Research Tank for Fabrication

[IanCullings]

Given the change in design from a PVC pipe (like the 1 L/s uses) to a prefabricated tank (for ease of construction and reduction in cost), we will need to find the optimal tank for construction. Considerations:

• Tank should be able to sustain full load of wastewater without any risk of collapse
• Tank choice should minimize cost
• Tank material is important, as we will need to drill holes and weld into the tank
• While not a problem for current tests, we will ultimately want to build these systems in Honduras from locally sourced materials. Consider this for future design work as well as tank choice
• Tank may not meet exact dimensions we are assuming, which is fine but will change design criteria slightly. This is why it is important to determine what tank we are using early.

Some suggestions from Monroe: http://www.plastic-mart.com/product/16398/rotoplas-400-gallon-tri-layer-water-tank-590286-590285 http://www.plastic-mart.com/product/16398/rotoplas-400-gallon-tri-layer-water-tank-590286-590285

[IanCullings]

After some research into tanks:

• The plastic mart tanks can be ordered with inlet ports placed wherever we would like. This could reduce costs and fabrication time significantly
• However, the plastic mart tanks are not designed to be pressurized, just for liquid storage. I have no idea what the tolerance level for this would be, they might still be safe under the pressures created by biogas? Does this matter since the biogas is contained entirely within the biogas lid, which we can fabricate out of a different material?
• It looks like we can source HDPE tanks of similar dimensions necessary in Honduras
• Talking with Ethan, he mentioned that HDPE tanks were very difficult to work with and to keep watertight. We will be able to weld them with the plastic welder in the lab however.

I called the company and requested a shipping quote, included below

@monroews what are your thoughts on the tank choice? Do you think the plastic tanks would be safe under the low pressures created by biogas production? If so they would make fabrication much simpler than working with PVC pipe.

[monroews]

Calculate the pressure in the tank with the biogas pressure vs what the pressure would be if the tank were full. Let me know what you learn! I'd like to know more about what was difficult with the HDPE tanks. You could see if there is an HDPE tank in the lab that you could practice welding on to see how easy or difficult it is. I visited a small factory in Guatemala about 6 years ago where they were fabricating wastewater treatment systems out of HDPE tanks. So we won't be the first ones using this approach.

[IanCullings]

Hi @ethan92429! We are currently working on picking what type of tank to use for our UASB system and we were hoping you could provide some insight based on your past experience. See the above comments for discussion on the tank choice.

Juan mentioned you had used HDPE tanks before and had troubles keeping them watertight. A couple of questions about this:

• Where did you source your tanks from them? About how large were they?
• Were you using plastic welding to keep the tanks watertight? If so how challenging was this? Was it similar to PVC welding?
• In your experience in Honduras did you see many areas using HDPE tanks, or just PVC? Would it be challenging to source HDPE tanks in Honduras?

[monroews]

Rotoplast sells tanks in Honduras. My understanding is that they are HDPE. The tanks are watertight. The question is what types of fittings will you be using. HDPE welding is a standard manufacturing technique.

[eak24]

This was before we were welding tanks together. Instead, we had used a product called "spinweld" to "weld" a bulkhead onto the bottom of the tank. It never made a good seal, so don't do this! Rotoplast is HDPE AFAIK. They have a very thin wall. If you have any alternative geometry to "round", you'll need something thicker so it doesn't buckle. Do you have a sketch of the geometry you are trying to make? I can ask a friend I have who is a plastic machinist for some advice on what he would use.

[IanCullings]

@ethan92429 We are just using simple cylindrical geometry, meaning we would probably not be altering the tank dimensions at all, just drilling into the sides. The rough dimesions are 3ft diameter and 7ft tall. The main concerns for the tank are the stability of the bottom and top, as the bottom will have a large settled bed of organic matter and bacteria, and the top will capture methane and keep it under pressure.

Any advice from your friend would be very helpful! Most of the tanks we've found are not designed for gas pressure, and the ones we've found are too small for us to use.

[monroews]

I'm puzzled by this issue. I've shared links to tanks with the right geometry. You can do the math on the effects of air pressure and solids. You will find that both are inconsequential. (if they weren't inconsequential, then your system of injecting waste into the bottom of the UASB wouldn't work!)

[eak24]

You could use Uniseals in place of bulkheads for the entrance tank!

[ananya-gangadhar]

@IanCullings, are we no longer having a sloping valley at the bottom of the reactor? The last I heard was that we were planning to use some sort of plate settler design for a sloped bottom inside the UASB reactor. If we are no longer planning to have a sloped geometry, we will have to modify most of our design code and reactor volume calculations.

[monroews]

I suggest that a few experiments with gelatin or other low density material are needed to see what happens when you inject water into the bottom of a flat bottom tank. If you can get uniform flow distribution with a flat bottom tank that will be ideal.

[IanCullings]

After talking with some representatives at Plastic-Mart about this 400-gallon tank:

• The prefabricated lid for the tank is threaded on and screws off. It is not gas tight, and is able to move up and down to vent gas
• We are able to order the inlet and outlet fittings to be moved and changed in size by sending them a technical drawing. It costs around $75-$100 per fitting depending on the sizing
• Shipping is around $200 and would take about 12 business days at most

Overall it is looking like the entire tank should cost around $700 - $800. Ordering this way and using uniseals should reduce fabrication time significantly.

[ananya-gangadhar]

Owing to the need for the tank to be able to withstand some amount of pressure, I was also looking into the pressure specs of the tanks. Most prefabricated tanks that we have been looking into are not designed to withstand any pressure and can collapse. I don't know how much of an issue that is but I decided to look into 3 feet wide HDPE pipes as well, just to see if a tank design similar to the 1 L/s would actually perform better.

Here are some suppliers that need to be contacted for price quotes:

For HDPE pipes

• [ ] ISCO
• [ ] JM Manufacturing Company
• [ ] Fabco Plastics

For PVC pipes

• [ ] Royal Building Products

[monroews]

I suggest you calculate the pressure you will be applying to the tank. The hydrostatic analysis will show you that you aren't applying any more pressure than the tank would be handling if it were full of the UASB sludge and open at the top..

[ananya-gangadhar]

@monroews We'd actually want a slightly pressurized tank in order to calculate the biogas flow rate in our prototype, wouldn't we?

[monroews]

I suggest doing the hydrostatic analysis.

[ananya-gangadhar]

So based on the geometry, the part of the tank that will be storing biogas has the following parameters:

Volume = 7100 cubic inches = 116 L Hydrostatic pressure on walls = 1.0123 atm = 15 psi This means that water would exert almost negligible pressure on the tank. But will such a low pressure even allow any biogas collection?

[monroews]

Why 15 psi? That is a LOT of pressure if it is gage pressure. Or did you mean absolute pressure in which case it is zero gage and applies no force to the tank that isn't balanced by atmospheric pressure on the outside of the tank.

[ananya-gangadhar]

This is absolute pressure. The gauge pressure would be 0.0123 atm

[monroews]

gage is what matters for the hydrostatic analysis. Compare that gage pressure with equivalent depth of water. Add that to the depth of water in the tank. Is that more than having the tank full of water? If not, then you aren't increasing the pressure above what it would be if the tank were full of water.