Dear Amin Hedayati

Environmental factors which influence biological reactions, such as pH, Temperature, nutrients and inhibitors concentrations, are amenable to external control in the anaerobic digestion process.

pH: Acetate and fatty acids produced during digestion  tend to lower the pH of digester liquor. However, the ion bicarbonate equilibrium of the carbon dioxide in the digester exerts substantial resistance to pH change. This resistance, known as buffer capacity, is quantified by the amount of strong acid (or alkali) added to the solution in order to bring about a change in pH. Thus the presence of bicarbonate helps prevent adverse effects on microorganisms (methanogens) which would result from low pH caused by excessive production of fatty acids during digestion. Proteins and other organic compounds, as well as bicarbonate, take a part in the buffering capacity and the resistance to changes in pH.

FACTORS THAT CAN AFFECT THIS COMPLEX SYSTEM:

Competition between different organisms for the same substrate Synergies between different groups of organisms.

Physical/Chemicals factors:

1.feedstock composition –

2. Fedstock structure –

3. pH –

4. pH  equilibriums –

5. Nutrients (trace elements)

6.  inhibition(VFA, ammonia) –

liquid/gas transfer etc. (H2 partial pressure) –

 Chemical reactions are catalyzed by biological enzymes (biochemical) Process.

Each group of organisms have an pH optima for maximum rate of reaction

methanogens: pH7-8 optimal, pH 6.5-8.5 operational

fermentation: pH 5-7 optimal, hydrolysis: pH 5-7 optimal

Optimal pH gives higest rate related Deviation from optimum value could be:

-Introduced with the influent -Consequenced by excess production and accumulation of acidic or basic conversion products such as VFA or Ammonia.

 e.g.   Ammonia NH3 + H2O  → NH4+ + OH

Free ammonia (non ionic) is more inhibitive to the AD process that the ammonium ion so for a given ammonia-nitrogen concentration lower pH is favored by the process.

Non ionic form of acetate is able to pass throughth emembrane. At low pH(8 )acetic acid Is in his ionic form, and  it is unable to pass the membrane  causing  an accumulation   outside the cell,

Is mainly present in protein (also Urea) In the AD nitrogen is converted to ammonia

Ammonia is known to be beneficial at low concentrations (about200mg/l)

But can be inhibitive at high concentration:

Complex inhibition mechanism Ammonia is antagonistic/synergistic with other substances  Anaerobic digesters can become acclimatized to high ammonia concentrations:

Hydrogen producers: acidogens and acetogens,thermodynamics unfavourable at high hydrogen concentration.Hydrogen consumers: hydrogenotrophic methanogens and homoacetogens, require hydrogen as substrate to produce methane and acetate.

Hydrogen plays an important intermediary role during  acetogenesis, as the reaction will only  occur if the hydrogen partial pressure is low enough to thermodynamically  allow the conversion of the acid such lowering of the partial pressure is carried out by hydrogen scavenging bacteria.

Please find attached herewith related documents

Regards,

Prem Baboo

Dr. Amin

may be your system is overloaded try to stop feeding couple of days and re-operate by lower concentration and higher retention time; it can help in many cases such yours.

best wishes

Dear Mr Hedaiati

I research the AD of wastes with low ratio alkalinity / acidity and I can control the stability by usie  limestone  in the acidogenic phase . Please found atached the file

It is possible that the retention time of 14 days is not adequate enough to allow completion of the acid forming processes. You may consider increasing the retention period. Overloading of the digester may also be a reason for the  considerable decrease in pH as this allows accumulation of acids which lower the pH. Consider reducing the loading rate, or raising the pH by adding lime

Dear Mr. Hedayati,

I will go with the aforementioned answers that overloading, low retention time and not enough alkalinity in your system are the main reasons for the decrease in pH. From practical experience, I would suggest adding more CaCO3 than reported in literature! Besides, even after correcting these parameters, it may be possible that your system takes some time to start functioning properly or does not function at all. The main reason being that a pH of 4.8 may already be fatal to the methanogens! In that case, you will need to re-seed the system.

Depending on how your system is, you could also use an automatic pH control system (feedback control). Otherwise, add small amount of CaCO3 daily to prevent any pH drop until you observe that the system has stabilised.

Alternatively, you could separate the methanogenic phase from hydrolysis, acidogenesis and acetogenesis by using 2 separate reactors.

You could also decrease the F:M ratio by either decreasing the inflow or increasing the concentration of microorganisms. This could stabilise your system.

Hope these suggestions help.

Dear Mr Hedaiati

A lower pH of the system means that volatile fatty acids are not effectively converted to CH4. This may be because the water retention time is too short or there is too little sludge in the reactor and there is not enough methane archaea.

Hi,

I am running a lab scale homoacetogenic reactor (batch) . It is fed with Hydrogen gas only in the head space (3 bar) and supplied CO2 as sodium bicarbonate. Objective is to produce acetic acid.

At the start of batch pH of the fermentation medium is around 7.5 to 8.0, but at the end (approximately after 10 days) pH raised to 9.5 or even some time goes to 10.2 , but at the end of batch I get higher concentration of acetic acid.

Why pH increases unexpectedly , even though it produces significant amount of acetic acid?