Parameter pH

The equation below gives the relationship between pH and hydrogen ion concentration in mol mL-1:

Water with a hydrogen ion concentration of 10-6molL-1 or 10-4gL-1 has, for instance, a pH value of 6.

Because of the hydrogen transportation by NAD, different products of fermentation are developed: the H+ ions isolated from the substrate are carried over to the uncharged NAD. The NAD molecules so charged (NADH + H+) regenerate (oxidize), by forming H2 molecules:

This reaction occurs independently of the hydrolysis and acidification of hydrocarbon and proteins. Hydrocarbons are easier to acidify, and no pH-buffering ions are released as with the degradation of proteins. Therefore the pH value decreases more easily. With the degradation of carbohydrates, the partial pressure of hydrogen increases more easily, as with other substances. This happens in combination with the formation of reduced acidic intermediate products.

Even when the hydrolysis and the acidification occur in different aparatuses and are separated from the methanation, complete suppression of the methanation is almost impossible.

The pH optimum of the methane - forming microorganism is at pH = 6.7-7.5. Therefore, it is important to adjust the pH-value in the second stage higher than that in the first stage of a two-stage biogas plant. Only Methanosarcina is able to withstand lower pH values (pH = 6.5 and below). With the other bacteria, the metabolism is considerably suppressed at pH < 6.7.

If the pH value sinks below pH = 6.5, then the production of organic acids leads to a further decrease of the pH value by the hydrolytic bacteria and possibly to cessation of the fermentation. In the reality, the pH-value is held within the neutral range by natural procedures in the fermenter. Two buffering systems ensure this.

A too strong acidification is avoided by the carbon dioxide/hydrogen carbonate/carbonate buffer system. During the fermentation, CO2 is continuously evolved and escapes into air. With falling pH value, more CO2 is dissolved in the substrate as uncharged molecules. With rising pH value, the dissolved CO2 forms carbonic acid, which ionizes. Thus, hydrogen ions are liberated.

At pH = 4 all CO2 is as free molecules; at pH = 13 all CO2 is dissolved in the form of carbonate in the substrate. The center around which the pH value swings with this system is at pH = 6.5. At a concentration of 2.5-5 gL-1, hydrogen carbonate gives particularly strong buffering.

A too weak acidification is avoided by the ammonia- ammonium buffer system (Figure 2.20). With falling pH value, ammonium ions are formed with release of hydroxyl ions. With rising pH value, more free ammonia molecules are formed.

Ammoniak Ammonium Buffer System

Figure 2.20 Ammonia-ammonium buffer system.

Figure 2.20 Ammonia-ammonium buffer system.

The center, around which the pH value swings with this system, is at pH = 10.

Both buffering systems can be overloaded by a feed of particularly rapidly acidifying waste water or organic material, by toxic substances, by a decrease in temperature, or by a too high volume load in the bioreactor; e.g., by feeding waste water out of a starch processing plant, which incurs the possibility of acetic acid toxification. Consequences are:19'

• increase in the amount of uncharged fatty acid molecules -this leads sometimes to increase in the hydrogen content in the substrate and CH4 production, sometimes to the detriment of the methanation

• inhibition of the methanation by increase in the proportion of unhydrolyzed inhibitors, e.g., sulfide

• rise of the pH value due to degradation from sulfate to H2S

• inhibition of reactions by rise in the proportion of free ammonia.

A drop in the pH)value and a rise of the CO ) in the biogas is an indication of a disturbance of the fermentation process. A first sign of the acidification is the rise of the propionic acid concentration. Measures for the prevention of excessive acidification are:

• Stoppage of the substrate supply, so that the methanogenic bacteria are able to degrade the acid

• Reduction of the organic space load (increase of the residence time'

• Increase of the buffering potential of the substrate by addition of selected co-substrates, in particular if the buffering potential of the substrate is small. It must be taken into consideration that the buffering potential changes because of the removal of the CO2

• Continuous removal of the acids

• Addition of neutralizing substances: milk of lime (CaO, Ca(OH) 2), sodium carbonate (Na2CO3), caustic soda solution (NaOH'

• Addition of diluting water

• Emptying and restarting the fermenter.

As a result of the feed of special caustic solutions for adjusting the pH-value or of the addition of cleaning and disinfecting agents, values of pH > 10 can arise in the reactor, which will lead to an irreversible loss of the activity of the bacteria.

Table 2.7 C/N ratio of organic wastes.201

Waste

DM content

Organic substances % of DM

C/N ratio

Straw

ca. 70

90

90

Waste from sawmills

20-80

95

511

Paper

85-95

75

173

Waste from housholds

40-60

40

18

Sewage sludge

0.5-5

60

6

Cleaning and disinfecting agents should therefore be tested for their inhibiting potential before their first application in the plant.

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