What’s In Your Substrate?

[Dan_P]

After several conversations with @taricha and @brandon429, I became interested in measuring the amount of organic material in the substrate of my fish only system. One method I found uses basic KMnO4 to estimate the chemical oxygen demand (COD) in a sample. The amount of KMnO4 consumed correlates to total organic carbon (TOC) present. I have just started to play around with this method and haven’t done anything serious about validating it or comparing the KMnO4 consumed to the TOC value from a Triton N-DOC test. I can hear @Rick Mathew tsk tsk’ing now What I did first was to look at the material that shakes loose from aquarium substrate. Here is the experiment.

I very gently scooped 20 mL of substrate from the aquarium, being careful to avoid losing anything. I dipped down about an inch which gave me a mixture of surface substrate and some deeper material. I can already see that a standard sampling is needed. I transferred the sand to a stoppered flask and very gently swirled it with 32 mL of tank water for 15 seconds, making sure I turned over all the substrate to free all the loose material. I followed this with 3-30 second very vigorous shakings. I measured the COD of the wash water after each wash. The plot below shows the cumulative COD. I also photographed the wash sample for the first three washes.

Observations and thoughts. The total change in absorbance 0.27 is 10x higher than the value for just aquarium water. This is more impressive when you consider that this amount of material being concentrated in the substrate and adjacent to a small amount of pore water. Quite a soup!

My first gentle wash is a model of substrate vacuuming. I tried to duplicate the turbulence I remember seeimg when I used to vacuum substrate. If I got this right, and I am way out on a limb here, vacuuming removes half the organic material. It takes very, very turbulent mixing to remove more material. I did not figure a good way to measure everything adhered to the substrate so I could determine when the washing was near completion. The measurement probably reflects a digestion of organisms (sorry guys) and organic matter In the substrate.

I would be interested in your thoughts that came to mind when you read this post.

Dan

Here is follow up testing on the combined substrate washings. In the first part of this experiment I used tank water to remove material from a sample of substrate In a series of rinses. I combined those rinses and measured the 5 day biological oxygen demand. I also measured the BOD5 of the tank water used for rinsing. By the end of the five days, the rinse sample had cleared and the bottom of the BOD flask was covered with a coating of fine particles. Tests were conducted on the clear liquid. In addition to measuring the oxygen consumed by these samples, I tested for PO4, alkalinity, NH3, NO3 and chemical oxygen demand.

As you can see in the table below, the “substrate rinse” was quite a digestible suspension. I think this in line with @brandon429 thinking. The “tank water” used to rinse the substrate contributed little if anything to the analytical results of the “substrate rinse“ sample. There was little chemical oxygen demand in the “substrate rinse”. It seems that all of the COD for the “substrate rinse” resides in the solid that settled out (last row in table). Adjusting the “suspended solids“COD for dilution, it accounts for about half of the initial COD In the “substrate rinse” sample. Whether this COD would be reduced further with a longer digestion period remains to be answered. @taricha has much evidence that certain types of detritus/grunge are not subject to reduction by biological digestion. Clearly, we have a can of worms to go through. Keep in mind that Jonathan’s observations are based on a large number of experiments whereas I have an N=1 database.