Integrated Environmental Biotechnology 241treatment has clear appeal. However, as was also pointed out in the same earlier
segment, it is not feasible to optimise conditions such that high levels of both
waste reduction and gas generation are deliverable. More commonly, in practice
only around a quarter of the potential biogas yield is actually achieved.
Using biogas
Although biogas from engineered AD processes share many similarities with
landfill gas (LFG), it is important to remember that it is of quite distinct quality,
being much cleaner and far less contaminated by traces of other gases. LFG may
contain a bewildering array of ‘others’, dependent on the exact nature of the waste
undergoing decomposition. The list includes the likes of 1,2-dichloroethene,
alkylbenzene, butylcyclohexane, carbon disulphide, propylcyclohexan, methane-
thiol, decane, dichlorobenzene, undecane, ethylbenzene, dodecane, trimethylben-
zene, tridecane, toluene, dimethyl disulphide, nonane and sulphur dioxide. Biogas,
by contrast, is relatively pure by comparison, since the bulk of the inorganic
matter and many potential pollutants are excluded from the bioreactor, either
by source or mechanical separation, as part of the waste preparation process.
This obviates the need for high temperature flaring, commonly used for LFG to
destroy residual pollutant gases, since they are highly hostile to the fabric of any
generation equipment intended to be used.
The main cause for concern in this respect is hydrogen sulphide (H 2 S), which is
a metabolic byproduct of sulphur-reducing bacteria. Unsurprisingly, the amount
present in the final gas produced depends largely on the relative abundance of
sulphur-containing compounds in the original biowaste. H 2 S is acidic and this
poses a major corrosion risk to gas handling and electrical generation equipment.
Scrubbing hydrogen sulphide out of biogas is possible, but in practice it is more
common to use a high-alkalinity lubricant oil which is changed often.
Biogas utilisation involves burning it, with some of the energy being trans-
formed to electrical. There are three basic types of engine which are suitable
generating motors for biogas uses, namely turbine, dual fuel and spark ignition.
For each there are numbers of different manufacturers worldwide. While, clearly,
it lies far outside of the scope of this book to discuss them, it is worth noting
that for any given application, the type of engine used will normally be decided
by a number of contextual issues. Hence, the quantity of the biogas produced, its
purity, the intended life of the plant, relevant pollution controls and other similar
site-specific considerations will need to be considered.
Generation processes are generally relatively inefficient thermodynamically,
and often much of the available energy is effectively lost as heat. However, the
nature of engineered AD processes are such that there is a ready built-in demand
for thermal energy to elevate and maintain the digester temperature. This may
account for between 20–50% of the total energy produced, dependent on system
specifics, in a typical temperate facility, with the remainder being available for