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 Post subject: Nitrate In Aquaponics
PostPosted: Nov 21st, '19, 16:08 
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I have a 5,000l aquaculture tank with bottom drain that gravity flows into two diy 200l radial flow filters, to a Cetus seive, then to a moving bed IBC with about 200l of knock-off kaldnes media and finally a IBC sump. This system in not currently in operation but in the past I have reared 1000's of Goldfish as well as livebearers and Angelfish in the summer months.

I am planning to convert the system to an aquaponics system primarily to reduce nitrates but also to grow vegies
for my family. At peak loading capacity I used to change 50% of the water per day to keep my nitrates at a desired level of <30ppm.

I have spent numerous hours researching papers by Rakocy and Lennard and I have a chemistry background and 30 years as a serious fish enthusiast so have a good grasp of water chemistry but at this stage no practical aquaponics experience. I have two queries:

1. I will grow a variety of vegatables, but as I alluded to nitrate reduction is my primary goal. Does anyone know what vegetables grow the fastest in aquaponics keeping in mind I will be maintaining relatively low nutrient levels?

2. I am considering adding an extra filter with bird netting like the UVI system. I note that that they use cleaning frequency to partially manipulate nitrate levels. I have never heard of any other RAS or aquaponics deliberately maintaining solids in the system to promote anaerobic decomposition. Anyone know why UVI use this specific technique?


Lastly, whilst on the nitrate train. This may be the only useful knowledge I can impart at this stage- I see API test kits are very popular on this board. Most of their kits are OK for hobby use but their nitrate kits are utter garbage. I have years of experience with these kits and have done side by side tests with 5 or so kits. Results vary wildly up to 100%. If you use results from an API nitrate test kit to diagnose problems or make management decisions, you can be led astray. The best thing is to make a few reference solutions of known concentration (KNO3 is probably the easiest to source for this). At least then you can be sure you are at least in the right ball park.


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PostPosted: Nov 21st, '19, 16:32 
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fish in the hills wrote:
2. I am considering adding an extra filter with bird netting like the UVI system. I note that that they use cleaning frequency to partially manipulate nitrate levels. I have never heard of any other RAS or aquaponics deliberately maintaining solids in the system to promote anaerobic decomposition. Anyone know why UVI use this specific technique?

Just my assumption, but I would suggest that it's to assist in controlling pH... ie: Aerobic activity lowers pH, anaerobic raises it.

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PostPosted: Nov 21st, '19, 19:01 
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With all that filtration a large Deep Water Culture Grow Bed (DWC) or a floating raft bed to grow your plants might work for you

Plants that use your nitrates will depend on time of year

Now Tomatoes and Cucumbers and Sweet Capsicums are my go to choices and Silverbeet for the chooks

I grow Peas and Brassicas during the cooler months

Leafy greens and grasses would be the obvious choices to use nitrates

But just grow what you like to eat

I like Suyo Cucumbers no mildew and love the hot weather

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PostPosted: Nov 22nd, '19, 07:05 
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Cheers for the replies :cheers:

Mr Damage- I'm guessing denitrification for pH control could save a fair bit on buffers in some systems. I'm just curious as to why they are using such a crude method . In the UVI system most of the fine solids in the net would be aerobically decomposed (costing money on extra aeration) with only a small fraction breaking down anaerobically. I think I'm still getting my head around the compromises between plant and fish growth that seem inherent to aquaponics.

Terra- I would love to play around with DWC but I live on steep terrain. I have about 20m of NFT channel so going to start with that. Leafy greens seem an easy starting point and we eat a heap of silverbeet, thanks for the suggestion....and peas, yes! Those things grow like lightning. Have you had good growth with them in aquaponics? They do however fix their own nitrogen so may be counter productive for my purposes.


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PostPosted: Nov 22nd, '19, 12:05 
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The other aspect to consider in regards to the UVI system and why they would employ the shadecloth method and anaerobic conditions, is possibly denitrification, as you suggested, to try and bring the Nitrogen level back to within a reasonable ratio with the mineral elements in the system.

They used a fairly crude, and I would suggest inefficient system of demineralising the fish waste solids collected, and being solely a DWC system, as opposed to the hybrid systems most commercial operations now run, there are no media filled grow beds, with all the benefits that come with them, to assist in the break down of the solids and release of mineral nutrients. Consequently, it's likely they would've ended up with an imbalance between Nitrogen and other elements, and high Nitrogen levels will lock out certain elements.

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PostPosted: Nov 23rd, '19, 17:57 
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Ive had peas turn into a jungle mess so yes they grow well in my system

I trellis them up on mesh now

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PostPosted: Nov 24th, '19, 10:04 
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fish in the hills wrote:
Lastly, whilst on the nitrate train. This may be the only useful knowledge I can impart at this stage- I see API test kits are very popular on this board. Most of their kits are OK for hobby use but their nitrate kits are utter garbage. I have years of experience with these kits and have done side by side tests with 5 or so kits. Results vary wildly up to 100%. If you use results from an API nitrate test kit to diagnose problems or make management decisions, you can be led astray. The best thing is to make a few reference solutions of known concentration (KNO3 is probably the easiest to source for this). At least then you can be sure you are at least in the right ball park.

I also read your other post.
I only use the API test for PH,Ammonia & Nitrite.I can't tell the difference between 40ppm & 160ppm on the API colour chart & when I compared it to a different brand,the the difference was huge.API showed dark red :dontknow: the exact number & the other one showed 15ppm.
fish in the hills wrote:
I am planning to convert the system to an aquaponics system primarily to reduce nitrates but also to grow vegies
for my family. At peak loading capacity I used to change 50% of the water per day to keep my nitrates at a desired level of <30ppm.

30ppm isn''t that low in aquaponics,so it's achieveble in your case.When Nitrates are too high it could be put down to 2 things,too many fish or not enough grow bed.
I would go for leafy greens to keep the Nitrates down.

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