Analysis and characterisation of biogas feedstocks - The Biogas: Science, Production and Applications - page 59

changed significantly ( pH 7.5), it is preferable to have a

neutralisation step before feeding to the biogas plant. If slight acidification
occurs during anaerobic digestion, the pH can be increased artificially by
adding base (e.g. Ca(OH) 2 ,Na 2 CO 3 , NaOH) in the reactor (Bischofsberger
et al., 2005).

< pH 6.8, >

3.3.3 Total solids (TS) and dry matter (DM)

For the estimation of the water content of a feedstock, the TS or the DM are
determined. Both parameters represent the same and are described in units
of percent or grammes per litre. This analysis involves drying the sample to
constant weight in a drying chamber at 103-105 C (standards EN 12880 and

APHA 2540 B). A drawback of this determination is that volatile substances
(e.g. volatile acids, alcohols) are not determined, although they can
represent a considerable percentage of the energy in the feedstock. This
difference is especially noticeable in wastewater and industrial by-products
with high levels of volatiles.
If feedstocks have a very high TS content, the addition of fresh water or
other liquid feedstocks to the biogas plant will be necessary. Alternatively,
process liquids such as the digestate or the liquid fraction of digestate after
solid-liquid separation can be used to dilute the feedstocks. If feedstocks
with a very high water content are used (or a lot of fresh water is added to
the process), the volume of digestate will be high and, consequently, its
nutrient concentration will be low.

8

3.3.4 Volatile solids (VS) and organic dry matter (ODM)

In order to determine the amount of organic matter in a sample, the VS (%)
or ODM (g/l) are determined. In general, this determination is carried out
together with the TS/DM determination just described. The sample is dried
to constant weight in a drying chamber at 103-105

￿ ￿ ￿ ￿ ￿ ￿

C. Then the sample is
ignited to constant weight in a muffle furnace at 550

8

8 C. (The relevant

standards are EN 12879 and APHA 2540 E.)
Although this is a useful and simple analysis, it is important to take into
account that the amount of organic matter in a sample does not directly give
sufficient information on the anaerobic degradability of the feedstock. In
addition, as mentioned above, VS/ODM determination can be inaccurate.
Substances that are volatile at 103 8 C leave the sample during the first drying

step.