CSTR using anaerobic mixed culture with coal as the substrate. HRT=SRT. Started at 40 day, dropped to 30 day, and ended on 20 day RT. Looking for other examples where increased growth rate improved the apparent yield in a chemostat or CSTR.
Dear Dr. Papendick, Is the experiment conducted at least in duplicate? It is quite strange that for 40 days the yield is 24 µmol/g, for 30 days 21µmol/g and for 20 days increases again until to 48µmol/g. Probably the HRT of 40 days leads to a insufficient OLR that affect the growth of methanogens, also linked to a low degradable substrate. Which kind of substrate did you feed to reactor?
Giorgia
why the choice of coal as substrate in the first instance? Is coal biodegradable? my thinking is that 20days may be the retention time for that particular substrate, since some materials have very short retention time. The micro-organisms that convert wastes to biogas have life span and may have reached the peak of their activity in 20 days. If the release of certain chemicals in the process of degradation of the coal is not conducive to the growth of the micro-organisms, they begin to die and this will affect the yield of biogas as experienced in the longer retention time. You need to also analyze the slurry at 30 and 40 days, check the pH to find out if the system has become acidic and experienced shock.
Thank you Giorgia and Akozuo. Unfortunately, there was not a repeat experiment so I'm currently searching the literature for potential reasons. Future experiments will need to confirm this trend and seek to justify the reasoning. If you want to see motivation for methanogenesis from coal, check out the Papendick et al. 2011 publication. Regarding pH, the reactor was reduced from 7.55 to 7.50 and 7.45 in the 40, 30 and 20 day RT. Although this is a narrow reduction, I have seen literature suggesting that 7.5 is an inhibitory threshold. I also like the concept of inhibitor production due to cellular death. Do you have any supporting literature for this?
Gas is stored in coal as an adsorbed component on or within the coal matrix and as free gas within the micropore structure or cleats within a coal bed. The gas is held in place mainly by reservoir pressure; reducing the reservoir pressure allows gas to be released from the coal. (http://www.ags.gov.ab.ca/energy/cbm/coal_and_cbm_intro2.html) This coludo be by the size and porosity of the coal you used. Also, as Akuzuo said, since its biodegradability can be very slow and probably requires specific bacteria to degrade it (microflora present in water leached from coal mines were shown to generate methane), you have to evaluate the quality of your inoculum in terms of its affinity and capacity to degrade the coal. In the article of Strapòc et al (http://aem.asm.org/content/74/8/2424.full) you can read about the enrichment of the adecuate inoculum that you require for the improvement of your experiment, so you do not have this yields. Also If you want to keep your bacteria, you can try to aclimate them by using a media with the adecuate mineral and nitrogen concentration, so they can grow normaly and have the chance to produce the enzymes they need to degrade the coal and generate methane. You can find other factors in the article of Jones et al Stimulation of Methane Generation from Nonproductive Coal by Addition of Nutrients or a Microbial Consortium
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2976240/
http://www.ags.gov.ab.ca/energy/cbm/coal_and_cbm_intro2.html
http://aem.asm.org/content/74/8/2424.full
As I understand you have a continuous feeding reactor?. You need to wait 3xTRH to reach a steady-state for your reactor and by this way you can expressed a real significant yield. Yes for a 40 days reactor you are steady state at > 120 days.
Normally, for conventionnal organic matter (food wate, manure..) if you raise SRT or HRT (CSTR) you raise the yield, but reduce the volumetric production of your reactor.
I don't know what is the experimental set-up, but do you measured the endogeneous methane production of your biomass without substrate? Because, here you've measured few milliter and endogeneous can affect systematically this value. Endogeneous production is affected by age, kind and amount of substrate received before, concentration of biomass etc...
I think It's better to run 3 experiment in parallel with 3 different SRT instead of serial test's to avoid modification of the biomass during time.
Also, in your results why if your X=0 you have different volume of methane (intercepts 616, 314 -111) is it logical in link with your experimental set-up???
Good luck!
Yann
Thank you for your response Yann. I now realize our run length was short. Consider the 3xRT noted for future design. It was a proof of concept run for the reactor, as biomethane potential assays on coal have been limited to serum bottle or Balch tube experiments. Coal is generally very low yielding in methane and is a solid/recalcitrant substrate, so I wouldn't consider it conventional. It has properties that make biomass quantification very difficult, so we haven't measured endogeneous metabolism. Also, the actual methanogenic pathway (coal-->intermediates-->methane) is not known due to the heterogeneous structure of coals. Regarding the intercepts, the headspace was never purged, so methane (and CO2) accumulated throughout the runs. I am just digging to see if anyone else has seen something like this, as a lead for future experimental designs. Thanks again!
There is obviously optimum conditions close to the 20 day SRT and the other SRTs if your system is being operated and monitored consistently for all conditions. Coal may be but is probably not, providing CO2 as an electron receptor if coal is a determinate in the oxidation and thermodynamics of the reaction. And/or coal may be the substrate-source providing some low level concentrations of light weight carboloxic organic acids at low concentrations for energy-NADH production; hence the low SRT. In any case coal is not normally considered a substrate for anaerobic growth. It can be a structural support for biological growth as failed activated carbon odor control systems can attest.
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