DWC ... Floating Raft...

[Nathanael]

DWC... has anyone been successful with it in a smaller backyard system and is it worth the go? 

This monday I will be finishing my 2nd 3'x6'x16" gravel growbed and I'm trying to decide if its worth adding a small DWC to complete my system for the time being. 

The order for the DWC would be as follows: Pond - Swirl filter (completed) - Bio filter (completed) - Floating Raft - back to pond via overflow. 

DWC system would be about 100gls. 

Any thoughts about this? Is it worth the go? 

If so, what should I focus growing? We love fast varieties of lettuce, strawberries, spinach, chard. to name a few.

[Sam Burton]

Jim, I'll give it a try.  I'm in the process of moving house and all but one small system is in pieces.  Will take pics and post them once everything is set back up.  In the meantime, my swirl filter is just a knockoff of the 'daveponics' one. 

[Jim Fisk]

Hey Rob,

I have the same underground common drain and quickly learned (first time more than one siphon went off at the same time) that you must have simple vents every few bed drains just like in any household plumbing to allow air in in order to avoid siphon issues. Every drain in your house would have the same issues without the vents and that is why they are in the plumbing code. (actually the plumbing code requires a vent for every trap) Seems much simpler (and cheaper) than the air blasters and allows for tight direct plumbing to the drains. This also avoids possible fungus gnat hang outs which is a problem wherever they can find water or dampness, ie open drains. You can place screens over the vents, which should terminate above the gbs to discourage any gnats from getting into the drain system. If you are having siphon issues on a common rail drain you need more vents.

Here is a pic showing a typical vent coming up from the 1 1/2" drain pipe to just above the deck and 90 between my ibc gbs and 90 again up the back wall. You can never have too many vents. That keeps the individual siphons from being affected by the other siphons. Vents can be much smaller than the main plumbing as air moves much easier than water. Mine are 1":

Rob Nash said:

the 'nash blaster' is to the return pipe under the grow beds that the bell siphon drains into. my intern named it that, because he insisted i take credit for coming up with it... at least as far as we know.

its just an air release pipe, with out it you will have air lock issues and the siphon will fail.

its a 4" section of pipe installed on the 2" drain pipe. This provides a place for the water to drain w/out splashing out of the pipe ...which is not glued to the bottom of bell siphon.

[Pat James]

Jim, For some reason I had thought your drain lines were larger than 1 1/2 inch. I know you use 1 inch drains on your growbeds. I got stopped again from doing more work on my plumbing (raining).

[Jim Fisk]

Hey Pat, Mine are 1 1/2" but you certainly could go 2". The bigger the drain line the less vents you need to avoid unwanted siphoning. (of course that happens when there is no air above the water in the drain line) I am only draining 5 ibcs per drain line which very rarely will all drain at once but the odds are it will happen eventually. Kind of like the electric service in your house. A 100A service might have 200A worth of breakers but the odds are so against all loads being on at once......

The only indication that I don't have quite enough vents is the times I walk in the GH and find one of my light weight bells (S&D grade) sitting 3" or so above the gravel guard and cocked to the side where it landed due to a strong burp in the drain line. In other words I need to add one more vent close to the last ibc gb. Or just add a little weight or cover, etc. to the bell. That pretty pink handle is only .97 at Lowes as is the powder blue so that's what my customers are getting these days.I am running the last 3 gbs with no vent. In your house EVERY drain requires a vent for a good reason so "what was I thinking?" Pretty easy to correct. Keep in mind that every siphon that is not in a siphon/drain phase acts as another vent IF you don't have a trap in the siphon drain line. (another reason I am against "traps") Not quite good enough though in real practice so venting the drain line very near each "T" would be the safest. Remember to end the vents above the media bed to avoid occasional back flow. Even 1/2" vent lines would do. It takes almost nothing to break that unwanted siphoning and back pressure.

[Jon Parr]

I avoid white on the planting surface, because the glare sucks to work around, and plants don't like light on their undersides. I paint mine a dirt-brown color, good results. I've had trouble with seedlings getting their roots  to cross the dry space between "raft" and water, but some things like bare-root strawberries are perfectly suited to that style. Quick crops like lettuce and leafy greens have done better on floating rafts.

Nathanael said:

@Jon Parr I also had one more question. When you mentioned a board supported on top of the DWC how would you describe a good way of doing that? I've seen the new pontoon version.... Would using 1/2" plywood painted white do the trick? Then just lay it on top of my DWC frame?

How much space around the edge would I need to leave so that there is plenty of oxygenation under the plants. And also, do you all recommend 2" or 3" net pots.

[John Malone]

Hello Rob,

I've started construction on the first of, hopefully, two DWC beds.

I have a follow up question regarding the polisher.   How often do you have to clean the netting and how difficult is it to do?

What is involved?

Thanks,

John

Rob Nash said:

John,

the polisher is a 30 gallon tuf tub with orchard netting in it.

flow into the bottom and out the top... it serves as a final filter to 'polish' the water before entering the rafts.

 

[Jon Parr]

John Malone, there are a lot of variables to answer that question. It depends a lot on fish type, fish density, feed rates, feed type, temp, systme design, and whether or not you are cultivating detritus eaters like scuds.

I have seen systems that literally need no cleaning of mesh tank or settling tank ever; using tilapia, low density, 75F, scuds, and high quality fish food (low ash and indigestible feather-meal and cellulose). On the other hand, I have had large tanks of carp that I fed rabbit food at high rates, and the mesh tank required weekly cleaning (which is a dirty and disgusting job). Scuds (gammarus) are your friends, btw.

[John Malone]

Thanks Jon.
Re-reading the question I see that it was of the "how long is a piece of string" genre. Silly me!
A better question would be, how do you clean the netting? My guess is that it is a simple hose-off on the lawn process, which I'm sure the grass would appreciate.
I'm running a low density system: 40 tilapia in a 500 gallon tank with 48 sq ft of gravel bed and soon an 8x4 DWC.
Given that the polisher would be after the gravel beds, which have worms breeding happily, I don't expect that there would be much for the polisher to catch. I guess it's one of those "try it and see" situations.
Thanks for the input.

[Jim Fisk]

John, if you are first passing thru your media beds before hitting the raft beds I do not believe you need anything at all. You won't see any solids for sure. They are a bio-filter. Different story altogether if you are going FT - Raft.

[John Malone]

Hello Jim,
There's a level of debate about whether there needs to be filtration after a grow bed before a DWC raft. From what I've read, I think it comes down maximizing efficiency and performance of the system.
I'll probably start without the filter, but if I have trouble with solids on the plant roots I will add a radial flow filter and/or a clarifier.
Given that I'm not doing this commercially, I'll keep it pretty basic at the moment.

I appreciate the input from all.
Now, back out to the garden and keep constructing...

[Jon Parr]

My opinion, and experience, says that you don't need any filtration between media and raft, IF the media beds are of sufficient size and design to accommodate the solids. A polisher between the two certainly doesn't hurt either, as long as it is kept aerobic. The UVI model uses settling and mesh tanks, in part to convert nitrates to N2 (anaerobic), thereby requiring a "de-gas" tank to get rid of N2, CO2, and H2S as well as re-oxegenate the water before the raft. We in backyard land usually want to conserve the nitrates for plant use, and can eliminate these UVI components.

Washing the mesh is as simple as spraying it on the lawn, and like I said, scuds may be able to do all the work of mesh cleaning.

[Vlad Jovanovic]

It seems like the crux of the matter lies in things Jon already mentioned...Fish stocking density, quality fish feed (like Skretting USA/SilverCup), size/volume of the grow beds, worms, scuds (it has been written that worms can take care of about 70% of the solids)...and one more seemingly secondary consideration, flow rates. Flow rates seem to effect fines settling. A polisher seems like a mighty fine way to trap some of those fines, especially if coupled with some scuds.

I went with 25-30% GB to 70%-75% rafts and plan to stock at about 0.25-0.3lbs of fish mass per sq.foot of grow space...possibly lower...and have sourced some quality food from Skretting. We'll see how that all works out in the long term (few years).

Now, there are those who feel that we are all "wonky sods" (or some such thing) for thinking that we can get away with using GB's to pre-filter DWC troughs long term (without all the swirl, net, clarifiers, etc...) no matter the set or setting...(at least there were a couple years back on some other forums) but, I don't really agree with that. I figure 2 years continually running with fish ought to be enough to lay the matter to rest in my mind.