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burningbaal · 5 years

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Anoxic fuge zone idea

I’ve got some extra space in that fuge, so I have an Idea. I like the idea of having an anoxic (ie anaerobic) zone for denitrification. This is the old idea of a big deep sand bed (like, 6″ deep). But they have a crippling downside...there’s usually some nasty sulfuric stuff in there we don’t want to EVER get loose in the tank because it can cause some nasty catastrophes. So, some people have opted for a remote DSB where you run the water over the top of this R-DSB, letting the sand soak up the nutrients and the anaerobic bugs to do their thing, but can close a couple ball valves and it is no longer ‘touching’ the system. The idea being that while you get the benefits of a DSB, you can take it offline cleanly to clean it out occassionally, and you don’t have some sand-sifting animal that mistakenly stirs up the wrong part of the sandbed.

The idea is that there are bacteria which, in the total absence of oxygen, will rob an oxygen off of nitrate (enzymatically binding that oxygen to a carbon to supply the sugar the bacteria needs). That effectively converts the nitrate (NO3) into nitrous oxide (NO2), or even molecular nitrogen (N2), which are gasses and will (probably invisibly) bubble out of the water. This entirely removes the nitrogen of the system. Presumably, this (as well as uptake by things like algae) is how nature manages nitrates in the ocean.

So here’s the idea; I think I’d use two that I do maintenance on in opposite cycles.

Build it:

Materials: a bucket and sand to fill it up.

Get a regular HDPE bucket and remove the metal handle (or get one with a plastic handle)

Fill it up with sand, maybe 1″ below the rim. I’m going to suggest a sand no smaller than 0.5 mm, but no huge grain either. Smaller grain holds the oxygen out a little better, but larger grain will be easier to clean.

drop it in the fuge (remember: I’m planning on having a huge stock tank as the fuge, so there’s lots of space in there) and forget about it

6-12 months later, do it again so you have two in there. This isn’t really necessary, but I’ll have the room and I think it will aid in stability because I’ll clean them alternately.

Maintain it:

Materials: about 50lbs of clean sand (assuming 5g buckets are used) a large PVC pipe, hammer, wood block, smaller pvc pipe and plug/cap that fits in the large PVC pipe (barely).

Timing is debatable, I’m thinking 1 year is a good starting point, but I’ll see how it goes, I’m hoping to wait long enough there are a few nasty sulfuric zones in there. But I don’t want to wait so long the whole thing is super nasty.

get a PVC or ABS pipe approximately 1/4 to 1/2 the diameter of the bucket (you want large enough to get a substantial core, but small enough it’s not a huge fraction of the sand in the bucket) and a big hammer, probably a block of wood. The pipe should be taller than the bucket, maybe by a few inches.

Remove the bucket from the fuge, gently pour (’decant’) the water off the top as you take it out. This can be into the fuge, there’s no risk regarding the water sitting on the top. Note you’ll need new saltwater to add to the tank while the bucket is out for a few days.

Set the bucket on something very stable, that provides good support on the bucket’s bottom.

Pick a spot (probably the middle) and push the PVC pipe straight down into the sand with your hands as far as you can, this just makes the next part a little more stable.

Holding the wood block on top of the pipe, hammer on the block to drive the PVC all the way down to the bottom of the bucket. You might pre-check heights so you know it got all the way down.

Leaving the pipe in place for now, scoop out the rest of the sand. As the pipe gets loose, you can remove it, but be careful to make sure all the sand stays in the pipe as a core. I suggest leaving it until you can get to its bottom and use a thing piece of plastic to hold the sand in the bottom while you lift it out. You want a nice clean core that’s not disturbed at all.

You can throw out the old sand, or you can probably clean it. I’m going to try using a couple 200 micron socks and a garden hose, or I might try a pillow case idea I heard about. Just make sure you’re cleaning is thorough, then leave it out to dry for next time.

Lay the pipe at an angle in the bucket so the bottom is on one side and top of the pipe is leaning out the other side of the bucket. This makes sure the old core of sand (populated with the anaerobic bacteria you have not disturbed and exposed to oxygen) are spread out across the bucket. Don’t worry if a little spills out the bottom of the pipe, as long as most of the sand in the core is undisturbed and you leave anything that comes out at the bottom (don’t mix into new sand), it’ll be fine.

Start pouring in new clean sand. I debate the next step. Do you need to remove the large PVC a few inches at a time, or can you wait until it’s full and do it all at once? I’d prefer the latter, but I’m unsure of how much force that might take.

Ideally, fill up the whole bucket with your new clean sand, and slowly add some saltwater (tank water is probably best) until the water stops seeping into the sand. I suggest stopping the sand 0.5-1″ below where you want it to be later.

Then leave it all sitting still for 1-2 days WITH the PVC pipe still in place, you can just leave it on the garage floor, it won’t matter to the bacteria in the core. The idea is to give the bucket time to become anoxic, mostly this will happen on its own. if you want, you could speed up the process by using live sand, or seeding the new sand with a little from your system. But just leaving it alone for a few hours may be enough, I’ll probably do a few days.

Then put the smaller PVC pipe with plug/cap into the big PVC pipe, then (this might be hard) keep the small pipe in place and pull the large pipe straight out of the bucket (at the angle it’s laying at). The idea is to pull that shell off around the core, leaving the core in place, then cover with the last 0.5-1″ of sand, especially covering the cavity that will be left where the pipe came out.

Once the sand is settled in the bucket, slowly lower the bucket into the fuge. Slowly to minimize disturbance to the sand. And remove the excess water (that you added when you took the bucket out)

Fuggetaboutit

The idea is that the core of sand you left will be the seed for the new sand, and it stayed anoxic the whole time so that bacteria is still ready to keep working. You’ve removed a lot of detritus from the bucket that’s been building up in the bucket’s sand, since you probably pulled at least 4g of sand. Plus you’ve removed a good portion of any nasty zones in that DSB. I think having two buckets is best, so one is about halfway through its maintenance cycle (and has hard-working bacteria) when the other one just got refreshed.

I suspect you could clean one bucket per year or even every other year with two buckets running, most people that reported substantial problems with DSB toxicity didn’t run into that until 4+ years in, and that was when the sand was disturbed in the tank water, which won’t happen here. So, filling two buckets might cost $50 on the first bucket, and $50 on the second bucket a year later. Then about $50/yr to replace the sand if you toss/replace the sand; I plan to try to clean my sand and put it back, making this free except the tap water used for cleaning.

I plan to initially do a bucket every year (so the bucket is in for two years before cleaning), but hope to extend that to a bucket every other year (bucket is in for four years) if all is going well.

Note: you can do this smaller. you could use little paint mixing pails (the all-plastic ones), there’s really only a few rules if you want to follow my idea:

It must fit at least 6″ of sand depth. if it’s less than 6″, you won’t get much anoxic zone as the top inch or two still has a fair amount of oxygen.

It should probably be 4-6″ wide or more. It doesn’t actually matter because the bucket wall is solid and won’t let oxygen in. The important part is to be able to get a core to preserve for seeding the new sand. in a 4″ pail, you can probably use a 1″ PVC pipe and be ok, though pushing the sand out of the 1″ pipe will be pretty tough.

Fits somewhere in your setup, preferably two of these fit. This is probably mostly a footprint issue, but it must be short enough that wherever it is, the rim is underwater. You could probably run the bucket somewhere else, letting a little water from a manifold run over the bucket surface and taking the overflow off the top somehow, but that seems like too much work.

You might want it dark, it certainly doesn’t need light. a little algae growing on the surface won’t hurt anything, might actually help with nutrient export if you pluck the algae off occasionally (without disturbing the sand).

If two don’t fit, you’re probably fine...just know that it may lack some stability since the whole solution is brand new instead of 50%. Make extra sure to keep the situation anoxic while refreshing the bucket, and you’ll still keep it going.

If you’re really needing a custom thing, you could build your own ‘bucket’ out of acrylic or something, actually having a clear panel would be fun to watch. Just be sure to seal it well so the whole depth is anoxic, and make sure it’s short enough to have water exposure on the top. Oh, and don’t do a divided reservoir like dosing containers; you need to take one out at a time while the other stays put.

#reef #tank #DIY #deep sand bed #dsb #nitrogen cycle #denitrification

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