Environmental Engineering Reference
In-Depth Information
optimisation of the process capacity can only be done by applying advanced
on-line monitoring; by identifying chemical and physical parameters, this
gives an early warning of process imbalances and ultimately leads to
improvements in plant efficiency. The monitoring tools must be robust,
technically as simple as possible and require little maintenance in order to be
of interest for the biogas sector.
Anaerobic digestion can be monitored in different process stages from
supply chain until final stage of produced biogas: substrate quality and its
biogas potential can be monitored by measurements of solids, TS (total
solids) and/or volatile solids (VS) or chemical oxygen demand (COD) and
their degradations during the process; intermediate products formation
(VFAs, H
2
, CO, NH
4
+
) and their influence on alkalinity and pH; and final
product formation by the overall biogas production or CH
4
/CO
2
rate.
Typically, for biological processes, microbial communities and activities are
also of interest (Boe, 2006).
Temperature and pH measurements are key quantifications and are
compulsorily monitored by all biogas plant operators to ensure a suitable
environment for the bacteria consortia. These are currently the most widely
measured real-time parameters at biogas plants, usually determined using
simple electronic pH meters and temperature sensors embedded in the
digester, but where to measure correctly to give a true picture of the process
is an interesting issue. Maintaining a constant system temperature is of
critical importance because of the strong influence of temperature on the
AD process from a kinetic and thermodynamic point of view (Kim et al.,
2002). Temperature fluctuations can adversely affect the digestion process,
especially for plants operating at thermopilic temperature ranges.
10.2.4 Monitoring the produced biogas
The most important measurement for biogas plant operators, from the
production point of view, is biogas volume. Biogas production measure-
ments can be expressed as overall biogas production or as methane yield,
since methane is the valuable product. These measurements only inform as
regards to the end-product (the produced biogas) without giving any
information about biological process such as imbalances occurring during
the previous biochemical stages. A low recorded biogas or methane yield
would only give an indication that the fermentation was disturbed. For
instance, a perm-selective gas sensor for determining the composition of
carbon dioxide/methane mixtures in the range 0-100% is described by Rego
and Mendes (2004). With a time response of less than 1.5minutes, such a
control will make it possible to optimise the operating conditions of the
methane recovery units, but not the fermentation itself. Measurement of the
exact gas composition might give some valuable information about the
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