Will Solids Removal Still be Necessary In DWC with a Media Bed?

[Gina Cavaliero]

The ultimate goal in integrating a media bed gravel filter with a DWC system is to eliminate the need for solids removal, essentially eliminating the labor factor as well as benefiting from the additional metabolization of those solids by introducing red worms.  Is it possible to remove all solids via the media bed or will some degree of solids removal, ie. settling tanks or swirl filters still be needed?  JD Sawyer just posed this question to me a couple days ago and quoted this from James Ebeling's book Recirculating Aquaculture,

"Carbon eating heterotrophic bacteria grow significantly faster than the autotrophic nitrifiers do. Their mass can double in an hour, while it takes nitrifiers days to double. This high growth and the associated oxygen demand consequently suffocate the nitrifiers buried deeper in the biofilms, resulting in death and sloughing of the biofilm from the bioreactor surfaces... You must have effective solids removal BEFORE the high ammonia water is transferred to the biofilter. As mentioned above, the heterotrophic growth will compromise the nitrifiers ability to oxidize ammonia, mostly because the heterotrophs consume the oxygen prior to the oxygen being able to diffuse into the biofilm to where the nitrifiers are. This results in nitrifiers being starved for oxygen and then they die off resulting in complete sloughing of the biofilm and loss of nitrification capacity. A recipe for disaster in RAS is to have poor solids removal"  (Ebeling, Biofilter design, Recirculating Aquaculture 2nd Ed).

My theory is that stocking densities and feed ratios will have  a tremendous effect on this and if managed correctly, should minimize the need for solids removal.

[Kobus Jooste]

Yes and no.  It is all going to be about your design.  If you stock high density and have only a few gravel beds, then you are looking for trouble soon and you are either going to remove solids directly or see them pour out of gravel beds.

 

In my design quest for a system that does not need solids removal but which operates roughly as a raft based set-up would, I have the follwing components:

• Nate's media filled towers draining back to the gravel beds
• Large and deeper than average gravel beds
• In heavily stocked systems, solids removal through clarification that is then passed to dedicated worm filled gravel beds through timed pumps.  This allows the overflow or normal stream of water to to go standard gravel beds, while the dedicated worm beds deal with the majority of the concentrated solids before returning the water stream to the standard gravel beds.  I have also noticed that even with a lot of gravel, there will be fines making it through.  I believe that a fines trap of sorts - such as a net filter, will be useful.  I will not clean it though, not unless there is signs of things going wrong (DO).

Thus to condense, I picture concentrated solids streams to worm beds, normal water to deep gravel, media towers and DWC seperated from the gravel with a simple net filter as my design components.  I will aerate in the media beds and use a U-bend aerator between the DWC and the fish tank.  I will not aerate the fish tank heavily as this will mess with setting up a circular current and pulling concentrated solids off the base of the tank. I will run the gravel on timer and allow gravity to return water from the DWC to the fish tank.

[Murray Hallam]

You are correct Gina, It all depends on the stocking density of the individual system. When I was in your fair country a few weeks ago it became apparent to me that Tilapia have very little commercial value there because of the abundance of low cost imports of Tilapia fillets.  I had not realised that before.
It makes sense that the average small farm would therefore carry low densities of fish, only sufficient to power the system and have the beauty of a balanced AP system, that is an ECO system.  Media beds if operated flood and drain (ebb and flow), draw in massive amounts of oxygen from the atmosphere every time the beds drain on each cycle. The oxygen requirements of the various bacteria and the plant roots are well provided.

If for any reason an individual decided to stock many more fish in the system the need may arise for additional filtration. My preferred solution to that would be to add more media beds = (more filtration)  Happily we grow great veggies in those media beds that are in reality, big very efficient bio filters.

[Chris Smith]

I have been using media beds as a filter for for my raft system for a year now. I have found that it is beneficial to have a form of fine solids removal before the rafts. My worms consume most waste in the media beds but some fines get through. Without filteration the fines collect on the first sets of roots in the first trough. I have a swirl filter between the media and the rafts.

I have 4 troughs that are 40' long, a 1000g FT with approximately 200-250lbs of fish. I first replaced my net tanks with a 4'x6'x12" media bed filled with cinder. This worked great for 6 months then I noticed sludge on roots in the first trough. Upon this discovery I added a swirl filter before the first trough and it began catching a lot of fine sediment. I then added a second media bed of the same size and the fines in the swirl filter cut in half. I am working on adding 2 more media beds and hope to have very little fines after they are complete. Through my trial and error I now think it to be best to have 25%-30% of the entire system grow space be media as a filter for rafts and towers. I still have a clarifier in the system which I am manually dumping into a media bed. I will soon add a pump and timer and send to poo to one of the beds.

Of course stocking densities and feeding schedules have a huge role in the amount of filtration needed. Worm population has a roll in the solids being processed, but they will self regulate their population according to the amount of food available in time. I have found that quality, small pellet, fish feed will be digested better by the fish resulting in less solids and require less filtration overall. When I feed the fish a lot of lettuce they make a huge mess that can clog up media short term, so the chickens get all lettuce scraps now.

[Kobus Jooste]

Just to clarify - I may have been a bit brief in the top post.  I saw the effect of lots of solids on nitrification in my test system when it was heavility stocked.  Always ammonia around in the tests.  For this purpose I would go with the clarifier fed worm beds to deal with the solids (primary filtration) and then have the lighter loads hitting the second bank of gravel beds first, and then the net filters.  This gives you the seperation between mineralisation and nitrification activity in your unit.  You can also add more gravel beds as Murray said, but I'm a fan of layering and segregation rather than expansive design.  Murray's suggestion will be simpler to construct and operate, but it may (or not) have a larger footprint.  In my test system I'm slotting net tanks (sump filters) underneath my gravel beds in order to save space.  I am a firm believer in catering for nitrifyers and mineralisation seperately though.  In the design I mentioned before, I will also ensure that the media bed that is clarifier-fed returns the nutrient enriched water to the system.  It is not an export.

 

That said, I fully agree with Murray in terms of the simplicity of a lower density gravel and DWC system.

[Sahib Punjabi]

Hello Murry Sir,

 

Thank you for visiting last month :-)

 

As you saw on your recent visit to my small Aquaponics research farm, I have established a "Hybrid" Aquaponics system, an ECO system that works very well. This has been established by using stocking lower fish density as learned from the Friendly's Micro System Manual, making minor use of filtration and mineralization techniques learnt from the UVI systems and by combing numerous methods of growing in Aquaponics, DWC, NFT's, Re-circulating Wicking beds, Vertigro Pots and other Vertical pots and towers, and Media beds (the use of which I had learned from both yourself and Joel Malcolm fantastic work in this field), and with minimum power use for the size of water pumps and oxygen, I have been able to establish and enjoy, as you say, "the beauty of a balanced AP system, that is an ECO system." Yes as you correctly state "Media beds if operated flood and drain (ebb and flow), draw in massive amounts of oxygen from the atmosphere every time the beds drain on each cycle. The oxygen requirements of the various bacteria and the plant roots are well provided."

 

I also use excess AP water to supplement the watering my external ground plants...you saw the results...just beautiful. So the solution is not only fishing densities but a combination of the numerous AP learning to establish a balanced AP system, that is an ECO system, regardless of the size of the system, and adjust for increases in solids by either adding additional grow beds or using the AP water for supplemental watering of ground organically grown plants.

 

God bless, 

Murray Hallam said:

You are correct Gina, It all depends on the stocking density of the individual system. When I was in your fair country a few weeks ago it became apparent to me that Tilapia have very little commercial value there because of the abundance of low cost imports of Tilapia fillets.  I had not realised that before.
It makes sense that the average small farm would therefore carry low densities of fish, only sufficient to power the system and have the beauty of a balanced AP system, that is an ECO system.  Media beds if operated flood and drain (ebb and flow), draw in massive amounts of oxygen from the atmosphere every time the beds drain on each cycle. The oxygen requirements of the various bacteria and the plant roots are well provided.

If for any reason an individual decided to stock many more fish in the system the need may arise for additional filtration. My preferred solution to that would be to add more media beds = (more filtration)  Happily we grow great veggies in those media beds that are in reality, big very efficient bio filters.

[Murray Hallam]

It just makes so much sense to retain the solids in the system. There is so much nutrient that can be captured and utalised. We have observed the collection of very fine material on the bottom of the grow troughs that it is about 1/8" deep after a year.
The lettuce roots are clean and the very fines never seem to get any deeper.  I believe there is bacterial breakdown happening although I do not have any science to back that notion. It is not a problem and not necessary to clean up.. Not yet anyway and this system I am talking about has been running for two and a half years.
Once again, we do not stock our systems heavily.

[Kobus Jooste]

Marray - I think that you will concur (perhaps?) that our views of water also ultimately influences our design philosophy.  Solids removal is associated with water wastage - something a South African or Australian would not dare do.  Part of the U.S. have lakes larger than the province in which I live thus perhaps their ample water supply influences their view on system design more than what us dry land lot can understand.  When I first designed my commercial unit concept, it was UVI style rafts with the filters substituted with gravel beds not just because of the nutrient retention, but because I was not willing to be washing filters every second day.

[Sylvia Bernstein]

Great discussion you have started here, Gina!  Kobus, can we assume that you are planting in all of your media beds - what you refer to as the "worm beds" and the second bank of beds?  If so,  do you see different growth results in each bed?  And if so, so you plant differently in them?

Kobus Jooste said:

Just to clarify - I may have been a bit brief in the top post.  I saw the effect of lots of solids on nitrification in my test system when it was heavility stocked.  Always ammonia around in the tests.  For this purpose I would go with the clarifier fed worm beds to deal with the solids (primary filtration) and then have the lighter loads hitting the second bank of gravel beds first, and then the net filters.  This gives you the seperation between mineralisation and nitrification activity in your unit.  You can also add more gravel beds as Murray said, but I'm a fan of layering and segregation rather than expansive design.  Murray's suggestion will be simpler to construct and operate, but it may (or not) have a larger footprint.  In my test system I'm slotting net tanks (sump filters) underneath my gravel beds in order to save space.  I am a firm believer in catering for nitrifyers and mineralisation seperately though.  In the design I mentioned before, I will also ensure that the media bed that is clarifier-fed returns the nutrient enriched water to the system.  It is not an export.

 

That said, I fully agree with Murray in terms of the simplicity of a lower density gravel and DWC system.

[Murray Hallam]

So very true Kobus !  Wasting water is an anathema ....where we come from that is for sure.

Kobus Jooste said:

Marray - I think that you will concur (perhaps?) that our views of water also ultimately influences our design philosophy.  Solids removal is associated with water wastage - something a South African or Australian would not dare do.  Part of the U.S. have lakes larger than the province in which I live thus perhaps their ample water supply influences their view on system design more than what us dry land lot can understand.  When I first designed my commercial unit concept, it was UVI style rafts with the filters substituted with gravel beds not just because of the nutrient retention, but because I was not willing to be washing filters every second day.

[Gina Cavaliero]

Thanks for your thoughtful responses.  Yes, ultimately the goal is to establish the balance in the system so that all waste is effectively metabolized and not removed.  The issues we have seen with DWC with low stocking densities without solid removal and capture is just what Chris explained he observed on the plant roots immediately after the gravel filters/beds, making either removal or efficient filtering optimal.  Dependent upon the longevity of the plant in the system at that point absolutely affects the plant's ability to take up nutrients as the root mass becoming occluded with solids.  A lettuce for instance with a 4 week span would be ok however plants with longer duration in the system eventually would become so clogged with waste, the plant would eventually die from the inability to take up nutrients. Root growth was observed above the water line demonstrating it was attempting to establish new root growth to make up for the unhealthy root mass below. This supports either the need for either solid removal even in low density DWC systems.  The same is being noted to a degree in a small test system we operate without solids removal.  However the small systems due to their minimal stocking densities are not well representative of large commercial sized systems, even though the ratios are consistent in density to system size.   They are good in practice and theory, but to really effectively evaluate the commercial application, we need to be able to consider it on the same scale.

[Kobus Jooste]

Sylvia, this is a system on paper that I am trying to get built at the moment (another meeting with the potential developer on Monday).  The worm beds are actually relatively small, and also designed to have worm feeding stations where all or some of the harvesting offcuts can be placed in order to recycle nutrients further.  The idea with them is to leave them unplanted at the moment, and only plant the next set of beds downstream of them.  If I have to think of a plant that I willwant to put in the worm beds, my current flavour of the day - passion fruit, definately comes to mind.  It is a nutrient hog and I can picture them doing really well in that first stage gravel filter. 

 

 

Sylvia Bernstein said:

Great discussion you have started here, Gina!  Kobus, can we assume that you are planting in all of your media beds - what you refer to as the "worm beds" and the second bank of beds?  If so,  do you see different growth results in each bed?  And if so, so you plant differently in them?

Kobus Jooste said:

Just to clarify - I may have been a bit brief in the top post.  I saw the effect of lots of solids on nitrification in my test system when it was heavility stocked.  Always ammonia around in the tests.  For this purpose I would go with the clarifier fed worm beds to deal with the solids (primary filtration) and then have the lighter loads hitting the second bank of gravel beds first, and then the net filters.  This gives you the seperation between mineralisation and nitrification activity in your unit.  You can also add more gravel beds as Murray said, but I'm a fan of layering and segregation rather than expansive design.  Murray's suggestion will be simpler to construct and operate, but it may (or not) have a larger footprint.  In my test system I'm slotting net tanks (sump filters) underneath my gravel beds in order to save space.  I am a firm believer in catering for nitrifyers and mineralisation seperately though.  In the design I mentioned before, I will also ensure that the media bed that is clarifier-fed returns the nutrient enriched water to the system.  It is not an export.

 

That said, I fully agree with Murray in terms of the simplicity of a lower density gravel and DWC system.

[Kobus Jooste]

Love your observation on scale.  I'm currently gearing up to take the 27 square meter bio-fertilizer unit I designed to full scale, and I'm already getting the funders used to the idea that what I have reported to them in terms of yields and operating parameters to date was really just proof of concept and that we need to press play on a full scale unit before we will know what will happen.

 

So many people think that what they see in a small system will also be the rule in a mega unit.  They fail to understand the dynamics of water movement and the forces that hold different size particles in suspension.  That is another reason why I like to have a couple of stages from fish tank to DWC - enough to break the velocity of the water and get as much of the suspended stuff as possible to settle.   

Gina Cavaliero said:

Thanks for your thoughtful responses.  Yes, ultimately the goal is to establish the balance in the system so that all waste is effectively metabolized and not removed.  The issues we have seen with DWC with low stocking densities without solid removal and capture is just what Chris explained he observed on the plant roots immediately after the gravel filters/beds, making either removal or efficient filtering optimal.  Dependent upon the longevity of the plant in the system at that point absolutely affects the plant's ability to take up nutrients as the root mass becoming occluded with solids.  A lettuce for instance with a 4 week span would be ok however plants with longer duration in the system eventually would become so clogged with waste, the plant would eventually die from the inability to take up nutrients. Root growth was observed above the water line demonstrating it was attempting to establish new root growth to make up for the unhealthy root mass below. This supports either the need for either solid removal even in low density DWC systems.  The same is being noted to a degree in a small test system we operate without solids removal.  However the small systems due to their minimal stocking densities are not well representative of large commercial sized systems, even though the ratios are consistent in density to system size.   They are good in practice and theory, but to really effectively evaluate the commercial application, we need to be able to consider it on the same scale.