Just dumped 16" deep sand bed. No signs of anarobic activity!

[LouH]

For several years I've had a 14"-16" deep remote deep sand bed (RDSB) in a 20 gallon Brute garbage can. I moved from southern California to northern Navada in October 2010. At that time I moved my tank/system in a U-Haul to its new location. I left the sand in the Brute garbage can and moved the whole thing without breaking it down. When I set up the system in its new home, I simply plumbed it (the RDSB) back into the system. Note that I attempted, with vairable success, to maintain salinity and temperature in each vessel containing live rock, sand, corals, fish, etc. I had one coral casuality and one cuke casualty (1 out of 5). When I got my cabinet assembled, put the tank put in place, and otherwise got the system up and running, I actually had very good growth for several months. This growth did slow down over time.

Fast forward to this morning. I decided to change out the Brute garbage can because over the years it has accumulated bulkheads of various sizes here and there as I experimented with circulation pumps, different plumbing configurations, etc. Having seen bulkheads fail radially at the flange, I decided that all of these unused bulkheads were liabilities, so it was time for a change.

I pulled the old Brute garbage can out of its home in the side cabinet and onto a thick visquine sheet that I had laid out on the floor. I had two plastic tubs on hand to receive sand as I scooped it from the old garbage can. To my amazement, at no point did I run into black patches of sand or detect any hint of sulfur small. The sand at the 16" inch (bottom) was just as clean as the sand on the surface. What does this mean? I've never been able to measure nitrates in my system, so I have to imagine that I'm getting de-nitrification somewhere. Is my RDSB doing anything, or is it simply clean? For comparison, I siphoned out some of the deeper sand (3") pockets in my main display, and it was nasty. Black spots in the sand were evident as was the familiar sulfur smell that accompanies anarobic microbial activity.

Notable facts include the use of sugar fine sand in the RDSB, and that the RDSB receives overflow water from a 20 gallon refugium, which recieves a slip stream from the sump's return line. I'd estimate the flow through the refugium and RDSB to be 1.5 gpm.

How do you guys interpret this observation?

Lou

 

[elegance coral]

It all makes sense to me. The fact that you can't see anaerobic activity, doesn't imply there's none taking place. After all, these are microbes.

I don't know your system, so all I can do is guess as to what's really taking place. From what you're saying, it sounds like the bulk of the organic matter is ending up in the sediments of the display. Then water moves relatively slow through the fuge, allowing more particles to settle out. By the time the water reaches the trash can full of sand, there's very little solid material to settle and rot. The microbes living here have to survive on dissolved nutrients that were released through decomposition in other areas of the system. This can produce plenty of nutrients to support microbial life in the can of sand without seeing visible signs of their existence.

For me, it's a lot easier to remove the solid organic matter, and the nutrients it contains, before those nutrients enter the water, than it is to try and fight the nutrients after they've made it into the water.

 

[elegance coral]

Sorry. double post

 

[LouH]

The refugium just upstream of the remote deep sand bed has a 3" deep sand bed. Prior to this, it simply held live rock with a bare bottom. Regardless the contents, the refugium does build up detrius. Because of the low flow rate through the vessel, it is essentially acting as a gravity clarifier. I've been contemplating removing all rock and sand from the refugium and using the vessel as an alternate location to run my GFO and GAC reactors. Right now they share a sump with my protein skimmer, and it is too crowded in there for my taste.

At any rate, the biggest issue in my system is the sand in the main display. The reason why it has pockets of deep sand is that I've added sand to areas where powerhead flow displaced the original sand. Over time this has led to areas of deep sand (2"-3") which clearly show accumulations of detritus and anarobic activity. I plan on siphoning that sand out of the display and using a more course grain sand to cover the tank bottom only in the front of the display.

I suppose that the ultimate question is whether or not the remote deep sand bed is performing significant denitrification activity for my system?

Lou

 

[mc-cro]

I agree, the sand in the brute was clean, because there were no major organics to break down in the sand.

A remote "DSB in a bucket", contrary to some people's understanding, is not the same thing as a functioning DSB in the tank. yes, your remote sand bed is doing something, a remote DSB will process nitrates, and will suck up a lot of phosphates. Generally, if set up like you have done, they will not have to process or accumulate the waste like a tradional DSB.

a DSB in your display is more like a litter box. fish poop in it and it gets buried. I'm not hating on it, I have always run a DSB. But those organics do get buried in the sand, that is where you get the black sand and the sulfer smell.

 

[reefnut2012]

I agree, the sand in the brute was clean, because there were no major organics to break down in the sand.

A remote "DSB in a bucket", contrary to some people's understanding, is not the same thing as a functioning DSB in the tank. yes, your remote sand bed is doing something, a remote DSB will process nitrates, and will suck up a lot of phosphates. Generally, if set up like you have done, they will not have to process or accumulate the waste like a tradional DSB.

a DSB in your display is more like a litter box. fish poop in it and it gets buried. I'm not hating on it, I have always run a DSB. But those organics do get buried in the sand, that is where you get the black sand and the sulfer smell.

I ran a DSB in a 300 gal reef with high fish/feeding load for years.Based on my experience, they do get buried to some extent. That is why it is imperative to have sand sifting creatures in your DT to maintain the DSB ie , cucumbers, brittle stars, etc.

 

[tmz]

The facultative heterotrophic bacteria that denitrify need organic carbon to thrive as well as nitrogen and phosphorous .
In a deep remote sand bed they get very little via diffusion so the bed is much less effective at removing PO4 and NO3 as you go deeper without a source of organics. Less NO3 and P4 will get down there too.
If a carbon source is buired in low flow deep sand , the bacteria will use up the available oxygen then turn to nitrate for it . When the nitrate is exhausted (anoxia) ,any remaining buried organic carbon will be used by sulfate reducing bacteria which use the sulfate(SO4) for oxygen and produce sufides( black spots) and hydorgen sufide as by products. So if the sand is clean there is nothing or very little to degrade permeating it.