Coil Denitrator Plans – for removing nitrates from an aquarium

NHolbrook October 17, 2011 Denitrification No Comments


Examine the drawing and notice the blue shaded area within the cylinder (C). BioBalls (F). Yup, and no they aren’t there so you can add another fish! These give additional surface area for anaerobic bacteria colonies. 

The orange rings are the coils of plastic tubing (E) running from within an inch of the lid, down to the bottom of the unit. Once you’ve wound the tubing , usually 1/4″, around and around inside the body of the tube, straighten the coils as much as possible. 


Leave the end of the coil exposed, just above what will be the base or bottom (D) of the device. Glue the bottom piece in place. A scrap of 1/2″ acrylic is best but 1/4″ will do. I recommend against 1/8″ as it is too fragile. 

base or bottom , again 1/4″ will do here. For your input (A) and effluent lines (output=B) you can simply run the tubing through these holes and seal with silicone or place fittings here and use nipple connectors… whatever you like. I recommend the fittings as there will be pressure in the unit and a good seal is required. Pour in the BioBalls at this point. Keep them about 1″ below what will be the top of the denitrator. 

At this point you should have a cylinder (C) with a bottom, coils of tubing wrapped around the inside walls of the acrylic tube, BioBalls inside the coils and a lid with fittings or holes drilled. Attach the upper end of the 1/4″ tubing to one of the nipples. Glue the lid/top on. Use Weld-On #16 thickened acrylic cement. It fills minor imperfections and sets within an hour. 

All done, except for one last detail. A proper drip rate is needed to maintain dwell-time within the unit so the bacterias can gobble up the nitrates. Too fast a flow and your tests will show nitrites, as the bacterias have too much O-2 and denitrification isn’t taking place. Too slow a drip or flow rate and hydrogen sulfides are produced, giving a rotten-egg smell that indicates trouble to the inhabitants of the reef or fish tank. I have experimented and found that a drip of just under a steady-stream is best. In other words, a very fast drip, but a definite drip just the same. Use a small air valve to regulate this on the output tube (B) running back to your sump or display tank. 

When finished the unit is sealed and not intended to be disassembled. Once up and running the coil denitrator will last for many, many years without any adjustments or fiddling. About the only area of attention is the drip valve. This requires cleaning periodically to remove salt crystal build-up! Cost to build this unit is around what you’d pay to BUY one through a catalog house, but where’s the satisfaction in that? If you don’t wish to DIY, there are several commercial units on the market that are very good and I recommend them all. The issue here is getting rid of nitrates, either through building or buying. I just like tinkering with the hobby and enjoy the challenges of DIY! Good luck, and any additional comments or questions may be directed to me, Don Carner. 

About Nitrate & How This Thing Actually Works 
The coil denitrator takes 5 to 6 weeks to cycle (yes, they cycle just like the tank). The quantity of product that is processed, (nitrate) is truly amazing considering how once established, there isn’t anything more to do! So how does this happen? As oxygen rich water is pumped into (G) and enters the top of the unit (A) it is forced to spiral down through the layers of plastic coil tubing (E) until exiting within the center of the cylinder (C). As the water level increases within the body of the unit, the BioBalls (F) become host to the millions of colonies of bacteria’s that commence multiplying. As the water reaches back up to the top, it exits through the other fitting (B), the one not internally connected that runs back to your sump or display tank. So? So, as the water slowly works it’s way down the spiral, the O-2 is consumed by AEROBIC bacterias, the same ones that are in your filter and make all the life possible. Somewhere around 3/4th’s of the way down however, the O-2 levels diminish within the spiral, having been consumed by the aerobic bacterias higher up the coil. (D=Base; G=Water Pump) 

Now what? Well, now the ANAEROBIC bacterias begin to flourish, the very ones that feed on nitrate, not O-2! As the water continues its travels it encounters the main interior chamber of the cylinder. All those BioBalls are just waiting to provide area for more anaerobic bacteria to consume all the nitrate that wasn’t converted inside the bottom 1/4 of coil. This is the “bank” that will allow the coil denitrator to continuously process more and more nitrate as it is produced within the display tank.


Some Pics from other Aquarists builds

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