Dear Cheng Ming

The four steps Fermentation not in step whole the process known as Fermentation

Stage 1- Hydrolysis:

 Hydrolysis is the breaking (“lysis”) of a large compound into small compounds by adding water (“hydro”). Insoluble components such as carbohydrates, fats and proteins undergo hydrolysis in this stage. Complex components are degraded into small soluble components by breaking their chemical bonds.

 Complex carbohydrates → simple sugars

Complex lipids → Fatty acids

Complex proteins → Amino acids

 Stage 2- Acidogenesis:

 In this stage, soluble components that were produced through hydrolysis; are degraded by facultative anaerobes and anaerobes. During degradation, carbon dioxide, hydrogen gas, alcohols, organic acids, some organic-nitrogen compounds and some organic-sulfur compounds are produced. Some of the other compounds are used to form new bacterial cells.

 Stage 3- Acetogenesis: Acetogenesis occurs in the acid-forming stage. Many of the acids and alcohols such as butyrate, propionate and ethanol may be degraded into acetate that will be used as a substrate by methane-forming bacteria and also carbon dioxide and hydrogen can form directly acetate by fermentative bacteria.

 Stage 4- Methanogenesis: In this step, methane is mainly produced from acetate and carbon dioxide and hydrogen gas. Here all of the compounds must be converted into compounds that can be used by methane forming bacteria. Acids, alcohols and other organic-nitrogen compounds cannot be used directly by methane-forming bacteria; as a result these components accumulate in the digester supernatant.

 Main Bacteria Which is Responsible for Biogas Process

Acetate-forming bacteria:

Sulphate-reducing bacteria

Methane-forming bacteria

Hydrogenotrophic methanogens( CO2 + 4H2   → CH4 + 2H2O)Please find attached herewith four steps fig,

Regards,

Prem Baboo

Dear Mr. Zhang,

As you may be aware, methanogens are the most vulnerable to pH changes among the several microorganisms required during the digestion process. Consequently, many studies have separated the methanogenesis phase from the other phases by using 2 reactors. As such, in the first reactor, hydrolysis, acidogenesis and acetogenesis take place and the end-products from the 1st reactor are then sent to the second reactor for methane production. Besides, in the 1st reactor, you'll expect a lower operating pH as compared to a much higher in the second reactor.

This practice is commonly employed for easily digestible substrates like food wastes which tend to reduce the pH upon rapid acid formation and these eventually result in suppression of methanogens.

Kind Regards,

Bundhoo Zumar

Dear Mr. Zhang,

in the literature you may find different divisions of the AD process: into 2, 4 or 5 stages (and also into 3 stages, but I didn´t find a proper source to link it). Therefore to avoid confusion in the future, I prepare also this comment for you:

The most common division, at least in my opinion, is the one explained in earlier post by Mr. Baboo (he also already answered the question concerning the fermentation….) .

Sometimes this division is extended by the 5th stage called disintegration: so conversion of complex matter into carbohydrates, proteins and lipids (just before hydrolysis), like this is done in the Anaerobic Digestion Model No. 1 (please have a look into ADM1 description from IWA task group; Batstone 2002 and see description of the parameter kdis).

Some older books describe AD process as 2 stages: because hydrolysis and acidogenic phases are linked closely to each other, just like acetogenic and methanogenic phases are also linked closely to each other [Deublein, Steinhauser. “Biogas from Waste and Renewable Resources”. 2011 Wiley-VCH Verlag GmbH & Co. KGaA. ISBN 978-3-527-32798-0].

However also in Deublein & Steinhauser [2011] the AD process for biogas production is described as 4 stages – and it is very good explained.

I hope this helps! Good luck! Best regards!  

If your question relates to the construction of the reactors; it is convenient to separate the acid formers from the gas formers since the reaction kinetics are very different for either.  Otherwise the reactions can be contained in one reactor that must be sized and controlled for both.  Regards,

It really depends on the perspective of the person who asks the question. Many years ago I wrote in the introducing paragraphs (p. 419) of a book chapter on acetogenesis that:

Substrate conversion in methanogenic environments is thought to occur in a multistep process (Fig; REFs). The number of subdivisions that can be made in these processes depends on the concept with which the overall process is approached. The reaction sequence can be divided into various “distinct” sub-processes (REF), or it can be dealt with at the level of stages (REF), or metabolic groups of organisms involved (REF), or of individual organisms (REF).

J. Dolfing (1987) Acetogenesis. In: A.J.B. Zehnder (Ed.) Biology of Anaerobic Microorganisms. John Wiley & Sons, pp. 417-468.

Nowadays we know a lot more details on these various steps, but the general observation is still valid.  The number of subdivisions or the level at which you lump reactions or processes together depends on your perspective or the research question.   

Build two reactors that can function in series, the first holding the acid formers and the second containing the gas formers.  There is lots of literature on how select for the two bacteria and how to build the reactors in Water Environment Federation research publications.

Simple classification i would suggest is the type of microorganisms, that function the fermentation process. Aerobes- hydrolysis, Facultative  anaerobes- acedogenesis & acetogenesis and anaerobes- methanogenisis.

to design a reactor, biphasic system is ideal, one involving open system to accomplish hydrolysis and acetogenesis. two, closed anaerobic system to perform partial acetogenesis and methanogenesis.

Regards

The AD process as a whole prefers to be warm, but in fact the first stages of the biology can very easily occur at lower temperatures, and often do. Thus depending on what you want to do, one means of 'separating' the stages is to establish a cold acid-forming unit/process followed by a mesophilic methanogenic stage. One difficulty you will encounter is that a slurry enriched by volatile organic acids will ordinarily not have a strong buffer system established, and of course at low pH, those same acids will be poisonous to the methanogens. Of course you can add a buffer at some point, but even so, you will need in any case to feed an acid enriched slurry to the methanogenic stage very slowly, and in the best instance, you should have characterized the acidity by titration or some other means, and feed according to a rate that your tests demonstrate are useful, remembering that the process can acclimate to increased feeding over time. Be sure to try a variety of inoculates.

If it were me, I would run both stages anaerobically.

David William House, author, The Complete Biogas Handbook, www.completebiogas.com

I could not add much more to David William House - this is how I design digesters for our more complex and variable feedstock.  The question of where does fermentation come in the series is simple, the decomposition process in the absence of free oxygen is the fermentation. In practice, for most feedstock it is only possible to separate into two phases.   The trick is to get a good separation between methanogens and the [acid forming + acetate forming] bacteria. Any free hydrogen is a bonus downstream.  This separation relies on the difference in Umax or the generation time of the organisms involved.  Hours for the acid forming bacteria, days for the Methanogenic archea.  So by half-filling say a 5 day buffer tank and operating normally at 2-3 days retention time, at a pH of 3.8 -5.6 the methanogens (inhibited by the acid condition's) are excluded from the hydrolysis tank.   While acetoclastic bacteria will create a methane/CO2 split from acetate in the Methane-producing primary digester, very large amounts of  carbon dioxide from the plant come from the hydrolysis tank. This may be useful if one is bottling CO2 but it can also be a hazard to the stability of gas engines or downstream gas to grid equipment if pulses of CO2 are created and fed directly to the engines or membranes without first blending with the methane rich fraction from the primary digester. The flow direction in the biogas pipe to the end user is a process sensitive issue. With short retention times in the buffer - acid hydrolysis tank, there will be by the laws of CSTR's a lot of carry over of short-circuited un-hydrolysed material and there are some benefits to putting two acid hydrolysis tanks in series in order to enhance the separation - especially useful if there are recalcitrant solids in the feedstock.  However, in practice, unless the feedstock is very regular (impossible in waste digesters), it is extremely difficult to maintain more separations than two phases even if there are two hydrolysis tanks in series. A possible exception is if they are operated at significantly different temperatures, say a cold and a mesophilic or cold and thermophilic unit. I prefer cold acid hydrolysis for most feedstock. Both phases of a two phase system need good quality seed for start up and the start up requires accurate Total VFA to bicarbonate ratio analysis as stated above. Two phase digesters are my favourite reactors - robust, responsive and very efficient.  It is important to remember that a spoon of vinegar in a digester is consumed in 15 seconds typically - hydrolysis or making the acetate / hydrogen is the rate limiting step.