first it is ridiculed, in the second it is opposed, in the third it is regarded as self-evident”

Arthur Schopenhauer

Gary

[nwestwood]

I believe Murray's filter box was designed to go with the floating raft beds. As I have found, floating raft crops don't like dissolved or suspended solids, and they don't do well without some sort of Bio Reactor to remove gases before it gets to the floating raft tank. The down side is that you are dumping and maintaining filters 2-3x daily.

For backyard aquaponics I prefer to have system that does not require so much maintenance. My next revision of my system will have more grow beds that process 100% of the water from the fish tank and then feed the floating raft tank as part of a bypass line attached to the main return line from the sump. The gravel beds will act as the filtration, and the sump with its heavy aeration will take care of off gassing. That way I get the clean water that the raft tank needs, without the need for extra equipment that takes a lot of extra time and attention.

Gravel Beds may not be the perfect filter for Aquaculture, but they are more than adequate for Aquaponics.

[GaryD]

Hi Neal,

I believe Murray's filter box was designed to go with the floating raft beds.

I acknowledged that when I said.....

While the new device is said to be for raft systems.....

I then went on to suggest that a similar device would be useful for any system.

The down side is that you are dumping and maintaining filters 2-3x daily.

For backyard aquaponics I prefer to have system that does not require so much maintenance.

That depends on the specific solids removal device and its design. A swirl tank or bubble jet filter, for example, takes just seconds to dump solids and that's a lot less time than it takes to remove dead fish from your system.

There will always be those people who are willing to assume a greater level of risk and I accept that. I do things that I would never recommend to a newcomer to AP.....because I'm making an informed decision.....because I have the resources to do it......and because I'm willing to accept responsibility if it goes belly up.

My next revision of my system will have more grow beds that process 100% of the water from the fish tank and then feed the floating raft tank as part of a bypass line attached to the main return line from the sump. The gravel beds will act as the filtration, and the sump with its heavy aeration will take care of off gassing. That way I get the clean water that the raft tank needs, without the need for extra equipment that takes a lot of extra time and attention. Gravel Beds may not be the perfect filter for Aquaculture, but they are more than adequate for Aquaponics.

I agree that, for lightly stocked aquaponics systems operated by competent people who test their water regularly, gravel grow beds (assuming you have enough of them), may be adequate.

The fact remains that an aquaponics system that has solids removed has the potential to be much more productive and far less risky than one that doesn't.

Gary

[Crusty]

I believe Murray's filter box was designed to go with the floating raft beds. As I have found, floating raft crops don't like dissolved or suspended solids, and they don't do well without some sort of Bio Reactor to remove gases before it gets to the floating raft tank. The down side is that you are dumping and maintaining filters 2-3x daily.

I get confused really easily when people use aquaculture terms in ways I do not understand, so please excuse me if I seem a little daft.

Are you able to clarify the use of a bio reactor to remove gasses? or a better way of asking is: What gasses are you removing? What sort of bio reactor removes these gasses?

Coming back to the down side you mentioned "dumping and maintaining filters 2-3x daily". I could be wrong but perhaps there is a better way to design your filters if they are causing you to work so hard with them. Mine are a ten minute job once a week with quite possibly greater loads.

For backyard aquaponics I prefer to have system that does not require so much maintenance. My next revision of my system will have more grow beds that process 100% of the water from the fish tank and then feed the floating raft tank as part of a bypass line attached to the main return line from the sump. The gravel beds will act as the filtration, and the sump with its heavy aeration will take care of off gassing. That way I get the clean water that the raft tank needs, without the need for extra equipment that takes a lot of extra time and attention.

I agree that any system design should be well thought out so that very little maintenance is required. In regards to your proposed new layout and keeping in mind the solids are still in your gravel beds, how do you clean a gravel bed?

Personally, I find the over simplification of aquaponics makes it either a great deal more work or a great deal less results. In the manner aquaponics is currently set out, you can not have both, high production, safety and ease of use.

Gravel Beds may not be the perfect filter for Aquaculture, but they are more than adequate for Aquaponics.

Which brings me right back to here. I often wonder about the definition of aquaponics and how it is seen to be so very different to its primary nutrient source, aquaculture. This is to say, that if a gravel bed is not suitable as a filter for aquaculture, how can it possibly be suitable for aquaponics?

As Gary has said, at low densities, they may work well. I believe they will only work well for a set amount of time until they will negatively effect the entire system be that through lower density of fish the next season, lots of dead fish, and low plant production. But from my point of view, if you are not removing solids out of the primary production loop, you are not getting the best out of your system or the space/electricity it is using.

[arachdog]

Hi crusty,
First off, I think your doing a great job re-teaching some aquaculture lessons here. I pretty much agree with everything you've posted and you seem to be going about it in a way that is actually gaining some traction. Not just spinning the wheels like my ill-considered attempts. But don't let Gary convince you to dumb down your posts too much, I'm sure not every backyard aquaponicist is afraid of a bit of mathematics.

But from my point of view, if you are not removing solids out of the primary production loop, you are not getting the best out of your system or the space/electricity it is using.

Although this statement is correct I don't think its quite the whole story. If people are only growing for their own needs then there maybe a situation where they actually need nutrients more than they need more fish ie. they prefer to produce a higher ratio of plants to fish. In this situation it would make sense to isolate the solids from the plants and fish, but not entirely remove them from the production loop until they had leached most of their nutrients. Of course in a commercial situation where any extra produce is extra cash, that makes no sense at all.