Senior Project- Need Help- Chemical Engineer!

MrMike999April 30, 2012

Hi All,

This is my first post. Thank you for reading. I am tasked with designing a 1 ft by 1 ft container that is to grow vegetables on top of our school parking garage for the cafeterias. Its depth can vary between 8 to 12 inches. The system should be as low maintenance as possible. That being said, I am looking to create a system that can be sustained by storing rain water only.

I am planning on using reflective tin on the sides and top of my system to reflect the suns energy away from the box, keeping the inside cool and hopefully causing less water to evaporate out of the soil (based on knowledge of thermal radiation).

PROBLEM: How do I keep as much water in the system as possible so that the one lettuce in the system always has enough water, but will not rot but also will have enough water.

SOLUTION: I am planning on using a mixture of soil and vermiculite (rather than perlite) to create a sponge like substrate that can store a lot of water. I am scared the use of vermiculite instead of perlite might cause root rot though.


1. I want to line the box (made of wood) with a plastic trash bag to prevent water from escaping. Will the trash bag rot if exposed to water for too long?

2. Is it necessary to have drain holes in any system? Is it possible to not have drain holes?

3. Should I use vermiculite or perlite?

BIG Q: If I used a soil mixture of soil and vermiculite, and used a plastic mulch and depended on rain as the only water source (roughly 2.2 gal per month) with NO drain holes at the bottom of the system, will there be root rot? Please let me know thoughts. I am on a deadline!

Thanks for your help! Its a very intricate project, but unfortunately I don't have the knowledge of gardening to complete it.


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I can answer some of your questions. First of all, use a soilless mix, not soil and vermiculite. Soil is not suitable for container medium. If you want a long lasting medium, consider the gritty mix discussed in this forum. Secondly, you do need drainage. Plants do not tolerate standing water, and building a water retentive soil is not the best for plants. Air to the roots is just as important as water.

A very low maintenance system might be a rain storage cistern connected to a drip fertigation system. This is the equivalent of drain to waste hydroponic system. You can use a totally inert hydroponic medium for this, but will still require drainage. Once fine tuned, this can provide the lowest maintenance, water and fertilizer usage.

    Bookmark   May 1, 2012 at 9:31AM
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DaMonkey007(10b - Miami)

Hey Mr. Mike,
As an engineer myself, I couldn't help read your post. I don't have all the answers, but I have a few thoughts for you. First, I think that your approach has a few flaws. Your trying to store your water supply IN the soil, this will not work. You WILL rot your roots, forget about that. What you want is to store the water in a reservoir and allow the soil to wick up what it needs, when it needs it. This is refered to as a "Self Watering Container". I don't personally use them, so I will not be the best person to advise you on the composition of your soil mix, however, you can find plenty of info regarding design and configuration on this and other forums here on GW. Additionally, if you are only required to grow one head of lettuce and you have 1 cu ft of allowable volume, I suggest that you dedicate as much of that volume as possible to the water reservoir. This will keep your supply high and your demand low. As an example you could try 6"x6"x4" (lwh) soil volume above a 12"x12"x8" (lwh) water reservior. Of course, if you wish to get the most out of the container, simply expand the growing area.

The big question is your water source. If you are relying on rain water only, are you planning on collecting that via a rain barrel (or otherwise) and manualy adding it to the system when needed, or are you trying to make the system collect the water itself? The former will be the obvious choice, as the later will be extremely tricky - especially if the collection system also falls within the 1 cu ft design constraint. If the later is the case, the success of your experiment relies completely on the weather, and it would have to rain an aweful lot for a system that size to collect enough water to sustain itself. Also remember that you will need water in the system on the day you plant, so relying completely on the system itself has many drawbacks and variables. I suggest collecting your rainwater elsewhere and manually adding it when your reservior gets low. However, to suppliment your manual additions you could certainly design the system to at least collect whatever it can, when it does rain. I might suggest something that resembles one of those cones that your dog gets at the vet - placed around the centrally located soil volume with a drain to the reservoir - and square, of course, to maximize the surface area of the "collection ring" within the confines of your design criteria.

Good Luck!


    Bookmark   May 1, 2012 at 9:33AM
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To Capoman and DaMonkey007, I appreciate you taking the time to read this post. I need all the help I can get, and hope more people jump on to share their ideas.

So to recap...

Capoman says:
1)Use a soiless mix, not soil and vermiculite.
2)I absolutely need a drainage system in my growing unit.
3)Building a water retentive soil system is not the best for plants. Air to the roots is just as important as water.

Does Capoman know:
1)I must start from a seed. The seed must be planted in the system and then grow into the vegetable. Our control vegetable is lettuce, but different types of vegetables may be grown in the system.

Questions for Capoman:
1)So should the soiless mix be mixed with vermiculite? Are you suggesting a soiless mix only and totally ditching the vermiculite?
2)Why is a soil and vermiculite mix a bad idea? (i.e. nutrient depletion or some other underlying reason) Fertilizer can be added to the system at the beginning of every growing season to re-fertilize the soil.

DaMonkey 007 says:
1)Create a self watering container (SWC). I am assuming SWC is a widely accepted term for certain types of systems.
2)I should focus on storing water in reservoir, not in the soil.

Does DaMonkey007 know:
1)The growing unit is 1 cubic foot at MAX. I cannot be any other shape than a 1 cubic foot box with an exposed top. Hoewever, I am allowed a separate water delivery system that can store as much water as I would like. It can be any size, but its weight when filled with water is one parameter being looked at in this project.


That was a quick recap. So I am thinking that attempting to store the water in the growing system itself is not the best idea. However please note these following numbers...

GROWING UNIT VOLUME: 1 cubic foot (1 ft x 1 ft x 1 ft)
... This means that while empty, this unit can hold 1 cubic foot of water. When full with mix, it is dependent on the absorptivity of the mix of how much water can be stored. However the amount of water stored in the unit with mix must be less than 1 cubic foot.

AVERAGE RAIN FALL: .30 - .37 feet (For Bronx, NY)
... This means that for each month, the total possible amount of rain fall that can be collected in the unit is between .30 - .37 cubic feet.

Volume Rain Collected = Rain Fall in Feet * Surface Area in Square Feet (Surface area is 1 square foot, this is a set dimension and cannot be changed)

As per the numbers I have just provided, the empty unit should be only 33% filled at the end of one month. This is without taking into consideration the max possible water storage when soil mix is in unit, the take up of water by the vegetable and the evaporation of water from the soil.

QUESTION: After reviewing these numbers, do I really have to worry about rotting the roots due to an excess of water in the system?

If so, is it possible to create a water delivery system that feeds water directly into the soil by a wicking mechanism?

Sorry for the long post. I am still in a cloud and trying to get all the facts straight. Thank you!

    Bookmark   May 1, 2012 at 12:10PM
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DaMonkey007(10b - Miami)

C'mon Mr. Mike - this is your senior project - it's meant for you to use the grey stuff between YOUR ears...not OURS!!!

Just kidding buddy. But seriously, use the old noggin. First, your assuming that you are going to get the exact average amount of rainfall equally spaced over the month...not going to happen. You might get all that rain in 1 hour, then none for the rest of the month. Or you might get 10 times that amount, 20 times, 30 even. How would you deal with that?

First research how a SWC works so you have some basic understanding. As for the design constraints - use the cube, fine. Simply separate your growing zone from your reservior within the cube, so that you have an enclosed reservoir below the grow zone. Construct the rest of the system - wicks, overflow, fill tube. I'm not going to tell you how - that's up to you to figure out ;)

If your allowed a separate holding tank, your options are endless. A 5 gal bucket with minimum static head greater than max fill line of the reservior comes to mind. I will leave those details up to you. If you really want to impress your professor, rig the delivery system to mechanically activate when the reservior is low. I'll give you a hint...use a ball float and an actuated valve.

That being said, for the sake of seniors everywhere and untimately for your own benefit, I will offer no more advice to you, our own budding engineer. Get those creative juices flowing buddy, and GET TO WORK!


    Bookmark   May 1, 2012 at 1:22PM
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Mr. Mike,

Here's a link to Raybo's Earthtainer guide - it's a SWC that Ray's refined over a number of years. It's larger than your specification, but the concepts are the same. This document should give you a better understanding of how to achieve your objectives.

Here is a link that might be useful: Earthtainer

    Bookmark   May 1, 2012 at 1:56PM
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Here's a link to a vid on the Groasis waterboxx. They do a pretty good job of diverting rainfall into a res/wicking system, if you need some inspiration :)

Here is a link that might be useful: Youtube video

    Bookmark   May 1, 2012 at 3:01PM
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Once again, thank you for your responses.

I am doing exactly what engineers are supposed to do, utilize their resources =). I know nothing of growing within containers, but I have spent most of the day studying SWCs and have a partial understanding of them. I understand that water moves up through the system by capillary action. I appreciate you pointing me in the direction of SWC.

Here is a question for you. I plan on using something like chinese-food containers turned upside down as my reservoirs and have my soil mix reach down to the bottom of the unit between these containers. I intend the soil to act as the wick itself. I have read a little bit about some folks doing it, but I have no experience in this field and would appreciate a word of advice on whether or not that can work.

I thank you for that link. It was extremely informational. I have been hearing a lot about this individual "Raybo" on this forum. Can you tell me more about his container mix of 3-2-1? I believe that means 3 parts bark, 2 parts growing mix, and 1 part perlite. Please confirm for me. Thank you!

Thanks for that email. It is a very interesting system and I like how they have sloped the top of it to collect water. Much appreciated.

    Bookmark   May 2, 2012 at 12:41AM
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DaMonkey007(10b - Miami)

I honestly can't give you any advice on that method, it doesn't seem plausible. I suggest a full partition separating the lower reservoir from the upper soil mass.

    Bookmark   May 2, 2012 at 8:23AM
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Sounds like you have a few options. I personally prefer a drip fertigation system, but a SWC will also work. Drip fertigation gives you more control, and all you need to do is fill a reservoir with nutrients so it is less maintenance, and plants love fertigation. That being said, a drip fertigation system does require a bit more cost. SWC is simpler and less costly, but will give you less control. But both require decent drainage and a reservoir to store rain so they aren't that different. I guess you'll have to decide what works best for your project. I do think though that your reservoir needs to be bigger to give you a better buffer.

    Bookmark   May 2, 2012 at 9:07AM
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About your question, I have no issue with vermiculite. It's soil in a container that I have an issue with. Soil is too water retentive and causes root rot in containers. I would consider using Tapla's 5:1:1 or gritty mix as a medium. If you go the fertigation route you can use hydroponic substrates such as pure perlite, coir, hydroton etc, although the mixes above should also work.

    Bookmark   May 2, 2012 at 9:12AM
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stev32k(Zone 8)

One thing you need to consider is evaporation from your rain water reservoir and a trick to help limit that loss is to use ping pong balls to cover the surface. The balls will let water in while reducing the evaporation rate.

Soil has been used to grow potted plants since pots were invented. For over ten years I have been growing tomatoes, strawberries, bell peppers, and some years carrots and onions in soil I dig from the yard or potting soil I get from Lowes and have never had root rot.

The container must have drain holes in the bottom. Cover the holes with a nylon shade cloth (or panty hose) then put in one layer of drainage rock. Then put in the soil. There are numerous soil mixtures you can use, but it should be loose and well draining.

You can place the container over a reservoir to collect excess water and use a wick to return it to the soil. If it is set up correctly you can add water directly to the reservoir and never add it to the soil (you can buy pots already set up that way) - just keep the reservoir full. The best wick I've found is a braided cotton wick used in kerosine lamps and you need several (maybe 5 or 6) in each container.

    Bookmark   May 2, 2012 at 4:09PM
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Can you tell me more about his container mix of 3-2-1? I believe that means 3 parts bark, 2 parts growing mix, and 1 part perlite.

It's 3 parts bagged growing mix such as ProMix, 2 parts pine bark fines (Ray specifies microbark - it's the same stuff) and 1 part perlite.

    Bookmark   May 2, 2012 at 8:50PM
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Hello All,

Thanks for all the help. Here is an update as well as more questions for you folk.
Here is my finished container. It is constructed out of .5 inch sheathing board. I am going to water proof it with the use of heavy duty plastic garbage bags. We have already designed a water proofing scheme that will keep water out even with a drainage hole drilled in the side (having a drainage hole makes water proofing with trash bags much more complicated). Does anybody have any reasons why black heavy duty garbage bags should not be used to water proof?

I would like to create a reservoir using recycled bottles like the ones seen in this next photo.
They are circular bottles. I am not going to separate the reservoir from the growing medium using any sort of divider. Rather I would like to pack my growing medium down in between the spaces between the bottles. The saturated growing medium will then act as the wick, and water will flow up through the mix itself.

I know the two are usually separated, but watch these two videos: (Go to 2:25)

To sum up, my questions are:
1) Can I use the soil mix packed between the bottle reservoirs as my wicking material?
2) Is it dangerous to use heavy duty plastic bags that to water proof my box? (Assume that the plastic will not be exposed to any UV rays)


    Bookmark   May 4, 2012 at 11:11AM
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They probably want you to define the aqueous throughput and the uptake of the lettuce. The material balance will reduce to an open system, with accumulation (i.e. the lettuce), without chemical reaction or heat transfer.

You would need to know the watering requirements of lettuce, the optimal spacing of lettuce and the minimum throughput of water required to prevent stagnation. Design the flow rate for the maximum uptake of properly spaced lettuce, and ensure the water is circulated enough to prevent stagnation. Then, design a collection/piping system. Verify your design using Bernoulli’s, or Hazen-Williams provided there are many nodes in the pipe system. Resolve any issues.

That’s all there is to it.This seems more like a weekend homework problem than a senior project.

In contrast, my senior project was a to design an entire ethylene plant, including a rigorous furnace design (radiant, conductive, and reactive heat balances, mass balances and momentum balances), quench tower sizing, compressor sizing and acid gas removal, column sizing and internals, refrigeration loops, pinch point analysis of heat exchangers, and an initial estimate of total plant costs. I had to define, size and specify the vessels and internals; the splits and recycles, the furnace tubing, pipes and burners; compressor staging and acid removal system, pump and valve sizing, cooling towers, power requirements, a layers of protection (LOPA) safety analysis, hydraulic sizing of all key flows, capital costs and proposed funding.

I would like to know more about how this project panned out. Hopefully, the project provided you an interest in gardening, which like home brewing, catalyst manufacture or cheesemaking, is an empirical science. Let me know how it went.

Calico Marty, P.E.

    Bookmark   May 25, 2013 at 1:14PM
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