Backyard Aquaponics
A place to discuss aquaponics

Hi Everyone,

I'm new here and also new to the concept of Aquaponics. I started researching the design part quite recently because I would like to implement the concept in my hometown. I'm more interested in DWC Aquaponics and would like some help on the calculations as I seem to be stuck in a rut.

If for instance, I wanted to grow 4000 lettuce/month on a staggered cycle of 1000 lettuce/week, as per my calculations I would require 200 sq.m. of growbed space, 1000 kgs. of fish (Tilapia) and 52,000 litres of water.

It seems to me like my calculations went wrong somewhere. Can someone please advise?

Thank you so much.

I'm not sure how much Tilapia eat. 60 to 100 gms of feed per square meter of DWC raft per day is what UVI uses as a goal for growing their crops. You should be able to back calculate from that if you have information on how much feed to give the Tilapia. You'll have to filter solids out before the DWC or they'll smother the roots (otherwise you could probably have fewer fish or feed less). Some of the nutrients in the excess solids can be recaptured using mineralization - these can then be added back in and would reduce the number of fish needed (I have no idea what size of reduction).

I think I've also read that they shoot for 1/2 lb of fish per gallon of water (roughly 226 grams in 3.8L). Recommended DWC growing area to fish tank volume that I've seen listed is 11.5 to 1.

Thanks for the feedback Scotty. Appreciate the help.

I'm not as certain about as I'd like to be about the last two numbers - I'm not sure where they came from anymore. The stocking density seems too high to me but I'm not used to Tilapia.

Here's some of the UVI information - http://www.uvi.edu/files/documents/Research_and_Public_Service/WRRI/UVIAquaponicSystem.pdf

Your example has a similar growing area to the UVI system - theirs is listed as 214 sq ft. Check the top link out once you hit the google search page. You'll notice some other information on variations in raft hole spacing for different kinds of lettuce that will affect the calculations as well.

https://www.google.com/search?q=dwc+racocy+aquaponics+stocking+levels&ie=utf-8&oe=utf-8#q=University+of+the+virgin+islands+bibb+lettuce+aquaponics+stocking+levels

The UVI model was based around fruiting crops, the feed rates would be much lower for lettuce you would think. The UVI is also very dated, its extreme good information, but dated. Many things have changed or advanced in the last 10 years, more so in the last 5.

Point taken on the age of these studies Charlie. I think that the lettuce study of theirs said the optimum feed rate was 57 gm/day/sq meter of lettuce growing area but I may be wrong. I think it's in the second link of my last post .

It would be interesting to see the plant specific information Dr. Lennard has and his approach to this but this might not be large enough scale for his targeted approach to calculating the information to make sense anyway. I can guess at the pH adjusting/buffering system but haven't seen how he does the calculations for his feeding method. There are probably some mods to the UVI system approach as well but I haven't seen them either .

I saw that UVI tried to carry Nitrates in the high 40's (48 or 49 I think) and that Dr. Lennard determined going lower wound up giving additional nutrient deficiencies (this is an oversimplification and not his actual wording). I think the other nutrient levels were listed, if so it might be possible to figure a close approximation for a specific approach based on this and the salts provided by the buffers/ph adjusters.

Cheers

Hi guys,

The reason I feel the calculation has gone wrong somewhere is because as per the same guidelines I would require 12 kgs fish feed / day

That doesn't feel right to me because those lettuce might turn out to be quite expensive. Not sure though.

I'm quite confused.

Would these guidelines be the same in DWC too?

Charlie can you suggest a better / accurate method to calculate this other than the UVI / Rakocy approach?

It would surely be a big help

bump

This thread and some of the links off it have some really good information on feed rates - http://www.backyardaquaponics.com/forum/viewtopic.php?f=1&t=26361