Volatile Fatty Acids monitoring for efficient and secured anaerobic digestion processes: POSTER
Chapman, J., Inizan, M., Pons, P and Myers, Hach, UK
(free)
Methane forming bacteria are sensitive to many process conditions, including temperature, pH, and the
presence of various toxins. Optimal performance occurs within a pH range of 6.8-7.2. If pH levels drop in
the digester, the methane-formers can be inhibited, halting the digestion process and biogas production
altogether. This is generally referred to as digester souring – an odor producing, time consuming, and
expensive failure of the system.Digester stability is greatly increased when high levels of alkalinity are present. Digester operators can
maintain a healthy balance between VFA (Volatile Fatty Acids) and alkalinity based on carefully monitored operational control of feed rates, mixing, and heating. The ratio FOS/TAC (VFA concentration divided by alkalinity available) is a key parameter to monitor anaerobic digestion units. Figure 1 illustrates the evaluation of FOS/TAC ratio according to empirical experience, with a suggestion to slow
down or stop feedstock addition when FOS/TAC ratio is going up 0.6.
While anaerobic digestion processes are well understood, digester upsets or inefficient energy recovery issues are still challenges commonly faced by operators.
CERES Germigny is a company located in France. Their digester that started up at the beginning of 2020(figure 2) is fed with agricultural materials : silage, hay and straw. Produced biomethane, generates on siteelectricity that is exported to the grid and is equivalent to the consumption of 3,000 pop. Equiv. A closemonitoring of digestion processes is performed to manage feedstock quantities and reactor settings withregular laboratory measurements (3 times per week) of feedstock organic content, pH, FOS/TAC ratio , ORPand temperature. pH, ORP and FOS/TAC measurements are performed thanks to a Hach titrator AT1000(figure 3) which allows a full set of results in just a few minutes.
