Environmental Engineering Reference
In-Depth Information
Carbon, nitrogen, phosphorous and sulphur must be present in the mixture
in optimal proportions; trace elements must also be present in adequate
amounts, above certain threshold concentrations as well as below toxic
levels, for the microbial processes to perform satisfactorily (Angelidaki,
2002).
The anaerobic digestion (AD) process has to become a much better
controlled process in the near future in order for biogas and biomethane to
gain a significant market share on the global energy scene. Even though AD
involves very complex and heterogeneous substrates, process analytical
technology (PAT) tools show promising prospective possibilities of being
able to serve as robust on-line monitoring and management tools. The
monitoring must be performed in real time and must be essentially non-
invasive in order to provide the needed insight into the process while at the
same time it must be effective and practical, especially in small- to medium-
scale plants. The AD process and biogas production are increasing in
importance. The AD sector is able to provide major biotechnological
solutions for European energy demands. Biogas production capacities in
Europe reached eight million tons of oil equivalent (toe) in 2009
(EurObserv'ER, 2010). The target for biogas is one third of all bioenergy
consumption in the EU, 15 million toe in 2015 (Holm-Nielsen and
Oleskowicz-Popiel, 2007). Biogas production has the potential to be one
of the most flexible and adjustable energy sources and at the same time takes
care of very problematic organic wastes and substrates. If organic waste is
used incorrectly it has major negative impacts on the water environment,
human and animal health, and air quality.
10.2 Process analysis and monitoring
10.2.1 Basic process analysis
Biological substrates - the biomass feedstock for biogas production - are
very heterogeneous by nature, and the AD process is a highly complex
multi-phased process. Process control has been developed and introduced in
the AD process in recent decades and the analysis tools are becoming
increasingly user-friendly over time. AD consists of several biochemical
steps catalysed by a mixture of microorganisms; thousands of different
species of bacteria are involved. The main steps are hydrolysis, acidogenesis,
acetogenesis and methanogenesis. The various steps of AD are highly
complex, but take place simultaneously when handled with good manage-
ment at the biogas plants. The limiting step can be hydrolysis if there is a lot
of material that is difficult to digest, solid organic and particulate material
like maize silage. On the other hand, when uncomplicated fermentable
feedstock is available, the accumulation of acids can occur and methano-
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