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Bioreactor Carbon Source Selection Process (8 replies)

Helena Russell
9 months ago
Helena Russell 9 months ago

I have read several texts and articles regarding bioremediation but have not run across a simple explanation of how one goes about selecting an effective yet economical carbon source for anaerobic and aerobic bioreactors. Currently I have seen the use of methanol, molasses, potato starch etc. as "feedstock" in bioreactors. Besides cost and local availability, how does one choose between carbon source alternatives? Is there a simple carbon bond strength metric or other simple starting point? I am sure this will result in a valuable discussion.

9 months ago
Unterstarm 9 months ago

A few years ago, I ran a lab that developed biologically-based treatment processes. Over the years, we evaluated a variety of carbon sources and came to learn that different applications call for different compounds.

For nitrate removal, methanol has been the reagent of choice, but ethanol was shown to be equally valid, on a dollar/mass of nitrate removed. According to the literature, temperature-sensitivity and process stability tend to favor ethanol.

I've seen this last issue (process stability) played out for the Nickel Plate Mine bioreactor, where methanol is used. Periodically, the denitrifying tank would become upset due to the invasion and establishment of filamentous fungi. These fungi altered the biomass/sludge bulk density and settle ability, causing all manner of trouble in this stirred tank. The bioreactor did not recover rapidly from these upsets.

In SRB bioreactors, I've used ethanol and ethylene glycol (EG) with equal success. I find that both reagents support stable sulfate reduction and their costs are comparable. I prefer EG because it is not flammable, has very low toxicity (as long as you get material without additives), and can be shipped anywhere in bulk quantities because it is used at airports for de-icing airplanes. Usually, the paperwork is also simpler.

Molasses is comparably cheap, but I worry that it is difficult to dose correctly, so that you get optimal biological activity and no excess BOD in the effluent. I've seen this problem (effluent with high BOD) twice and it killed the projects in both cases. I also know of commercial bioreactors that use molasses without a problem, so it is not a universal problem.

Other carbon sources are more project-specific, in terms of process stability, dosage optimization, year-round availability, cost, bulk shipping, etc. I would be disinclined to use them unless they offered a compelling advantage. 

Bill Fraser
9 months ago
Bill Fraser 9 months ago

The following papers/presentations might be a good starting point if you are considering solid carbon sources (as opposed to liquid feed stocks). As you alluded to, all "organics" are not created equal, and there should be a thoughtful and logical process to arrive at the correct organic source based on the site specific conditions. http://inside.mines.edu/~lfiguero/Figueroaetal2004IMWA.pdf https://www1.villanova.edu/content/dam/villanova/engineering/vcase/sym-presentations/2011/62_3hagerty.pdf

Marshal Meru
9 months ago
Marshal Meru 9 months ago

We have used the COD to sulfate ratio for comparison of carbon sources. Our mine water bioreactors are fed ethanol initially with a COD/Sulfate of 0.67 (theoretical ratio required for conversion). Since most of the ethanol is accounted for by increases in alkalinity, this is a fairly reliable baseline. Next a different carbon source is used, such as molasses, and a new COD/Sulfate ratio is determined which yields the same percent reduction as the ethanol. Provides a nice relative ranking of carbon sources.

9 months ago
Oberstorm 9 months ago

Being a practical guy and barring someone once again calling out my ignorance, I think you hit the nail on the head: logistical price and availability being the primary driver. Not to mention climate. Molasses is a pain on a hot day and you already know on a cold day.

While some organic carbon sources may work better than others and need some intermediate digestive activities to get to sulfate reduction, bugs will adapt, also considering the practical issues of cellulose or other indigestible or unintended feed contents. Based on this last thought, hoping it adds to the value of the subject, has anyone had unintended consequences from using a "creative" carbon source?

Bob Mathias
9 months ago
Bob Mathias 9 months ago

I have always viewed the carbon source question for BCRs as a continuum running from "candy & booze" to "main course ", to "heavy roughage" if you'll excuse the food & drink analogy. As it says above, the very easily utilized carbon sources are great but there are drawbacks to be considered. Pure animal manure fits in there somewhere too.

As we've discovered, you can reduce nitrate with solid phase organic sources like straw and corn Stover. You just need to make sure you account for the slower kinetics of these solid bio-sources. This leads to the inevitable consideration of organic substrate longevity. Some materials have faster degradation kinetics and are consumed rapidly, so you need less space for the BCR. But this means that you need to replenish/refurbish your bioreactor more frequently, which makes it less passive. The ultimate end point condition is when you're feeding your microbes constantly with molasses or alcohol; your microbial population is addicted and if you cut off their supply, they go into serious withdrawal. This is less likely with solid phase organic sources.

I agree with re: Figueroa et al. It's the best paper I've seen on this issue (at least for solid phase carbon sources). The authors evaluated five different species of wood, corn cobs, manure, and alfalfa hay among others. The reference (2003) is also downloadable for free from the American Society for Mining and Reclamation (ASMR) website.http://asmr.us/Publications/Conference%20Proceedings/2003/1112-Seyler.pdf

I encourage you to join ASMR.

There are some other "generic" references available too, such as "The American Muck Book" by J.D. Browne (I didn't make this up) that was published in 1852. Download a free copy of the out-of-print tome from Google Books (I think). I had a lot of great information. Mr. Browne was a century and a half ahead of his time.

With regard to comment above, I've only had unintended consequences once when we tried using mesquite wood chips in a BCR (our client had a large supply). This stuff was loaded with salts that were released in a series of bench scale tests. The TDS levels in the bench effluents were through the roof. We suspect that Tamarac wood might behave similarly but we've never dared to try it. We switched to pine wood chips; less plentiful at the site but it solved the problem.

I've rambled quite a bit. I hope you find some of this useful.

Marshal Dienes
9 months ago
Marshal Dienes 9 months ago

Are you planning on designing a passive or active treatment system? You might need to take this into consideration for choosing the correct substrate. Also from my research on anaerobic SRB passive treatment systems, I noticed a significant decrease in efficiency during low temperature months. If building an anaerobic system in a region that experiences harsh seasonal variations, a decline in biologic activity during winter months can reverse the redox environment of your bioreactor. Then your remediation "sink" becomes your new "source".

9 months ago
Hauptsturm 9 months ago

We tried formic acid on a high pH feed for nitrate removal, the acetic acid performed better.

Helena Russell
9 months ago
Helena Russell 9 months ago

I should have been a bit more specific. I am mainly interested in liquid carbon source alternatives for bioreactors in a plant environment.

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