Environmental Engineering Reference
In-Depth Information
Table 2.6 Specific feedstock compounds and their effect on the AD process
(digestibility, disturbing effects and inhibitory effects)
Compound
Digestibility Feedstock
source
Process
disturbing
effects
Process inhibition
Fats
Very good
Slaughterhouses
Rendering plants
Foaming
Layering
Poor water
solubility
High VFA levels
Low pH
Proteins
Very good
Dairy processes
Pharmaceutical
industry
Foaming
pH decrease
High ammonia
concentrations
Carbohydrates
Sugars
Starch
Cellulose
Agro-industries
Crop residues
Animal manures
Foaming
Lignin
incrustation
pH decrease
Very good
Very good
Poor
Volatile fatty
acids (VFA)
Very good
Rendering plants
Oil mills
Poor water
solubility
High VFA levels
Organic
pollutants
Pesticides
Antibiotics
Detergents
Poor
Animal manures
Crop and crops
residues
Organic wastes
Foaming
Antibiotic effects
Source: Adapted after Steffen et al. (1998).
mixture, but also on other local conditions inside the digester (Steffen et al.,
1998). For example, an increased amount of volatile fatty acids (VFA) can
cause process imbalance if their concentration inside the digester exceeds the
pH buffer capacity of the AD process, reaching so-called shock-levels
(Steffen et al., 1998). Increased levels of VFA can occur as a consequence of
rapid degradation of large amounts of organic macromolecular matter
(lipids, carbohydrates or proteins). In 'healthy digesters', microbial
adaptation to increased concentrations of VFA occurs eventually. High
concentrations of end products such as free ammonia can also have
inhibitory effects through accumulation inside the digester. Residual
amounts of antibiotics, disinfectants, pesticides and toxic compounds like
heavy metals can be supplied with agricultural feedstock or organic wastes
as shown in Table 2.6, but their inhibitory effect is reported by Steffen et al.
(1998) to be minor. Their presence in digestate can nevertheless compromise
its amenability as crop fertilizer. Information on the evaluation of possibly
inhibitory feedstock can also be found in Chapter 3 of this topic.
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