Environmental Engineering Reference
In-Depth Information
3.4.4 Total organic carbon (TOC)
The total carbon in a sample is made up of total inorganic carbon (TIC)
(such as dissolved carbon dioxide, carbonate and bicarbonate) and total
organic carbon (TOC) - organic matter that can either be dissolved or
particulate matter. Consequently, TOC can either be determined by itself or
by subtracting TIC from the total carbon. Standard procedures for TOC
measurements can be found in EN 1484 or APHA 5310. For biogas
feedstocks, sample preparation is important. Solid or practically solid
substances are normally dried before TOC analysis. Lightly contaminated
watery samples (without particulate matter) can be injected directly into a
TOC analyser suitable for liquid samples. However, most biogas feedstocks
that are not solid contain many particulates. Therefore, solid-liquid
separation and separate analysis of the two phases can make sense. In
practice, the drying of these liquid or semi-liquid samples will often be
preferred.
For TOC analysis, the sample is acidified or purged with CO
2
-free gas for
TIC removal. The next step is oxidation of the TOC. The oxidation of
medium- and high-TOC samples is often done by combustion; in liquid
samples, chemical oxidation (often supported by UV photo-oxidation) is
also applied. The produced CO
2
is quantified by infrared sensors or thermal
conductivity sensors.
Although TOC gives exact information on the amount of organic carbon
in a feedstock sample, it does not give any information on its biodegrad-
ability. Lignocellulose, for example, will be determined as TOC, yet it is not
anaerobically degradable. In addition, the TOC does not give any
information on the oxidation state of the carbon in the feedstock; a COD
analysis provides better information in this regard. To sum up, although a
TOC analysis of a biogas feedstock sample can give good information on
carbon content, in practice a COD analysis is often preferred.
3.4.5 Continuous fermentation tests
The performance of a biogas feedstock under practical conditions can be
influenced by various factors (Braun, 2007) - inadequate substrate
concentration and nutrient composition, inhibiting substrate components,
metabolite formation and inhibiting process operational conditions.
Continuous fermentation tests give reliable information on the long-term
performance of a substrate in a biogas reactor. However, these continuous
tests are quite expensive due to their duration and the high amount of
analysis involved.
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