Volatile fatty acids (VFA) in a biogas plant: profile, thresholds and the significance of propionate

VFA, that is volatile fatty acids (acetic, propionic, butyric, valeric and others), are an intermediate product of the degradation of organic matter. In a healthy process they are consumed almost immediately, so their concentration is low; the accumulation of VFA means that the rate of acid formation exceeds the ability of methanogens to consume them. The best indicator is not the sum but the profile: the share of acetate, propionate and higher acids and the direction of change over time.

What we measure and how

The standard is chromatographic determination (GC with an FID detector or HPLC) in the filtrate of a slurry sample, after acidification and centrifuging. The result is given in mg/l or g/l for individual acids. FOS/TAC titration gives a sum expressed in acetic acid equivalents, but does not distinguish between acids. Samples must be protected from further degradation: rapid cooling, centrifuging, freezing if analysis does not take place the same day.

The role of individual acids

Indicative values

In the literature and in operators' practice it is assumed that stable agricultural plants have total VFA on the order of below approx. 1–3 g/l (depending on the substrate), with acetate dominant. The following are regarded as warning signals:

• propionate above approx. 1 g/l (in some sources 0.8–1.5 g/l),
• a propionate/acetate ratio above approx. 1–1.4 (the threshold of Hill et al. is often cited, but should not be applied mechanically),
• a sharp rise in the VFA sum by tens of per cent within a day.

Plants operating stably at higher concentrations (e.g. with high ammonia and SAO) show that the absolute value depends on adaptation, and what counts is the deviation from the plant's own norm.

Why the trend matters more than the number

Example: a plant has a constant acetate level of approx. 0.4 g/l and propionate of 0.1 g/l. After the silage feed is increased, within four days acetate rises to 1.1 g/l and propionate to 0.5 g/l. Neither of these values exceeds a "textbook" threshold, yet the profile has changed markedly. A reaction (reducing the feed) at this point prevents the acidification that would appear after the following days.

What causes a rise in VFA

1. Too abrupt an increase in the feed rate or a change of mix to a more easily degradable one.
2. A drop in temperature, a failure of heating or mixing.
3. Inhibitors: ammonia, H₂S, antibiotics, disinfectants, oxygen.
4. A deficiency of trace elements.
5. A shortened HRT or washout of biomass.

Response

Good practices

• Determine VFA regularly (e.g. once a week, more often during start-up and mix changes).
• Record results in one table together with pH, FOS/TAC, feed rate and gas production.
• Ensure a consistent methodology, including sample preparation.
• Use the profile as evidence when assessing the cause, e.g. of a fall in gas production.

VFA is an indicator suited to the problem: it makes it possible to distinguish overload from inhibition, and also to answer whether you only need to slow down feeding, or change the mix or supplement additives.

How to convert results

Chromatography results are given in mg/l of individual acids. For comparison with titration the sum is converted into acetic acid equivalent, taking molar masses into account (acetic 60, propionic 74, butyric 88 g/mol). Example: 800 mg/l of acetate and 300 mg/l of propionate, converted to acetic equivalent, is 800 + 300 × 60/74 ≈ 1,043 mg/l. Values converted in this way can be compared with the FOS result from titration to check the agreement of the methods; a discrepancy of several dozen per cent is normal, because titration also covers other compounds of an acidic nature.

Frequency of measurements and risk

In a stable plant once a week or every two weeks is enough. During start-up, after a change of mix, when introducing new substrates or in a crisis it is worth measuring 2–3 times a week, and in the event of abrupt changes — daily. The cost of analyses is small compared with the losses from a few days of disturbed gas production, which is why many plants have contracts with laboratories for periodic determinations of VFA, TAN, pH and FOS/TAC, and collect the results in one database together with substrate feed rates.