A modified version of a biosensor (DENICON) used to measure nitrates in wastewaters is presented which allows for the determination of VFA concentration as readily biodegradable COD (rbCOD). A sample of the digested effluent is mixed to denitrifying microorganisms in excess of nitrates. As a consequence, a fraction of NO3, proportional to the biodegradable organic carbon, is reduced and may be measured by titration.

The concentration of rbCOD is then derived from the consumption of nitrates as Biodegrable Nitrate Demand (BND). Taking into account that the prevailing fraction of rbCOD in digested anaerobic liquors is made of VFA, this paper investigates the possibility of measuring VFA concentration with the proposed biosensor. Extensive tests have been carried out on a laboratory instrument, using acetate, butyrate and propionate as model compounds.

A satisfactory correlation (R2 > 0.95) between organic carbon and reduced nitrate has been found for the three substrates. Mixtures of acids, especially acetic + propionic have also been tested because this association is the one often found when unstable conditions arise in a digester. It has been observed that the reaction rate increases with respect to acetic acid only when a solution of acetic + propionic acids is denitrified. This effect might allow for a qualitative evaluation of incoming instability.

The instrument can be easily automated and might allow a VFA determination every 20-40 minutes, which is sufficient for most anaerobic applications.

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