This ‘mythconception’ would be amusing if not for the fact that it is responsible for the premature death of so many fish.
The purpose of grow beds in a media-based aquaponics system is to facilitate nitrification and to grow plants……in that order.
There’s a prevailing belief among some aquaponicists that an additional function of grow beds is to trap and process solid wastes. These solid wastes consist of fish faeces and uneaten food.
This belief ignores the fact that a bio-filter functions most effectively when sedimentary solids are removed…….something that every trainee wastewater treatment operator knows. It also flies in the face of advice from leading aquaponics researchers like Dr Wilson Lennard, Dr James Rakocy and Dr Nick Savidov.
Advocates of retaining the solids argue that the solids need to be mineralised so that the nutrients that are bound up in the solids can be made available to the plants. They cite the existence of composting worms in their systems as evidence of the fact that mineralisation is occurring.
For the sake of clarity, I agree with the basic proposition that the solid wastes in an aquaponics system should be processed and returned to the system. Where I differ with advocates of grow bed worm farming is that I don’t believe that this processing should happen in the grow beds.
I’m not just suggesting that solids should not be trapped in the grow beds. The fact is that there are suspended solids that cannot be trapped by the grow bed media. .
According to Dr Wilson Lennard, up to 75% of the solids in an aquaponics system may be in solution. Some of this may precipitate out if the water is allowed to become still for long enough and some of it will only be removed by exposure to nitrifying bacteria. Incidentally, this information should put paid to the suggestion that the bacteria won’t have enough to ‘eat’ if the solids are removed.
I first became conscious of the impact of suspended solids when I removed the fish from one of my aquaponics units. Although I continued to recirculate the water, I noticed that (after about four hours) there was a substantial build up of fine sediment on the floor of the fish tank. I vacuumed it out of the tank. The next day, there was a further build up of sediment on the tank floor which I also removed. This continued for four days.
The oft-cited University of Virgin Islands aquaponics unit employs clarifiers to capture sedimentary solids….and mineralisation tanks to trap the suspended solids. During their brief stay in the mineralisation tanks, the solids yield up the nutrients. The gunk that remains in the mineralisation tanks is physically removed twice weekly.
It’s also important to understand that, not only are there different types of solids to be dealt with in an aquaponics system, there are also different types of bacteria to be considered.
Let me explain…..there are two main types of aerobic micro-organisms that inhabit aquaponics systems ….autotrophic and heterotrophic. Autotrophic bacteria convert the ammonia in the water into nitrates. Heterotrophic bacteria convert fish faeces and uneaten food into ammonia.
Heterotrophic bacteria are fast-growing and most autotrophic bacteria are (comparatively speaking) slow growing. Heterotrophic bacteria will breed up in much greater numbers if the feedstock is available
……and herein lies a problem.
The purpose of bio-filtration is the conversion of prospectively toxic ammonia and nitrites to less harmful nitrates……but, if heterotrophic bacteria are in disproportionately large numbers, they may be producing more ammonia than the nitrifying bacteria can convert. It’s a matter of simple deduction, therefore, that vigorous growth of nitrifying bacteria should be encouraged while the conditions that promote the growth of heterotrophic bacteria should be avoided.
But maintaining a predominance of autotrophic bacteria is not the only issue.
The conversion of these solids depletes the system of oxygen. High levels of dissolved oxygen are in the interests of the fish, plants and bacteria so anything that diminishes dissolved oxygen levels is also to be avoided.
Nitrates, when exposed to low dissolved oxygen levels can be converted by bacteria back to nitrites. At certain concentrations, nitrites (like ammonia) are toxic to fish.
As the solids (or the residue of spent solids) build up, anaerobic patches are created within the grow bed. This leads to denitrification which, in turn, causes the pH of the system to rise.
With high concentrations of ammonia, diminished oxygen levels and rising pH levels, we have the circumstances for a fish kill.
Toxic grow bed conditions can be avoided in two ways:
- Regular cleaning of the grow beds coupled with modest stocking and feeding rates.
- The use of appropriate mechanical and biological filtration devices.
As I said earlier, I’m supportive of the idea that the solid wastes should be mineralised and the nutrients returned to the system.
The best way to handle solids in a small aquaponics system is to remove them before they get into the bio-filters….or grow beds.. This is achieved through the use of sedimentation tanks, clarifiers, swirl filters, screens or similar mechanical filtration devices.
For very small systems, mechanical filtration may be as simple as running the water from the fish tank through a piece of filter foam or even an orphan sock. Grow bed maintenance can be much less frequent if sedimentary solids are removed.
Mineralisation of the solids can be effected by either aerobic or anaerobic digestion.
Aerobic digestion can be facilitated by putting the solids into drum full of water. Place an air stone in the drum and leave it for a few days. Remove the air stone and allow the spent solids to settle out before decanting the nutrient-rich water.
Anaerobic digestion is a bit more complicated but may provide a more useful effluent. Take an airtight container and attach an airlock device (a home brew fermenter is ideal). Put the diluted solids into our simple bio-digester and fit the airtight lid. After a few days, air bubbles will issue from the airlock. About forty days later (the exact digestion period will depend on the ratio of solids to water and the ambient temperature), the digestion of the solids will be completed. You can then separate the liquor containing the nutrients from the sludge and use them in your system.
As the arguments in support of using a grow bed to capture and process solids fall prey to science and logic, its advocates often flee to a last bastion called Balance.
The claim of the balance advocates is that a properly designed flood & drain system… stocked and fed at suggested rates… doesn’t need cleaning.
“It’s all about balance,” they cry. “You have to reconcile your stocking density, feeding rates and plant production with each other.” They then go on to say that if you are experiencing problems with water quality, it must be that you have too many fish or that you are giving the fish too much food.
While this seems logical enough, it’s not the complete picture. You see, any consideration of the performance of an aquaculture or aquaponics system has to take account of productivity and sustainability.
Sustainable production of fish requires that you grow a pre-determined amount of fish biomass within a given time while minimising the energy and food costs that it takes to do it.
The electricity that it takes to run an aquaponics system is a fixed cost. You’ll spend the same amount of money on energy to power the water and air pumps on a system that is producing at optimum levels that you will on one the same system that is under-performing.
If, for example, the accumulation of solids in a system limits your capacity to grow the greatest possible amount of fish in the shortest possible time, then the energy cost per kg of fish or plants may increase to the point where it is no longer economical to grow them.
The other productivity issue arises out of the heightened risk of operating an aquaponics system outside of the optimum water quality parameters. Producing fish that cost too much is one thing; producing fish that die before you get them to the dinner table is another kettle of (prematurely dead) fish entirely.
To summarise:
- Bio-filters (including grow beds) function more efficiently when solids are removed.
- The digestion of large volumes of solids diminishes dissolved oxygen levels. Fish, plants and nitrifying bacteria all do best when dissolved oxygen levels are optimised.
- Built up fish wastes (or other sediment) create pockets of anaerobic (without oxygen) activity resulting in denitrification.
- Denitrification causes the pH of the system to rise. Ammonia in the presence of high pH levels is toxic to fish.
- Removal of the solids, and their subsequent mineralisation, is a simple (and far less risky) thing to do.
- The question of balance in an aquaponics system is more about productivity and sustainability and less about accommodating dogma.
Only by optimising the water quality in an aquaponics system, can you maximise its productivity, and you can only optimise water quality in a small aquaponics system by removing the solids.
You might wonder why people would seek to perpetuate a myth that so obviously defies such logic.
Those most responsible are some of the peddlers of aquaponics products …..and the reason that they do it is because they know that there’s a larger market for their offerings if they create the impression that running an aquaponics system is easy.
Running an aquaponics system doesn’t need to be complicated but it’s not quite as easy as some people would have you believe.
Sounds like the stuff of Aquaponics Mythconception #6 – Aquaponics is Easy! Stay tuned.
-o0o-
{ 18 comments… read them below or add one }
hhhmmm, thats a lot to think about. My system is 9 months old, The fish were all fingerlings when I started, Solids were not a problem. But I do see now that there are more and more solids going into the grow bed. Is it possible to divert the outlet of the grow bed and periodically “wash” the bed with higher pressure water to break loose the solids and flush them out of the system? Is it necessary to remove all the plants and physically remove the grave to clean it out? Any experience with this?
Neal……the easier way to go is to ensure that the solids don’t get into the grow bed in the first place. This is as simple as using a sedimentation tank or clarifier (for the settleable solids) and a mineralisation tank fitted with filter pads to trap the suspended solids.
If you have to use gravel, keep it to a depth of about 150mm (6″) and clean it out periodically. Incorporating a trickling bio-filter into your system (at the end of the water cycle) will optimise nitrification and aeration.
Gidday Gary,
As you know Im extremely interested in maintaining as clean & s…t free system as possible. The more I research aquaponics the more I realize the critical & essential need for solids removal,as well as, the reinstating, of the vital bits to keep the system producing vegys & fish.
Noted the 2 ideas for areobic & anerobic mineralization.
A couple of questions spring to mind.
In a 200 litre drum would a 50/50 ratio of solids & water be suitable for either anerobic or aerobic mineralization?
Which system is a more efficent ?
Is there a place in our back yard systems for both?
Do either or both mineralization/digester systems work better with more or less water to solid imputs?
Can you aerobicly digest an amount of solids,decant the result, then anerobic the sludge,or is this a bit over the top for Backyarders ???Thanks for any help you can give,keen to see the Queenslander up & running.By ghost youve give that Backyard of yours,a real working over, Dude !!!!!!
Steve A……..I find it remarkable that a relative newcomer to aquaponics like yourself can so quickly grasp the need to keep solids out of aquaponics systems when people who have been around much longer grapple with the concept.
Both work……aerobic digestion happens faster and is easier to facilitate……and anaerobic digestion produces bio-gas (so greater skills and consideration of risk is called for) and produces a better effluent.
I’d suggest that you try aerobic digestion initially………and you can do it with much less that 50/50 solids to water.
For backyard purposes, anaerobic digestion is probably of limited practicality – particularly given the gas that it produces.
Once you’ve digested the solids, allowed them to settle and drawn off the liquor, you’ll find that there’s not much left in the sludge. It’s good for the compost heap or worm farm.
The backyard is taking shape and I’m stiff and sore in equal measure……but it’s going to be worth the effort.
Hi,
Just a comment about your article which does cite my name and an apparent argument I make for using the gravel bed in a backyard aquaponic system as a mineralisation filter. As always in media, when things are taken out of context or not completely included, then the argument required by the writer is supported. In this case, my name (and that of my close friends and collegues) has been used in the context of being an “advocate for leaving solids in backyard systems”. What is sadly left out here is the fact that I also advocate that backyard systems are configured correctly in terms of fish to plant ratio’s. If this was the case (which it isn’t at all!; EVERY backyard aquaponic system I have ever seen, whether home built or purchased from a supplier, has the WRONG fish to plant ratio’s employed – please note that I use “FISH to PLANT ratio”, which is not “system volume ratios” or any similar erroneous ratio!) then we would have far fewer fish in systems than we do now, and then, the chance of “overwhelming” the gravel bed with solids would be far lower.
The gravel bed WILL mineralise solids at acceptable rates without impacting on dissolved oxygen, biofiltration etc…in systems. BUT, if the amount of solids entering the system is too high (most likely due to the wrong fish to plant ratio being employed and the subsequent excess release of solid fish waste) then yes, the bed will be overwhelmed with solids. Therefore, gravel beds will mineralise solid fish wastes at acceptable rates IF the system is designed with the correct fish to plant ratio in mind. What happens mostly is that the fish to plant ratio is incorrect, so too many fish inhabit the system, and subsequently, too much solid waste enters the gravel bed.
In addition, I have NEVER said “…up to 75% of the solids in an aquaponics system may be in solution.” What I have said is that up to 75% of the wastes produced by the fish in the system are in the dissolved form, because they dissolve directly into the water across the fish gills, OR, up to 75% of the nutrients in the water are in dissolved form. This may seem a trivial reading of what you have said, and it may even seem pedantic. BUT, the fact is that the statement “…up to 75% of the solids in an aquaponics system may be in solution.” is completely factually different from what I have ever said!
So, I am not against the information you are supplying to the public; much of what you have to say seems based in reality and uses common sense. In addition, there is much (but not ALL) that I agree with. However, if using peoples names to make arguments, it would be nice to stay true to that persons beliefs and NOT to use that persons name, and more importantly, your interpretation of what they may have said, as an argument to support your own thoughts or feelings.
Thanks for your time,
Wilson Lennard
Wilson Lennard……..I’ve addressed your comments on my forum because the formatting features of the forum make it easier to do so. I invite you to visit here……where I’ve provided a detailed response.
Found the article good and supportive of my own system ideals for a small home based system. The removal of solids prior to the media beds in my own system has proven far better than the plug and play purchased system we first purchased. I’d been ridiculed over comments on sludge and find the article good advice for others. My next move is swirl filtering for sediment removal to promote growth in both sides of my system. Thanks again for the excellent guide to new users.
Ken…….the fundamentalists still get flustered whenever I speak about solids removal but I find increasingly, that their dark mutterings are becoming fainter, as time goes by…….and as more people like yourself begin to grasp and demonstrate the logic of solids removal.
Even some of the die-hards are starting to qualify their insistence that solids removal is not necessary by saying that they only recommend light stocking rates.
Gary,
Enjoyed the post. One thing that I think changes perspective on the solids debate is whether the system is intended to be a fish centric system or a plant centric one. I load fish extremely heavily (one could even say excessively) in my systems. Plants and growbeds are more of a secondary concern to me because they are just one part of a bigger filtration and nutrient handling scheme in my goal of producing the maximum amount of fish mass possible. I find it absolutely required practice to remove solids as quickly as possible from my system (by design). My solution to the problem of “dumping” solids is to compost them and feed to traditionally grown plants as well as standalone shallow duckweed trays which go back into the system to supplement the other feed given to the fish. This, in it’s own right, is a way of “closing the loop” and keeping things sustainable, while, in my opinion, allowing a system to far outperform one that attempts to deal with solids cycling solely within the aquaponics system itself. These tradionally grown plants and duckweed trays are considered part of my overall filtration system (with a bonus of some free fish food), and this type of system will greatly outperform a closed loop aquaponics system processing solids solely in the growbeds. Perhaps it sometimes pays to think outside of the “loop”.
Kellenw……if the well-being of the fish is the paramount issue (and it should be) then the issue of solids removal is resolved in my mind. While I acknowledge that a very lightly stocked aquaponics system can function with the solids being processed in the grow beds, that shouldn’t be taken to mean that it’s the best way to do it.
I agree that returning the nutrients (that are recovered from the external processing of solids) is a good practice. If you read my posts on our Queenslander design, you’ll see that we’re proposing exactly what you’ve described for the reasons that you so clearly articulate.
I’m pleased to encounter a fellow traveller. While our numbers are still few, they’re growing as commonsense overtakes dogma.
Found this article on the net :
“Certain Conditions Having Great Influence In The Process Of Nitrification”
http://chestofbooks.com/crafts/scientific-american/sup3/Notes-On-Nitrification-Continued.html
(excerpt below, please read it first)
this article not only confirms your theories and findings (which correspond with mine) but also seems to lead to the interesting conclusion that growbeds should be kept moist and not wet as in an autosiphon system: short floods and prolonged drains seem to be the way to go.
I continue to theorise and would appreciate your comments.
With a continuously running pump and a short loop over the clarifier/sedimentation tank, shortcircuited to the fish tank, the extra volume pumped in this loop (not going to the grow beds) will not only eliminate solids (the major culprits!) more quickly but also will certainly benefit aeration of the fish tank AND of the clarifier/sedimentation tank.
The longer interruptions between floods will better aerate the media, the bacteria and the plant roots in the grow beds.
Additionally lots of plants do not particularily like their feet in the water all day.
It also explains why floating raft systems need so much aeration and the consequent extra energy consumption: oxygen is of the utmost importance to plant roots and bacteria.
Continuing this theory it would seem that a mixture of course sand (which offers the most surface to bacteria, aerates well and offers support for the plants) with vermiculite (to keep in the moisture) would be the best media choice on the (resolved) condition that solids are removed to avoid clogging. This would highly enhance the biofilter performance of the growbeds.
Covering the growbeds with styrofoam (thinner, as it doesn’t have to keep the plants floating) would keep out light in general, keep out excessive sunlight in summer, thus keep out algae, and keep in warmth in winter. Again this option offers an extra advantage: shallow thus cheaper and still more efficient growbeds.
excerpt of the article:
“If we suppose that a solution containing a nitrifiable substance is supplied with the nitrifying organism, and with the various food constituents necessary for its growth and activity, the rapidity of nitrification will depend on a variety of circumstances:
1. The degree of concentration of the solution is important. Nitrification always commences first in the weakest solution, and there is probably in the case of every solution a limit of concentration beyond which nitrification is impossible.
2. The temperature has great influence. Nitrification proceeds far more rapidly in summer than winter.
3. The presence or absence of light is important. Nitrification is most rapid in darkness; and in the case of solutions, exposure to strong light may cause nitrification to cease altogether.
4. The presence of oxygen is of course essential. A thin layer of solution will nitrify sooner than a deep layer, owing to the larger proportion of oxygen available. The influence of depth of fluid is most conspicuous in the case of strong solutions.
5. The quantity of nitrifying organism present has also a marked effect. A solution seeded with a very small amount of organism will for a long time exhibit no nitrification, the organism being (unlike some other bacteria) of very slow growth. A solution receiving an abundant supply of the ferment will exhibit speedy nitrification, and strong solutions may by this means be successfully nitrified, which with small seedings would prove very refractory. The speedy nitrification which occurs in soil (far more speedy than in experiments in solutions under any conditions yet tried) is probably owing to the great mass of nitrifying organisms which soil contains, and to the thinness of the liquid layer which covers the soil particles.
6. The rapidity of nitrification also depends on the degree of alkalinity of the solution. Nitrification will not take place in an acid solution; it is essential that some base should be present with which the nitric acid may combine; when all available base is used up, nitrification ceases.”
another excerpt from the same article:
“The Distribution Of The Nitrifying Organism In The Soil
Three series of experiments have been made on the distribution of the nitrifying organism in the clay soil and subsoil at Rothamsted. Advantage was taken of the fact that deep pits had been dug in one of the experimental fields for the purpose of obtaining samples of the soil and subsoil. Small quantities of soil were taken from freshly-cut surfaces on the sides of these pits at depths varying from 2 inches to 8 feet. The soil removed was at once transferred to a sterilized solution of diluted urine, which was afterward examined from time to time to ascertain if nitrification took place. These experiments are hardly yet completed; the two earlier series of solutions have, however, been examined for eight and seven months respectively. In both these series the soil taken from 2 inches, 9 inches, and 18 inches from the surface has been proved to contain the nitrifying organism by the fact that it has produced nitrification in the solutions to which it was added; while in twelve distinct experiments made with soil from greater depths no nitrification has yet occurred, and we must therefore conclude that the nitrifying organism was not present in the samples of soil taken.
The third series of experiments has continued as yet but three months and a half; at present no nitrification has occurred with soil taken below 9 inches from the surface. It would appear, therefore, that in a clay soil the nitrifying organism is confined to about 18 inches from the surface; it is most abundant in the first 6 inches. It is quite possible, however, that in the channels caused by worms, or by the roots of plants, the organism may occur at greater depths. In a sandy soil we should expect to find the organism at a lower level than in clay, but of this we have as yet no evidence. The facts here mentioned are in accordance with the microscopical observations made by Koch, who states that the micro-organisms in the soils he has investigated diminish rapidly in number with an increasing depth; and that at a depth of scarcely 1 meter the soil is almost entirely free from bacteria.”
Hi Gary & everyone
I was very glad to discover this peripheral galaxy of the forum universe i am accustomed to – thank you all for the very useful information.
following your advise for a long time now, Gary, I can confidently say that from what i have been experiencing this last year, on the issue of solid removal (& stocking ‘policy’) the only way to go is removing the solids, especially when the fish specie involved are Tilapia, which are second only to Americans in producing waste (once the fish exceed 600 grams, it’s a wasteoponics system your running). my system being a Constant Hight One Pump system, utilizing gravitation to keep the flow from an elevated fish tank to the growbeds – the pipes (1.5″, or 40 mm) got clogged roughly every fortnight. I have not yet dug down into the Tuff (volcanic rock) growbeds to see how clogged they actually are (300 mm is a lot of digging..) but i notice that in some parts the water takes longer to peculate, which i assume indicates clogging.
just recently i have installed a synthetic media filter (working in recycling greywater i used a MATALA greywater diversion device which i had around – NOT a cheap solution but very effective) and the results seem very satisfying. i will build for my second system a trickling filter (good info on making one of these on this page) –
http://www.biofilters.com/TrickFilter2.htm
I have been toying with the idea mentioned regarding using a mix of vermiculite, peat moss & compost as grow material in a system operating on a timer initiating fast fill and very slow drain – wonder if any might have any comments about this
my second system which is a very simple fishtank – pump – growbeds setup uses 200 mm growbeds and the water quality seems none the worst for it.
last and not least – has anyone ever tried incorporating a Grander living-water device in an aquaponics system? curious..
all the best
regards from Israel
Tomer…..good to hear from you again.
I’m receiving increasing numbers of reports of people who are going the way of solids removal in aquaponics systems. While clogging of beds is possible in extreme situations, the bigger issue with solids is the effect that they have on oxygen levels. Advocates of using grow beds to process sedimentary and suspended solids ignore the fact that those solids consume oxygen in their decomposition.
Most fish that die in aquaponics systems do so due to lack of oxygen in the water. Increasingly, we’re finding that fish kills are the result of several factors (rather than one big problem like equipment failure or power interruptions) that allow oxygen levels to drop below those required to sustain the health and well-being of the fish.
A colleague and I installed two of those Matala greywater devices to his aquaponics system and they are doing an excellent job of trapping sedimentary and suspended solids. Given that the Matala mat is available separately, it shouldn’t be too difficult to make up a similar (perhaps larger) device at much lower cost.
The vermiculite/peat moss/compost mix will work very well as a growing medium…..although I’d substitute coco coir for the peat moss on sustainability grounds. Rather than attempt to recirculate water through that mix, however, I’d suggest that you set it up on a timer so that it gets a controlled amount of water (just enough to run from the drain hole) at intervals of 8 – 12 hours. This will provide for adequate feeding with plenty of opportunity to drain.
You might also like to consider using this growing mix…..in a wicking bed configuration. That way, you’ll only need to water it every few days. You can still water wicking beds from your fish tanks….which means that you still get two crops for the same amount of water.
I use both Deep Water Channel (styrofoam rafts) and Ebb and Flow aquaponics systems. And while I do use settling tanks, bird netting, matala mats, etc. for mechanicsl filtration on my DWC systems I do not use any mechanical filtration on my E+F systems besides the growbed media. Let me just say that having to drain (and waste) hundreds of gallons of water to clean the filters on my DWC systems is a major reason I’m an advocate of media based Ebb and Flow AP systems.
As far as E+F systems go a properly proportioned system is what is really needed for solids accumulation to be dealt with. When the correct ratio is used from the beginning then the system has a greater capacity to deal with solids. Its like putting a small filter on a big fish tank. If its too small its not going to cut it. Its that simple.
I also believe for most people adding another filter onto their AP system is not practical. Space is always a consideration as well as maintaining the filter. Not that people are lazy but the more ‘chores’ you add the less involved people will become.
Also, its interesting to me that you say that earthworms are what all the ‘leave-in the solids’ folks (like myself) are advocating yet you do not address this aspect of media based aquaponic systems in your article. It would be beneficial for your readers to know that earthworms like to eat organic matter just like the excess fish feed and ’solid’ fish waste that accumulate in the top of the growbed. Now if there’s something that likes to eat what I’m trying to get rid of then how would that not be a benefit to me. If anyone looks into vermiculture and worm compost tea they will see the benefits of having earthworms in their systems.
Your articles are very informative but in this instance I believe you left some important issues out.
Law One……Thanks for your considered comments. I’ve dealt with them in detail on my forum. I invite you to read them…….here.
hello gary,
i have only just started to set up a system and want to remove solids
before they get into the growbeds.
do you know of any sites that show designs and pictiures of
swirl filters, sedimenters and clarifiers.
thanks.
peter.
Peter……you’ll find photos and diagrams of such devices if you use Google Images. You can also register on Aquaponics HQ forum and I’ll post some photos there.