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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.

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Thanks for clarifying that, Kobus.  That was what I thought was happening based on what TC had said but I wanted to hear your thoughts as well.

 

When I had one gravel bed and all the flow going through it I had a lot of fines making it to the first roots in the trough. I added a swirl filter to catch the fines and act as surge buffer. When I added a second gravel bed I had to split the flow between the two beds. I had to make smaller siphons because the larger one would not work with the reduced flow. Once I had the second GB installed I had far less fines in the swirl filter. After reading some of the posts I am wondering if I'm getting less fines because of smaller siphons? I always assumed the larger gravel bed space and more worms were eating  up more poo which left fewer fines in general.

The fines that I get out of the gravel beds is very different from what is leaving the fish tank. The stuff in the bottom of the swirl seems more like worm castings than fish poo. When I clean out the filter I put the muck in the gravel bed and the worms eat it up by the next day so I guess its not castings.

 

The swirl filter has been a very important component of my system as it serves two functions. It does the obvious, letting fine sediment to fall out of the water before reaching the troughs. It also does another very important function of regulating or buffering the rush of water leaving the gravel beds. The filter has the capacity to take the flood from both beds and slowly let the water flow to the troughs. This keeps a near constant flow of nutrient rich water entering the troughs. This buffering effect makes the filter more effective as it keeps the water velocity slow.




Chris Smith said:

 

One thing to remember when you reduce your flow back to the FT is that you still need to be thinking about nutrient export from the FT and nitrification of the FT water whether the plants need the nutrient boost or not. Just take tests while you reduce the flow and see where the breaking point is for your load of fish (which I am sure you planned on doing anyways).

As for cheaper ways to recirculate the trough water you may look at adding a couple small airlift lines off of your existing air supply. Big diameter pipe can move a ton of water with very little air. Basically keep your water pump line the same but reduce the size of the water pump to something that suites your new desired flow for the FT, then add a "recirc line" for the troughs where the line comes out of the sidewall or one tank, down to the ground, over to the next trough, then back up and through the wall just atthe water surface(half of the pipe under water, half above). Add an airlift line in the bottom of the trough you want to return the water to and you're done. A 4" line and 2cfm into the bottom of the pipe will give you massive flow :)

Sorry for the ramble, just a thought if you are gonna experiment with this modification due to your crazy high electricity cost. 

Great idea Ryan. I think I will make an airlift to help loop some of the trough water. I am actually in the process of installing a solar powered looping pump to save on power.
Has anyone done an analysis of what nutrients "solids" are primarily made up of?

Having a DWC I think (hypothesising) would increase the presence of beneficial bacteria, due to aeration and addtional habitat surface area of the media. Adding worms is genius, and would make for more efficient nutrient utilization. Fresh water muscles could also be a useful introduction, thereby making nitrates and phosphates more soluble. 

Nutrient cycle and sludge production during different stages of red tilapia (Oreochromis sp.) growth in a recirculating aquaculture system

Gholamreza Rafiea and Che Roos Saad

aDepartment of Fishery and Environmental Sciences, Faculty of Natural Resources, University of Tehran, P.O. Box: 31585-4314, Karaj, Iran bDepartment of Agrotechnology, Faculty of Agriculture, Universiti Putra Malaysia, Serdang, Selangor, Malaysia

 

Abstract

The nutrient cycle of input feed and sludge production was evaluated for five stages of red tilapia growth in a recirculating aquaculture system. Five weight groups of red tilapia, 20±0.00 (20), 39.70±0.44 (40), 80.38±0.41(80), 113.62±1.92 (120), and 177.67±1.81 (180) g in triplicates were selected as treatments and randomly introduced to the experimental units (75 fishes/unit) and cultured for a 3-week period. The body weight and biomass of the fish were correlated with the assimilation rates of some minerals supplied by the input feed. It was estimated that red tilapia could capture on average, 11.46% Fe, 13.43% Zn, 6.81% Mn, 3.55% Cu, 26.81% Ca, 20.29% Mg, 32.53% N, 7.16% K, and 15.98% P of input feed during a culture period (from 20–200 g). The sludge settled over the hydroponic troughs could capture average rate of 23.93% Fe, 86.05% Mn, 46.17% ZN, 21.49% Cu, 15.71% Ca, 88.87% Mg, 5.55% N, 5.85% K, and 17.90% P of input feed in each experimental unit. The dry matter of sludge showed significant differences (P<0.05) among treatments and ranged from 5.00% to 10.00% of dried input feed. The concentrations of total nitrogen, phosphorus, and magnesium in water were not significantly different (P>0.05) among experimental fish groups at the end of experimental period and continuously increased during the 21-day experimental period. The electroconductivity (EC) of water continuously increased during the experimental period. The pH of water decreased in all treatments at the end of experiment. The results of this study predicted the fate of input feed nutrients in a representative recirculating system where the particular diet was used. It was also demonstrated that the aquaculture effluent carries out a large amount of nutrients, including solids form which can be accumulated in the hydroponic(s) troughs.
PRELIMINARY EVALUATION OF ORGANIC WASTE FROM TWO AQUACULTURE SYSTEMS AS A SOURCE OF INORGANIC NUTRIENTS FOR HYDROPONICS

 

J.E. Rakocy, D.S. Bailey, R.C. Shultz, J.J. Danaher

 

Abstract

 

Aquaponic (AP) and greenwater tank culture (GW) systems have been developed at the University of the Virgin Islands for the intensive production of fish. Aquaponics is the combined culture of fish and hydroponic plants in recirculating systems. Greenwater tank culture utilizes a mixed suspended growth process involving autotrophic and heterotrophic bacteria and phytoplankton for water treatment. Large quantities of organic waste are removed from these systems daily. This organic waste is a potential source of inorganic nutrients for hydroponics. Samples of AP and GW sludge were continuously aerated for 29 days to facilitate mineralization. EC increased from 4.6 to 6.0 mS/cm (3200 to 4200 ppm TDS) in AP sludge and from 1.8 to 3.4 mS/cm (1260 to 2380 ppm TDS) in GW sludge. Thirteen nutrients (Ca, Mg, K, NH4, NO3, PO4, SO4, Fe, Mn, Zn, Cu, B and Mo) were compared to a standard hydroponic lettuce formulation (Resh, 1995). Final concentrations of six AP nutrients and eight GW nutrients exceeded standard hydroponic values while concentrations of seven AP nutrients and five GW nutrients were less than standard hydroponic values. Na and Cl concentrations in AP and GW sludge exceeded the recommended values for hydroponics. Hydroponic nutrient formulations derived from aquaculture waste may require dilution, supplementation and source material (fish feed) without salt.



Miguel Afonso said:

Having a DWC I think (hypothesising) would increase the presence of beneficial bacteria, due to aeration and addtional habitat surface area of the media. Adding worms is genius, and would make for more efficient nutrient utilization. Fresh water muscles could also be a useful introduction, thereby making nitrates and phosphates more soluble. 

 

Here is a pic of the fines filter i use, just after install. I used a 30 gallon tub with orchard netting. The gravel beds are on a timer with a "perforated standpipe", so the flow from the media beds is nice and slow. The original one of these i tried was a 55 gallon drum on its side, it worked great. ...for this redo i wanted to keep the fine filter or "polisher" closer to the media beds, so i ran the inflow  pipe under the rafts to the far end, there i have a T and two 90s to direct the flow back toward the drain.

  

 

 

 

 

 

 

perforated stand pipe

Sorry Kobus hadn't been able to jump back into the discussion for the last few days, I was busy installing trough liner and putting holes in rafts. I see you refer to using a clarifier and net filter, I was certainly hoping with the inclusion of media beds with a DWC the need for those would have been eliminated. Shucks! Oh well. I need to read further on into the threads to catch up. My experimental system is a 300 gallon tank, 100 tilapia (slightly above fingerling size), 4- 8x4 troughs and an md2 water pump. I was hoping to include probably 2-3 x100 gallon rubbermaid containers as media beds between the tank and troughs. I'm not settled yet on the methodology for the entry and exit of the water though.

Kobus Jooste said:

Nigel - I also enjoy it as I am busy trying to salvage a large system design that I started a few years ago but which ran out of momentum when the current business partners I have pulled out.  It was fortunate in a way, as I had time to study my small mixed design system and make some important observations.  The one that I think will bug large scale mixed systems the most is water velocity.  I'm not so sure what the average person in this forum has in terms of experience with large flood and drain, but it is an impressive sight.  With enough of a head, you can wash gravel with such a system (as I ended up doing during tests).  A single line of gravel beds will not stop the solids.  You should also not have too much of a drop between your components because of the velocities you create.

 

 

These beds are 7.5 meters long, about 1.5 meters wide and just under a meter deep.  When those siphons let go, the outflow almost fills a 110 mm pipe.  Nothing but the largest solids will stay behind.

 

My modification: Run clarifiers on fish tanks.  Send clarifier solids via timed pumps to the first bank of gravel beds.  Clarifier outflow and the water that drain back out of the first bank of gravel beds goes to a second bank of gravel beds.  Between the second gravel beds and the DWC, I am going to put a fine net tank or similar to try to trap the smallest fines.  I will steer away from siphons between beds.  They are just too powerful in this design.  Simply use a perforated stand pipe design and use small level changes to "push" the water along from the gravel beds through to the DWC.  Any serious drainage flow and you will likely end up with sediments / solids in the DWC.

Nigel - I do not have a full scale system to base my observations on, thus you are welcome to consider it opinion and not an absolute truth.  I think that a relatively well stocked commercial system, you are still going to end up with fines that may make it through mutiple gravel beds if the water flow is high.  I see two options - slow the water down in extensive gravel beds before you get to DWC, or have spome setting tanks.  My design for the clarifier sends the solids to primary gravel beds daily on timer, thus you do not have to clean and discard from the clarifier, and the design for the net filyer is also no clean, thus it will e lower maintenance than a UVI style set-up.  I am personally going back in design philosophy to multi-stage, but not to the high maintenance way of doing it.

Nigel Clement said:
Sorry Kobus hadn't been able to jump back into the discussion for the last few days, I was busy installing trough liner and putting holes in rafts. I see you refer to using a clarifier and net filter, I was certainly hoping with the inclusion of media beds with a DWC the need for those would have been eliminated. Shucks! Oh well. I need to read further on into the threads to catch up. My experimental system is a 300 gallon tank, 100 tilapia (slightly above fingerling size), 4- 8x4 troughs and an md2 water pump. I was hoping to include probably 2-3 x100 gallon rubbermaid containers as media beds between the tank and troughs. I'm not settled yet on the methodology for the entry and exit of the water though.

After opperating and modifying my Friendly style systems and various other media systems I now have a new design philosiphy for intrgrating larger systems that will likely reduce much of the solids issue. If/when I get the chance to build another system this is what I would do...

I would combine the fish tank and sump and place it in the ground so the top tank is just below the trough elevation. The water pump can only draw water off the top 1/3 of the tank so the fish can never run out of water in the bottom 2/3. The tank has to be sized so there is enough capacity in the top 1/3  to function as the sump normally would. This means enough water to float the system and be able to handle filling media beds while having some extra working reserve.

Aeration and retention time in the tank helps break down the solids. Solids that go through the impellers of a pump are nearly liquefied taking care of a lot of the problem. The pump sends water to a series of gravel beads via an indexing valve. The indexing valve would be advanced by using a timed(1 min on & 15 min off) solenoid teed prior to the indexing valve and return to tank. The gravel beds will have a an overflow and a slow bleed back allowing full drainage within 10-15 minutes. The slow bleed back should be slow enough for the solids to be trapped and minimal fines to get past. There will likely need to a polishing off filter but it will likely be minimal. A net tank with gammarus would likely do. The water them goes to the rafts and returns to tank. NFT, towers, and other sub systems can be run by pumping out of a trough and returning the water to the trough slightly down stream.

Since I do not have the luxury of building a new system I have to continue to improve on what I have. I am seriously considering creating another in ground fish tank and turning my existing tank into a gravel bed. Don't tell my wife though.

Just keep in mind that most solenoid valves require a lot of pressure to operate properly.  The solenoids need far more pressure than the low flow Aquaponics Indexing Valves and definitely more pressure than the Gravity modified versions that I'm able to run with a 50 watt pump being turned on/off by a timer. 

 

Anyway, thanks for that description there Chris.  I had been thinking that if I were to do such a system, I would probably do something pretty similar but I would run the pump on a timer (My 300 gallon system has 600 gallons of media beds and the water level in it doesn't fluctuate much with the pump running a 9 minutes on, 1 minute off cycle so each of the 6 beds gets flooded once per hour.)  The flow out of the beds would provide constant water flow to rafts easily enough and a settling/net tank of sorts would likely handle the fines coming out of the beds.

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