Analysis and characterisation of biogas feedstocks - The Biogas: Science, Production and Applications - page 78

be inhibitory. In addition, S 2 is known to precipitate many metal ions
which can have a negative effect on the bioavailability of trace elements.
According to Bischofsberger et al. (2005), inhibition starts at about
30mgH 2 S/l, whereas Braun (1982) states that inhibition does not normally
occur below 100mgH 2 S/L and even 200mgH 2 S/l can be tolerated after
sufficient adaptation time. According to Chen et al. (2008), the range of
inhibitory thresholds for undissociated H 2 S in literature is even broader: 40
to 400mgH 2 S/l. The range Speece (1996) reports, where about 50%
inhibition occurred, is 60 to 240mgH 2 S/l. However, practical experience
showed that H 2 S can become problematic at much lower concentrations,
especially when coupled with other inhibitory components such as ammonia
(Chen et al., 2008) or low iron concentrations.

Light and heavy metal ions

In low to medium concentrations, many light metal ions show no inhibitory
effect on anaerobic digestion. According to Chen et al. (2008), the ions Na,
K, Mg, Ca and Al can become inhibitory at very high concentrations. For
Na, for example, the inhibitory concentration is 5.0 g/l although, after
adaptation, concentrations up to 15 g/l were tolerated (Speece, 1996).
With regard to heavy metals, the situation is similar to other biological
organisms - at low concentrations they can be essential for microbial
wellbeing and at higher concentrations they can be toxic. In practice, heavy
metals will be tolerated quite well due to the fact that they form poorly
soluble precipitants with sulphide and carbonate. The lowest thresholds of
reported negative effect are Cu (40mg/l), Cd (20mg/l), Zn (150mg/l), Ni
(10mg/l), Pb (340mg/l) and Cr (100mg/l), according to Bischofsberger et al.
(2005). Braun (1982) found the following concentrations to cause 20%
inhibition at pH8: Cd (157mg/l), Ni (73mg/l), Cu (113mg/l) and Zn
(116mg/l). Most light and heavy metals can be detected using standard trace
element analysis (see Section 3.4.1).

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Antibiotics and disinfectants

It is obvious that most detergents and chemicals that are designed to inhibit
or kill microbes will have a negative effect on anaerobic digestion.
Antibiotics can be present in manure or other animal residues, whereas
disinfectants can be part of wastewater from, for example, the food industry.
These substances should be avoided in biogas feedstocks, although small
amounts may be tolerated.