Biofuels production 163Table 4.5 Inhibitors of biomethanation (U.S. EPA 1979)
Parameters Inhibiting concentration (mg/L)
Volatile acids >2,000 (as acetic acid)a
Ammonia nitrogen 1,500 - 3,000 (at pH>7.6)
Sulfide (soluble)b >200;
3,000 toxic
Calcium 2,500 - 4,500;
8,000 strongly inhibitory
Magnesium 1,000 - 1,500;
3,000 strongly inhibitory
Potassium 2,500 - 4,500;
12,000 strongly inhibitory
Sodium 3,500 - 5,500;
8,000 strongly inhibitory
Copper 0.5 (soluble metal)
Cadmium 150c
Iron 1,710c
Chromium+6 3
Chromium+3 500
Nickeld 2
a Within the pH range of 6.6 to 7.4 and with adequate buffering capacity, volatile acids
concentrations of 6,000 to 8,000 mg/L may be tolerated.
bOff-gas concentration of 6% is toxic.
c Millimoles of metal per kg of dry solids.
d Nickel promotes methane formation at low concentrations. It is required by methanogens.
4.4.1 Modes of operation
Batch operation
In this mode of operation the digester is filled completely with organic matter
and seed inoculum, sealed, and the process of decomposition is allowed to
proceed for a long period of time until gas production decreases to a low rate
(duration of process varies based on regional variation of temperature, type of
substrate, etc.), it is unloaded, leaving 10-20% as seed, then reloaded and
operation continues. In this type of operation the gas production is expected to
be unsteady and the production rate varying from high to low, and digestion
failures due to shock load is not uncommon. This mode of operation, however,
is suitable for handling large quantities of organic matter at distant intervals. It
may need separate gas holders if a steady supply of gas is desired.