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Like Scott (Esshup) says I've done both -- a floating RBC and one where the axles go through the sides of a large tank with the ends capped. I'll take the floating hands down. Not only will it adjust with the water level there is no weight pushing down on the axle to speak of, and PVC inside PVC that is lubricated with water lasts indefinitely.

One of the downsides to typical nonfloating RBC's is the weight the axle has to endure. As bacteria builds up on the surfaces of the RBC it really adds up. It's a tremendous amount of weight and historically a lot of RBC axles have snapped if they didn't float. Not that it can't be done though.

Thanks for the pointers Cecil. I'll work on the design some more. I have some ideas for modifying my design to be free floating that I want to plan out some more. I'll post here when I have a more complete plan.

Here is the latest plan that I have drawn up. The dark grey is 1" sch 80 PVC, the light grey is 1.5" sch 40. The main outer shaft is 2" sch 40. The blue is foam, the green is the media, and the light grey outline is the cups.

The design is going to be free floating attached to a 1.5" T fitting that is going to have a 1.5" sch 40 piece about 1/2 in longer than the bottom of the rbc. This way if it bottoms out it will still be free floating and spinning. The 1" sch 80 shafts will connect to the outside of the barrel/barrel housing and the 1.5" sch 40 will slide up and down these 2 shafts freely. I decided to use 1" to 1.5" bushing rather than 1" couplers to keep the needle bearings in place since the outside diameter of the bushing is about perfect.

Nothing in the image is to scale; it is just something I whipped up in mspaint to visualize my idea.

Let me know what you think.

The concept looks quite reasonable.

The needle bearing idea will probably work, but not as easily as you think. In a perfect world, that being, all the materials came in at size, you will have some interference. A bit of machining will solve the problem. I would also consider spherical UHMW bearings, instead of the roller, if you think you need a bearing at all. All you really need is a retainer if it is floating.

Don't expect your schedule 80 grey pvc to be super rigid.

PVC is also a very poor bearing material (not even listed as a bearing material). In Cecil's application, the PVC is acting more in the fashion of a retainer to keep the axis of rotation, well, retained, and not as an actual bearing. Put a cylinder in water with external forces without retaining the axis of rotation and it will be uncontrollable. I personally would not worry about PVC as a retainer, but don't use it in a load carrying bearing application.

What media are you using?

Since you posted this. I was playing around with the idea of using 10MM twin wall corrugated polypropylene as media. That would work quite well. A 2ft diameter x 4ft long RBC would be in excess of 1100 sq ft, plus it is buoyant.

Look into the history of RBC's and you will find that they were Engineered for treating municipal waste water, with good success. The problem with the axles snapping and bearing/mechanical failures came about when some fish guy's, without engineering skills tried to duplicate this. They use to use plywood for media. These however, were the pioneers in Aquaculture, and a Big Thanks to them!!!

RBC's for Aquaculture have come a long way since then, and everyone has their flavor.

Nice device for doing the task at hand. And as I said earlier, you can thank the Aquaculture Pioneers for this.

Let us know how things are going.

DMRBG,

I like it! Give it a shot! The only thing I can warn about is RBC's with media end up heavier than plates due to more surface area for the biofilm to collect on. Of course more surface area is better right? However, Dr. VanGorder actually warned me that they sound great on paper but can be incredibly heavy over time.

He said they even tried rectangular plate ones for more surface area and ending up scrapping them.

I have heard of some trying square plates, and have seen some utilizing octagon shapes. Round takes less power to rotate in water because it is not trying 4 times (square) per revolution to create a new path thru the water. That takes a bit more power. If you rotationally staggered them, it would be less power, but now you are basically back to a circle, If you get what I mean, which takes even less power, and creates more surface area.

Yeah Phil I think they burned up the motors or something if I recall correctly.