162 Organic waste recycling: technology and management
4.3.5 Presence of toxic compounds
For anaerobic digestion of organic wastes such as human excreta, animal
manure and other agricultural residues, accumulation of volatile fatty acids, H 2
and undissociated ammonia are commonly associated with digester failure. The
presence of molecular oxygen is also inhibitory to the methanogenic bacteria.
A list of common inhibitors to anaerobic digestion is given in Table 4.5.
Inhibition caused by excess concentrations of certain ions can be
counterbalanced by some other ions (antagonistic ions), while it can also be
exacerbated by others (synergistic ions). Organic wastes containing those
inhibitors as listed in Table 4.5 should be pre-treated or diluted so that the
inhibitors' concentrations are below the inhibiting concentrations prior to being
fed to anaerobic digesters
4.3.6 Mixing
Mixing of digester slurry is important to provide better contact between the
anaerobic bacteria and the incoming organic wastes, so that biogas production is
enhanced. It reduces the settling of solids or accumulation of digested solids at
the bottom of the digester and helps to prevent/break-up scum formation at the
slurry surface. For small-scale digesters, mixing of the digester's slurry can be
accomplished manually as described in section 4.4. In large-scale digesters,
mixing can be done mechanically by stirring and recirculation of the gas and/or
the digested slurry.
4.4 Modes of operation and types of biogas digesters
There are different types of anaerobic digesters for experimental purposes, pilot
plant investigations and actual field use. Their design, materials, system
performance, price, etc. naturally vary a great deal. Operationally, it is required
that air is excluded from the content of the digester, and sufficient volume is
provided within the digester for the biological reactions to take place. Major
modes of digester operation can be classified into three groups as follows: