Fundamental science and engineering of the anaerobic digestion process for biogas production - The Biogas: Science, Production and Applications - page 109

mesophilic temperatures will have higher buffering capacities if hydroxide
ions are available than if treated at thermophilic temperatures.
Ammonia nitrogen concentrations in the range of 1500-3000mg/l at pH
levels above 7.4 are said to be toxic, while levels above 3000mg/l are
supposed to be toxic at all pH levels (Van Velsen and Lettinga, 1979). It has
been found, however, that anaerobic processes have operated efficiently
with higher levels of ammonia nitrogen (Sugrue et al., 1992: Van Velsen and
Lettinga, 1979); this is a reflection of the ability of the methanogenic
population to acclimatise to toxins. Nizami et al. (2011b) found ammonia
nitrogen levels of 2400mg/l in a wet digestion process treating grass silage.
Marin-Perez et al. (2009) state that the inhibition level for grass digestion is
4700mgN/l. A slaughterhouse waste digester in Austria (International
Energy Agency (IEA), undated) found ammonia nitrogen levels varied
between 4500 and 7500mg/l depending on substrate composition. At higher
ammonia levels, gas production decreased. Undissociated ammonia (NH 3 )
is believed to be the toxic form of ammonia nitrogen and its production is
temperature dependent. At lower temperatures, less NH 3 is produced. The
slaughterhouse waste digester in Austria maintained the temperature of the
digester at or below 35 8 C to minimise production of NH 3 (IEA, undated).

5.3.3 Nutrients, trace elements and inhibition

An average microorganism cell contains nitrogen, phosphorous and sulphur
in quantities of 12, 2 and 1% of dry bacteria biomass respectively.
Anaerobic metabolism is such that their growth rate is approximately 10%
of that of aerobes. Thus, for wastewater treatment, nitrogen (N) and
phosphorous (P) are required in the ratio of 700:5:1 (biological oxygen
demand:N:P) as compared with aerobic metabolism, which requires the
ratio of 100:5:1 (Wheatley, 1990).
Sulphur, potassium, calcium, magnesium, chloride and sulphate ions are
also required for proper digestion. Trace amounts of iron, copper, zinc,
manganese, molybdenum and vanadium are also required for cell growth
(WPCF, 1987). Sulphides, which are required by many methanogens as a
sulphur

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source for growth, are inhibitory to methanogens in high

concentrations (Zeikus, 1979).
Sulphur compounds tend to be particularly problematic as they cause
precipitation of essential trace nutrients such as iron, nickel, copper and
molybdenum, which are insoluble at low redox potentials. SRB have lower
doubling times than methanogens and utilise similar substrates and hence
divert some of the anaerobic process to H 2 S production. H 2 S interferes with
intracellular microbial metabolism by binding with essential enzymes and
co-enzymes (Wheatley, 1990).
Heavy metal ions such as Cu þþ and Zn þþ , alkaline and earth alkaline