**2.2.5 Inhibitory substances**

In anaerobic digestion systems a characteristic phenomenon can be observed. Some substances which are necessary for microbial growth in small concentrations inhibit the digestion at higher concentrations. Similar effect can have high concentration of total volatile fatty acids (tVFA's). Although, they represent the very substrate that methanogenic archaea feed upon the concentrations over 10,000 mgL-1 may have an inhibitory effect on digestion (Mrafkova et al., 2003; Ye et al., 2008).

Inorganic salts can significantly affect anaerobic digestion. Table 4 shows the optimal and inhibitory concentrations of metal ions from inorganic salts.


Table 4. Optimal and Inhibitory concentrations of ions from inorganic salts

In real operating systems it is unlikely that inhibitory concentrations of inorganic salts metals would occur, mostly because in such high concentrations insoluble salts would precipitate in alkaline conditions, especially if H2S is present. The most real threat in this case is sodium inhibition of anaerobic digestion. This can occur in cases where substrates are wastes with extremely high salt contents (some food wastes, tannery wastes…) or when excessive use of sodium substances were used in neutralization of the substrate or the digester liquid. Study done by Feijoo et al. (1995) shows that concentrations of 3000 mgL-1 may already cause sodium inhibition. However, anaerobic digestion can operate up to concentrations as high as 16,000 mgL-1 of sodium, which is close to saline concentration of seawater. Measures to correct the sodium inhibition are simple. The high salt substrates must be pre-treated to remove the salts (mostly washing). The use of sodium substances as neutralizing agents can be substituted with other alkaline substances (such as lime).

Heavy metals also do have stimulating effects on anaerobic digestion in low concentrations, however higher concentrations can be toxic. In particular lead, cadmium, copper, zinc, nickel and chromium can cause disturbances in anaerobic digestion process. In farm wastes, e.g. in pig slurry, especially zinc is present, originating from pig fodder which contains zinc additive as an antibiotic. Inhibitory and toxic concentrations are shown in Table 5.

Other organic substances, such as disinfectants, herbicides, pesticides, surfactants, and antibiotics can often flow with the substrate and also cause nonspecific inhibition. All of these substances have a specific chemical formula and it is hard to determine what the behaviour of inhibition will be. Therefore, when such substances do occur in the treated substrate, specific research is strongly advised to determine the concentration of inhibition and possible ways of microorganisms adaptation.

In anaerobic digestion systems a characteristic phenomenon can be observed. Some substances which are necessary for microbial growth in small concentrations inhibit the digestion at higher concentrations. Similar effect can have high concentration of total volatile fatty acids (tVFA's). Although, they represent the very substrate that methanogenic archaea feed upon the concentrations over 10,000 mgL-1 may have an inhibitory effect on

Inorganic salts can significantly affect anaerobic digestion. Table 4 shows the optimal and

Sodium 100-200 3500-5500 16000 Potassium 200-400 2500-4500 12000 Calcium 100-200 2500-4500 8000 Magnesium 75-150 1000-1500 3000

In real operating systems it is unlikely that inhibitory concentrations of inorganic salts metals would occur, mostly because in such high concentrations insoluble salts would precipitate in alkaline conditions, especially if H2S is present. The most real threat in this case is sodium inhibition of anaerobic digestion. This can occur in cases where substrates are wastes with extremely high salt contents (some food wastes, tannery wastes…) or when excessive use of sodium substances were used in neutralization of the substrate or the digester liquid. Study done by Feijoo et al. (1995) shows that concentrations of 3000 mgL-1 may already cause sodium inhibition. However, anaerobic digestion can operate up to concentrations as high as 16,000 mgL-1 of sodium, which is close to saline concentration of seawater. Measures to correct the sodium inhibition are simple. The high salt substrates must be pre-treated to remove the salts (mostly washing). The use of sodium substances as

neutralizing agents can be substituted with other alkaline substances (such as lime).

Heavy metals also do have stimulating effects on anaerobic digestion in low concentrations, however higher concentrations can be toxic. In particular lead, cadmium, copper, zinc, nickel and chromium can cause disturbances in anaerobic digestion process. In farm wastes, e.g. in pig slurry, especially zinc is present, originating from pig fodder which contains zinc additive as an antibiotic. Inhibitory and toxic concentrations are

Other organic substances, such as disinfectants, herbicides, pesticides, surfactants, and antibiotics can often flow with the substrate and also cause nonspecific inhibition. All of these substances have a specific chemical formula and it is hard to determine what the behaviour of inhibition will be. Therefore, when such substances do occur in the treated substrate, specific research is strongly advised to determine the concentration of inhibition

**Moderate inhibition [mgL-1]** 

**Inhibition [mgL-1]** 

**2.2.5 Inhibitory substances** 

shown in Table 5.

and possible ways of microorganisms adaptation.

digestion (Mrafkova et al., 2003; Ye et al., 2008).

inhibitory concentrations of metal ions from inorganic salts.

**Optimal concentration [mgL-1]** 

Table 4. Optimal and Inhibitory concentrations of ions from inorganic salts


1As inhibitory concentration it is considered the first value that shows diminished biogas production and as toxic concentration it is considered the concentration where biogas production is diminished by 70 %.

Table 5. Inhibitory and toxic concentrations of heavy metals
