Dear Cristiano,

the answer to your question is YES, but you need also to measure the change of pressure inside your batch reactor over time, to produce a cumulative hydrogen production curve you can later on model using the Gompertz Equation.

The change in pressure over time will tell you how much biogas is produced by fermentation. Being the head space (V) of the batch reactor constant as well temperature (T), the pressure change is equal to mole change over time (pV=nRT). Knowing the number of moles you can transform it to a volume knowing that 1 mole = 22.414 litres at 1 atm and 0°C. Some math is in between.

As soon as you have the entire volume of biogas produced over time, you use the data on hydrogen percentage to evaluate the volume of hydrogen in the sistem.

To monitor the pressure in the reactor you should use a pressure-meter system.

You can find something already in the market looking for systems similar to those used for BOD (Biochemical Oxygen Demand) measure. Oxytop, Oxycup, something like that.

Some new equipments are able to track not only negative but also positive pressure.

Take anyway info from all those systems used for BMP tests (Biochemical Methane Potential). They can be used for Hydrogen production test as well.

Please do not hesitate to contact me for further info ([email protected]).

I would be happy to help and may see whether we can excahge some experiences.

We are busy in hydrogen production too.

Looking forward to hearing from you,

ciao

Luca

I would be delighted to explain the microbiological activity of the bioconversion process in the course of biogas production; which is predominantly producing methane and carbon dioxide. For the analysis of gas, GC is an analytical tools to define gas composition. The anaerobic process in early stage would deliver hydrogen as a result of hydrolysis of organic compounds occurs in primary stage. This stage can be expanded by controlling the process parameters; Microbial fuel cell using wastewater as substrate is partially achieved. In fact 7 clear stages were identified in the entire biogas process as hydrolysis proceeds; specific bacteria or hydrogen formers may appear. As pH of the culture dropped by acid formers the concentration of organic acids gradually increases. Control of pH and having defined ratio of C/N can assist us to precede methanogenic bacteria to lead the microbial pathway to methane formation. All biochemical reactions are catalyzed by mix consortium of anaerobic bacteria in a fast speedy hybrid systems. Study of rate equation can help us to project microbial mass production rate as Gompertz rate model like Logistic or even Monod are projecting the same expression with different assumptions. I would say by material balance in early stage with control of process parameter can achieve hydrogen production; however, it is not a sustainable process.

Dear Luca,

thank you very much for your comprehensive answer.

As you mentioned some systems have been derived from BMP tests, and actually that's exactly the point which I am more concerned about, when working with H2 production.

Most hydrogen-producing bacteria are inhibited by hydrogen partial pressure, while as far as I know, accumulation of methane does not have the same effect on methanogens.

So I am not sure if measuring the cumulative H2 curve without releasing the biogas would change the maximum H2 production, due to inhibition.

If this was the case, then measuring the productivity with the water displacement (releasing the biogas) and the pressure (without releasing the biogas) might lead to different results..

What do you think about it?

Better to measure H2 production with the more precise pressure method (with possible H2 production inhibition) or the less precise water displacement (but with a lower occurrence of inhibition)?

Dear Cristiabo,

Yes, it is available on the market

http://puregasproducts.com/h2sensorandalarm.htm

http://www.sciencedirect.com/science/article/pii/S0925400511003674

best wishes

Gabor

Dear Christiano,

I do not think ppH2 inhibition would be so important in this case (Clostridia could accumulate metabolic end-products easily). With appropriate operating conditions, you could also avoid the growth of methanogens.

HOWEVER, with no release of the partial pressure, you may favor specifically the growth of homoacetogenic bacteria and consequently underestimate the total amount of cumulated H2.

Do not hesitate to contact me if you have further question.

Best Regards

Eric

Ciao Cristiano,

you are right. Hydrogen producing bacteria are sensitive to high H2 partial pressure while methanogenic bacteria do not worry about biogas pressure at all.

And I also agree with your concern about the methods of measurements.

We use for example the water displacement method to evaluate the entire biogas production from batch tests (we use salt water at pH of 2 as measuring solution) and then we measure the quality of biogas evolution in the head space of reactors using a GC. We use this systems because they are quite cheap to be prepared. You buy a glass bottle, you use plugs with a hole in the midle and a silicon disc as sampling point. With few money you have the reactor ready to work.

Some collegues from Hamburg University (TUHH) were using the Oxytop systems.

My suggestion in this case is to use a "small" liquid volume and a "large" head space in order to keep the over-pressure as low as possible. Such systems have anyway a max pressure they can retain. Small liquid volume also means small amount of substrate to be fermented and therefore, small biogas production.

My experience and my feeling is that you can actually use the system you prefere for such a kind of experiments. You are right that both systems have advantages and disadvantages, but at this stage of the research they do not represent the main concern from my point of view.

The difference in data obtained from one system to the other is more related to "measurement errors" (volumes, GC analysys, weight of substrates, mistakes of students...) more than real effects on partial pressure onbiological methabolism.

So the suggestion is: use the system you prefere or the one you already have in your lab. Use the same systems for all the experiments you will run.

If your porpouse is to select some high-fermentative bacteria, put all of them is equal conditions. Say: test 1, low sustrate concentration (so small partial pressure); test 2, medium substrate conc and test 3, large substrate conc.. Once you select some best performing bacteria, you can better evaluate their specific behaviour using a system where you try to keep the partial-pressure of hydrogen as small as possible.

For example you can try to flush contantly nitrogen in the reactor.

Anyway, if you think to the application of dark fermentation in a full scale reactor, bacteria will have to deal with high hydrogen partial pressure anyway. Strategies for having low H2 partial pressure might be too expencive to be applied.

You can have anyway a look to pubblications of Prof. William Clarke, from the Queensland University in Australia. They used some membranes and developed ways of keeping H2 partial pressure as low as possible with promising results.

Hope this is useful and I remain available for suggestions and discussion.

Ciao!

Luca

Hi, we have some data on continues release and cumulative h2 production, both in batch and there is some difference. It's better to use a continue release system like mariotte tubes. Cristiano I will

E mail you this data

Gonzalo

Hi Christiano,

We use a simple syringe method to measure H2 cumulatively in serum bottles with black rubber stoppers. You pierce the stopper and remove 0.2 ml from the gas phase (be careful that the plunger is not pressed from the syringe, because the pressure may be high). Then you pull the syringe out of the bottle, until the needle is half-way the stopper. Then pull the plunger to the end of the syringe and then remove the syringe from the stopper. This is done to make sure that no hydrogen leaks from the syringe because of the overpressure in the bottle. Directly inject the entire 1-ml into the GC (molecular sieve column). If the GC is not near the sampling area, you can stick the syringe in a big stopper and then walk with it to the GC.

A standard curve can be prepared by adding known amounts of H2 to similar sized bottles and use the same sampling procedure. In this way you should be able to measure hydrogen production quantitatively. In this way you measure the H2 concentration per volume. Pressure in the bottle doesn't affect the calculation. It is realy very simple and we do it already for many years.Feel free to contact me for further questions.

Servé

Its relatively easy for the small lab scale batch reactors, if you know exact volume of the head space then increase in the percentage of gas over the time can be measured by taking sample with air tight syringe and injecting in GC, the volume can be calculated by ideal gas equation taking care of the temperature.

Dear Cristiano,

At first look, you can be right in the light of your explanation below.

"Most hydrogen-producing bacteria are inhibited by hydrogen partial pressure, while as far as I know, accumulation of methane does not have the same effect on methanogens.

So I am not sure if measuring the cumulative H2 curve without releasing the biogas would change the maximum H2 production, due to inhibition. "

But the solubility of H2 in water is very low. Specially under the conditions of biogas in anaerobic digestor, it's solubilty is more negligible in slurry medium and at the temperature of 50-60 C. So I think that the cumulative increase on the partial pressure of hydrogen in gas phase at the top of the digestor by time does not cause any increase on solved hydrogen concentration in slurry -aqueous phase by time, it means that there will be no change on hydrogen inhibitory effect since it's concentration will be at a fixed /saturated value. As a result, we can say that there will be no change on inhibition degree over time and you can measure hydrogen concentration cumulatively . Plase feel free for further questions if you have. Kind regards,

Celebi

Thank you all for your highly interesting comments and suggestions!

It\s really nice to have the possibility of such an exchange of experiences and expertise here.

Best regards,

Cristiano.

Increasing ppH2 inside the reactor might increase the pH as well and relatively affects the H2 yield..

The answer is a simple NO. It is mainly because Hydrogen is not only released by the cellulolytic bacteria, but by other organisms too.

So finally owns the "no" ?? :)

Well, actually I am not working with cellulosic substrates at all..

Not all fermentation process are carbohydrate-based..

as the FP7 project GRAIL (http://cordis.europa.eu/projects/rcn/110949_en.html) might show.