Environmental Engineering Reference
In-Depth Information
32.3.2.2.5 Inhibitory Substances
In anaerobic digestion systems, a characteristic phenomena can be observed. Some substances, which are
necessary for microbial growth in small concentrations, inhibit the digestion in higher concentrations. A
similar effect can be had from a high concentration of total volatile fatty acids (tVFAs). Although they
represent the very substrate that methanogenic bacteria feed upon, the concentrations over 10,000 mg/L
may have an inhibitory effect on digestion (Mrafkova et al. 2003; Ye et al. 2008).
Inorganic salts can significantly affect anaerobic digestion. Table 32.5 shows the optimal and
inhibitory concentrations of metal ions from inorganic salts.
In real operating systems, it is unlikely that inhibitory concentrations of inorganic salts of metals
would occur, mostly because in such high concentrations ions would precipitate in nonsoluble salts in
alkaline conditions, especially if H
2
S 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, etc.) or when excessive use of sodium substances were
used in neutralization of the substrate or the digester liquid. A study done by Feijoo et al. (1995)
show that concentrations of 3000 mg/L already show occurrence of sodium inhibition. However,
anaerobic digestion can operate up to concentrations as high as 16,000 mg/L of sodium, which is
close to the saline concentration of seawater. Measures to correct the sodium inhibition are simple.
The high salt substrates must be pretreated 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 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. For farm waste especially, zinc
is present (in pig slurry) and it usually originates from pig fodder that contains zinc additive as an
antibiotic. Inhibitory and toxic concentrations are shown in Table 32.6.
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 behavior 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 microorganism adaptation.
32.3.3 a
naEroBic
d
igEStion
t
Echnology
Basic block schematic of anaerobic digestion (Figure 32.3) shows that technological process of
anaerobic digestion is relatively simple. It consists of three basic phases: substrate preparation and
pretreatment, anaerobic digestion itself, and posttreatment of digested material with biogas use.
In this section all of the processes will be elaborated in detail.
32.3.3.1 substrates and Pretreatment
In general, all types of biomass can be used as substrates as long as they contain carbohydrates,
proteins, fats, cellulose, and hemicellulose as main components. It is, however, important to
taBle 32.5
optimal and Inhibitory concentrations of Ions from Inorganic salts
optimal concentration [mg/l]
moderate Inhibition [mg/l]
Inhibition [mg/l]
Sodium
100-200
3500-5500
16,000
Potassium
200-400
2500-4500
12,000
Calcium
100-200
2500-4500
8000
Magnesium
75-150
1000-1500
3000
