polypropylene for "tray-ponics"-type "rafts"

[Paul Trudeau]

Glenn Martinez has described a nice small raft system where he uses 24 inch x 36 inch x 8 inch-deep plastic masonry mixing tubs (trays) (available at Home Depot) as plant troughs.  Instead of floating foam rafts, he lays a 24 inch x 36 inch piece of plywood on the top edge of the tray, an inch or two above the water level.  I built my system last year on this model.  As he did, I stained the plywood with some white stain.  It's worked OK for me, but I've had a few problems with the plywood:

(1) fairly labor-intensive to drill and stain

(2) warping

(3) not easy to clean plant debris, mold, or algae off the stained wood

The morning after the US election in November, I collected as many plastic campaign lawn signs as I could find and have been experimenting with using them instead of plywood.  They are made of 4 mm corrugated (aka "fluted") polypropylene (same material used in a number of food packaging items yogurt cups).  Some of these signs are exactly 1/2 the size of one of those masonry trays, so two of the signs cover one tray perfectly.  Each half is supported on 3 of its 4 edges by the rim of the tray.  The 4th edge needs a little support in the middle, so I use a 7 inch scrap of PVC pipe standing on end in the tray.  So far it's working out great.  These re-purposed signs do of course have printing on them, and I'm finding that the printing can scrape off.  So I'm planning to buy sheets of new (unpainted) corrugated polypro and cut my "rafts" out of that.  Good news is that I just found out my local independent plastics store sells this stuff for about US$12 per 4 foot x 8 foot sheet--that's less than 1/2 the price of a sheet of plywood.  No staining is required, there's no warping, and the material wipes clean very easily.  Also lots easier to drill with a hole saw, and the material is easily cut with a utility knife.  Lots lighter than plywood too, so it's a cinch to handle.

Any thoughts on all this, anyone?

[Paul Trudeau]

Hi RW,

From what I can find on the web, unmodified HDPE is definitely susceptible to UV degradation.  There's a modified HDPE that has increased UV resistance.  Apparently polypro is generally susceptible to UV degradation over time as well, showing up as fine cracks.  Some corrugated polypro sheets are made with an additive to increase UV resistance, specifically for sustained outdoor use (even greenhouse construction).  I'm not sure how much of an issue UV degradation will be in my particular application.  I'm going to try it and see over time.  Ditto re algae growth (none observed so far).  There is a high-opacity version of corrugated polypro as well.  Coroplast is a big brand name for the polypro sheets - lots of info at coroplast.com.

Are you going to actually float your HDPE rafts, or have them suspended above the water a bit?

[RW]

They are going to be suspended above the water about a half inch.

I have some polypro sheets and HDPE sheets.  I'm going to do a side by side comparison.  We'll see how they hold up in the sun.  I can't imagine the UV degradation is any worse than painted styrofoam rafts.

 

 

Paul Trudeau said:

Hi RW,

From what I can find on the web, unmodified HDPE is definitely susceptible to UV degradation.  There's a modified HDPE that has increased UV resistance.  Apparently polypro is generally susceptible to UV degradation over time as well, showing up as fine cracks.  Some corrugated polypro sheets are made with an additive to increase UV resistance, specifically for sustained outdoor use (even greenhouse construction).  I'm not sure how much of an issue UV degradation will be in my particular application.  I'm going to try it and see over time.  Ditto re algae growth (none observed so far).  There is a high-opacity version of corrugated polypro as well.  Coroplast is a big brand name for the polypro sheets - lots of info at coroplast.com.

Are you going to actually float your HDPE rafts, or have them suspended above the water a bit?

[Jim Troyer]

Hi All:

first post in this group ;-) 

@ Paul -

If you feel the need to coat that sign material, use boat varnish, it has UV properties, it will need a re-coat every year however.  This is what you should have used on the plywood too.  Stain does not seal the wood, just colors it.

You could seal those tubes with a hot soldering gun attachment, melt and press them together before it cools.

Also, most ink is now food grade.  The printers were worried about ink leaching into the groundwater below landfills so they made the switch to water borne pigments.

It appears those grow trays are connected with PVC tubing, is that correct?  If so, can we have a better pic of how you did that?  Seems like a good method for a starter system and eliminates blue barrel syndrome...

Jim

[Paul Trudeau]

Hi Jim,

Thanks for the info.

Yes, I connected those trays with 2" PVC pipe.  It was tricky.  I will start another thread here regarding that, because it's starting to veer off the original topic.

Paul

[Paul Trudeau]

thread re connecting trays is here.

Paul Trudeau said:

Hi Jim,

Thanks for the info.

Yes, I connected those trays with 2" PVC pipe.  It was tricky.  I will start another thread here regarding that, because it's starting to veer off the original topic.

Paul

[Paul Trudeau]

two-month update on using corrugated polypropylene to suspend net pots over water trays:

--sealing the holes in the corrugated polypropylene with silicone didn't seem to stop a bit of algae from growing inside the sheet.  My sealing job must not have been perfect.  But it would be way too much work to do it perfectly.

--white 4mm corrugated polypropylene is not completely opaque, and as such some sunlight reaches the water underneath, and I'm getting algae growing in there.  Considering backing the white with an underlayer of black.

--the surface of the corrugated polypropylene cleans up very easily with plain water and a brush or rag. 

Haven't given up on this material for this application yet...

[Paul Trudeau]

Am now using a double layer of corrugated polypropylene as I talked about doing in my last post:  white on top of black.  Seems to have entirely eliminated algae.  Double thickness is of course stronger than a single layer, which is good.  With a single layer, I was having to prop up the center of each 24" x 36" sheet ("raft", although it's not floating) with a vertical piece of 2" PVC about 7" long.  Not necessary with the double layer.  Sheet of black cost about 50% more than white, but it's still pretty inexpensive. 

[Steve (from England)]

Hi.

I've just been reading through this discussion, and have found it really interesting. I am still constructing my system, and was planning to use trays. I was unsure whether using trays in an aquaponic system was actually even a viable option, so I'm now pleased to find others already using them. My polytunnel should be around 10 feet tall, and I'm hoping to have the trays on shelving in two or three tiers. The trays I've purchased are 36" x 24" and come with lids. The plan is to use some trays with media and have them flood and drain. But the majority of trays will be used as raft, with holes in the lids to hold the pots,( with a gap between water and lid ).

I would really appreciate your advice, tips, experiences etc, to help me on my way or even avoid making a huge mistake.

Thanks.

[Dave Lindstedt]

I also have purchased several of these tubs at a local Harbor Freight  store. Cost with tax was about $7.00 each.  I drilled drain holes in the bottom, have taken sludge from my grow troths mixed with compost and planting plants that do not lend well, such as carrots and other root vegtables, to my raft system.  What is nice is they are small and portable and I can move them indoors if need be.

[Marlena Nel]

Thank you so much for this picture!!!! it is exactly the info I have been looking for without complicated language!!!! or acronyms I don't know yet.!!! hahah... Thank you

Vlad Jovanovic said:

Eric, what Paul is describing sounds more like an updated Gericke method, as described by Kratky,B.A., 1996, Non-Circulating Hydroponic Methods, DPL Hawaii, Hilo, HI.

I've liked it before in other discussions, but here it is again... http://www.ctahr.hawaii.edu/hawaii/downloads/Three_Non-circulating_...

From a nutrient standpoint the PDF doesn't deal with AP or anything, but it's got some pretty cool hydroponic sub-system ideas in it nonetheless. And passive at that... I remember you had brought up the "passive hydro" topic to TC about a year ago, so you might dig the link Eric...

At any rate...It's nice to see those campaign signs actually fulfilling a useful purpose. Way to go Paul  No staining, no warping, no glues/binders...nice.

You may want to protect them from the suns UV rays somehow (IDK, a sheet of something or another...tin foil...whatever), they'll probably last longer that way.

[Joni Solis]

So Vlad, are you saying that the non-circulating rafts do not work with fish water? Sorry, I am super new to all of this and trying to under stand. If not fish water what fertilizer did you use in your raft trays?

Vlad Jovanovic said:

Yes, it would be tough to come up with a non-circulating AP design (I threw in the passive examples for Eric's sake, I figured he'd appreciate it). But really this method has worked for me both circulating, and non-recirculating, with extra air, and with no extra air...It works great.

[Vlad Jovanovic]

Hi Joni...no I'm not saying it wouldn't work, there would be a way(s) to do it...like by using the 'fish water' that has already gone through a biofilter, and using it in a 'stand alone' Kratky set up...but because of the biological oxygen demand (BOD) that the fish (and perhaps bacteria) place on a system, circulating your water would be important. 

As long as the plant essential elements are in solution, it would work...hydroponic salts need no bacterial action for the salts to dissociate into there plant bio-available form. You could still use fish water or humonia water...it would just need to pass through a biofilter first (so that the nutrients would be present in solution).