Backyard Aquaponics
A place to discuss aquaponics

There are 2 version of the RSG (“Really Smart Guy”) filter. One to provide iron that the plants can absorb and one to off-gas nitrates. They both use the same construction to provide an anoxic mini-environment. Bacteria that can convert nitrate to nitrogen + water and bacteria that can chelate iron will only live in an anoxic environment. In essence, the RSG filters are a sand-filled length of PVC sunk vertically into the growbed. Water flows in from the distribution grid at the top, and out through notches at the bottom.

Basic construction:
Cut a length of 4-inch (100mm) PVC to a length equal to the depth of your growbed media. Cut 3-4 small notches out of one end of it. This will just provide an easy exit for the water. Notched end down, sink the PVC into the growbed media, so it is standing vertically, with the notched end all the way on the bottom of the growbed. As you sink the PVC, remove the media from the inside of the PVC. Pick a location in the growbed so that you can get water from the distribution grid to flow into the top of the PVC. Also make sure that the RSG filter is located at least 12” (300mm) away from the drain for the growbed. This will allow water to re-oxygenate before it returns to the fishtank.

For the Nitrate Off-gassing version of the RSG filter, fill the PVC with sand with a few handfuls of activated charcoal mixed in. If you want to use the RSG filter to chelate iron, add a couple handfuls of rusty metal into the mix. Fill the PVC up to about a ½ inch (12mm) from the top edge.

In operation, the water flowing in at the top of the pipe flows down through the sand/charcoal, and also overflows the top edge of the standpipe. That way you can easily see that you are not introducing air into the column when the bed drains. It will take some time (2-3 weeks) for your anaerobic bacteria to establish and get going. The sand drains very slowly, so even if your pump cycles on and off, you will not introduce air into more than the top inch of sand.

At this point, Les and I have constructed the nitrate-offgassing versions and we both see evidence that they are working. Although I have some rusty metal in the bottom of my growbed, and that should be anoxic enough, it is harder to measure to determine if it really works.

A picture of my RSG filter in action.

Nice summary thanks JP

Hey JP, Why is the charcoal necessary?

The bacteria need a carbon source. They couple the reduction of nitrate to N2 to the oxidation of reduced carbon.

Hmm, maybe that's why I'm not seeing a change yet. My sandy bits need better water flow, but I guess carbon is on order as well.

Would the usual aquarium filter stuff work? I have some of that around.

Edit: Done.

Aquarium filter carbon is what I used. Go for it.

Good one Janet. My only concern is that the sand may gradually find its way out of the bottom of the RSG and into the grow-bed. Have you found this at all. How big are your 4 notches?

I haven't really had a settling problem, so I think the sand is staying put. However, I didn't figure a little sand in the bottom of the GB would be too much of a problem. After all, if any of us didn't wash our gravel extremely well, there's a bit of sand or dirt already there.

My notches are perhaps 3/4 inch (20mm) deep.

I am newbie so please be kind when I ask silly Q's.

I thought we wanted nitrates for the plants to grow? Will there still be enough for the plants with this running, does it reach some sort of equilibrium like the other bacteria in the system based on load? finally am I correct in assuming you may want to do this to increase you fish load without increasing grow bed space?

Skygazer, Janet has been experimenting with RSG filters as a means of lowering excessively high nitrate levels.

There are times during harvest, initial plant growth, seasonal aspects etc that nitrate levels in a system may exceed the ability of the growbeds to extract them... especially if the fish are also at a stage of growth and high feeding levels.

By incorporating one or two RSG filters (which can be removed or temporarily deactivated) Janet has found what appears to be a reliable and adjustable method of controlling nitrate levels.

Exactly, Rupe. My nitrates were over 1000--not really good for the fish. Although I was able to get the nitrates down with water changes, I didn't want to be discarding water like that. The RSG filter is keeping my nitrate at 10 or below right now.

I am able to increase or decrease RSG activity somewhat by twisting the water distribution pipe to increase or decrease water flowing into the RSG filter.

Janet - wouldn't water also be getting into the filter throught the notches as the bed floods and drains.

When you say the notches are 3/4 inch deep - how long are they?

I put in a couple of small crenellations on the base of the pipe and left in some gravel to impeded the sand outflow.

Bit difficult for water to come in the bottom vb as the vertical pipe is full of water.

Re sand going out the bottom...I marked the level of the sand and it had not sunk in the 2 weeks I have been running it.

The drain cycle may be pulling water through the RSG filter but I have "experimentally" removed the water flow to the filter and "observed"...didn't notice any difference in the speed of the water level dropping in the pipe.

Nice write-up by the way Janet

VB, the notches are isosceles triangles with a base of 1/2 inch (12mm) and a height of 3/4 inch (20mm). Thereabouts. Me and my trusty hacksaw are not precise.

One thing I did notice about the sand was that if I had a strong flow of water in at the top of the RSG, that the stream of water tended to wash a bit of sand over the top edge of the RSG. If you are seeing that sort of problem, just put a little gravel on top.