Targeted control of biogas plants with the help of FOS/TAC
PRACTICE REPORT LABORATORY ANALYSIS TITRATION FOS/TAC
A  biogas plant  functions most efficiently when substrates are added in amounts that are tailored to the  fermentation process . For this purpose, the exact status of the fermentation in the digester must be known and documented over a long period of time. This is achieved by regular, easily performed in-house laboratory analyses of the  FOS/TAC ratio . The operator is provided with exact information of the biodegradation performance of the digester and therefore of the  biogas production . Any interference with the process can be quickly identified and eliminated in a targeted manner. The plant is operated more efficiently and cost-effectively.
Authors: Ulrich Lossie
- Biogas consultant - DEULA Nienburg
Petra Pütz
- “Diplom-Ingenieur”, chemistry -  A pplications, laboratory products, HACH LANGE Biogas Plant Wambeln
Source: EnerCess
FOS/TAC: Reliable assessment of the fermentation process
Carrying out a FOS/TAC measurement
T ake a representative sample of the fermentation substrate.  R emove any coarse components from the sample. It is crucial that sample preparation (filter, tea strainer, kitchen strainer or centrifuge) should always be carried out in the same way.    W eigh out 20 ml substrate and fill up with distilled water if necessary.  P lace the sample on a magnetic
stirrer and homogenise it continuously during the titration process.  T itration with 0.1 N H 2SO 4 to pH 5; note the volume (ml) of acid added.    T itration with 0.1 N H 2SO 4 to pH 4.4; note the volume (ml) of acid added.  C alculate FOS/TAC using the empirical formula (see Fig. 2).To measure the FOS/TAC ratio with the TIM 840 titrator, 5 ml prepared sample are introduced into a titration beaker containing a follower bar. 50 ml distilled water are then added, the beaker is positioned under the
measuring electrode of the TIM and the measurement is started. The following work steps ( , , ), which are usually rather tricky in practice, are performed by the titrator. After about 5 minutes the (automatic) titration is complete and the results are displayed.
The TAC and FOS values are calculated directly using a pre-programmed formula.
All measured values can be stored in the autotitrator and/or sent to a printer or PC.
FOS/TAC_ANALYSIS
What is the FOS/TAC ratio?The Federal Agricultural Research Centre (Bundesforschungsanstalt für Landwirtschaft/FAL) developed the FOS/TAC analysis from a titration test (Nordmann-Methode) in order to determine the quotient of the acid concentration and the buffer capacity in the fermentation substrate. FOS  stands for F lüchtige O rganische S äuren, i.e. volatile organic acids, and is measured in mg HAceq/l, while TAC  stands for T otales A norganisches C arbonat, i.e. total inorganic carbonate (alkaline buffer capacity), and is measured in mg CaCO 3/l.
The FOS/TAC ratio has long been recognised as a guide value for
assessing fermentation processes. It enables process problems extending as far as the imminent inversion of the digester biology to be detected at an early stage, so that countermeasures can be initiated.
How is the FOS/TAC ratio determined?
Either by manual titration or, more easily and quickly, with a titrator, e.g. the
TIM 840 (see Fig. 1 and Fig. 4). Relative to manual titration, this is more accurate and saves several minutes per sample.
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Fig. 2: Calculation of the FOS/TAC ratio
Fig. 1: The TIM 840 titrator for FOS/TAC
determination at the Wambeln biogas plant.
Assessment and use of the FOS/TAC ratio
In practice, a FOS/TAC ratio of 0.3 to 0.4 is normal, although each plant has its own optimal ratio. This can only be determined by long-term observation and regular checks, as there is a
strong dependence on the substrate. For example, plants that make use of renewable raw materials usually require a FOS/TAC ratio of 0.4 to 0.6 for stable operation.
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“At the end of 2006 we started up our cofermentation plant. The fermentable material is a selective m
ixture of food residues, pig slurry and chicken droppings. We control the fermentation process by carrying out regular FOS/TAC measurements with the TIM 840 from HACH LANGE. Usually the ratio is between 0.3 and 0.4 in the digester and between 0.2 and 0.3 in the post-digester. We can recognise a threat of acidification immediately, because the ratio increases, and can take appropriate corrective action. In such a case we change the composition of the biomass input, e.g. by increasing the proportion of pig slurry or chicken droppings, which increases the buffer capacity (TAC). Thanks to this simple monitoring method, we need have no fear that the process will crash.”Dipl.-Ing. Christian Nölle Quality Manager
Wambelner Bioenergiegesellschaft mbH
Rules of thumb for the assessment of FOS/TAC ratios (empirical values provided by DEULA-Nienburg).The point at which the plant is operating most efficiently – i.e. when gas
production is at a maximum and there is no danger that the process will crash can only be determined by testing out a variety of options. Such a crash is extremely costly; several weeks without gas production and the huge amount of work (pumping out, emptying the
digester, etc.) required to make the plant operational again can endanger the profitability of a whole y
ear.
Fig. 3: FOS/TAC curve of the Wambeln biogas plant over a period of 3 months
Technical data
D O C 042.52.20011.M a r 08
polarised
TIM 840/845 TECHNICAL DATA
Literature -  P rof. Dr. Peter Weiland, Christa Rieger, Institut für Technologie und Biosystem-technik, Abt. Technologie, Bundes-forschungsanstalt für Landwirtschaft (FAL): …Prozessstörungen frühzeitig erkennen“, BIOGAS Journal 4/06 -  D r. Jürgen Wiese EnerCess GmbH, Ralf König HACH LANGE GmbH: …Prozessbegleitende Fermenterüber-wachung auf Biogasanlagen“, DVGW energie  wasser-praxis 09/2006-  H ACH LANGE Information …Rou-tine-Analytik für Biogas“, Art.Nr. DOC032.72.20007.APR07 -  K urzbedienungsanleitung: FOS/TAC-Messung mit dem HACH LANGE
TITRALAB
Table: Technical data, TIM 840/845
Fig. 4: HACH LANGE TIM 840 titrator for the determination of FOS/TAC, pH and redox potential
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