Have you measurements on pH, temperature, how long has this AD been running since startup, the amount of feedstock and digestate generated?

Total Volatile Fatty Acids (measured as acetic acid equivalents) 928 mg/L

- Total FOS (measured as acetic acid equivalents) 13,753 mg/L

- Total TAC (measured as Calcium Carbonate equivalents) 19,698 mg/L

- FOS/TAC = 0.7

Hi,

TAC values are ok , but could be increased, for example by adding sodium bicarbonate or other alkaline agents.

If you have an access to cow manure, this will help increase the buffer capacity of the mix.

Regarding Fos, lactate is a common acid that is produced under anaerobic conditions which has a low pKa and can cause high FOS.

Hi James,

I had a similar problem with a CSTR: low VFA measurements (using HPLC) but high FOS values form the FOS/TAC determination. I found in the publication of Purser et al. (2014) ( https://doi.org/10.1016/j.watres.2014.05.020 ) that FOS is highly overestimated in relation with measured total VFA content (fig 2 on the article). It was suggested that carbonate content may produce an interference during titration of acetate (fig 3 on the article). A correction of the determination of FOS, from the Nordmann method, is then needed for a more accurate estimation of VFA content.

I hope this information may give you a clue to solve your question.

Best Regards,

Henry Fisgativa

It can be related to the high humic and fulvic acids present in the feedstock. Titration methods also measuring those, as far as I concern. You should be aware that FOS/TAC is only an indicator and shouldn't be used as absolute measurements. However, changing trends can be used as a stability monitoring.